Method and system for delivering cardiac resynchronization therapy with variable atrio-ventricular delay

[0001] This invention pertains to methods and apparatus for treating cardiac disease with electrical therapy.

[0002] Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction.

[0003] Pacing therapy can also be used in the treatment of heart failure, which refers to a clinical syndrome in which an abnormality of cardiac function causes a below normal cardiac output that can fall below a level adequate to meet the metabolic demand of peripheral tissues. When uncompensated, it usually presents as congestive heart failure due to the accompanying venous and pulmonary congestion. Heart failure can be due to a variety of etiologies with ischemic heart disease being the most common. It has been shown that some heart failure patients suffer from intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) such that their cardiac outputs can be increased by improving the synchronization of ventricular contractions with electrical stimulation. In order to treat these problems, implantable cardiac devices have been developed that provide appropriately timed electrical stimulation to one or more heart chambers in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy (CRT). Ventricular resynchronization is useful in treating heart failure because, although not directly inotropic, resynchronization can result in a more coordinated contraction of the ventricles with improved pumping efficiency and increased cardiac output. Currently, a most common form of CRT applies stimulation pulses to both ventricles, either simultaneously or separated by a specified biventricular offset interval, and after a specified atrio-ventricular delay interval with respect to the detection of an intrinsic atrial contraction and/or an atrial pace. Appropriate specification of these pacing parameters is necessary in order to achieve optimum improvement in cardiac function, and it is this problem with which the present invention is primarily concerned.

[0004] The present invention relates to a method or system for computing and/or setting optimal cardiac resynchronization pacing parameters as derived from intrinsic conduction data. Such intrinsic conduction data may be collected via the sensing channels of an implantable cardiac device for delivering cardiac resynchronization therapy. One of the resynchronization parameters which may be set in this manner is the atrio-ventricular delay interval used in atrial tracking and AV sequential pacing. In accordance with the invention, an optimum value for this parameter is computed at a plurality of atrial rates from intrinsic conduction data. An implantable device may be programmed to vary the atrio-ventricular delay interval with atrial rate as resynchronization pacing is delivered by setting the AVD interval to an optimum value stored in a table which is associated with the present atrial rate. The implantable device may be further programmed to initialize or update the table by taking an intrinsic conduction measurement, computing an optimal value for the AVD interval as a linear function of the intrinsic conduction measurement, and storing the computed optimal value and an associated atrial rate corresponding to the present atrial rate in the table. Optimum values of the atrio-ventricular delay interval which have not been computed from intrinsic conduction data may derived using a linear or non-linear interpolating function.

[0008] The present invention relates to a method or system for setting the pacing parameters and/or pacing configuration of a cardiac rhythm management device for delivering resynchronization pacing to the left ventricle (LV) and/or the right ventricle (RV) in order to compensate for ventricular conduction delays and improve the coordination of ventricular contractions. In accordance with the present invention, optimum pacing parameters may be computed based upon intrinsic conduction data derived from measurements of intra-cardiac conduction times using the sensing channels of an implanted device. Algorithms for computing and/or setting these pacing parameters may be implemented in either the programming of an external programmer, in the programming of the implanted device itself, or as a manually implemented procedure (e.g., by using a printed table lookup to compute optimum parameters from intrinsic conduction data). In one embodiment, the external programmer communicates with the implantable device over a telemetry link and receives either raw electrogram data, markers corresponding to particular sensed events, or measurements of the intervals between particular sensed events as computed by the implantable device. The external programmer may then compute optimal settings for pacing parameters which are either transmitted to the implantable device for immediate reprogramming or presented to a clinician operating the external programmer as recommendations. Alternatively, the external programmer may present the intrinsic conduction data to the clinician who then programs the implantable device in accordance with an algorithm. In another embodiment, the implantable device is programmed to automatically set certain pacing parameters in accordance with information gathered from its sensing channels.

[0009] As will be explained in more detail below, one aspect of the present invention involves the computation based upon intrinsic conduction data of optimum atrio-ventricular delay (AVD) interval for delivering ventricular resynchronization therapy in an atrial tracking and/or atrio-ventricular sequential pacing mode to patients having some degree of AV block or having an atrial conduction delay. Another aspect relates to the optimal pacing configuration for delivering ventricular resynchronization therapy to patients having a right ventricular conduction deficit.

1. Exemplary Hardware Platform

[0010] The following is a description of exemplary hardware components used for practicing the present invention. A block diagram of an implantable cardiac rhythm management device or pulse generator having multiple sensing and pacing channels is shown in FIG. 1. Pacing of the heart with an implanted device involves excitatory electrical stimulation of the heart by the delivery of pacing pulses to an electrode in electrical contact with the myocardium. The device is usually implanted subcutaneously on the patient's chest, and is connected to electrodes by leads threaded through the vessels of the upper venous system into the heart. An electrode can be incorporated into a sensing channel that generates an electrogram signal representing cardiac electrical activity at the electrode site and/or incorporated into a pacing channel for delivering pacing pulses to the site.

[0011] The controller of the device in FIG. 1 is made up of a microprocessor 10 communicating with a memory 12 via a bidirectional data bus, where the memory 12 typically comprises a ROM (read-only memory) and, or a RAM (random-access memory). The controller could be implemented by other types of logic circuitry (e.g., discrete components or programmable logic arrays) using a state machine type of design, but a microprocessor-based system is preferable. As used herein, the programming of a controller should be taken to refer to either discrete logic circuitry configured to perform particular functions or to the code executed by a microprocessor. The controller is capable of operating the pacemaker in a number of programmed modes where a programmed mode defines how pacing pulses are output in response to sensed events and expiration of time intervals. A telemetry interface 80 is provided for communicating with an external programmer 300. The external programmer is a computerized device with an associated display and input means that can interrogate the pacemaker and receive stored data as well as directly adjust the operating parameters of the pacemaker. As described below, in certain embodiments of a system for setting pacing parameters, the external programmer may be utilized for computing optimal pacing parameters from data received from the implantable device over the telemetry link which can then be set automatically or presented to a clinician in the form of recommendations.

[0012] The embodiment shown in FIG. 1 has four sensing/pacing channels, where a pacing channel is made up of a pulse generator connected to an electrode while a sensing channel is made up of the sense amplifier connected to an electrode. A MOS switching network 70 controlled by the microprocessor is used to switch the electrodes from the input of a sense amplifier to the output of a pulse generator. The switching network 70 also allows the sensing and pacing channels to be configured by the controller with different combinations of the available electrodes. The channels may be configured as either atrial or ventricular channels allowing the device to deliver conventional ventricular single-site pacing with or without atrial tracking, biventricular pacing, or multi-site pacing of a single chamber. In an example configuration, a left atrial sensing/pacing channel includes ring electrode 53a and tip electrode 53b of bipolar lead 53c, sense amplifier 51, pulse generator 52, and a channel interface 50, and a right atrial sensing/pacing channel includes ring electrode 43a and tip electrode 43b of bipolar lead 43c, sense amplifier 41, pulse generator 42, and a channel interface 40. A right ventricular sensing/pacing channel includes ring electrode 23a and tip electrode 23b of bipolar lead 23c, sense amplifier 21, pulse generator 22, and a channel interface 20, and a left ventricular sensing/pacing channel includes ring electrode 33a and tip electrode 33b of bipolar lead 33c, sense amplifier 31, pulse generator 32, and a channel interface 30. The channel interfaces communicate bi-directionally with a port of microprocessor 10 and include analog-to-digital converters for digitizing sensing signal inputs from the sensing amplifiers, registers that can be written to for adjusting the gain and threshold values of the sensing amplifiers, and registers for controlling the output of pacing pulses and/or changing the pacing pulse amplitude. In this embodiment, the device is equipped with bipolar leads that include two electrodes which are used for outputting a pacing pulse and/or sensing intrinsic activity. Other embodiments may employ unipolar leads with single electrodes for sensing and pacing. The switching network 70 may configure a channel for unipolar sensing or pacing by referencing an electrode of a unipolar or bipolar lead with the device housing or can 60.

[0013] The controller 10 controls the overall operation of the device in accordance with programmed instructions stored in memory. The controller 10 interprets electrogram signals from the sensing channels and controls the delivery of paces in accordance with a pacing mode. An exertion level sensor 330 (e.g., an accelerometer, a minute ventilation sensor, or other sensor that measures a parameter related to metabolic demand) enables the controller to adapt the atrial and/or ventricular pacing rate in accordance with changes in the patient's physical activity, termed a rate-adaptive pacing mode. The sensing circuitry of the device generates atrial and ventricular electrogram signals from the voltages sensed by the electrodes of a particular channel. An electrogram is analogous to a surface EKG and indicates the time course and amplitude of cardiac depolarization and repolarization that occurs during either an intrinsic or paced beat. When an electrogram signal in an atrial or ventricular sensing channel exceeds a specified threshold, the controller detects an atrial or ventricular sense, respectively, which pacing algorithms may employ to trigger or inhibit pacing.

2. Cardiac Resynchronization Pacing Therapy

[0014] Cardiac resynchronization therapy is most conveniently delivered in conjunction with a bradycardia pacing mode. Bradycardia pacing modes refer to pacing algorithms used to pace the atria and/or ventricles in a manner that enforces a certain minimum heart rate. Because of the risk of inducing an arrhythmia with asynchronous pacing, most pacemakers for treating bradycardia are programmed to operate synchronously in a so-called demand mode where sensed cardiac events occurring within a defined interval either trigger or inhibit a pacing pulse. Inhibited demand pacing modes utilize escape intervals to control pacing in accordance with sensed intrinsic activity. In an inhibited demand mode, a pacing pulse is delivered to a heart chamber during a cardiac cycle only after expiration of a defined escape interval during which no intrinsic beat by the chamber is detected. For example, a ventricular escape interval for pacing the ventricles can be defined between ventricular events, referred to as the cardiac cycle (CC) interval with its inverse being the lower rate limit or LRL. The CC interval is restarted with each ventricular sense or pace. In atrial tracking and AV sequential pacing modes, another ventricular escape interval is defined between atrial and ventricular events, referred to as the atrio-ventricular pacing delay interval or AVD, where a ventricular pacing pulse is delivered upon expiration of the atrio-ventricular pacing delay interval if no ventricular sense occurs before. In an atrial tracking mode, the atrio-ventricular pacing delay interval is triggered by an atrial sense and stopped by a ventricular sense or pace. An atrial escape interval can also be defined for pacing the atria either alone or in addition to pacing the ventricles. In an AV sequential pacing mode, the atrio-ventricular delay interval is triggered by an atrial pace and stopped by a ventricular sense or pace. Atrial tracking and AV sequential pacing are commonly combined so that an AVD starts with either an atrial pace or sense. When used in CRT, the AVD may be the same or different in the cases of atrial tracking and AV sequential pacing.

[0015] As described above, cardiac resynchronization therapy is pacing stimulation applied to one or more heart chambers in a manner that compensates for conduction delays. Ventricular resynchronization pacing is useful in treating heart failure in patients with interventricular or intraventricular conduction defects because, although not directly inotropic, resynchronization results in a more coordinated contraction of the ventricles with improved pumping efficiency and increased cardiac output. Ventricular resynchronization can be achieved in certain patients by pacing at a single unconventional site, such as the left ventricle instead of the right ventricle in patients with left ventricular conduction defects. Resynchronization pacing may also involve biventricular pacing with the paces to right and left ventricles delivered either simultaneously or sequentially, with the interval between the paces termed the biventricular offset (BVO) interval (also sometimes referred to as the LV offset (LVO) interval or VV delay). The offset interval may be zero in order to pace both ventricles simultaneously, or non-zero in order to pace the left and right ventricles sequentially. As the term is used herein, a negative BVO refers to pacing the left ventricle before the right, while a positive BVO refers to pacing the right ventricle first. In an example biventricular resynchronization pacing mode, right atrial paces and senses trigger an AVD interval which upon expiration results in a pace to one of the ventricles and which is stopped by a right ventricular sense. The contralateral ventricular pace is delivered at the specified BVO interval with respect to expiration of the AVD interval.

[0016] Cardiac resynchronization therapy is most commonly applied in the treatment of patients with heart failure due to left ventricular dysfunction which is either caused by or contributed to by left ventricular conduction abnormalities. In such patients, the left ventricle or parts of the left ventricle contract later than normal during systole which thereby impairs pumping efficiency. In order to resynchronize ventricular contractions in such patients, pacing therapy is applied such that the left ventricle or a portion of the left ventricle is pre-excited relative to when it would become depolarized in an intrinsic contraction. Optimal pre-excitation of the left ventricle in a given patient may be obtained with biventricular pacing or with left ventricular-only pacing. Although not as common, some patients have a right ventricular conduction deficit such as right bundle branch block and require pre-excitation of the right ventricle in order achieve synchronization of their ventricular contractions.

3. Optimal Adjustment of Pre-excitation Timing Parameters

[0017] Once a particular resynchronization pacing configuration and mode is selected for a patient, pacing parameters affecting the manner and extent to which pre-excitation is applied must be specified. For optimum hemodynamic performance, it is desirable to deliver ventricular pacing, whether for resynchronization pacing or conventional bradycardia pacing, in an atrial tracking and/or AV sequential pacing mode in order to maintain the function of the atria in pre-loading the ventricles (sometimes referred to atrio-ventricular synchrony). Since the objective of CRT is to improve a patient's cardiac pumping function, it is therefore normally delivered in an atrial-tracking and/or AV sequential mode and requires specification of AVD and BVO intervals which, ideally, result in the ventricles being synchronized during systole after being optimally preloaded during atrial systole. That is, both optimal interventricular synchrony and optimal atrio-ventricular synchrony are achieved. As the term is used herein for biventricular pacing, the AVD interval refers to the interval between an atrial event (i.e., a pace or sense in one of the atria, usually the right atrium) and the first ventricular pace which pre-excites one of the ventricles. The AVD interval may be the same or different depending upon whether it is initiated by an atrial sense or pace (i.e., in atrial tracking and AV sequential pacing modes, respectively). The pacing instant for the non-pre-excited ventricle is specified by the BVO interval so that it is paced at an interval AVD+BVO after the atrial event. It should be appreciated that specifying AVD and BVO intervals is the same as specifying a separate AVD interval for each ventricle, designated as AVDR for the right ventricle and AVDL for the left ventricle. In patients with intact and normally functioning AV conduction pathways, the non-pre-excited ventricle will be paced, if at all, close to the time at which that ventricle is intrinsically activated in order to achieve optimal preloading. In patients with normal AV conduction, the optimal AVD and BVO intervals are thus related to both the intrinsic atrio-ventricular interval and the amount of pre-excitation needed for one ventricle relative to the other (i.e., the extent of the ventricular conduction deficit).

[0018] In order to optimally specify the AVD and BVO parameters for a particular patient, clinical hemodynamic testing may be performed after implantation where the parameters are varied as cardiac function is assessed. For example, a patient may be given resynchronization stimulation while varying pre-excitation timing parameters in order to determine the values of the parameters that result in maximum cardiac performance, as determined by measuring a parameter reflective of cardiac function such as maximum left ventricular pressure change (dP/dt), arterial pulse pressure, or measurements of cardiac output. Determining optimal pacing parameters for an individual patient by clinical hemodynamic testing, however, is difficult and costly. It would be advantageous if such optimal pacing parameters could be determined from measurements of intrinsic conduction parameters which reflect how excitation is conducted within the patient's heart during intrinsic beats. In the approach of the present invention, therefore, intrinsic conduction data is collected from a surface EKG or from the sensing channels of the implantable cardiac resynchronization device and then used to compute optimum values of resynchronization pacing parameters.

[0019] As noted above, the objective of CRT is to restore a normal or near-normal conduction sequence to ventricular contractions by using pacing pulses to compensate for interventricular and/or intraventricular conduction deficits. CRT is most commonly used to treat left ventricular dysfunction brought about by parts of the left ventricle contracting later than normal during an intrinsic cardiac cycle. Biventricular (or left ventricle-only) pacing accomplishes this by pre-exciting the left ventricle with a first pace delivered to the left ventricle followed by a pace to the right ventricle at the BVO interval (or intrinsic activation of the right ventricle in the case of left ventricle-only pacing). The left ventricle electrode excites the left ventricular free wall, while the right ventricle electrode excites the ventricular septum. The desired situation is simultaneous contraction of the left ventricular free wall and septum (septum- free wall fusion). When clinical hemodynamic testing is performed on a population of subjects with intact AV pathways to determine the optimum values of the AVD and BVO intervals, there is found to be a correlation between the optimum AVD and BVO intervals for a particular subject and that subject's measured conduction delay between the right and left ventricles during an intrinsic beat. The optimum AVD interval and the intrinsic atrio-ventricular interval are also correlated from patient to patient. Therefore, the optimum AVD and BVO intervals for a particular patient may be estimated from intrinsic conduction data in terms of specified coefficients k_n as:
BVO=k₁·Δ_RL+k₂
and
AVD=k₃AV_R+k₃·Δ_RL+k₄
where Δ_RL is a measurement reflective of the interventricular conduction delay between the right and left ventricles such as the interval between right and left ventricular senses or the QRS width on a surface ECG, and AV_R is the right intrinsic atrio-ventricular interval measured as the interval between an atrial sense (or pace) and a right ventricular sense. It should be appreciated that these equations can also be expressed in terms of separate AVD intervals for the right and left ventricles, designated as AVDR and AVDL, respectively, and separate measured intrinsic atrio-ventricular intervals for the right and left ventricles, designated AV_R and AV_L, respectively. The intervals are thus related as:
AV_R−AV_L=Δ_RL (if determined by right and left ventricular senses)
and
AVDL−AVDR=BVO
The equations for computing optimal values of AVDR and AVDL are thus:
AVDR=k₅AV_R+k₆AV_L+k₇
and
AVDL=k₈AV_R+k₉AV_L+k₁₀
In certain implementations of the techniques described herein, separate intrinsic atrio-ventricular intervals are measured following an atrial sense and following an atrial pace which are then used to compute AV delay intervals for pacing in atrial tracking and AV sequential modes, respectively. Also, unless otherwise specified, the term biventricular pacing should be taken to include left ventricle-only pacing. If the computed optimal BVO interval is a large negative value which is longer than the right intrinsic atrio-ventricular interval, the right ventricle will be activated intrinsically anyway so that the biventricular pacing is effectively left ventricle-only pacing.

[0020] In order to pre-derive the specified coefficients for later programming into the system or for use by a clinician, clinical population data is obtained that relates particular values of the measured intrinsic conduction parameters to an optimum value of the pre-excitation timing parameter as determined by concurrent measurement of another parameter reflective of cardiac function (e.g., maximum dP/dt or minimum atrial rate). A linear regression analysis is then performed to derive values of the specified coefficients used in the formula for setting the pre-excitation timing parameter, the specified coefficients thus being regression coefficients.

[0021] The techniques for setting resynchronization pacing parameters as described above, as well as others to be described below, may be implemented in a number of different ways. In one implementation, a system for setting the pacing parameters includes an external programmer. In an example embodiment, one or more intrinsic conduction parameters, as measured from electrogram signals generated by the sensing channels of an implantable cardiac resynchronization device during intrinsic beats, are transmitted to the external programmer via a wireless telemetry link. The measured intrinsic conduction parameters may represent averages of values obtained during a specified number of intrinsic beats. The external programmer then computes a pre-excitation timing parameter such as the AVD or BVO in accordance with a formula that equates an optimum value of the pre-excitation timing parameter to a linear sum of the measured intrinsic conduction parameters multiplied by specified coefficients. In an automated system, the external programmer then automatically programs the implantable device with the computed optimum parameter values, while in a semi-automated system the external programmer presents the computed optimum values to a clinician in the form of a recommendation. An automated system may also be made up of the implantable device alone which collects intrinsic conduction data, computes the optimum parameter values, and then sets the parameters accordingly. In another embodiment, which may be referred to as a manual system, the external programmer presents the collected intrinsic conduction data to a clinician who then programs the implantable device with parameters computed from the intrinsic conduction data by, for example, using a printed lookup table and procedure. Unless otherwise specified, references to a system for computing or setting pacing parameters throughout this document should be taken to include any of the automated, semi-automated, or manual systems just described.

4. Computation of Optimal Pacing Parameters Based Upon Atrial Conduction Delay

[0022] The above-described methods for computing optimal resynchronization pacing parameters from intrinsic conduction data are based upon measured intrinsic conduction data which include intrinsic atrio-ventricular intervals and/or interventricular conduction delays. Another measurable intrinsic conduction value which may be used in computing optimal pacing parameters is the conduction delay between the right and left atria during an intrinsic beat. One use of this technique is to compute optimal resynchronization pacing parameters for patients with AV block. AV block refers to an impairment in the AV conduction pathways such that either no intrinsic conduction from the atria to the ventricles occurs (complete or 3_rd degree AV block) or the intrinsic atrio-ventricular interval is longer than normal. In patients with some degree of AV block, the above techniques cannot be used to estimate the optimum AVD interval for CRT because either no measured intrinsic atrio-ventricular interval can be obtained or, if it can, it does not reflect optimum hemodynamics. In one aspect of the present invention, an optimum AVD interval for pre-exciting the left ventricle in such patients may instead be computed from a linear function of the intrinsic delay between right atrial and left atrial activation, referred to as the RA-LA interval:
AVD=k₁₁(RA-LA)+k₁₂
where the AVD interval in this situation is the interval from an atrial event to a left ventricular pace. The coefficients of the equation may be obtained as before from a regression analysis of clinical population data. The technique may be implemented with the device in FIG. 1 using the right and left atrial sensing channels to measure the RA-LA interval as the interval between right atrial pace or sense and a left atrial sense. After obtaining the optimum AVD interval, the BVO interval may be set to either a nominal value or computed from intrinsic conduction data as described above.

[0023] The rationale for employing the RA-LA interval to compute an optimum AVD is as follows. When intrinsic depolarization is absent, ventricular synchrony is optimized by delivering biventricular stimulation with or without an offset to result in septum-free wall fusion. Atrial-ventricular synchrony for the left atrium and left ventricle in this situation is optimized when the onset of left ventricular pressure rise coincides with the peak of left atrial contraction. To achieve optimal atrio-ventricular synchrony in the treatment of left ventricular dysfunction, stimulation to the left ventricle should therefore be applied at a certain time ahead of peak contraction in the left atrium. FIG. 2 is a timing diagram which illustrates this point. During an intrinsic beat, a right atrial sense RA_S is followed by a left ventricular sense LA_S which is then followed by the peak of left atrial contraction X. In a subsequent beat where the left ventricle is paced at an optimal AVD interval with respect to a right atrial sense, the left ventricular pace LV_P results in the onset of left ventricular pressure rise Y coinciding with the peak of left atrial contraction X. The optimum AVD interval is thus proportional to the interval between the right atrial sense RA_S and the peak of left atrial contraction X or its surrogate, the RA-LA interval between the right atrial sense RA_S and the left atrial sense LA_S.

[0024] The RA-LA interval may also be used to compute optimal resynchronization parameters in patients with normal AV conduction. For example, if it is determined that intrinsic left ventricular activation occurs later than left atrial activation by a specified threshold amount, it may be desirable to compute the resynchronization pacing parameters based upon the intrinsic atrio-ventricular interval, the interventricular conduction delay, and the RA-LA interval. Thus, the right ventricle may be paced at an AVDR interval computed as described above as:
AVDR=k₅AV_R+k₆AV_L+k₇
or
AVDR=k₃AV_R+k₃·Δ_RL+k₄
The AVDL interval for pacing the left ventricle (or the BVO interval) may then be computed as a function of the RA-LA interval:
AVDL=k₁₃(RA-LA)+k₁₄
5. Variable AVD Interval

[0025] The optimal values of the AVD interval discussed above have been computed without regard to the atrial rate. For optimum hemodynamics, however, the AVD interval should vary with atrial rate in a manner similar to the way the intrinsic AV interval varies with atrial rate in normal individuals. Such physiological varying of the intrinsic AV interval is due to neural and hormonal influences acting on the AV node which cause the interval to shorten when sympathetic nervous activity is high and lengthen when parasympathetic nervous activity is high, which factors also act on the sino-atrial node to result in high and low atrial rates, respectively. Since improvement in hemodynamics is the objective of CRT, it would be desirable for a programmed AVD interval used in delivering CRT to mimic the physiological varying of the intrinsic AV interval. Varying of the AVD interval would be especially useful during high atrial rates caused by rate-adaptive pacing modes at high exertion levels or atrial overdrive pacing algorithms.

[0026] The AVD interval may be varied by programming the implantable CRT device with a look-up table or function which maps particular atrial rates to particular optimal AVD values. As the atrial rate changes, either due to atrial pacing or intrinsic atrial activity, the device may then dynamically compute the optimal AVD interval. For atrial rates which are not in the table, a linear or non-linear interpolating function may be used. Such a table or function may be generated in various ways. One way is during clinical hemodynamic testing where the AVD intervals which result in a maximized cardiac function parameter are determined for different atrial rates. Examples of cardiac function parameters which could be used in this procedure include dP/dt, arterial pulse pressure (e.g., finger pulse pressure), and measurements of cardiac output. Determining multiple optimal AVD intervals in this manner, however, is even more time-consuming and difficult than determining a single optimal non-varying AVD interval using clinical hemodynamic testing. Accordingly, a system made up of an external programmer and an implantable device or the implantable device alone may use intrinsic conduction data collected by the implantable device's sensing channels to compute optimal AVD intervals for a plurality of atrial rates.

[0027] In an exemplary embodiment, the system computes optimal atrio-ventricular delay (AVD) intervals to be used for delivering cardiac resynchronization therapy to a particular patient by collecting intrinsic conduction data at different atrial rates. The atrial rate may vary intrinsically or as a result of pacing during the data collection. Such data may be an intrinsic AV interval measured as the interval from an atrial sense or pace to a ventricular sense in the case of patients with normal AV conduction, or the RA-LA interval measured as the interval from a right atrial sense or pace to a left atrial sense in the case of patients with or without normal AV conduction. The system computes an optimal value of the AVD interval for delivering a left ventricular pace following an atrial event in an atrial tracking or AV sequential pacing mode as a linear function of the measured intrinsic AV interval or the RA-LA interval in the same manner as described earlier. The computed optimal AVD interval value is associated with an atrial rate corresponding to the measured atrial rate which was present when the intrinsic AV interval or RA-LA interval was measured, with the optimal AVD interval value and its associated atrial rate being stored in a table contained in the memory of the implantable device. After generating a table in this manner (e.g., after at least two such optimal AVD intervals have been stored in the table), the implantable device may vary the AVD interval as the atrial rate changes (by, e.g., rate-adaptive pacing of the atria or pacing the atria in accordance with an overdrive algorithm) by determining a present atrial rate and setting the AVD interval to the optimum value stored in the table which is associated with the present atrial rate. In one variation of the embodiment, the atrial rate associated with each optimal AVD value stored in the table is a range defined by upper and lower limits. The implantable device then varies the AVD interval with the atrial rate by selecting an optimum AVD interval value from the table which is associated with a range which encompasses the present atrial rate. In another variation, the implantable device varies the AVD interval if there is no associated atrial rate in the table corresponding to the present atrial rate by interpolating between two optimum AVD values stored in the table which have associated atrial rates above and below the present atrial rate using a linear or non-linear interpolation function. In still another variation, the implantable device is programmed to update the table if there is no associated atrial rate in the table corresponding to the present atrial rate by measuring an RA-LA interval or intrinsic AV interval, compute an optimal value for the AVD interval as a linear function of the RA-LA interval or intrinsic AV interval, and store the computed optimal value and an associated atrial rate corresponding to the present atrial rate in the table.

[0028] FIG. 3 illustrates an exemplary algorithm which may be implemented in the programming of an implantable cardiac rhythm management device with a plurality of sensing/pacing channels for sensing cardiac electrical activity and delivering pacing pulses to selected cardiac chambers and a controller programmed to deliver biventricular pacing in accordance with an atrial tracking or AV sequential pacing mode and a programmable atrio-ventricular delay (AVD) interval. The controller is programmed to determine a present atrial rate at step S1 and, if an appropriate table entry for that atrial rate exists as determined at step S2, vary the AVD interval by setting the AVD interval to an optimum value stored in a table which is associated with the present atrial rate at step S3. The controller may be further programmed to vary the AVD interval if there is no associated atrial rate in the table corresponding to the present atrial rate by interpolating between two optimum AVD values stored in the table which have associated atrial rates above and below the present atrial rate. If an appropriate table entry for the present atrial rate does not exist as determined at step S2, the controller is further programmed to initialize or update the table by taking an intrinsic conduction measurement at step S4, where the intrinsic conduction measurement may be either an RA-LA interval or an intrinsic AV interval. If the latter is used, collection of that parameter requires a temporary cessation of pacing therapy so that intrinsically conducted ventricular beats can occur. An optimal value for the AVD interval as a linear function of the intrinsic conduction measurement is then computed at step S5, and the computed optimal value and an associated atrial rate corresponding to the present atrial rate are stored in the table at step S6.

[0029] Although the invention has been described in conjunction with the foregoing specific embodiments, many alternatives, variations, and modifications will be apparent to those of ordinary skill in the art. Other such alternatives, variations, and modifications are intended to fall within the scope of the following appended claims.