Implantable defibrillator system

Abstract: The present invention uses switching networks for the predetermined combination of capacitor-discharge waveforms in order to approximate a particular waveform for application to the heart muscle in a defibrillation procedure. These are usually biphasic waveforms, or waveforms exhibiting a positive pulse followed immediately by a negative pulse. Examples of the discharge-process possibilities are those of two capacitors in parallel, two in series, two in sequence, or a capacitor or capacitors with polarity inverted. Adding capacitors and switches expands the range of possibilities and provides waveforms "tailoring" under the guidance of a switching network that can be digitally programmed from an external programming console, much as in the digital programming of a pacemaker system. Digital signals are transmitted to the implanted system using infrared, visible, RF electromagnetic radiation, or ultrasound radiation. The use of multiple capacitors rather than just a single capacitor makes it possible to pack the components more densely, and in the case of some waveforms, to use less total capacitance than in the prior art, both factors contributing to very desirable size reduction for the implantable defibrillator. One or more implanted batteries are used to meet the heavy energy requirements of defibrillation, and these are recharged, one at a time, and after full discharge, by means of implanted series-array photovoltaic devices, preferably monolithic.

Description: The present invention relates to devices for energy delivery to a heart of a human during defibrillation. Defibrillating the human heart is accomplished by applying an electrical waveform to the cardiac muscle with appropriate electrodes, causing the cessation of rapid uncoordinated contractions of the heart and a restoration of normal beating of the heart. The optimal voltage-versus-time waveform for this purpose is known approximately, but not precisely. There is wide agreement in the prior art that, at a minimum, it incorporates a rectangular or approximately rectangular positive-going pulse. The adjective "positive-going" has meaning after one has determined the point on or near the heart that best serves as voltage reference for a particular patient, a matter that varies from person to person. In the simplest case, a single such pulse is used for defibrillation. This option is termed the "monophasic" waveform. For the sake of a concrete description, let one choose a specific waveform that falls in the ranges of accepted values. Such an example is shown in idealized form in FIG. 3. This pulse has an amplitude of +400 volts, and a duration of 7 milliseconds. The literature on this subject is extensive, with a typical example being Feeser, et al., Circulation , volume 82, number 6, page 2128, December, 1990, an article that includes an extensive bibliography. The electrical resistance presented by the human heart muscle to the passage of defibrillation current is quite low, usually falling in the range from 40 to 100 ohms. These low values are a combined result of the large area electrodes employed and nonlinearity to the heart muscle as an electrical resistor, with resistance declining as current increases. Because of this fact, large current values are required, typically several amperes, at the typical but arbitrarily chosen amplitude of 400 V. It is a considerable challenge to generate a rectangular waveform at such a high current, especially in a battery-powered implantable defibrillator system, which must be small and light in weight. Fortunately, an approximation to the idealized waveform is able to accomplish the desired result, but with an effectiveness that is believed to be considerably lower. A simple and extensively used approximation to the ideal waveform can be achieved by charging a capacitor to 400 volts in the present example, and connecting it directly from one electrode to the other on the heart muscle, and letting the heart serve as the load resistance that discharges the capacitor. When a resistor is used in this manner to discharge a capacitor, the result is a voltage-versus-time waveform that declines in exponential fashion from the initial capacitor voltage, +400 volts at present. The nonlinearity mentioned above distorts the exponential waveform somewhat, but not significantly. For purposes of approximating a rectangular waveform, it is necessary to interrupt the discharging process while the voltage remains at some significant fraction of its original value. This can be accomplished by simply breaking, or "opening", the circuit formed by capacitor and resistor, the latter in this case being the heart. At that instant the voltage experienced by the heart drops abruptly to zero. Assuming for specific illustration that in the 7-ms duration of the pulse, the waveform voltage has dropped to 200 volts, or half its initial value, we see that the "characteristic time" of the discharging process, or the time it takes the voltage to decline to about 36.8% of its initial value, must exceed 7 ms in the present case. The characteristic time in seconds is determined by the product of the resistance R in ohms and the capacitance C in farads. For the assumed requirements, one must achieve a characteristic time, or "RC time constant" of 10 ms. Continuing with specific values for illustrative purposes, let one assume a cardiac resistance of 70 ohms, requiring therefore a capacitance of 143 microfarad. Relationships are illustrated in FIG. 4A for the assumed conditions. The qualitative difference between FIGS. 3 and 4A is known in the jargon of the specialty as the degree of "tilt", and a "low-tilt" waveform is preferred. The approximate waveform is unsatisfactory if the voltage at the end of the pulse interval is a small fraction of the initial voltage, a condition that would be described as a "high-tilt" waveform. The connection and disconnection of the capacitor to the heart muscle is accomplished in the present state of the art by means of a switching network employing the power field-effect transistor (or FET), by way of example and for purposes of illustration only and not to be construed as limiting of the present invention. This is a three-terminal, solid-state device, with one terminal being a control electrode, and the other two terminals being power electrodes. Resistance between the power terminals can be either very low (the "closed" condition), or very high (the "open" condition). Thus, the FET is functionally equivalent to the familiar mechanical single-pole, single-throw switch, and is described as such for purposes of brevity and illustration. A circuit employing a single such switch can deliver the waveform illustrated in FIG. 4A. Both the waveform and the circuit that produces it, shown in FIG. 4B, are discussed in more detail later. The first adjective, i.e. "single" of the term single-throw switch indicates that this switch incorporates a single pivoted armature or movable lever, and the second adjective ("throw") indicates that there is one stable position in which current is conducted. Description of the switching network is made more transparent by choosing the mechanical-switch option, as illustrated in FIG. 4B, which schematically presents the network that delivers the result shown in FIG. 4A as later discussed in detail. Another waveform of the prior art is the "biphasic" waveform, which is capable of achieving defibrillation with less energy than that necessary with the monophasic waveform. This difference is of prime importance to implantable systems because the total energy deliverable by a battery over its life is approximately proportional to its size and weight. In a biphasic waveform, a positive pulse of the kind described earlier is followed by a negative pulse. Ideally, the negative pulse should be rectangular, should be comparable in amplitude to the positive pulse, and should have a duration somewhere between 10% and 90% that of the total pulse width. Continuing with specific examples, let one adopt as the positive waveform that of FIG. 5A, where the duration of the negative pulse is shown to be 3 ms. The simplest biphasic switching network of the prior art delivers a negative pulse with an initial amplitude equal to that at the end of the positive pulse, as is illustrated in FIG. 5A. One embodiment of a network for delivering this combined waveform is illustrated in FIG. 5B. This circuit employs two single-pole, triple-throw switches. Such a switch can be realised by using three FETs that all have one power electrode in common. Since the two switches are linked in order to function in coordination, they can be described as a single double-pole, triple-throw switch. Here, c designates the charging position, p the positive-pulse position, and n the negative-pulse position. US-A-4 800 883 discloses an apparatus for energy delivery to a heart of a human during defibrillation, the apparatus comprising at least two capacitors, electronic switching circuitry for connecting said capacitors to the heart and control means connected to the electronic switching circuitry to deliver the capacitor energy to the heart, wherein said capacitors are electrically connected only in series to the other or others of said capacitors for energy delivery to the heart for a first phase of energy delivery. US-A-4 637 397 discloses a method and apparatus for stimulation of cells in arrhythmic myocardial tissues includes depolarizing the cells by application of a first electrical pulse or shock followed by a second electrical pulse having a polarity opposite to the first pulse. Finally, a third pulse having the same polarity as the first pulse is applied to the cells. The present invention provides a device for energy delivery to a heart of a human during defibrillation, comprising the features defined in claim 1. The present invention overcomes the disadvantages of the prior art by providing switching networks that provide, for example, discharging of two capactiors in parallel during the positive pulse, and in series during the negative pulse, and that can also be discharged in sequence, if desired. Extensions of such principles provide switching networks that employ two capacitors, and that provide arbitrary combinations of parallel, series, and sequential discharging of the capacitors, with appropriate control-signal patterns, in order to tailor the resulting waveform into an approximation of any desired ideal waveform, within wide ranges. By similar manipulation of capacitor interconnections, it is possible to capture and use pulse portions that are simply discarded in the prior art, thereby permitting one to achieve certain waveforms with less capacitance than was required before. Because the capacitor is physically the largest component in a prior art defibrillator, reduction in its value and hence size is significant. Such a waveform is illustrated in FIG. 8. It constitutes a better approximation to the ideal monophasic waveform of FIG. 3 than does the prior art waveform of FIG. 4A, but yet it requires 11% less capacitance than the prior art case. To accomplish control and setting of the switching network from outside the body, it is foreseen that digital programming through, for example, RF electromagnetic radiation, will be used as it now is for other systems such as pacemakers. It is further provided that a telemetry relay positioned at or on the body can be employed to facilitate the digital programming. It is further proposed that photovoltaic devices be subcutaneously implanted to charge the implanted batteries. Particularly useful here are series-array "solar" cells, with the monolithic series-array versions of the prior art being especially well-adapted. A further option, of course, is the use of an implanted coil as in the prior art that is powered by an external source of electromagnetic radiation. The general purpose of the invention is to realize an implantable defibrillator system that provides waveform optimization before or after implantation, that has a long useful implanted life, and that is smaller in size than previous implantable defibrillators. Part of the novelty resides in the design of switching networks for waveform tailoring. One embodiment of the present invention provides two capacitors that can by a switching network be discharged in parallel, doubling the current and reducing the tilt, or else can be discharged in series, or in sequence. When the capacitors are discharged in series to provide the negative pulse, it is possible to boost negative-pulse amplitude to approximate that of the positive pulse. Or, the capacitors can be discharged in parallel for less tilt when a diminished amplitude is acceptable. In extensions of the present invention, two capacitors are used, and series, parallel, and sequential capacitor-discharge configurations can be optionally combined to tailor a waveform conforming as nearly as possible to the ideal waveform that will be perceived as the defibrillation art progresses. A novel feature of replacing one large capacitor by two smaller capacitors of the same aggregate capacitance bring design flexibility advantages. For example, the smaller components can be packed more densely, leading to smaller overall package size. This is because the large capacitors used in this work have an inflexible cylindrical form factor. Another novel feature is a reduction in the amount of capacitance needed to generate certain waveforms, and hence a further size reduction. Added to the novelty of the implanted defibrillator system is the combination of the above features with digital programming through rf communication of the kind used in pacemakers. Further combinational novelty includes a telemetry relay to permit the programming electronics and its operator to be spatially removed from the operating theater where implantation of the defibrillator system is performed. A still further novelty resides in the use of implantable batteries, because of the appreciable energy demands over time of defibrillator systems. Because of the relatively high voltage requirements of a defibrillator system, the invention further employs implanted series-array (and hence high-voltage) photovoltaic devices, with monolithic series-array varieties being particularly favored for reasons of reliability and efficiency. Recharging by this means brings with it the novel and attractive features of being noninvasive, inexpensive, and simple to the point of being simplistic with respect to the demands that recharging places on the patient or the medical attendant. One significant aspect and feature of the present invention is the discharging of defibrillation capacitors in parallel, in series, or in sequence, as desired, through novel switching networks. Another significant aspect and feature of the present invention increases the number of capacitors further, and through novel switching networks, provides arbitrary combinations of parallel, series, and sequential discharging of arbitrary combinations of the capacitors, thereby tailoring the defibrillation waveform with a high degree of flexibility. A control system controls the switching network to achieve the desired waveform. An additional significant aspect and feature of the present invention is the realization of certain waveforms with less total capacitance than required in the prior art, thereby making possible a smaller implantable defibrillator because the capacitor is such a dominant component in an implantable defibrillator. Thus, the use of two smaller capacitors contributes in two ways to the size reduction of an implantable defibrillator. Still another significant aspect and feature of the present invention is the digital programming of the control system from outside the body, with an RF transmission being one possible form of communication for the purpose. A telemetry system can be placed at or near the body of the person having the implant. Having thus described the embodiments of the present invention, it is one object of the present invention to achieve defibrillator-waveform flexibility, to adapt to changes in the patient's needs, or to adapt to advancing knowledge of the defibrillation art. Another object of the present invention is to achieve a system of long life and high reliability for the implantable defibrillator system. An additional object of the present invention is to realize a smaller implantable defibrillator package than that of a prior art defibrillator of equal capability. Yet another object of the present invention is to ensure that the patient's portion of the routine-maintenance technology is simple, easy-to-use, nonhazardous, and low-technology in character, especially as to battery recharging. There are now described, by way of example and with reference to the accompanying drawings, devices according to preferred embodiments of the present invention. In the drawings: FIG. 1 illustrates an implantable defibrillator system implanted in a patient and an external programming console; FIG. 2 illustrates a block diagram of the implantable defibrillator system; FIG. 3 presents the idealized waveform for monophasic defibrillation; FIG. 4A shows a prior art monophasic waveform achieved through capacitor discharge and switching; FIG. 4B schematically illustrates a circuit for achieving the prior art monophasic waveform; FIG. 5A presents a prior art biphasic waveform; FIG. 5B schematically illustrates a circuit for generating a prior art biphasic waveform; FIG. 6 illustrates a monophasic waveform resulting from the sequential discharging of two capacitors; FIG. 7A illustrates one biphasic waveform of the present invention; FIG. 7B schematically illustrates a circuit for producing one biphasic waveform of the present invention; and FIG. 8 illustrates yet another monophasic waveform. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates an implantable defibrillator system 5, such as an implantable defibrillator 10 in a patient 11, the implantable defibrillator 10 in turn including an electronic switching network 12 for the flexible combination of capacitor discharge waveforms in order to approximate a particular waveform for application to the heart muscle in a defibrillation procedure. A programming console 13 that is able to compute and transmit instructions to the electronic switching network 12 is comparatively remote from the patient 11 who is undergoing implantation surgery, and a telemetry relay or repeater is employed near or on the patient's body. Digital signals are transmitted to the implanted device using infrared, visible or RF electromagnetic radiation or ultrasound radiation. FIG. 2 illustrates the implantable defibrillator 10 where all numerals correspond to those elements previously described, including a capacitor section 14 electrically connected through electrodes to a heart 15, an electronic switching network 12, a committed or noncommitted fibrillation detector 16, a voltage sensing and control unit 17, batteries 18 and 19 connected through the voltage sensing and control unit 17 to the electronic switching network 12, and a charging unit 20. As illustrated, battery 18 is connected through the voltage sensing and control unit 17 to the electronic switching network 12 and its associated components. The battery 19 is being recharged through the voltage sensing and control unit 17 by the charging unit 20. When battery 18 is deeply discharged, the voltage sensing and control unit 17 switches the output of the charging unit 20 to battery 18 and switches the output of the battery 19 to the electronic switching network 12, as represented by the reversal of representative switches 21 and 22. At this time, battery 18 is charged and battery 19 powers the implantable defibrillator 10. The charging unit 20, when it employs photovoltaic devices, can be a subcutaneously implanted device that is powered from an external light source, and can be implanted subcutaneously or more deeply when it is a coil to which energy is delivered from an external source of electromagnetic radiation. FIG. 3 illustrates a voltage-versus-time waveform 23 that is of idealized rectangular character, having typical, but arbitrarily chosen, amplitude of +400 volts and duration of 7 milliseconds. The waveform consists of a single pulse and it is termed monophasic. FIG. 4A illustrates prior art voltage-versus-time monophasic waveform 24, the type generated by a charged capacitor, appropriately switched. Realistic and arbitrary pulse duration 25 are again employed. The initial pulse voltage 26 is fixed by the voltage to which the capacitor is charged, and the final pulse voltage 27 is fixed by the discharge rate, the RC time constant 28, which can be graphically determined by linear extrapolation of the initial tangent 29 to the point 30 where the tangent intersects the time axis 32. FIG. 4B illustrates a prior art schematic diagram 40 of a circuit placed between a charging source 42 and a heart 44, for generating the prior art monophasic waveform 24 of FIG. 4A, employing a capacitor 46, a single-pole, single-throw switch 48, and a diode 50. FIG. 5A illustrates a biphasic waveform 60 of the prior art, wherein the final height 62 of the positive pulse, +200 volts, is equal in magnitude and opposite in sign to the initial height 64 of the negative pulse, -200 volts. Typically, and arbitrarily, by way of example and for purposes of illustration only and not to be construed as limiting of the present invention, chosen durations for the two pulses are 7 ms 66, and 3 ms 68, respectively. FIG. 5B illustrates the schematic diagram 70 of a circuit placed between a charging source 72 and a heart 74, for generating the biphasic waveform 60 of FIG. 5A, employing a capacitor 76 and the single-pole, triple-throw switches 80 and 90 (which switches in combination can alternatively be described as one double-pole, triple-throw switch). The three positions of the switch 80 are c, the charging position 82, b, the positive-pulse position 84, and n, the negative-pulse position 86. The corresponding positions of the switch 90 are c, the charging position 92, b, the positive-pulse position 94, and n, the negative-pulse position 96. FIG. 7A illustrates a biphasic waveform 140 of the present invention, wherein the initial height 142 of the negative pulse is comparable in magnitude to the initial height 144 of the positive pulse, and is greater in magnitude than the final height 146 of the positive pulse, thus providing a more nearly ideal waveform than the prior art waveform 100 of FIG. 6. FIG. 7B illustrates the schematic diagram 150 of the circuit placed between a charging source 152 and a heart 154 for generating the waveform 140 of FIG. 7A, employing the capacitors 156 and 158, two single-pole, triple-throw switches 160 and 170 (which switches in combination can alternatively be described as one double-pole, triple-throw switch), and also two single-pole, double-throw switches 180 and 190 (which switches in combination can alternatively be described as one double-pole, double-throw switch). The three positions of the switch 160 are c, the charging position 162, p, the positive-pulse position 164, and n, the negative-pulse position 166. The corresponding positions of the switch 170 are the c, charging position 172, p, the positive-pulse position 174, and n, the negative-pulse position 176. The two positions of the switch 180 are c, the charging and positive pulse position 182, and n, the negative-pulse position 184. The corresponding positions of the switch 190 are c, the charging and positive pulse position 192, and n, the negative-pulse position 194. The interconnecting leads 196 and 198 achieve the capacitor "stacking" (series connection) and polarity reversal required for the negative pulse. MODE OF OPERATION Waveform tailoring is accomplished by instantaneous switching that converts parallel-connected capacitors into series-connected capacitors, or the reverse, and/or interchanges of the capacitor-terminal connections. Such manipulation of capacitor interconnection also makes possible a reduction in the total capacitance required to achieve certain waveforms, and hence a reduction in defibrillator size. Power FETs can be used for switching, and are controlled by digital signals. The necessary high or low control voltage is distributed from the switching network that can be digitally programmed. The electronics needed for programming of the implantable defibrillator system need not be implanted, but can be a part of the programming console. Input information to the programming console takes the form of the desired initial voltage of the positive pulse, pulse duration and the tilt, and likewise corresponding data for the negative pulse. In the programming console is the necessary logic and memory, or if needed, ancillary microcomputer hardware and software, that converts the waveform information into digital instructions. The programming console can be comparatively remote from the patient who is undergoing implantation surgery when telemetry relay or a telemetry repeater is employed near or on the patient's body. Foreseen is the use of infrared or radio-frequency electromagnetic radiation, or ultrasound radiation for remote programming purposes. The radiation can be directed along a single line of sight, or can be diverted by means of passive reflector repeater mirrors. THE SWITCHING NETWORKS Novel switching networks are a part of the present invention of the implantable defibrillator system. The principles can be illustrated by observing the waveform features illustrated in FIG. 7A. Here, each of the two capacitors of FIG. 7B are used in a novel manner. During the positive pulse, the two capacitors are discharged in parallel, producing a positive pulse result equivalent to that in FIG. 4A and FIG. 6. The negative pulse is generated by placing the two capacitors in series, providing an initial negative pulse height equal in magnitude to the initial positive pulse height. Because two identical capacitors in series display one quarter of the capacitance of the same two capacitors in parallel, the discharge rate of the negative pulse in FIG. 7A is four times that of the positive pulse in FIG. 7A. The increased tilt is believed to be tolerable because the negative pulse is of relatively short duration. The schematic diagram in FIG. 7B illustrates a circuit that delivers the waveform of FIG. 7A, with the designations c for charging, p for positive-pulse position, and n for negative-pulse position having the same meanings as before. By using more capacitors, additional to the two of the invention, one can clearly see that the principles just illustrated can be combined in a wide variety of ways, and that the resulting capacitor networks can deliver varied waveforms. Placing the FETs of the switching network under the rule of a control network permits a practitioner to adapt defibrillation to changing patient needs, or to further enlightenment concerning optimal waveforms. It is further possible to carry out noninvasive digital programming of the control network by programming means that have been developed for implanted-pacemaker programming. One prominent method uses digitally encoded RF electromagnetic radiation projected through the skin and into the implanted electronic system. A telemetry repeater further improves the use convenience of this kind of system in the operating theater. Since a defibrillator system typically requires that several amperes and hundreds of volts be delivered to the heart muscle, the power requirements are unusual by the standard of ordinary implanted electronic systems. Because the pulses are of brief duration, fortunately, the energy requirements are brought within tractable limits. Nonetheless, the energy that must be implanted in battery form is significant, especially as duty cycle increases, and is a foremost consideration in the engineering and application of this kind of system.

Claims: A device (10) for energy delivery to a heart of a human (15; 154; 214) during defibrillation, comprising: (a) two capacitors (14; 156, 158; 216, 218); (b) electronic switching circuitry (12; 160, 170, 180, 190; 220, 230, 240, 250, 260) for connecting said capacitors (14; 156, 158; 216, 218) to the heart (15; 154; 214); and (c) control means (17) connected to the electronic switching circuitry (12; 160, 170, 180, 190; 220, 230, 240, 250, 260) to deliver the capacitor energy to the heart; characterised in that; for a first phase of energy delivery one of said two capacitors (14; 156, 158; 216, 218) is connected in either of the two modes parallel and series to the other of said two capacitors and during a second phase of energy delivery said one of said two capacitors is connected in the other of said two modes to said other of said two capacitors. A device (10) according to Claim 1, wherein said one of said two capacitors (14; 156, 158; 216, 218) is connected in parallel to the other capacitor for a first phase of energy delivery; and the device (10) includes control means (17) connected to the electronic switching circuitry (12; 160, 170, 180, 190; 220, 230, 240, 250, 260) to deliver the capacitor energy to the heart with said one of said two capacitors (14; 156, 158; 216, 218) connected in series to the other capacitor for a second phase of energy delivery, said second phase being either the same or opposite polarity of said first phase. A device (10) according to Claim 1, wherein said one of said two capacitors (14; 156, 158; 216, 218) is connected in series to the other capacitor for a first phase of energy delivery; and the device (10) includes control means (17) connected to the electronic switching circuitry (12; 160, 170, 180, 190; 220, 230, 240, 250, 260) to deliver the capacitor energy to the heart with said one of said two capacitors (14; 156, 158; 216, 218) connected in parallel to the other capacitor for a second phase of energy delivery, said second phase being either the same or opposite polarity of said first phase.

Inventors: ADAMS THEODORE P, KROLL MARK W, ADAMS, THEODORE P., KROLL, MARK W.
Applicants: ANGEION CORP, ANGEION CORPORATION
Application year: 1992
Main classification: A61N1
IPC codes: A61N1, H02M3