Defibrillation pacing circuitry

[0001] The invention of the present application may find application in systems such as are disclosed in U.S. patent application entitled “SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,” having Ser. No. 09/663,607, filed Sep. 18, 2000, now U.S. Pat. No. 6,721,597, and U.S. patent application entitled “UNITARY SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,” having Ser. No. 09/663,606, filed Sep. 18, 2000, now U.S. Pat. No. 6,647,292, of which both applications are assigned to the assignee of the present application, and the disclosures of both applications are hereby incorporated by reference.

[0002] In addition, the foregoing applications are related to the U.S. patent application Ser. No. 09/940,283 and entitled “DUCKBILL-SHAPED IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTER AND METHOD OF USE,” U.S. patent application Ser. No. 09/940,371 and entitled “CERAMICS AND/OR OTHER MATERIAL INSULATED SHELL FOR ACTIVE AND NON-ACTIVE S-ICD CAN,” U.S. patent application Ser. No. 09/940,468 and entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH IMPROVED INSTALLATION CHARACTERISTICS,” U.S. patent application Ser. No. 09/941,841 and entitled “SUBCUTANEOUS ELECTRODE WITH IMPROVED CONTACT SHAPE FOR TRANSTHORACIC CONDUCTION,” U.S. patent application Ser. No. 09/940,356 and entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH HIGHLY MANEUVERABLE INSERTION TOOL,” U.S. patent application Ser. No. 09/940,340 and entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH LOW-PROFILE INSTALLATION APPENDAGE AND METHOD OF DOING SAME,” U.S. patent application Ser. No. 09/940,287 and entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH INSERTION TOOL,” U.S. patent application Ser. No. 09/940,377 and entitled “METHOD OF INSERTION AND IMPLANTATION OF IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTERS,” U.S. patent application Ser. No. 09/940,599 and entitled “CANISTER DESIGNS FOR IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS,” U.S. patent application Ser. No. 09/940,373 and entitled “RADIAN CURVE SHAPED IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTER,” U.S. patent application Ser. No. 09/940,273 and entitled “CARDIOVERTER-DEFIBRILLATOR HAVING A FOCUSED SHOCKING AREA AND ORIENTATION THEREOF,” U.S. patent application Ser. No. 09/940,378 and entitled “BIPHASIC WAVEFORM FOR ANTI-BRADYCARDIA PACING FOR A SUBCUTANEOUS IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR,” and U.S. patent application Ser. No. 09/940,266 and entitled “BIPHASIC WAVEFORM FOR ANTI-TACHYCARDIA PACING FOR A SUBCUTANEOUS IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR,” the disclosures of which applications are hereby incorporated by reference.

[0003] The present invention relates to apparatus and methods useful in connection with performing electrical cardioversion/defibrillation and optional pacing of the heart.

[0004] Defibrillation/cardioversion is a technique employed to counter arrhythmic heart conditions including some tachycardias in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with electrical impulses or shocks, of a magnitude substantially greater than pulses used in cardiac pacing.

[0005] Defibrillation/cardioversion systems include body implantable electrodes that are connected to a hermetically sealed container housing the electronics, battery supply and capacitors. The entire system is referred to as implantable cardioverter/defibrillators (ICDs). The electrodes used in ICDs can be in the form of patches applied directly to epicardial tissue, or, more commonly, are on the distal regions of small cylindrical insulated catheters that typically enter the subclavian venous system, pass through the superior vena cava and, into one or more endocardial areas of the heart. Such electrode systems are called intravascular or transvenous electrodes. U.S. Pat. Nos. 4,603,705, 4,693,253, 4,944,300, 5,105,810, the disclosures of which are all incorporated herein by reference, disclose intravascular or transvenous electrodes, employed either alone, in combination with other intravascular or transvenous electrodes, or in combination with an epicardial patch or subcutaneous electrodes. Compliant epicardial defibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and 5,618,287, the disclosures of which are incorporated herein by reference. A sensing epicardial electrode configuration is disclosed in U.S. Pat. No. 5,476,503, the disclosure of which is incorporated herein by reference.

[0006] In addition to epicardial and transvenous electrodes, subcutaneous electrode systems have also been developed. For example, U.S. Pat. Nos. 5,342,407 and 5,603,732, the disclosures of which are incorporated herein by reference, teach the use of a pulse monitor/generator surgically implanted into the abdomen and subcutaneous electrodes implanted in the thorax. This system is far more complicated to use than current ICD systems using transvenous lead systems together with an active can electrode and therefore it has no practical use. It has in fact never been used because of the surgical difficulty of applying such a device (3 incisions), the impractical abdominal location of the generator and the electrically poor sensing and defibrillation aspects of such a system.

[0007] Recent efforts to improve the efficiency of ICDs have led manufacturers to produce ICDs which are small enough to be implanted in the pectoral region. In addition, advances in circuit design have enabled the housing of the ICD to form a subcutaneous electrode. Some examples of ICDs in which the housing of the ICD serves as an optional additional electrode are described in U.S. Pat. Nos. 5,133,353, 5,261,400, 5,620,477, and 5,658,321 the disclosures of which are incorporated herein by reference.

[0008] ICDs are now an established therapy for the management of life threatening cardiac rhythm disorders, primarily ventricular fibrillation (V-Fib). ICDs are very effective at treating V-Fib, but are therapies that still require significant surgery.

[0009] As ICD therapy becomes more prophylactic in nature and used in progressively less ill individuals, especially children at risk of cardiac arrest, the requirement of ICD therapy to use intravenous catheters and transvenous leads is an impediment to very long term management as most individuals will begin to develop complications related to lead system malfunction sometime in the 5-10 year time frame, often earlier. In addition, chronic transvenous lead systems, their reimplantation and removals, can damage major cardiovascular venous systems and the tricuspid valve, as well as result in life threatening perforations of the great vessels and heart. Consequently, use of transvenous lead systems, despite their many advantages, are not without their chronic patient management limitations in those with life expectancies of >5 years. The problem of lead complications is even greater in children where body growth can substantially alter transvenous lead function and lead to additional cardiovascular problems and revisions. Moreover, transvenous ICD systems also increase cost and require specialized interventional rooms and equipment as well as special skill for insertion. These systems are typically implanted by cardiac electrophysiologists who have had a great deal of extra training.

[0010] In addition to the background related to ICD therapy, the present invention requires a brief understanding of a related therapy, the automatic external defibrillator (AED). AEDs employ the use of cutaneous patch electrodes, rather than implantable lead systems, to effect defibrillation under the direction of a bystander user who treats the patient suffering from V-Fib with a portable device containing the necessary electronics and power supply that allows defibrillation. AEDs can be nearly as effective as an ICD for defibrillation if applied to the victim of ventricular fibrillation promptly, i.e., within 2 to 3 minutes of the onset of the ventricular fibrillation.

[0011] AED therapy has great appeal as a tool for diminishing the risk of death in public venues such as in air flight. However, an AED must be used by another individual, not the person suffering from the potential fatal rhythm. It is more of a public health tool than a patient-specific tool like an ICD. Because >75% of cardiac arrests occur in the home, and over half occur in the bedroom, patients at risk of cardiac arrest are often alone or asleep and can not be helped in time with an AED. Moreover, its success depends to a reasonable degree on an acceptable level of skill and calm by the bystander user.

[0012] What is needed therefore, especially for children and for prophylactic long term use for those at risk of cardiac arrest, is a combination of the two forms of therapy which would provide prompt and near-certain defibrillation, like an ICD, but without the long-term adverse sequelae of a transvenous lead system while simultaneously using most of the simpler and lower cost technology of an AED. What is also needed is a cardioverter/defibrillator that is of simple design and can be comfortably implanted in a patient for many years.

[0013] Moreover, it has appeared advantageous to the inventor to provide the capability in such improved circuitry to provide a signal suitable for pacing when the circuitry is not operating in a defibrillation mode.

[0014] Accordingly, the invention relates in various aspects to methods and apparatus for selectively converting a defibrillator circuit or circuit for delivering a defibrillating pulse to a patient into circuitry suitable for providing a constant current, useful, e.g., in pacing applications.

[0022] FIG. 1 illustrates a conventional “H-bridge” defibrillator circuit 11. The circuit 11 includes a capacitor C₁ which is charged to a high voltage V_HV and four switches H₁, H₂, L₁, L₂. The capacitor C1 and switches H₁, H₂, L₁, L₂ are used to create either a monophasic voltage pulse or a biphasic voltage pulse (FIG. 2) across a patient represented by resistance R_PATIENT. In various applications, the switches H₁, H₂, L₁, L₂, may be MOSFETs, IGBTs, or SCRs (silicon controlled rectifiers).

[0023] To create a biphasic waveform such as that shown in FIG. 2, a first pair of switches, e.g., H₁ and L₂, may be closed to create a positive pulse 13. Then all of the switches, H₁, H₂, L₁, L₂, are turned off during a “center pulse” delay period 15. At the end of the delay period 15, the switches H₂ and L₁ are both closed, thereby reversing the current through the patient R_PATIENT to produce a negative voltage pulse 17. Typically, digital logic is employed to control the sequencing of the switches H₁, H₂, L₁, L₂. In such cases, the order of the pulses can be inverted, i.e., the negative pulse 17 can be produced before the positive pulse 13. In illustrative applications, the duration of the pulses 13, 17 is, e.g., 1 to 20 milliseconds and the inter-pulse delay d₁ is, e.g., one millisecond.

[0024] FIG. 3 illustrates circuitry which may operate as a defibrillator circuit during a first selected interval and as a constant current source during a second selected interval. The constant current may be useful, for example, in providing a “pacing” current to a patient R_PAT.

[0025] As in FIG. 1, the high side switches H₁, H₂ employed in FIG. 3 may be IGBTs, MOSFETs, SCRs, or other suitable switches. Such high side switches H₁, H₂ may be controlled in any suitable manner such as, for example, with pulse transformers, opto-couplers, photo-voltaic generators, or in accordance with the teachings of application filed Ser. No. 10/011,957, filed on Nov. 5, 2001 on behalf of the same inventor and entitled Simplified Defibrillator Output Circuit, herein incorporated by reference. Digital logic suitable for controlling such circuitry to achieve switching may comprise a programmed microprocessor, microcontroller, discrete logic, or other forms of digital logic control.

[0026] In the circuit of FIG. 3, a resistor R₁ is inserted in series with the emitter or the source leg of a first low side transistor Q₂, which is preferably an IGBT or MOSFET. Similarly, a resistor R₂ is inserted in series with the emitter or source leg of the second low side transistor Q₁. A constant voltage is applied across the resistor R₁ via a voltage source, which applies a voltage V_DRIVE to the gate (or base) of the first low side transistor Q₂. During operation of the circuit of FIG. 2 as a defibrillator, the transistors Q₁, Q₂ serve the purposes of low side switches, e.g., L₁, L₂ of FIG. 1, and the resistors R₁, R₂ are switched out of the circuit by suitable means, e.g., switches SW₁ and SW₂. During pacing operation of the circuit of FIG. 3, a suitable switching signal is applied to switch resistor R₁ into the circuit.

[0027] In an illustrative application of the circuitry of FIG. 3, the low side transistors Q₁, Q₂ may be high voltage IGBTs or MOSFETs, ranging from 500 volts to 3,000 volts capacity or greater. In the circuit of FIG. 3, the voltage across the resistor R₁ is defined by the equation:
V_R1=V_DRIVE=V_T  (1)
where V_T is the fixed (constant) threshold voltage of the low side transistor Q₁. Thus, if V_DRIVE is 15 volts, and V_T is in the range of 2-6 volts, V_R1 is in the range 13 to 9 volts. Accordingly, a constant voltage is applied across the resistor R₁, resulting in a constant current I_R1 through the resistor R₁, and hence through the patient R_PATIENT.

[0028] As those skilled in the art may appreciate, the threshold voltage V_T of the transistor Q₁ may vary from device to device. Hence, it is typically necessary to calibrate the circuit in production. In calibrating a circuit like that of FIG. 3, a known voltage is applied and the current through R₁ is measured, typically resulting in a large offset, which is compensated for by the system software.

[0029] In order to avoid calibration, the voltage source may be constructed using a feedback circuit employing an operational amplifier A as shown in FIG. 4. The op-amp A is connected to directly drive the low side transistor Q₂, which may comprise, e.g., a MOSFET or IGBT. Use of the operational amplifier A removes the uncertainty of the threshold voltage V_T so that the current that passes through the resistor R₁ is equal to simply V_DRIVE divided by R₁. Thus, one can either drive the transistor Q₂ with a voltage source and calibrate the system for the V_T of the transistor Q₂ or use an op-amp circuit to remove the error created by the threshold voltage V_T of the transistor Q₂.

[0030] During constant current source operation of the circuit of FIG. 4, the appropriate high side switch is on to permit current flow. In addition, the capacitor voltage V_C needs to be appropriately selected according to a number of considerations. First, the current that is programmed to go through the patient will generate a voltage V_PATIENT across the patient. Then, in order to make the current source work, the voltage compliance V_COMP of the current source must be appropriately set. In the case of FIG. 4, the voltage compliance V_COMP is the voltage V_R1 across the resistor R₁ plus the minimum operating voltage V_T of the low side transistor Q_2. Accordingly, the minimum voltage V_HV across the capacitor C₁ is defined by the relation:
V_HV(min.)=V_PATIENT+V_COMP  (2)
The higher V_HV is above V_HV (min.), the closer the current source will approach an ideal current source. Another consideration in setting V_HV is power consumption.

[0031] The amount of current I_R1 can be varied by varying the voltage V_DRIVE or by switching in different resistors, e.g., in series with or for R₁. From an implementation point of view, it is less attractive to switch in a resistor because such switching requires adding transistors or other switching devices. It is more efficient to simply vary the voltage V_DRIVE. Suitable logic circuitry may be provided to select the value of V_DRIVE. A DAC (digital to analog converter) is one example of such logic circuitry. As those skilled in the art will appreciate, a DAC is a circuit that generates different voltages in response to corresponding digital codes provided to it. Such a DAC could be used to drive either an input of the op-amp A (as illustrated in FIG. 4) or the input (gate) of the transistor Q₁. As noted above, an advantage of the op-amp is that it removes the V_T term from the V_HV equation. Particular parameter ranges for circuitry as configured in FIGS. 3 and 4 include 1 to 50 ohms for the resistance R₁ and 1 to 20 volts for a V_DRIVE resulting in a current ranging from 0 to 500 milliamps.

[0032] Another illustrative circuit for implementing a current source is illustrated in FIG. 5. This circuit employs a resistor R₃ connected between the high voltage capacitor C₁ and the high side switches H₁, H₂. The resistor R₃ is switched out of the circuit by a switch SW₃ for defibrillator operation and into the circuit for pacing.

[0033] The circuit of FIG. 5 is somewhat more energy wasteful but will work with the use of a high voltage switch for SW₃. In the circuit of FIG. 5, the switches H₁, H₂, L₁, L₂ are manipulated so as to place the resistor R₃ in series with the output. The amount of current may then be selected by the voltage to which the capacitor C₁ is charged. As an example, assuming the patient resistance R_PATIENT varies from 30-150 ohms, selecting a resistor R₃ of anywhere from 500-5000 ohms, i.e., a resistance that is much larger than that of the patient, results in an approximation of an ideal current source. The approximation is: $\begin{array}{cc}i=\frac{{\mathrm{VH}}_V}{R_3+R_{\mathrm{PAT}}}& \left(3\right)\end{array}$

[0034] While creation of a current source according to FIG. 5 is relatively easy, switching the circuit to the defibrillation mode is more complex. As shown in FIG. 6, a high voltage switch SW₃ is connected across the series resistor R₃ to switch R₃ out of the circuit in order to enter the defibrillation mode. Since the high voltage switch SW₃ is a floating switch, a high side driver 19 is also needed. These considerations render the circuit of FIG. 6 more difficult to implement in an implantable device.

[0035] In contrast, the circuits of FIGS. 3 and 4 require a switch, e.g., SW₁ to switch to the defibrillation mode, but the switch SW₁ does not have to be a high voltage switch. Instead, the switch SW₁ need only be a smaller, low voltage device having the capacity to pass the defibrillation current. In an illustrative circuit, there may be on the order of only 10 volts across SW₁, which is advantageous.

[0036] Thus, only a low voltage switch need be used in the circuits of FIGS. 3 and 4. No low voltage driver is necessary since the switch SW₁ is referenced to ground and can therefore be driven directly. A high side driver circuit is unnecessary. In either of the circuits of FIG. 3 or FIG. 4, the voltage V_DRIVE is preferably implemented by a DAC, either connected to directly drive the resistor R₁ (FIG. 3) or to drive the resistor R₃ through an op-amp A (FIG. 4).

[0037] Provision of a constant current has the advantage of maintaining a constant current density across the heart, irrespective of the electrode interface impedance.

[0038] While the present invention has been described above in terms of specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the following claims are intended to cover various modifications and equivalent methods and structures included within the spirit and scope of the invention.