Simplified defibrillator output circuit

[0001] The invention of present application may find use in systems such as are disclosed in the U.S. patent application entitled “SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,” having Ser. No. 09/663,607, filed Sep. 18, 2000, pending, 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, pending, 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] Applications related to the foregoing applications include a U.S. patent application Ser. No. 09/940,283 entitled “DUCKBILL-SHAPED IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTER AND METHOD OF USE,” U.S. patent application Ser. No. 09/940,371 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 entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH IMPROVED INSTALLATION CHARACTERISTICS,” U.S. patent application Ser. No. 09/941,814 entitled ‘SUBCUTANEOUS ELECTRODE WITH IMPROVED CONTACT SHAPE FOR TRANSTHORACIC CONDUCTION,” U.S. patent application Ser. No. 09/940,356 entitled ‘SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH HIGHLY MANEUVERABLE INSERTION TOOL,” U.S. patent application Ser. No. 09/940,340 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 entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH INSERTION TOOL,” U.S. patent application Ser. No. 09/940,377 entitled “METHOD OF INSERTION AND IMPLANTATION FOR IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTERS,” U.S. patent application Ser. No. 09/940,599 entitled “CANISTER DESIGNS FOR IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS,” U.S. patent application Ser. No. 09/940,373 entitled “RADIAN CURVE SHAPED IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTER,” U.S. patent application Ser. No. 09/940,273 entitled “CARDIOVERTER-DEFIBRILLATOR HAVING A FOCUSED SHOCKING AREA AND ORIENTATION THEREOF,” U.S. patent application Ser. No. 09/940,378 entitled “BIPHASIC WAVEFORM FOR ANTI-BRADYCARDIA PACING FOR A SUBCUTANEOUS IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR,” and U.S. patent application Ser. No. 09/940,378 entitled “BIPHASIC WAVEFORM FOR ANTI-TACHYCARDIA PACING FOR A SUBCUTANEOUS IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR,”

[0003] The subject invention relates generally to electronic circuitry and finds particular application in defibrillator circuitry.

[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] Circuitry for controlling so-called “high side” switches in defibrillator circuitry has exhibited considerable complexity, necessitating use, for example, of transformers, opto-couplers and/or photo voltaic generators. Such complexity is generally undesirable and particularly undesirable in devices intended for implantation within the human body.

[0014] According to one aspect of the invention, high side defibrillator driver circuitry is provided which employs silicon controlled rectifiers serving as high side switches. Applying a control current to a selected gate of one of the high side SCR's turns on that SCR. According to another aspect of the invention, positive turn-on of the high side SCRs is ensured by inserting a constant current source into the low side activation current path at start-up.

[0019] FIG. 1 illustrates an electrical circuit including a first illustrative embodiment according to the invention. The circuit includes a high voltage capacitor C₁ grounded at one terminal and connected at its opposite terminal to respective anodes of two silicon controlled rectifiers SCR₁, SCR₂.

[0020] The respective cathodes of the respective rectifiers SCR₁, SCR₂ are connected to respective first terminals of first and second low side switches SW₁, SW₂. The respective cathodes of the silicon controlled rectifiers SCR₁, SCR₂ are additionally electrically coupled to respective physical locations on a patient on either side of a patient resistance denoted R_PAT.

[0021] The gate or trigger terminal of the first silicon controlled rectifier SCR₁ is connected through a capacitor C₃ to a first terminal 101 of the second switch SW₂. The gate or trigger terminal of the second silicon controlled rectifier SCR₂ is connected through a second capacitor C2 to the first terminal 103 of the first low side switch SW,. Respective second terminals of the switches SW₁, SW₂ are connected to ground in the embodiment illustrated in FIG. 1.

[0022] The respective first terminals 103, 101 of the respective low side switches SW₁, SW₂ are those which, in response to application of a switching signal, cause the switches SW₁, SW₂ to close. Hence, the first terminals 101, 103 may comprise, e.g., the gates of respective switching transistors or respective SCRs.

[0023] The embodiment of FIG. 2 particularly illustrates respective transistors Q₁, Q₂ used as the switching devices in a circuit according to FIG. 1. These transistors may be, for example, IGBTs or MOSFETs. The switches SW₁, SW₂ can also comprise silicon controlled rectifiers (SCRs).

[0024] To illustrate operation of the circuit of FIG. 1, assume a control voltage signal is applied to the first terminal 101 of the second switch SW₂. Such application closes the switch SW₂ and creates a current through the coupling capacitor C₃ into the gate of the first silicon controlled rectifier SCR₁, which current turns on SCR₁. Activation of the first silicon controlled rectifier SCR₁ applies a voltage to the patient resistance R_PAT and causes a current to flow through SCR₁, R_PAT, and SW₂ to ground. In one application, this current may be terminated by appropriately turning off the switch SW₂ to thereby create a monophasic waveform.

[0025] To create a biphasic waveform, the switch SW₂ is opened for a selected interval, and a control voltage signal is then applied to the first terminal 103 of the first switch SW₁. This control voltage signal closes the switch SW₁ and creates a drive current into the gate of the second silicon controlled rectifier SCR₂, thereby turning SCR₂ “on.” A current path is thus created from the high voltage capacitor C₁ through SCR₂, the patient resistance R_PAT, and the switch SW₁, resulting in a negative going pulse, i.e., the second phase of a biphasic waveform.

[0026] The circuit of FIG. 1 has a drawback in that it exhibits an extremely rapid change of current with respect to time (“dI/dt”), which may present control problems. Another drawback is that it takes a finite amount of time for the second switch SW₂ to come “on,” which presents the possibility that the SCR1 will not turn on because a silicon controlled rectifier typically requires a certain amount of current flow through it in order to maintain the “on” state. In such case, the circuit will not switch correctly.

[0027] The embodiment of FIG. 3 improves over those of FIGS. 1 and 2 by providing positive control of switching of the silicon controlled rectifiers SCR₁, SCR₂ by causing the low side switches SW₁, SW₂ to operate as current sources.

[0028] Thus, in the embodiment of FIG. 3, the capacitors C₂, C₃ are arranged to have control signals selectively applied to respective terminals 102, 104, which are not connected to the first terminals 101, 103, e.g., gates, of the switching devices SW₁, SW₂. Additionally, respective constant current source circuits 107, 109 are created in the respective lower legs 111, 113 of the switches SW₁, SW₂. Third and fourth switches SW₃, SW₄ are provided to selectively short out the respective constant current sources 107, 109, i.e., create a short circuit around them to ground.

[0029] With respect to the operation of the circuit of FIG. 3, various components are selectively activated (“turned on”) in order to deliver a monophasic pulse, if desired, or both phases of a biphasic waveform. In order to create a monophasic pulse, for example, the switch SW₂ is turned on and enabled to work as a current source. Then the gate of SCR₁ is pulsed with a signal applied to the first terminal 104 of the capacitor C₃. Once the pulse triggers SCR₁, SCR₁ is guaranteed to stay on because the current source 107 is activated to supply an amount of current selected to hold SCR₁ on.

[0030] An example of a current source circuit 107, 109 is illustrated in FIG. 4, where the transistor Q₂ functions as one of the low side switches SW₁, SW₂ of FIG. 3. The constant voltage applied across the resistor R₁ may be supplied, for example, by a digital to analog converter or other techniques including those shown and described in co-pending application Ser. No. 10/011,955 by this inventor entitled Defibrillation Pacing Circuitry filed Nov. 5, 2001 and incorporated by reference herein.

[0031] The current supplied by the current sources 107, 109 can be programmed to any desired amount, e.g., anywhere between 10 to 500 milliamps, but is particularly set to the value of the holding current required by SCR₁. Thus, for example, if the holding current is 100 milliamps, the current through R_PAT rises from 0 to 100 milliamps very quickly, but only to 100 milliamps. Thus, dI/dt is fairly limited.

[0032] At that point (100 ma), the monophase pulse is initiated by turning on the first silicon controlled rectifier SCR₁. As soon as SCR₁ turns on, the current source 107 is switched out of the circuit, e.g., by creating a short across the resistance R₁ of FIG. 4, which results in a very high current (≅V_HV/R_pat) with the dI/dt and the dV/dt being controlled by the controlled slew rate of the turn-on and turn-off time of the drive signal. To truncate the monophase pulse, the low side switch SW₂ is then turned off, the current goes to 0, and the silicon controlled rectifier SCR₁ turns off.

[0033] In order to create the second phase of a biphasic waveform, a similar procedure is followed. The switch SW₁ is enabled, e.g., by application of a 15 volt pulse with respect to ground, as is the current source I₁ in its leg. Then, via the capacitive coupling provided by the capacitor C₂, the second silicon controlled rectifier SCR₂ is triggered, for example, by application of a 15 volt pulse with respect to ground, but only to the programmed holding current provided via the current source 109. Next, the current source 109 is removed and the current through R_PAT ramps up in a controlled manner by controlling the slew rate of the drive signal to create the second phase of the biphasic waveform.

[0034] As those skilled in the art will appreciate, the illustrative embodiments employ a high side bridge wherein the drivers each include only two components, the SCRs and the respective capacitors C₂, C₃. The control voltages applied to the capacitors may range from, for example, 5 to 15 volts. Thus, the high side driver circuitry has been made smaller, simpler, with fewer components. As those skilled in the art appreciate, SCRs are typically smaller devices than IGBTs or MOSFETs, resulting in an even more efficient use of silicon. In addition to these advantages, the dV/dt and the dI/dt are controllable.

[0035] 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.