Methods and apparatus for preventing atrial arrhythmias by overdrive pacing and prolonging atrial refractoriness using an implantable cardiac stimulation device

[0002] The invention generally relates to implantable cardiac stimulation devices such as pacemakers, and in particular, to techniques for overdrive pacing heart tissue to prevent or terminate dysrhythmia.

[0003] A dysrhythmia is an abnormal heart beat pattern. One example of a dysrhythmia is a bradycardia wherein the heart beats at an abnormally slow rate or where significant pauses occur between consecutive beats. Other examples of dysrhythmias include tachyarrhythmias where the heart beats at an abnormally fast rate, e.g., atrial tachycardia where the atria of the heart beat abnormally fast.

[0004] One technique for preventing or terminating dysrhythmias is to overdrive pace the heart where an implantable cardiac stimulation device, such as a pacemaker or implantable cardioverter defibrillator (ICD), applies electrical pacing pulses to the heart at a rate somewhat faster than the intrinsic heart rate of the patient. For bradycardia, the implantable cardiac stimulation device may be programmed to artificially pace the heart at a rate of 60 to 80 pulses per minute (ppm) to thereby prevent the heart from beating too slow and to eliminate any long pauses between heart beats. To prevent tachyarrhythmias from occurring, the implantable cardiac stimulation device artificially paces the heart at a rate of at least five to ten pulses per minute faster than the intrinsic tachyarrhythmia heart rate of the patient. In other words, a slight artificial tachycardia is induced and maintained in an effort to prevent an actual tachycardia from arising.

[0005] It is believed that overdrive pacing is effective in at least some patients for preventing or terminating the onset of an actual tachycardia for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, additional portions of the heart also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse, has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods which, in turn, can trigger a tachycardia. By overdrive pacing the heart at a uniform rate, the likelihood of the occurrence of ectopic pulses is reduced and the refractory periods within the heart tissue are rendered uniform and periodic. Thus, the dispersion of refractory periods is reduced and tachycardias triggered thereby are substantially avoided.

[0006] Thus, it is desirable with patients prone to tachyarrhythmias to ensure that most beats of the heart are paced beats, as any unpaced beats may be ectopic beats. A high percentage of paced beats can be achieved simply by establishing a high overdrive pacing rate. However, a high overdrive pacing rate has disadvantages as well. For example, a high overdrive pacing rate may be unpleasant to the patient, particularly if the artificially-induced heart rate is relatively high in comparison with the heart rate that would otherwise normally occur. Also, a high overdrive pacing rate may be a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia. Also, the need to apply overdrive pacing pulses operates to deplete the implantable cardiac stimulation device's power supply, perhaps requiring frequent surgical replacement of the power supply. Typically, the power supply is located within the implantable cardiac stimulation device and thus this requires surgical replacement of the implantable cardiac stimulation device.

[0007] A high overdrive pacing rate may be especially problematic in patients suffering from heart failure, particularly if the heart failure is due to an impaired diastolic function. A high overdrive pacing rate may actually exacerbate heart failure in these patients. Also, a high overdrive pacing rate may be a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia. Also, the need to apply overdrive pacing pulses operates to deplete the implantable cardiac stimulation device's power supply, perhaps requiring frequent surgical replacement of the power supply. Typically, the power supply is located within the implantable cardiac stimulation device and thus this requires surgical replacement of the implantable cardiac stimulation device.

[0008] Problems associated with overdrive pacing are particularly severe for certain aggressive overdrive techniques which trigger an increase in the pacing rate based upon detection of a single intrinsic heart beat. With such techniques, a significant increase in the pacing rate is triggered by detection of a single intrinsic heart beat so as to promptly respond to the occurrence of a high rate tachycardia, such as an SVT. As a result, even in circumstances where a high rate tachycardia has not occurred, the detection of a single intrinsic heart beat can cause a significant increase in the overdrive pacing rate, which may be reduced only gradually. If a second intrinsic heart beat is detected before the overdrive pacing rate has been gradually lowered to a standard overdrive pacing rate, a still further increase in the pacing rate occurs. As can be appreciated, the foregoing can cause the overdrive pacing rate to increase significantly, perhaps to 150 ppm or more, even though a high rate tachycardia has not occurred. The aforedescribed problems are addressed by a copending, commonly-assigned patent application to Florio et al., entitled “Methods and Apparatus for Overdrive Pacing Heart Tissue using an Implantable Cardiac Stimulation Device,” U.S. patent application Ser. No. 09/471,788, filed Dec. 23, 1999, the contents of which are incorporated herein by reference in their entirety.

[0009] The prolongation of atrial refractoriness is also known to reduce the likelihood of atrial arrhythmias in some patients. Certain drugs, such as amiodarone or beta-blockers, like sotalol, help to maintain sinus rhythm by prolonging atrial refractoriness and thus act to reduce atrial arrhythmias in some patients. Other drugs help slow the ventricular rate once atrial fibrillation has occurred. These drugs reduce ventricular rate by AV nodal inhibition. These include verapamil, diltiazem, beta-blockers, and/or digoxin. However, drugs can cause side effects and many patients are resistant to drug therapy.

[0010] It would therefore be desirable to have some means other than the ingestion of drugs available to this patient population for prolonging atrial refractoriness. To that end, preemptive electrical stimulation of the atrium is known to prevent atrial arrhythmias in some patients. The use of such electrical stimulation is described in a copending, commonly-assigned patent application to Bornzin et al., entitled “Implantable Cardiac Stimulation Device and Method for Prolonging Atrial Refractoriness,” U.S. patent application Ser. No. 09/488,284, filed Jan. 20, 2000, the contents of which are incorporated herein by reference in their entirety. The Bornzin application describes an approach that uses an implantable cardiac stimulation device capable of pacing the heart of a patient while pacing the atria in an improved manner which assists in prolonging atrial refractoriness and treating atrial arrhythmias. Furthermore, this approach provides therapeutic benefit to patients with hypertension, heart failure, acute myocardial infarction, etc.

[0011] Another alternative approach to reduce atrial arrhythmias is described in U.S. Pat. No. 5,403,356 to Hill et al. (the Hill patent). The Hill patent describes placing at least two electrodes in the atrium, preferably in the triangle of Koch and/or an area of prolonged effective refractory period elsewhere in the atrium. Pacing pulses are then applied to the multiple electrodes either simultaneously or separated by a short delay.

[0012] While each approach offers benefits in avoiding incidences of arrhythmia, e.g., atrial arrhythmia, it is believed that treatment can be further improved by combining selected aspects of these approaches to achieve an improved result. Accordingly, the present invention is directed to that end.

[0013] The present invention is directed toward a method and apparatus for reducing the incidence of atrial arrhythmias by using an overdrive algorithm to determine the application of overdrive stimulation pulses to a patient's heart, e.g., in the atria. In a first aspect of the invention, the apparatus first determines an overdrive pacing rate and then applies pairs of temporally spaced (staggered), i.e., primary and secondary, pacing pulses at the determined overdrive pacing rate. In a further aspect of the invention, the pairs of pacing pulses are applied at the overdrive pacing rate to multiple spatially spaced electrodes, i.e., electrodes distributed among multiple sites in a patient's heart, e.g., in the atria. In accordance with a first preferred embodiment, the electrodes may be distributed within a single atrium, e.g., the right atrium, of the patient's heart. Alternatively, a first electrode may be placed in the right atrium and a second electrode may be placed in the coronary sinus or the left atrium or multiple electrodes may be placed proximate to the left atrium.

[0014] In accordance with a further aspect of invention, a method is provided for overdrive pacing a patient's heart using an implantable cardiac stimulation device wherein an increase in an overdrive pacing rate is performed only in response to detection of at least two intrinsic beats within a predetermined search period. Initially, an overdrive pacing rate is determined and the heart is paced at the overdrive pacing rate. Intrinsic heart beats arising during overdrive pacing are detected. If at least two intrinsic heart beats are detected within a first predetermined search period, then the overdrive pacing rate is increased by a predetermined rate increment. If at least two intrinsic heart beats are not detected within a second predetermined search period, the overdrive pacing rate is decreased by a predetermined amount. By increasing the overdrive pacing rate only in response to the detection of at least two intrinsic heart beats within the first predetermined search period, an excessively high heart rate is avoided as might otherwise occur if an overdrive pacing rate increase were based upon detection of only a single intrinsic heart beat.

[0015] In an exemplary embodiment, the first predetermined search period extends for X number of cardiac cycles following detection of a first intrinsic beat wherein X is in the range of eight to forty cycles. The second predetermined search period Z is also within the range of eight to forty cardiac cycles. If, after detection of a first intrinsic heart beat, a second heart beat is detected within X cardiac cycles, then the overdrive pacing rate is increased by Y ppm wherein Y is five, ten, fifteen, twenty or twenty-five. If Z cardiac cycles occur without a rate increase, then the rate is decreased by an amount W ppm per cardiac cycle wherein W is one, two, three, four or five.

[0016] In a second exemplary embodiment, the first predetermined search period is N consecutive cardiac cycles, wherein N is, for example, ten. Thus, if there are at least two intrinsic heart beats within a set of N consecutive cardiac cycles, the overdrive pacing rate is increased. Otherwise, the overdrive pacing rate is decreased. With the second embodiment, it is easy to program a minimum percentage of paced beats. For example, to attain at least a minimum of 90% paced beats, N is set at ten. If fewer than 90% of the beats are paced beats (i.e., at least two beats out of every ten beats are intrinsic beats), the overdrive pacing rate is increased; otherwise it is decreased. This provides a feedback loop which maintains the pacing rate at a rate sufficient to provide about 90% paced beats on the average.

[0017] In still other embodiments, the implantable cardiac stimulation device may periodically suspend overdrive pacing to permit detection of three consecutive intrinsic heart beats. The intrinsic heart rate is calculated based upon those three heart beats and overdrive pacing resumes at a rate corresponding to the intrinsic heart rate. In another embodiment, the implantable cardiac stimulation device periodically determines the intrinsic atrial rate and compares the atrial rate with the current overdrive pacing rate. If the difference between the atrial rate and the overdrive pacing rate exceeds a predetermined threshold amount, the implantable cardiac stimulation device adjusts the overdrive pacing rate to equal the atrial rate. Otherwise, the implantable cardiac stimulation device continues to pace at the current overdrive pacing rate.

[0018] In accordance with a second aspect of the invention, a method is provided for adaptively varying overdrive pacing characteristics so as to achieve a predetermined degree of pacing. Overdrive pacing pulses are applied to the heart in accordance with programmed values specifying overdrive pacing characteristics. An actual degree of pacing resulting from the overdrive pacing pulses is determined. The programmed values are then varied based upon the degree of pacing resulting from the overdrive pacing pulses.

[0019] In an exemplary embodiment of the second aspect of the invention, overdrive pacing is performed by periodically determining an intrinsic atrial rate, then pacing the heart at a rate equal to the intrinsic rate plus an additional overdrive pacing margin. The overdrive pacing margin is thereafter selectively increased or decreased so as to maintain the actual degree of pacing at about 95% paced beats. To this end, the overdrive pacing margin, which may initially be set to five beats per minute above the intrinsic heart rate, may be incrementally increased or decreased to maintain the percentage of paced beats at the target rate of about 95%.

[0020] In a second exemplary embodiment, overdrive pacing is performed in accordance with a dynamic atrial overdrive technique which operates to periodically increase a pacing cycle length (i.e., to decrease the pacing rate) to permit detection of intrinsic paced beats. The pacing cycle length is automatically extended every N_MAXcardiac cycles by a predetermined amount. Initially, N_MAXmay be, for example, ten cycles. In accordance with the invention, the value for N_MAXis periodically increased or decreased in accordance with the actual degree of pacing so as to maintain the actual degree of pacing at about 95% paced beats. Hence, possible disadvantages associated with increasing the overdrive pacing rate in response to detection of only a single intrinsic beat are substantially avoided and the average overdrive pacing rate is kept reasonably low. Other programmable values defining the dynamic atrial overdrive algorithm may also be adaptively varied in accordance with the actual degree of pacing.

[0021] Hence, with the second aspect of the invention, overdrive pacing characteristics are adaptively varied so as to reduce the average overdrive pacing rate while still maintaining, on the average, the target percentage of paced beats. The degree of risk or discomfort to the patient resulting from overdrive pacing is thereby minimized and the longevity of the power supply of the implantable cardiac stimulation device is increased while still achieving a sufficiently high percentage of paced beats to reduce the risk that a naturally occurring tachyarrhythmia will occur within the patient.

[0022] Once the overdrive pacing rate has been determined, the pacing pulses are applied as pairs of temporally spaced pacing pulses, i.e., primary and secondary pacing pulses, where the primary pacing pulse is applied according to the overdrive pacing algorithm and the secondary pacing pulse, delayed in time from the primary pacing pulse, is additionally applied to extend the refractoriness of the cardiac tissue and thus further limit the potential for a tachycardia to occur.

[0023] The invention therefore provides an implantable cardiac stimulation device, e.g., a pacemaker or ICD, including generating means for delivering pacing pulses to an atrium of a heart and control means coupled to the generating means for causing the generating means to deliver a primary pacing pulse to the atrium at the overdrive pacing rate and causing the generating means to deliver a secondary pacing pulse to the atrium a delay time after the delivery of the primary pacing pulse.

[0024] In accordance with further aspects of the present invention, the implantable cardiac stimulation device may further include a detector that detects atrial activations of the heart, wherein the generator control inhibits the generator from providing the primary pacing pulse when an atrial activation is detected during an escape interval and wherein the generator control further causes the generator to deliver the secondary pacing pulse to the atrium the delay time after an atrial activation is detected during the escape interval.

[0025] The present invention also discloses a means to automatically establish the delay timing for delivery of the secondary pacing pulse. The advantage of automated adjustment of the delivery of the secondary pacing pulse is that there is no necessity to manually adjust the interval. Additionally, if there are physiologic changes to the patient's heart that would necessitate adjustment of the delay, it will be automatically performed by the device, thus automatically optimizing therapy.

[0026] In a significant additional aspect of the present invention, the aforedescribed overdrive algorithm used in combination with the pacing pulse pairs may be performed via multiple sites in a patient's heart. In a first embodiment, two or more electrodes are distributed in the atria, e.g., the right atrium of the patient's heart, and a combined signal received from the electrodes is used as input to the aforedescribed overdrive algorithm. The algorithm then triggers a single primary/secondary pulse generator which simultaneously drives multiple electrodes for simultaneously stimulating multiple sites in the patient's heart. Alternatively, multiple primary/secondary pulse generators can be used to individually drive the multiple electrodes, thus enabling a programmed time delay between stimulation of each of the sites in the patient's heart. Additionally, cardiac tissue depolarizations can be individually sensed from each of the electrodes and the sensed signal indicating the highest heart rate can be used as input to the overdrive algorithm. Again, pacing pulses can be applied either simultaneously or staggered with a programmed time delay to multiple sites in the patient's heart.

[0027] Apparatus embodiments of the invention are also provided. Other aspects, features, and advantages of the invention will be apparent from the detailed description which follows in the combination with the attached drawings.

[0028] FIG. 1A is a schematic diagram illustrating an implantable cardiac stimulation device, a pacemaker/defibrillator, internally configured in accordance with the invention, and connected to a patient's heart.

[0029] FIG. 1B is a schematic diagram of an alternative embodiment of a portion of the implantable cardiac stimulation device of FIG. 1A showing the use of multiple pulse generators for individually driving each of the implanted electrodes.

[0030] FIG. 1C is a schematic diagram of an alternative embodiment of a portion of the implantable cardiac stimulation device of FIG. 1A showing the use of a pulse delay circuit in conjunction with a pulse generator for individually driving each of the implanted electrodes.

[0031] FIG. 1D is a schematic diagram of an alternative embodiment of a portion of the implantable cardiac stimulation device of FIG. 1A showing the use of multiple sense amplifiers for individually sensing depolarizations from the multiple electrodes.

[0032] FIG. 2 is a timing diagram illustrating paced beats and unpaced beats within the heart of FIG. 1A.

[0033] FIG. 3 is a flow chart illustrating an overdrive pacing method wherein an overdrive pacing rate is increased only if at least two intrinsic events are detected within X cardiac cycles of one another.

[0034] FIG. 4 is a flow chart illustrating an overdrive pacing method wherein an overdrive pacing rate is increased only if at least two intrinsic heart beats are detected within a block of N consecutive cardiac cycles.

[0035] FIG. 5 is a flow chart illustrating a method for adaptively varying programmable values defining overdrive pacing characteristics so as to maintain a target degree of pacing.

[0036] FIG. 6 is a flow chart of the method of FIG. 5 configured for adaptively varying an overdrive pacing rate.

[0037] FIG. 7 is a flow chart illustrating a method for adaptively modifying the automatic pacing cycle length adjustment to maintain a target degree of pacing.

[0038] FIGS. 8 and 9 are cardiac electrograms that illustrate the effects of adjusting the delay between the primary and secondary atrial pacing pulses.

[0039] FIG. 10 is a simplified flowchart of an exemplary implementation of the method of the present invention.

[0040] FIG. 11 is a simplified flowchart of an exemplary process for automatically adjusting the atrial delay time between a primary atrial event and delivery of the secondary atrial pacing pulse if a ventricular depolarization occurs in response to the secondary atrial pacing pulse.

[0041] The present invention is directed toward a method and apparatus for reducing the incidence of atrial arrhythmias by using an overdrive algorithm to determine the application of overdrive stimulation pulses to a patient's heart, e.g., in the atria. In a first aspect of the invention, the apparatus first determines an overdrive pacing rate and then applies pairs of temporally spaced (staggered), i.e., primary and secondary, pacing pulses at the determined overdrive pacing rate. In a further aspect of the invention, the pairs of pacing pulses are applied at the overdrive pacing rate to multiple spatially spaced electrodes, i.e., electrodes distributed among multiple sites in a patient's heart, e.g., in the atria. In accordance with a first preferred embodiment, the electrodes may be distributed within a single atrium, e.g., the right atrium, of the patient's heart. Alternatively, a first electrode may be placed in the right atrium and a second electrode may be placed in the coronary sinus or the left atrium or multiple electrodes may be placed proximate to the left atrium. Furthermore, the invention relates to techniques for controlling overdrive pacing to multiple sites in a patient's heart so as to achieve and maintain a target degree of pacing and thus suppress tachycardias. The techniques will first be described with reference to FIGS. 1-4 wherein an overdrive pacing rate is increased only in response to detection of at least two intrinsic heart beats within some predetermined number of cardiac cycles. Then, techniques of the invention will be described with reference to FIGS. 5-7 wherein programmable values specifying overdrive pacing characteristics are adaptively varied so as to maintain a target percentage of paced beats. Next, the techniques associated with the delivery of temporally spaced pacing pulse pairs to extend the refractoriness of the cardiac tissue will be described in reference to FIGS. 8-11. Finally, the use of these techniques in a multiple site environment will be discussed.

[0042] In FIG. 1A, a simplified block diagram is shown of a dual-chamber implantable cardiac stimulation device 10 which is capable of treating both fast and slow arrhythmias with stimulation therapy, including cardioversion, defibrillation, and pacing stimulation. While a dual-chamber device is shown, this is for illustration purposes only, one of skill in the art could readily eliminate or disable the appropriate circuitry to provide a single-chamber stimulation device capable of treating one chamber with cardioversion, defibrillation and pacing stimulation or appropriate circuitry may be added to provide a three or four chamber stimulation device.

[0043] To provide atrial chamber pacing stimulation and sensing, the implantable cardiac stimulation device 10 is shown in electrical communication with a patient's heart 12 by way of implantable atrial leads 20aand 20b, respectively having atrial tip electrodes 22aand 22band atrial ring electrodes 24aand 24b. The electrode pairs 22a, 24aand 22b, 24bare preferably distributed among two sites in the right atrium for dual site pacing or one pair is positioned in the right atrium, e.g., in the patient's atrial appendage, and the second pair is positioned in the coronary sinus or left atrium for bi-atrial pacing. For example, a lead designed for placement in the coronary sinus region could be implanted to deliver left atrial pacing. For a complete description of a coronary sinus lead, see U.S. patent application Ser. No. 09/196,898, “A Self-Anchoring Coronary Sinus Lead” (Pianca et al.), and U.S. Pat. No. 5,466,254, “Coronary Sinus Lead with Atrial Sensing Capability” (Helland), which patents are hereby incorporated herein by reference. Alternatively, both electrode pairs may be located proximate to the left atrium, i.e., both electrode pairs may located in the left atrium, one electrode pair may be located in the left atrium with the second electrode pair located in the coronary sinus, etc. It is believed that by pacing, either simultaneously or staggered, to multiple (two or more), e.g., atrial, sites in the patient's heart, spontaneous depolarizations of ectopic foci can be suppressed and thus the potential for tachycardia, e.g., atrial tachycardia, can be minimized. Such treatment is described in copending, commonly assigned application entitled “Method and Apparatus for Preventing Atrial Arrhythmias by Overdrive Pacing Multiple Heart Tissue Sites Using an Implantable Cardiac Stimulation Device” (Park et al.), U.S. patent application Ser. No. 09/548,116, filed Apr. 11, 2000, the teachings of which are incorporated herein in their entirety. In contrast, the present invention is further directed toward the application of multiple, i.e., primary and secondary, temporally spaced pacing pulses to an atrial site, as described further below, at an overdrive pacing rate. It is believed that this combination further suppresses spontaneous depolarizations of ectopic foci beyond the contribution of the individual techniques. Furthermore, embodiments of the present invention may additionally include the application of these pulse pairs to multiple atrial sites, as described in the aforementioned Park et al. application, to even further limit the possibility of spontaneous depolarizations causing an arrhythmia.

[0044] The implantable cardiac stimulation device 10 is also shown in electrical communication with the patient's heart 12 by way of an implantable ventricular lead 30 having, in this embodiment, a ventricular tip electrode 32, a ventricular ring electrode 34, a right ventricular (RV) coil electrode 36, and an SVC coil electrode 38. Typically, the ventricular lead 30 is transvenously inserted into the heart 12 so as to place the RV coil electrode 36 in the right ventricular apex, and the SVC coil electrode 38 in the superior vena cava. Accordingly, the ventricular lead 30 is capable of receiving cardiac signals and delivering stimulation in the form of pacing and shock therapy to the right ventricle.

[0045] The housing 40 (shown schematically) for the implantable cardiac stimulation device 10 includes a connector (not shown) having atrial tip terminals 42aand 42band atrial ring terminals 44aand 44b, which are adapted for connection to the atrial tip electrodes 22aand 22band the atrial ring electrodes 24aand 24b, respectively. The housing 40 further includes a ventricular tip terminal 52, a ventricular ring terminal 54, a right ventricular (RV) shocking terminal 56, and an SVC shocking terminal 58, which are adapted for connection to the ventricular tip electrode 32, the ventricular ring electrode 34, the RV coil electrode 36, and the SVC coil electrode 38, respectively. The housing 40 (often referred to as the “can”, “case” or “case electrode”) may be programmably selected to act as the return electrode, or anode alone or in combination with one of the coil electrodes, 36 and 38. For convenience, the names of the electrodes are shown next to the terminals.

[0046] At the core of the implantable cardiac stimulation device 10 is a programmable microcontroller 60 or other processor which controls the various modes of stimulation therapy. As is well known in the art, the microcontroller 60 includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy and may further include RAM or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, the microcontroller 60 includes the ability to process or monitor input signals (data) as controlled by program code stored in a designated block of memory. The details of the design and operation of the microcontroller 60 are not critical to the present invention. Rather, any suitable microcontroller 60 may be used that carries out the functions described herein. The use of microprocessor-based control circuits for performing timing and data analysis functions is well known in the art.

[0047] As shown in FIG. 1A, an atrial pulse generator 70 and a ventricular pulse generator 72 generate pacing stimulation pulses for delivery to the patient's heart by the atrial leads 20aand 20band the ventricular lead 30, respectively, via a switch bank 74. The pulse generators, 70 and 72, are controlled by the microcontroller 60 via appropriate control signals, 76 and 78, respectively, to trigger or inhibit the stimulation pulses. The microcontroller 60 further includes timing circuitry that controls the stimulation device's timing of such stimulation pulses.

[0048] The switch bank 74 includes a plurality of switches for switchably connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, the switch bank 74, in response to a control signal 80 from the microcontroller 60, determines the polarity of the stimulation pulses (e.g., unipolar or bipolar) by selectively closing the appropriate combination of switches (not shown) as is known in the art.

[0049] An atrial (ATR) sense amplifier 82 and a ventricular (VTR) sense amplifier 84 are also coupled to the atrial and ventricular leads 20 and 30, respectively, through the switch bank 74 for detecting the presence of cardiac activity. The switch bank 74 determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches, as is also known in the art. In this way, the clinician may program the sensing polarity independent of the stimulation polarity.

[0050] Each sense amplifier, 82 and 84, preferably employs a low power, precision amplifier with programmable gain and/or automatic gain control, bandpass filtering, and a threshold detection circuit, known in the art, to selectively sense the cardiac signal of interest. The automatic gain control enables the implantable cardiac stimulation device 10 to deal effectively with the problem of sensing the low frequency, low amplitude signal characteristics of ventricular fibrillation.

[0051] The outputs of the atrial and ventricular sense amplifiers, 82 and 84, are connected to the microcontroller 60 which, in turn, inhibit the atrial and ventricular pulse generators, 70 and 72, respectively, in a demand fashion whenever cardiac activity is sensed in the respective chambers. The sense amplifiers, 82 and 84, in turn, receive control signals over signal lines, 86 and 88, from the microcontroller 60 for purposes of controlling the gain, threshold, polarization charge removal circuitry (not shown), and the timing of any blocking circuitry (not shown) coupled to the inputs of the sense amplifiers, 82 and 84, as is known in the art.

[0052] For arrhythmia detection, the present invention utilizes the atrial and ventricular sense amplifiers, 82 and 84, to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. As used herein “sensing” is reserved for the noting of an electrical depolarization, i.e., a P-wave or an R-wave, and “detection” is the processing of these sensed depolarization signals and noting the presence of an arrhythmia. The timing intervals between sensed events (e.g., the P—P and R—R intervals) are then classified by the microcontroller 60 by comparing them to a predefined rate zone limit (i.e., bradycardia, normal, low rate VT, high rate VT, and fibrillation rate zones) and various other characteristics (e.g., sudden onset, stability, physiologic sensors, and morphology, etc.) in order to determine the type of remedial therapy that is needed (e.g., bradycardia pacing, anti-tachycardia pacing, cardioversion shocks or defibrillation shocks, also known as “tiered therapy”).

[0053] Cardiac signals are also applied to the inputs of an analog to digital (A/D) data acquisition system 90. The data acquisition system 90 is configured to acquire intracardiac electrogram signals, convert the raw analog data into a digital signal, and store the digital signals for later processing and/or telemetric transmission to an external device 102. The data acquisition system 90 is coupled to the atrial and ventricular leads, 20a, 20b, and 30, through the switch bank 74 to sample cardiac signals across any pair of desired electrodes.

[0054] The microcontroller 60 is further coupled to a memory 94 by a suitable data/address bus 96, wherein the programmable operating parameters used by the microcontroller 60 are stored and modified, as required, in order to customize the operation of the implantable cardiac stimulation device 10 to suit the needs of a particular patient. Such operating parameters define, for example, pacing pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each shocking pulse to be delivered to the patient's heart 12 within each respective tier of therapy.

[0055] When executing the operating instructions stored in memory 94, the microcontroller 60 implements a number of functional stages in accordance with the present invention. Those stages include a generator control stage 182 which includes an atrial generator inhibiting stage 184, a ventricular generator inhibiting stage 186, a primary atrial pacing pulse stimulation control 188 (A_S1), a secondary atrial pacing pulse stimulation control 190 (A_S2), and a ventricular pacing pulse stimulation control 192 (V). The stages of microcontroller 60 further include an atrial delay timer 194 and an AV delay timer 196.

[0056] In accordance with a primary aspect of the present invention, whether the implantable cardiac stimulation device 10 is in a single or dual chamber pacing modality, the generator control stage 182, in response to the overdrive pacing algorithm described in detail below, causes the atrial pulse generator 70 to deliver a secondary atrial pacing pulse to the patient's heart a secondary pacing pulse delay time after causing the atrial pulse generator 70 to deliver a primary atrial pacing pulse to the patient's heart. When atrial pacing is in a demand mode, the generator control stage 182 inhibits the atrial pulse generator 70, and thus the delivery of the primary atrial pacing pulse, if an atrial event, i.e., depolarization signifying activation of the atrium, is detected during an atrial escape interval. However, the generator control stage 182 still causes the atrial pulse generator 70 to deliver the secondary atrial pacing pulse after the secondary pacing pulse delay time following detection of the atrial event. The secondary atrial pacing pulse prolongs the refractoriness of the atria. The secondary pacing pulse delay time period, i.e., the period of time between the primary pacing pulse or the detected atrial depolarization and the secondary atrial pacing pulse, is preferably between about 180 to 300 milliseconds. The atrial refractory period is typically about 130 milliseconds in normal patients but may be as short as 80 milliseconds in some patients with recurrent atrial arrhythmias. Accordingly, the secondary atrial pacing pulse will serve to prolong the period during which the atria are refractory and thus decrease the likelihood of a spontaneous reentrant arrhythmia. The ventricle and the AV node will be refractory when the secondary atrial pacing pulse is delivered. As a result, the secondary atrial pacing pulse will have no affect on the ventricle. Atrial kick will be provided by the primary depolarization and it is likely that the atria will be in hemodynamic block. Thus, the hemodynamics will not be disturbed by the secondary atrial pacing pulse.

[0057] As previously mentioned, the atrial pacing may be in a demand mode. To that end, the atrial generator inhibiting stage 184 preferably includes an atrial delay timer 194 for timing an atrial escape interval. If, during the atrial escape interval, the atrial sense amplifier 82 detects an atrial event, the atrial generator inhibiting stage 184 will preclude the primary atrial pulse stimulation control 188 from issuing a control signal to the atrial pulse generator 70 and thus will inhibit delivery of the primary atrial pacing pulse. However, upon detection of an atrial event by the atrial sense amplifier 82, the atrial delay timer 194 begins timing the secondary pacing pulse delay period. At the end of the secondary pacing pulse delay time period, the secondary atrial pacing pulse stimulation control 190 issues control signal 76 to the atrial pulse generator 70 to cause it to provide the secondary atrial pacing pulse.

[0058] If an atrial event is not detected by the atrial sense amplifier 82 during the atrial escape interval, the primary atrial pacing pulse stimulation control 188 issues control signal 76 to the atrial pulse generator 70 at the end of the atrial escape interval to cause it to provide the primary atrial pacing pulse. When the primary atrial pacing pulse is provided by atrial pulse generator 70, the atrial delay timer 194 commences to time the secondary pacing pulse delay time period. At the end of the secondary pacing pulse delay time, the secondary atrial pacing pulse stimulation control 190 then issues control signal 76 to the atrial pulse generator 70 to cause the generator 70 to provide the secondary atrial pacing pulse.

[0059] The atrial pacing described above may be provided in a single chamber atrial pacing modality. In such a modality, the primary atrial pacing pulses, absent spontaneous atrial activations, will occur at an overdrive pacing rate determined by the overdrive pacing algorithm. However, the atrial pacing may be provided in association with ventricular pacing as well.

[0060] When dual chamber pacing is provided, the provision of a primary atrial pacing pulse or the detection of an atrial activation during the atrial escape interval causes the AV delay timer 196 to time an AV delay time period. At the end of the AV delay time period, the ventricular pacing pulse stimulation control 192 issues control signal 78 to the ventricular pulse generator 72 causing it to provide a ventricular pacing pulse to the right ventricle. Additionally, the ventricular pacing may further be in a demand mode. To that end, if a ventricular activation is detected by the ventricular sense amplifier 84 during the AV delay time period timed by the AV delay timer 196, the ventricular generator inhibiting stage 186 will preclude the ventricular pacing pulse stimulation control 192 from issuing control signal 78 to the ventricular pulse generator 72, thus inhibiting the delivery of the ventricular pacing pulse.

[0061] In view of the foregoing, it may be seen that the present invention provides an implantable pacemaker which extends the atrial refractory period of a patient's heart without the use of drug therapy. The secondary pacing pulse follows either a spontaneous atrial activation or a primary pacing pulse determined by the overdrive pacing algorithm by a predetermined time period, typically with a delay between 180 to 300 milliseconds. This timing will not disturb the hemodynamics of the heart. The secondary atrial pacing pulse may slow the sinus rate of the heart, thus increasing the likelihood of dynamic atrial overdrive pacing wherein overdriving the intrinsic atrial rhythm may occur. Lastly, the second atrial stimulus will not be conducted because the ventricle and AV node will be refractory due to a conducted ventricular depolarization or because of a paced ventricular depolarization.

[0062] Advantageously, the operating parameters of the implantable cardiac stimulation device 10 may be non-invasively programmed into the memory 94 through a telemetry circuit 100 in telemetric communication with an external device 102, such as a programmer. The telemetry circuit 100 is activated by the microcontroller via control signal 106. The telemetry circuit 100 advantageously allows intracardiac electrograms and status information relating to the operation of the implantable cardiac stimulation device 10 in addition to the data contained in the memory 94 relating to the interaction of the device with the patient's heart to be sent to the external device 102 through an established communication link 104. For examples of such devices, see U.S. Pat. No. 4,809,697, entitled “Interactive Programming and Diagnostic System for use with Implantable Pacemaker” (Causey, III et al.) and U.S. Pat. No. 4,944,299, entitled “High Speed Digital Telemetry System for Implantable Device” (Silvian).

[0063] In the preferred embodiment, the implantable cardiac stimulation device 10 further includes a physiologic sensor 110. Such sensors are commonly called “rate-responsive” sensors. The physiological sensor 110 is used to detect the exercise state of the patient, to which the microcontroller 60 responds by adjusting the rate and AV delay at which the atrial and ventricular pulse generators, 70 and 72, generate stimulation pulses. A common type of rate responsive sensor is an activity sensor, such as an accelerometer or a piezoelectric crystal, which is mounted within the housing 40 of the implantable cardiac stimulation device 10. Other types of physiologic sensors are also known, for example, sensors which sense the oxygen content of blood, respiration rate and/or minute ventilation, pH of blood, ventricular gradient, etc. However, any sensor may be used which is capable of sensing a physiological parameter which corresponds to the exercise state of the patient. The type of sensor used is not critical to the present invention and is shown only for completeness.

[0064] The implantable cardiac stimulation device 10 additionally includes a battery 114 which provides operating power to all of the circuits shown in FIG. 1A. For the implantable cardiac stimulation device 10, which employs shocking therapy, the battery 114 must be capable of operating at low current drains for long periods of time, and, in the case where the pacemaker also performs as a cardioverter/defibrillator, the battery must also be capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse. The battery 114 must also have a predictable discharge characteristic so that elective replacement time can be detected. Accordingly, the present invention preferably employs lithium/silver vanadium oxide batteries, as is presently true for many such devices.

[0065] The implantable cardiac stimulation device 10 further includes a magnet detection circuitry (not shown) coupled to the microcontroller 60. It is the purpose of the magnet detection circuitry to detect when a magnet is placed over the implantable cardiac stimulation device 10, which magnet may be used by a clinician or patient to perform various functions controlling the implantable cardiac stimulation device 10.

[0066] As further shown in FIG. 1A, the present invention preferably includes an impedance measuring circuit 120 which is enabled by the microcontroller 60 by a control signal 122. The known uses for an impedance measuring circuit 120 include, but are not limited to, lead impedance surveillance during the acute and chronic phases for proper lead positioning or dislodgment; detecting operable electrodes and automatically switching to an operable pair if dislodgment occurs; measuring respiration or minute ventilation; measuring thoracic impedance for determining shock thresholds; detecting when the device has been implanted; measuring stroke volume; and detecting the opening of the valves, etc. The impedance measuring circuit 120 is advantageously coupled to the switch bank 74 so that any desired electrode (including the RV and SVC coil electrodes, 36 and 38) may be used. The impedance measuring circuit 120 is not critical to the present invention and is shown for only completeness.

[0067] FIG. 1B shows a schematic diagram of an alternative embodiment of a portion of the implantable cardiac stimulation device 10 of FIG. 1A showing the use of multiple pulse generators 70aand 70bfor individually driving each of the implanted atrial electrode pairs. Each of the pulse generators 70aand 70bare individually controlled by the microcontroller 60 via control signals 76a, 76b.Accordingly, in this embodiment, the pacing of each site can be individually controlled. In operation, this embodiment allows either simultaneous pacing or pacing staggered between the two sites by a determined time delay, e.g., pacing the right atrium in advance of the left atrium.

[0068] FIG. 1C is a schematic diagram of an alternative embodiment of a portion of the implantable cardiac stimulation device 10 of FIG. 1A showing the use of a pulse delay circuit in conjunction with a pulse generator for individually driving each of the implanted atrial electrodes. This embodiment performs similarly to that described in reference to FIG. 1B except that a single pulse generator 70 is used in conjunction with a pulse delay circuit 106 which is used to stagger the pacing between the multiple sites. Preferably the pulse delay circuit 106 is programmable via control line 108 to enable the microcontroller 60 to alter the stagger amount.

[0069] FIG. 1D is a schematic diagram of an alternative embodiment of a portion of the implantable cardiac stimulation device 10 of FIG. 1A showing the use of multiple sense amplifiers 82aand 82bfor individually sensing depolarizations from the multiple atrial electrode pairs. Accordingly, in this embodiment, the depolarization rate can be individually sensed at each of the sites in the patient's heart 12. In a first case, the depolarization rate may be the same at each site with a time delay reflecting the conduction speed from the depolarized tissue to each electrode. In a second case, the distribution of ectopic foci may result in a different depolarization rate at each site. In a preferred embodiment, the present invention senses the highest depolarization rate and uses that rate as input to the overdrive algorithm described below. Furthermore, even if the sensed depolarization rates are essentially equivalent, it is preferable that the site at which depolarization is sensed earliest be the site that is stimulated first. For example, if an atrial tachycardia is caused by ectopic foci in the right atrium, an electrode in the right atrium will sense the depolarization before an electrode in the left atrium or coronary sinus. Accordingly, to suppress depolarizations of the ectopic foci, it may be preferable to stimulate right atrial electrode before stimulating the left atrial electrode. Accordingly, the sensing embodiment of FIG. 1D is preferably used in conjunction with the pulse generator embodiment of either FIG. 1B or FIG. 1C to enable this function.

[0070] FIG. 2 illustrates a sequence of pacing pulses 152 administered by the implantable cardiac stimulation device 10 of FIG. 1A. Each electrical pacing pulse triggers an evoked response 154 representative of an artificially induced heart beat. FIG. 2 also illustrates a single unpaced beat 156 not preceded by a pacing pulse 152. Unpaced beat 156 may be, for example, an ectopic beat caused by a naturally occurring electrical signal generated within the heart from a location other than the sinus node from which normal sinus rhythm heart beats are naturally generated. As discussed above, ectopic beats have been found to sometimes trigger tachyarrhythmias and hence it is desirable to minimize the number of ectopic beats. Accordingly, the implantable cardiac stimulation device 10 of FIG. 1A performs an overdrive pacing algorithm intended to generate overdrive pacing pulses 152 at a sufficiently high rate to minimize the number of unpaced beats 156 without causing an unnecessarily high heart rate. To this end, the implantable cardiac stimulation device 10 determines the actual degree of pacing resulting from the overdrive pacing pulses and adaptively modifies the overdrive pacing rate to maintain the actual degree of pacing at about a target degree of pacing wherein about 95% of the total beats are paced beats. Furthermore, as described in reference to FIGS. 8-11, each paced beat (as determined by the overdrive pacing algorithm described below) is replaced by a pair of staggered primary and secondary pacing pulses to additionally limit the potential for an atrial tachycardia to occur.

[0071] FIG. 3 illustrates a technique for overdrive pacing a heart wherein an overdrive pacing rate is increased only in response to the detection of at least two intrinsic P-waves occurring within X cardiac cycles of one another wherein X is, typically, between eight and forty cardiac cycles. Briefly, the technique is summarized as follows:

[0072] 1. Identify a P-wave.

[0073] 2. If another P-wave occurs within X cardiac cycles, increase the pacing rate by Y bpm.

[0074] a) X is preferably programmable from about 8 to 40 cardiac cycles.

[0075] b) Y is the programmable rate increase and is preferably programmable to 5, 10, 15, 20 or 25 ppm.

[0076] 3. If Z cardiac cycles occur without a pacing rate increase, then decrease the pacing rate by W ppm.

[0077] a) Z is the dwell time before the pacing rate is decreased and is preferably programmable from 8 to 40 cardiac cycles.

[0078] b) W is preferably programmable at 1, 2, 3, 4, or 5 ppm.

[0079] Additionally, the implantable cardiac stimulation device 10 periodically suspends pacing to permit detection of three consecutive P-waves. At that time, the sinus rate is computed based upon the detected P-waves and the overdrive pacing rate is reset to be equal to the sinus rate.

[0080] This technique will now be explained more fully with reference to FIG. 3. Initially, at step 200, the intrinsic sinus rate is detected by detecting three consecutive P-waves. At step 204, a current overdrive pacing rate is set to be equal to the detected sinus rate. At step 206, the implantable cardiac stimulation device 10 begins to count the number of paced cycles (I_RESET) since the overdrive pacing rate was set based upon the intrinsic sinus rate. This count of paced cycles is eventually compared with a rate recalibration value in step 218 (described below) and if it exceeds the recalibration value, step 200 is repeated to detect a new intrinsic sinus rate for resetting of the overdrive pacing rate.

[0081] At step 208, the atria of the heart is paced at the current overdrive pacing rate. At step 210, the number of cycles (I_CYCLES) since pacing began at the current rate is counted. Note that, I_RESETand I_CYCLESare initially equal to one another. However, as will be described, the values typically diverge from one another with further execution of the method steps. I_CYCLESis eventually compared against a rate recovery value Z at step 214 (described below) and if I_CYCLESexceeds Z, the overdrive pacing rate is decreased.

[0082] At step 212, the implantable cardiac stimulation device 10 detects any intrinsic P-waves. If a P-wave is not detected, processing proceeds to step 214 wherein the count of paced cycles since pacing began at the current rate (I_CYCLES) is compared with the rate recovery value (Z). If I_CYCLESexceeds Z, then step 216 is performed wherein the current overdrive pacing rate is decreased by a pacing decrement amount W, preferably preset to 1, 2, 3, 4, or 5 ppm. Processing then returns to step 206 for continued pacing at the newly reduced overdrive pacing rate. Thus, if at least I_CYCLESof pacing occurs before detection of a single intrinsic P-wave, the current overdrive pacing rate is reduced to provide for a rate of recovery. If, at step 214, I_CYCLESdoes not exceed Z, then step 218 is performed wherein the implantable cardiac stimulation device 10 determines whether the count of paced cycles since the current rate was originally set (I_RESET) exceeds a rate recalibration value (N_{RECALIBRATION}). If I_RESETexceeds N_{RECALIBRATION}, then step 200 is again executed wherein a new sinus rate is detected and the overdrive pacing rate is reset to the new sinus rate. This ensures that the overdrive pacing rate does not remain significantly different from the sinus rate for any extended period of time. If, at step 218, I_RESETdoes not exceed N_{RECALIBRATION}, then processing returns to step 206 for additional pacing at the current overdrive pacing rate.

[0083] What has been described thus far with respect to FIG. 3 are circumstances wherein no intrinsic P-waves are detected. Steps performed in response to the detection of P-waves will now be described. More specifically, if at step 212 an intrinsic P-wave is detected, then step 220 is performed wherein a determination is made as to whether a count has already begun of the number of overdrive pacing cycles since detection of the last detected P-wave. During the first execution of step 220 following detection of the first P-wave, the count has not yet begun and hence processing continues to step 222 wherein the implantable cardiac stimulation device 10 begins to count the number of overdrive pacing cycles since the last detected P-wave (I_P-WAVE). Processing then returns to step 206 for additional pacing at the current overdrive pacing rate while incrementing I_P-WAVE(along with I_RESETand I_CYCLES) with each additional pacing cycle.

[0084] If another P-wave is detected at step 212, then execution proceeds through step 220 to step 224 wherein I_P-WAVEis compared with a pacing cycle increment value (X). If I_P-WAVEis less than X, indicating that the last two detected intrinsic P-waves are within X cardiac cycles of one another, then step 226 is performed wherein the current overdrive pacing rate is increased by a predetermined pacing increment amount (Y) set to, for example, 5, 10, 15, 20, or 25 ppm. Thereafter, pacing continues from step 206 at the new higher overdrive pacing rate. If, however, at step 224, I_P-WAVEwas found to be greater than X, meaning that the last two detected intrinsic P-waves were more than X cycles apart, then the overdrive pacing rate is not immediately increased. Instead, processing proceeds to step 222 wherein I_P-WAVEis reset to begin a new count of the number of overdrive pacing cycles since the most recently detected P-wave.

[0085] Thus, FIG. 3 is a flow chart illustrating one technique for implementing an overdrive pacing algorithm which, among other features, (1) increases an overdrive pacing rate if two P-waves are detected within X cardiac cycles of one another, (2) decreases the overdrive pacing rate if a rate increase does not occur within at least Z cardiac cycles, and (3) resets the overdrive pacing rate to be equal to a detected sinus rate every N_{RECALIBRATION}number of cardiac cycles regardless of the extent to which the overdrive pacing rate is modified during the interim.

[0086] FIG. 4 illustrates a method for controlling overdrive pacing wherein an overdrive pacing rate is increased only if at least two intrinsic atrial beats are detected within a block of N consecutive cardiac cycles. The technique of FIG. 4 is summarized as follows:

[0087] 1. At the conclusion of a block of N cardiac cycles, the implantable cardiac stimulation device determines if there is more than one P-wave in the block of cardiac cycles. If there is more than one P-wave, the pacing rate is increased by Y ppm.

[0088] a) Y is a programmable rate increment and is preferably programmable to 5, 10, 15, 20 or 25 ppm.

[0089] 2. If Z cardiac cycles occur without increasing the pacing rate, then the pacing rate is decreased by W ppm.

[0090] a) Z is the dwell time before the pacing rate is decreased and is preferably programmable from 8 to 40 cardiac cycles.

[0091] b) W is preferably programmable at 1, 2, 3, 4, or 5 ppm.

[0092] Additionally, the implantable cardiac stimulation device 10 periodically suspends pacing to detect the intrinsic atrial rate and compare the intrinsic atrial rate with a current overdrive pacing rate. If the difference between the atrial rate and the overdrive pacing rate exceeds a predetermined threshold (N_THRESHOLD), then the overdrive pacing rate is reset to the detected atrial rate. Otherwise, overdrive pacing continues at the current pacing rate.

[0093] The technique will now be described in greater detail with reference to FIG. 4. Certain steps of FIG. 4 are similar to those of FIG. 3 and, accordingly, will not be re-described in detail. Initially, at steps 300 and 302, the implantable cardiac stimulation device 10 detects the current intrinsic atrial rate and sets a current overdrive pacing rate based on the detected atrial rate. Initially, the overdrive pacing rate is set to be equal to the detected atrial rate. During subsequent iterations of steps 300 and 302, the overdrive pacing rate is set to the atrial rate only if the atrial rate minus the current overdrive pacing rate exceeds N_THRESHOLD.

[0094] At step 304, the implantable cardiac stimulation device 10 begins to count all paced cycles (I_RESET) since the overdrive pacing rate was set at step 302. At step 305, the implantable cardiac stimulation device 10 paces the atria at the current overdrive pacing rate while detecting any intrinsic P-waves. At step 306, all paced cycles since pacing began at the current rate are detected and counted (I_CYCLES). At step 308, the implantable cardiac stimulation device 10 also counts every group of N consecutive paced cycles (I_N) wherein N is, for example, ten. Initially, the counts initiated at steps 302, 306 and 308 will be the same. As will be seen, however, these counts may diverge from one another with further processing of the method steps.

[0095] At step 310, the implantable cardiac stimulation device 10 determines whether N pacing cycles have elapsed by examining I_N. If I_Nequals N, then step 312 is performed wherein the implantable cardiac stimulation device 10 determines whether at least two intrinsic P-waves have been detected within the group of N paced cycles. If so, the current overdrive pacing rate is increased at step 314 by an amount Y wherein Y is equal to, for example, 5, 10, 15, 20 or 25 ppm. Thereafter, processing returns to step 304 for additional atrial pacing at the new overdrive pacing rate. If, at step 312, at least two intrinsic P-waves were not detected within the group of N paced cycles, then step 316 is performed wherein the count of N paced cycles (I_N) is reset such that the next set of N consecutive paced cycles may be counted. In this manner, the overdrive pacing rate is increased if, and only if, at least two P-waves are detected within a group of N consecutive cardiac cycles.

[0096] If, at step 310, N paced cycles have not yet elapsed (i.e., the count I_Nis less than N), then step 318 is performed wherein the implantable cardiac stimulation device 10 determines whether the count of paced cycles since pacing began at the current rate (I_PACED) exceeds a rate recovery value (Z). If so, then at step 320, the overdrive pacing rate is decreased by a pacing decrement amount W, wherein W is preset to, for example, 1, 2, 3, 4 or 5 ppm. Hence, if the overdrive pacing rate has not been increased as a result of the detection of at least two P-waves within a block of N cycles, then the overdrive pacing rate is decreased to provide rate recovery.

[0097] If, at step 318, I_CYCLESdoes not exceed Z, then step 322 is performed wherein the implantable cardiac stimulation device 10 determines whether the count of paced cycles since the current rate originally set (I_RESET) in step 302 exceeds a rate calibration value N_{RECALIBRATION}. If so, then steps 300 and 302 are repeated wherein overdrive pacing is suspended to permit detection of the intrinsic atrial rate and the overdrive pacing rate is then set based upon the intrinsic atrial rate. As noted above, within step 302, a determination is made as to whether the difference between the intrinsic atrial rate and the overdrive pacing rate exceeds a threshold N_THRESHOLDand, if not, the overdrive pacing rate is not reset to be equal to the atrial rate. If, at step 322, I_RESETdoes not exceed N_{RECALIBRATION}, then processing merely returns to step 304 for additional pacing at the current overdrive pacing rate.

[0098] Thus, FIG. 4 illustrates an overdrive pacing technique wherein, among other features, (1) an overdrive pacing rate is increased only if at least two P-waves are detected within a block of N consecutive cardiac cycles, (2) the overdrive pacing rate is decreased if the overdrive pacing rate is not increased within Z consecutive cardiac cycles, and (3) the overdrive pacing rate is periodically reset to an intrinsic atrial rate if the difference between the atrial rate and the current overdrive pacing rate exceeds a predetermined threshold. By increasing the overdrive pacing rate only in response to the detection of at least two P-waves within a block of N consecutive cardiac cycles, excessively aggressive overdrive pacing rate increases are avoided. Additionally, with appropriate selection of N, a minimum percentage of paced cycles can be achieved on the average. For example, by setting N equal to ten, the average percentage of paced cycles will be maintained at about 90%. If more than ten percent of the cardiac cycles are intrinsic cycles, then the overdrive pacing rate is increased. Otherwise, the overdrive pacing rate is periodically decreased. Hence, an average of about 90% is sustained.

[0099] With reference to FIG. 5, techniques for adaptively varying overdrive pacing characteristics are summarized. Initially, at step 400, a particular overdrive pacing technique or algorithm is selected by the implantable cardiac stimulation device 10. Then, at step 401, programmable values, i.e., control values, are input from a memory 402 for control of the operation of the algorithm. (If the implantable stimulation device 10 is capable of performing only a single overdrive pacing technique, step 400 is not necessary.) Depending upon the overdrive pacing technique, the programmable values may be representative of: an overdrive pacing rate, an overdrive pacing margin, a pacing cycle length, a number of pacing pulses prior to pacing cycle length extension (Z), an amount of time prior to pacing cycle length extension, a number of unpaced beats prior to pacing cycle length extension, a rate increment magnitude (Y), a rate decrement magnitude (W), a search window duration (X), a pacemaker base rate, and a sensor modulated base rate.

[0100] At step 403, the implantable cardiac stimulation device 10 applies overdrive pacing pulses to the heart in accordance with the requisite programmable values. While overdrive pacing is performed, the implantable cardiac stimulation device 10 performs steps 404-412 to adjust the programmable values so as to reduce any difference between an actual degree of pacing and a target degree of pacing. More specifically, at step 404, the implantable cardiac stimulation device 10 determines the actual degree of pacing resulting from the pacing pulses. The actual degree of pacing may be represented by a percentage of paced beats (determined as a function of time or as a function of cardiac cycles) or by any other appropriate factor. At step 406, a target degree of pacing is input from a memory 408 and, at step 410, the implantable cardiac stimulation device 10 compares the actual degree of pacing with the target degree of pacing. At step 412, the implantable cardiac stimulation device 10 adjusts the values used from memory 402 so as to reduce any difference between the actual degree of pacing and the target degree of pacing. The specific adjustment depends upon a particular programmable value being adjusted. In some cases, a value may need to be increased so as to cause a decrease in the degree of pacing. In other cases, a value may need to be decreased so as to cause a decrease in the degree of pacing. The direction of the adjustment and the magnitude of the adjustment are set so as to achieve a negative feedback loop which converges the actual degree of pacing to the target degree of pacing. To this end, routine experiments are performed to determine optimal values for adjusting the various parameters to achieve the desired feedback loop and to eliminate adjustment values, if any, which may result in a positive feedback loop causing the actual degree of pacing to deviate from the target degree of pacing, rather than to converge to the target degree of pacing. The resulting adjustment in the values may be linear or non-linear, depending upon the particular programmable values and depending upon the amount of difference, if any, between the actual degree of pacing and the target degree of pacing. As can be appreciated, a wide range of possible adjustments can be employed consistent with the invention depending upon the characteristics of the overdrive pacing technique being implemented. In many cases, two or more programmable values are adjusted simultaneously. For example, both the overdrive pacing margin and the number of pacing pulses prior to a pacing cycle length extension may be adaptively adjusted.

[0101] A first specific example of the technique of FIG. 5 will now be described with reference to FIG. 6. In this specific example, the implantable cardiac stimulation device 10 operates to maintain the overdrive pacing rate at a rate equal to the intrinsic rate plus a programmable rate margin. The rate margin is adaptively varied so as to maintain a target degree of pacing. Initially, at step 500, the implantable cardiac stimulation device 10 inputs an initial overdrive pacing margin from a memory unit 502. The margin may be, for example, five beats per minute (bpm)—indicating that the heart is to be paced at a rate equal to the intrinsic heart rate plus five bpm. At step 504, the implantable cardiac stimulation device 10 periodically determines the intrinsic heart rate and administers overdrive pacing pulses to the heart at a rate equal to the intrinsic rate plus the overdrive pacing margin. For example, if the intrinsic rate is found to be 60 bpm, the implantable cardiac stimulation device 10 overdrive paces the heart at a rate of 65 ppm. If the intrinsic rate is found to increase to 80 bpm, then the overdrive pacing rate automatically increases to 85 ppm. In this manner, the implantable cardiac stimulation device 10 seeks to maintain the overdrive pacing rate at a rate slightly higher than the intrinsic rate at all times.

[0102] A determination of the intrinsic rate at step 504 may be performed, for example, by periodically deactivating overdrive pacing, thereby permitting detection of intrinsic beats from which the intrinsic heart rate is determined. In this regard, an estimate of the intrinsic heart rate may be calculated based upon the time between the detected intrinsic beats. The greater the number of intrinsic beats that are detected, the more precise the determination of the intrinsic heart rate.

[0103] Step 506 is periodically performed wherein the implantable cardiac stimulation device 10 counts the number of paced beats and the number of unpaced beats until a predetermined period of time, such as 60 seconds, has elapsed. Alternatively, the implantable cardiac stimulation device 10 counts the beats until a predetermined number of total beats, such as 100 beats, have been counted. The implantable cardiac stimulation device 10 then calculates a percentage of the number of paced beats out of a total number of beats. In the example of FIG. 2, with nine paced beats and one unpaced beat, the percentage of paced beats is about 90%. At step 508, the implantable cardiac stimulation device 10 inputs a target degree of pacing from a memory unit 510. The target of pacing may be, for example, 95% paced beats. At step 512, the implantable cardiac stimulation device 10 determines whether the actual percentage of paced beats determined at step 506 is greater than the target percentage of paced beats input at step 508. If so, then step 514 is performed wherein the implantable cardiac stimulation device 10 automatically decreases the overdrive pacing margin by a predetermined amount, such as one ppm. If not, then step 516 is performed wherein the implantable cardiac stimulation device 10 automatically increases the overdrive pacing margin by the predetermined amount.

[0104] Thereafter, step 504 is performed using the adjusted overdrive pacing margin. Hence overdrive pacing may now occur at a rate of six ppm above the intrinsic rate or perhaps only at a rate of four ppm above the intrinsic rate. With repeated iterations of steps 504-516, the degree of overdrive pacing is thereby periodically, adaptively adjusted so as to maintain the actual percentage of paced beats at an amount about equal to the target degree of pacing, e.g., at about 95%. Hence, if the initial overdrive pacing adjustment factor was too high such that substantially 100% of heart beats were paced beats, the overdrive pacing adjustment factor is decreased somewhat to permit occasional detection of an unpaced beat. This helps ensure that the overdrive pacing rate is not so high so as to possibly adversely affect the health of the patient. Also, avoidance of an unnecessarily high overdrive pacing rate helps preserve battery longevity. Moreover, in embodiments wherein the implantable cardiac stimulation device 10 relies upon detection of an occasional intrinsic beat so as to determine the intrinsic heart rate, a reduction of the overdrive pacing rate helps ensure that intrinsic beats are occasionally detected. On the other hand, if the actual degree of overdrive pacing was found to be significantly less than 95%, then the overdrive pacing rate is increased so as to prevent too many intrinsic beats from occurring which might trigger a tachyarrhythmia.

[0105] Although not specifically shown in FIG. 6, if, at step 512, the actual percentage of pacing is found to be exactly equal to the target degree of pacing, then the implantable cardiac stimulation device 10 may be configured to not adjust the overdrive pacing adjustment factor either up or down. Also, the predetermined amount by which the overdrive pacing margin is increased may differ from that in which it is decreased. Also, the predetermined amounts may vary depending upon the current overdrive pacing rate or upon the current overdrive pacing adjustment factor. For example, if the overdrive pacing margin is currently set to 20 ppm, the factor may be increased or decreased by a greater amount than if the overdrive pacing margin was currently set to two or three ppm. Likewise, if the current overdrive pacing rate (i.e., the sum of the current intrinsic heart rate and the current overdrive pacing margin) is particularly high, then the predetermined amounts may also be relatively high. Also, note that the overdrive pacing margin may, at times, be negative. As can be appreciated, a wide range of alternatives may be provided consistent with the principles of the invention.

[0106] Another specific example of the technique of FIG. 5 will now be described with reference to FIG. 7. In this specific example, the implantable cardiac stimulation device 10 performs a dynamic atrial overdrive technique wherein detection of a single P-wave triggers an immediate, significant increase in the overdrive pacing rate and wherein, after an increase, the pacing cycle length is periodically extended to gradually reduce the overdrive pacing rate. More specifically, the dynamic atrial overdrive technique operates as follows. The implantable cardiac stimulation device 10 monitors the atria of the heart and detects P-waves and, in response to detection of a single P-wave, increases the overdrive pacing rate by a programmable increment value which depends upon whether the current overdrive base rate is within: 1) a lower rate overdrive (LRO) regime of between 25 and 59 ppm, 2) a middle rate overdrive regime (MRO) of between 60 and 149 ppm, or 3) an upper rate overdrive (URO) of between 150 and 185 ppm.

[0107] Within the LRO regime, the implantable cardiac stimulation device 10 increases the overdrive pacing rate with each sensed P-wave by an LRO increment programmable value, e.g., 5, 10, 15, 20 and 25 ppm. Within the URO regime, the implantable cardiac stimulation device 10 increases the overdrive pacing rate with each sensed P-wave by a URO increment programmable value, e.g., 5 or 10 ppm. (Typically, the LRO increment value is programmed to a high value, such as 25 ppm, whereas the URO increment is programmed to a low value such as 5 ppm.) Within the MRO regime, the implantable cardiac stimulation device 10 increases the overdrive pacing rate with each sensed P-wave by an MRO increment which is a blended value between the LRO increment and the URO increment. The MRO increment is equal to the LRO increment when the base rate is 60 ppm. The MRO increment varies gradually when the base rate is in the range of 60 ppm to 150 ppm until the increment is equal to the URO increment when the base rate is equal to 150 ppm.

[0108] The implantable cardiac stimulation device 10 also exploits a dynamic rate recovery technique wherein the overdrive base rate is decreased if a predetermined number of pacing cycles occur without any detected P-waves. The predetermined number of cycles and the amount of the decrease are both programmable. The amount of the decrease varies depending upon whether the base overdrive pacing rate is within one of two regimes.

[0109] The specific operation of the implantable cardiac stimulation device 10 within the various regimes is described with reference to the following examples.

[0110] As an example of operation within the LRO regime, if the current overdrive pacing rate is 45 ppm, the LRO increment value is 5 ppm, and a P-wave is sensed, the current overdrive pacing rate is immediately increased to 50 ppm. If the P-wave arises from intrinsic atrial activity occurring at a rate of 53 bpm, then a second P-wave will be detected before a paced beat can be generated (because the overdrive base rate is still below the intrinsic rate). Hence, another P-wave is detected and the overdrive pacing rate increases to 55 ppm.

[0111] As another example of operation within the LRO regime, if the current overdrive pacing rate is 55 ppm, the LRO increment is 25 ppm, the URO increment is 5 ppm, and the patient experiences an SVT at 160 bpm, then the overdrive pacing rate increases by 25 ppm with each sensed atrial beat until the overdrive pacing rate exceeds 60 ppm. Then, any further increments begin at slightly less than the LRO increment of 25 ppm and are gradually reduced to the URO increment of 5 ppm when the overdrive pacing rate exceeds 150 ppm.

[0112] To provide rate recovery, the implantable cardiac stimulation device 10 counts the number of pacing pulses delivered at a current overdrive pacing rate and, if the number of cycles exceeds a threshold value N_MAx, the implantable cardiac stimulation device 10 decreases the overdrive pacing rate by increasing a pacing cycle length (CL) equal to the amount of time between individual pacing pulses. N_MAXis preferably programmable within a range of 1 to 32 cycles. Thus, if N_MAXis programmed to 10 cycles and the overdrive pacing rate has remained constant for 10 cycles, then the CL is increased by a programmable rate recovery value. In this manner, so long as no intrinsic activity is detected, the overdrive pacing rate gradually decreases. Whenever intrinsic atrial activity is sensed, the counter associated with N_MAXis reset and, in accordance with the techniques already described, the overdrive pacing rate is incremented. Exemplary programmable CL increment values are:

[0113] Milliseconds/Cycle

[0114] 6;13

[0115] 6;19

[0116] 13;19

[0117] 19;25

[0118] As noted, an increase in the pacing cycle length causes a corresponding decrease in the overdrive pacing rate. In the foregoing, the first value represents the increase in CL in milliseconds per cycle to be used if the current base rate is over 100 ppm. The second value represents the increase in CL in milliseconds per cycle to be used if the current base rate is 100 ppm or less. Thus, two base CL increment regimes are used.

[0119] In a specific rate recovery example, if the current overdrive pacing rate is 102 ppm, the intrinsic atrial rate is 90 ppm, and the dynamic rate recovery values are programmed to 6;19 milliseconds/cycle, then the pacing cycle length decreases after every N_MAXas follows:

[0120] Thus, the implantable cardiac stimulation device 10 employs a dynamic atrial overdrive technique which increases an overdrive base rate very promptly in response to detection of intrinsic atrial activity (i.e., P-waves) and provides a rate recovery technique for reducing the overdrive pacing rate when overdrive pacing is no longer needed. The degree of increment or decrement to the overdrive pacing base rate depends, as described above, upon the current base rate regime. Additional variations to the overdrive pacing rate may be based upon detection of premature atrial contractions (PACs) or other intrinsic events.

[0121] In the technique of FIG. 7, N_MAXis adaptively varied to maintain a target degree of pacing. Initially, at step 600, the implantable cardiac stimulation device 10 inputs both an initial pacing cycle length (CL) and N_MAXfrom a memory unit 602. CL may be, for example, one second (corresponding to an overdrive pacing rate of 60 ppm) and N_MAXmay be, for example, initially set to ten.

[0122] At step 604, the implantable cardiac stimulation device 10 repeatedly administers overdrive pacing pulses to the heart in accordance with the dynamic atrial overdrive algorithm described above wherein the CL is automatically extended every N_MAXpulses to allow occasional detection of intrinsic heart beats or other intrinsic activity. While step 604 is performed, the implantable cardiac stimulation device 10 additionally performs steps 606-616 as follows. The implantable cardiac stimulation device 10 counts the number of paced beats and unpaced beats for either a predetermined period of time or a predetermined number of pulses and then calculates the percentage of paced beats at step 606. A target degree of pacing is input at step 608 from a memory 610 and, at step 612, the implantable cardiac stimulation device 10 determines whether the actual percentage of pacing is greater than the target percentage of pacing. If so, then N_MAXis decreased by a predetermined amount, such as one cycle, at step 614. If not, then N_MAXis increased by a predetermined amount, such as one cycle, at step 616. Thereafter, the overdrive pacing performed by the implantable cardiac stimulation device 10 during step 604 is performed using the adjusted value for N_MAX. With repeated iterations of steps 604-616, the actual degree of pacing is maintained substantially at or near the target degree of pacing so as to prevent excessive overdrive pacing while still minimizing the number of non-paced beats. In this regard, N_MAXis decreased when the actual percentage of pacing is greater than the target percentage of pacing so as to permit a more prompt detection of an intrinsic pulse from which a new intrinsic heart rate is determined. By permitting a more prompt detection of a next intrinsic beat, the overdrive pacing rate can thereby be adjusted in accordance with the dynamic atrial overdrive algorithm to more closely match the intrinsic rate. In contrast, by increasing N_MAXif the actual percentage of pacing is found to be less than 95%, a greater amount of time elapses prior to detection of a next intrinsic pulse, thereby delaying readjustment of the overdrive pacing rate. This may result in a generally higher overdrive pacing rate. In any case, regardless of whether the adjustments to N_MAXresult in an increase or decrease in the overall average overdrive pacing rate, the adjustments to N_MAXwill typically operate to maintain the percentage of paced beats at about the target percentage and the advantages set forth above are achieved.

[0123] In another specific example of the technique of FIG. 5, the implantable cardiac stimulation device 10 employs a technique for modulating the base rate of the implantable cardiac stimulation device 10 based upon circadian rhythms of the patient. The technique for modulating the base rate is described in U.S. Pat. No. 5,476,483 to Bornzin et al. which is incorporated by reference herein. Briefly, in accordance with the technique of the Bomzin et aL. patent, the base rate associated with a transfer function of a rate-responsive cardiac pacemaker is modulated. Activity sensor measurements are used to derive activity variance measurements, which in turn are used to modulate the base pacing rate. In one embodiment, a histogram is used to store activity variance measurements collected over a period of about. one week. A histogram is used to derive an activity variance threshold, which is compared to current activity variance measurements to determine if the patient is asleep. If the patient is deemed to be asleep, the pacing rate is set to a rate that comfortably meets the low metabolic demands of the patient during sleep. In alternative embodiments, the activity variance measurements are applied to a base rate slope to modulate the base pacing rate.

[0124] Thus, the base rate can be modulated and as such, the percentage of time that underlying, intrinsic P-waves are detectable during a search period can be adjusted by adaptively adjusting the base rate in accordance with the adaptive techniques described above. With proper selection of appropriate adaptive adjustment values (when the implantable cardiac stimulation device 10 extends an atrial escape interval to permit detection of an underlying, intrinsic P-wave), the adjusted base rate will limit the extension of the escape interval so that the base rate is in fact higher than the underlying atrial rate. Accordingly, there will be few, if any, emerging P-waves. More specifically, parameters sleep_rate and BPR_slope within Equation 9 of the Bomzin et al. patent are adaptively adjusted so as to achieve a target degree of pacing. Increasing sleep_rate and BPR_slope has the effect of increasing the base rate and thus increasing the percentage of atrial pacing.

[0125] FIGS. 8 and 9 show examples of the use of a pair of staggered pacing pulses with the aforementioned overdrive pacing algorithm. In the figures, A1 refers to a primary atrial pacing pulse and A2 refers to a secondary atrial pacing pulse. FIG. 8 shows surface ECG, atrial pacing event markers, atrial IEGM and ventricular IEGM during the use of pairs of staggered atrial pacing pulses in AOO mode at a base rate of 90 bpm with an A1-A2 delay of 220 ms. The subject was a dog under the influence of a beta blocking drug (esmolol) given to slow AV nodal conduction. Note that all atrial stimuli captured the atrium as indicated by evoked responses evident in the atrial IEGM. However, only the A1 initiated atrial depolarizations conducted to the ventricle. This is because the A2 initiated depolarization arrives at the AV node while it is still refractory due to conducting the A1 initiated depolarization.

[0126] FIG. 9 depicts the same conditions as FIG. 8 except that the A1-A2 interval is 260 ms. Note that all atrial stimuli captured the atrium. Note also that the first two and possibly the third A2-initiated atrial depolarizations conduct to the ventricle. This is because the AV node has had time to recover from conducting the A1 initiated depolarization and is no longer refractory when the A2 initiated depolarization arrives. AV nodal conduction is intermittent, with 6 out of 10 consecutive A2 stimuli conducting in the test that produced this figure.

[0127] FIG. 10 is a simplified flow chart showing an exemplary implementation of a dual chamber embodiment of the present invention showing the operation of the microcontroller 60 during each cardiac cycle. Initially in step 700, the atrial sense amplifier 82 is used to determine whether an atrial event has been sensed. If there has not yet been an atrial event, the microcontroller 60 determines in step 702 using atrial delay timer 194 whether the atrial escape interval has timed out wherein the atrial escape interval is determined according to the aforementioned overdrive algorithm. If the atrial escape interval has not timed out, the process continues with step 700. If the atrial escape interval has timed out, the process continues with the primary atrial pacing pulse stimulation control 188 causing the atrial pulse generator 70 to generate a primary atrial pacing pulse in step 704. Next, the AV delay timer 196 is set in step 706A and a secondary pacing pulse AA (atrial to atrial pulse) delay, e.g., 180 to 300 ms., is started in step 708A. Alternatively, step 706B is processed if an atrial event was sensed in step 700 and a secondary pacing pulse PA (P-wave to atrial pulse) delay, e.g., 180 to 300 ms., is started in step 708B.

[0128] In either case, the process continues with step 710 if either (1) an intrinsic atrial event was sensed or (2) if a primary atrial pacing pulse was delivered at the overdrive pacing rate. In step 710, the secondary atrial pacing pulse stimulation control 190 causes the atrial pulse generator 70 to deliver a secondary atrial pacing pulse following the time out of atrial delay timer 194. In step 712, it is determined whether a ventricular event, i.e., depolarization signifying activation of the ventricle, has occurred using the ventricular sense amplifier 84. If there has not been a ventricular event sensed, the process continues with step 714 which determines using AV delay timer 196 whether the AV delay has expired. If the AV delay has not expired, the process returns to step 712 to continue looking for a ventricular event. If the AV delay has expired, the ventricular pacing pulse stimulation control 192 causes the ventricular pulse generator 72 to deliver a ventricular pulse in step 716. The atrial escape interval is then started in step 718 and the process continues with step 700 for the next cardiac cycle. Alternatively, the atrial escape interval begins following detection of a ventricular event in step 712.

[0129] As has been discussed, the atrial refractory period can be extended by providing a secondary atrial pacing pulse a delay time following either an intrinsic atrial event, i.e., a P-wave, or the delivery of a primary atrial pacing pulse at the overdrive pacing rate. However, the delay time must be within an appropriate range for each patient. If the delay time is too short, e.g., less than 120 to 150 ms., the secondary atrial pacing pulse may cause atrial fibrillation. If the delay time is too long, e.g., greater than 300 ms., the AV node will no longer be in its refractory period and the ventricle may be stimulated. Accordingly, an exemplary auto-adjustment mode for determining optimal AA (primary atrial pacing pulse to secondary atrial pacing pulse) and PA (intrinsic P-wave to secondary atrial pacing pulse) delay time values, is presented in reference to FIG. 11.

[0130] Following, a predetermined time period (see step 720), e.g., once an hour to once a day, the AA and PA delay time values are extended to a MaxDelay value, e.g., 350 ms., (see step 722) to restart an auto-adjustment process. These delay time values are used in step 710 (see FIG. 10) to determine when the secondary atrial pacing pulse should be delivered. However, as shown in FIG. 11, optimal delay time values are determined. Following the delivery of the secondary atrial pacing pulse in step 710A, the microcontroller 60 determines in step 724 using the ventricular sense amplifier 84 whether a ventricular activation, i.e., depolarization occurred. If ventricular depolarization occurs, then the appropriate delay time value is decreased in step 726 by a predetermined amount Δ, e.g., 5 ms. The appropriate delay time value is selected by determining by whether the primary atrial event was either the delivery of a primary atrial pacing pulse (in which case the AA delay time is decreased) or whether the primary atrial event was the detection of a P-wave (in which case the PA delay time is decreased). However, as previously discussed, the delay time values are not allowed to decrease below a MinDelay time, e.g., 120 to 150 ms. Accordingly, it is determined in step 728 whether current delay time value +Δ exceeds the MinDelay value. If this condition is not satisfied, step 726 is bypassed.

[0131] The present invention provides a therapy that can be advantageously used by an implantable cardiac stimulation device for prolonging atrial refractoriness to avoid the potential for atrial fibrillation and flutter. However, the use of an multiple atrial pacing pulses delivered at an overdrive pacing rate will increase the battery consumption and thus limit the life of the implantable cardiac stimulation device 10. Accordingly, it may be desirable to restrict the use of the aforedescribed therapy to periods of time when prolonging atrial refractoriness would be most beneficial to a patient. For example, when a patient is at rest there is greater tendency for vagal-mediated tachycardia to occur. Accordingly, it may be desirable to only enable secondary atrial pacing pulses during periods of rest. Such periods of rest can be determined as described in U.S. Pat. No. 5,476,483 to Bornzin, et al. which is incorporated herein by reference. Additionally, it may be desirable to sense the occurrence of a high incidence of PACs (premature atrial contractions), e.g., greater than 2 PACs per minute, and use this occurrence as a triggering event to enable secondary atrial pacing pulses. In an exemplary use, the previously described method of providing secondary atrial pacing pulses would be enabled for a predetermined period of time, e.g., an hour, following a high incidence of PACs. Following this predetermined period, secondary atrial pacing pulses would again be disabled. If a high incidence of PACs should reoccur, the therapy would again be enabled. Accordingly, battery consumption would be minimized by only providing this therapy when it is needed.

[0132] What has been described are various techniques for providing multiple time-delayed pacing pulses to a patient's heart which are delivered at an overdrive pacing rate. While the technique has been specifically described for providing a primary pulse followed by a single time-delayed secondary pulse, one of ordinary skill in the art would recognize that this technique could also include the application of a series of secondary time-delayed pulses, i.e., a pulse train where, for example, a secondary′ pulse is delayed from the secondary pulse, a secondary″ pulse is delayed from the secondary′ pulse, etc. Furthermore, these techniques may also be used for applying these pulses to multiple sites in a patient's heart. Such multiple site atrial pacing may be provided by two or more electrode pairs, e.g., 22a, 24aand 22b, 24b, being placed in the right atrium, the left atrium, or one electrode pair being placed in each atrium. For example, a first atrial electrode pair may be located in the right atrium and a second atrial electrode pair may be implanted in the coronary sinus near the left ventricular free wall or, during open heart surgery, being placed on the left atrial myocardium. The second atrial electrode pair may be paced simultaneously with the atrial electrode pair implanted in the right atrium. Alternatively, the electrode pair which first sensed an intrinsic atrial event may be paced prior to the other electrode pair according to a determined delay time. Furthermore, when the technique for providing multiple time delayed pacing pulses is applied to multiple electrodes (even without overdrive pacing), the ability to prevent atrial arrhythmias can be improved.

[0133] The described overdrive pacing algorithms include techniques for adaptively adjusting overdrive pacing programmable values so as to maintain a desired or target degree of overdrive pacing, or to maintain any other desirable characteristic of the overdrive pacing. The overdrive pacing techniques and the multiple pulse pacing techniques have been described primarily with reference to flow charts illustrating steps performed by components of an implantable cardiac stimulating device. Each method step of the flow charts additionally represents a functional component for performing the method step. The functional component may comprise individual hardware devices or may comprise software components. In some cases a single functional component will perform two or more method steps. In other cases two or more functional components in combination will perform a single method step. In general, the techniques described herein may be implemented using any appropriate technology such as hardware, software, firmware, or the like.

[0134] The examples described herein are merely illustrative of the invention and should not be construed as limiting the scope of the invention. Accordingly, the scope of the present invention is defined by the following claims.