Abstract:
In an ICD, a highly efficient biphasic defibrillation pulse is generated by switching at least two charged capacitors, e.g., three capacitors, from a parallel connection to various combinations of a parallel/series connection or a series connection during the first phase of the defibrillation pulse. Such mid-stream parallel/series connection changes of the capacitors steps up the voltage applied to the cardiac tissue during the first phase. A stepped-up voltage during the first phase, in turn, gives an extra boost to, and thereby forces additional charge (current) into, the cardiac tissue cells, and thereby transfers more charge to the membrane of the excitable cardiac cell than if the capacitors were continuously discharged in series. Phase reversal is timed with the cell membrane reaching its maximum value at the end of the first phase.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/046,610, filed May 14, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to implantable medical devices, and more particularly to an implantable cardioverter defibrillator (ICD) configured to provide a high efficiency defibrillation waveform. 
     BACKGROUND 
     AN ICD continues to be a relatively large device for implantation in the human body. The size of the ICD is primarily determined by the battery and capacitors used therein. The size of the battery (or batteries, in some instances) and capacitors, in turn, is determined by the shock energy requirements for a defibrillation pulse. Thus, a design approach which reduces the energy requirements for defibrillation results in a direct reduction in the overall ICD size. 
     In existing ICD devices, the defibrillation waveform or pulse used to deliver a defibrillation shock to the heart is generated by first charging the equivalent of a single capacitor (most ICDs use two capacitors connected in series to function as a single capacitor, thereby reducing the working voltage requirements for each capacitor of the series stack, as explained below) to a desired charge level (voltage) and then discharging the single capacitor through the cardiac tissue for a prescribed period of time during a first or positive phase of the defibrillation waveform, and then reversing the polarity of the discharge for a second prescribed period of time during a second or negative phase of the defibrillation waveform, thereby producing a biphasic stimulation pulse or waveform. It should be noted that in this context the term “single capacitor” is used to refer to a single capacitance, which may be, and usually is obtained by a hardwired connection of two capacitors in series such that the two series capacitors always function and act as though they were a single capacitor. (Two capacitors are connected in series in this manner in order to achieve a higher working voltage for the series-connected capacitor. That is, when two capacitors are connected in series, and each has a working voltage of, e.g., 375 volts (V), then the overall or total working voltage of the series combination becomes 750 V.) 
     The purpose of applying a defibrillation shock to the heart is to shock the heart out of a state of fibrillation, or other non-functional state, into a functional state where it may operate efficiently as a pump to pump blood through the body. To this end, the positive phase of the biphasic waveform is preferably a very high voltage that serves to synchronously capture as many heart membrane cells as possible. See, Kroll, “A minimum model of the signal capacitor biphasic waveform”  Pace , Nov. 1994. The negative phase of the biphasic waveform, in contrast, simply serves to remove the residual electrical charge from the membrane cells and bring the collective membrane voltage back to its original position or value. See, e.g., Kroll, supra; Walcott, et al., “Choosing The Optimal Monophasic and Biphasic Wave-Forms for Ventricular Defibrillation,”,  Journal of Cardiovascular Electrophysiology  (September 1995). A biphasic pulse generator of the type used in an ICD device is shown, e.g., in U.S. Pat. Nos. 4,850,357, issued to Bach, Jr.; and 5,083,562, issued to de Coriolis et al. 
     When a voltage shock is first applied to a membrane cell, the membrane does not respond to the shock immediately. Rather, the cell response lags behind the applied voltage. This time lag is more or less predictable in accordance with the Blair membrane model. See, e.g., Blair, “On the intensity-time relations for stimulation by electric currents. I”  J. Gen Physiol ., Vol. 15, pp. 709-729 (1932), and Blair, “On the intensity time relations for stimulation by electric currents. II”,  J. Gen Physiol ., Vol. 15, pp. 731-755 (1932); Pearce et al., “Myocardial stimulation with ultrashort duration current pulses”,  PACE , Vol. 5, pp. 52-58 (1982). When the applied voltage comprises a biphasic pulse having a constant voltage level for the duration of the positive phase (a condition achievable only when the voltage originates from an ideal battery), the membrane cell response to the positive phase reaches a peak (i.e., is at an optimum level) at the trailing edge of the positive phase. Unfortunately, when the applied voltage originates from a charged capacitor, as is the case for an ICD device, the applied voltage waveform does not remain at a constant voltage level, but rather has a significant “tilt” or discharge slope associated therewith. Such tilt or slope causes the peak membrane cell response to occur at some point prior to the trailing edge of the positive phase, which is less than optimum. What is needed, therefore, is a way to optimize the applied voltage waveform so that a maximum membrane cell response occurs coincident with, or nearly coincident with, the trailing edge of the positive phase. 
     It is known in the art to switch the capacitors of an ICD from a parallel configuration during the positive phase of a biphasic defibrillation pulse to a series configuration during the negative phase of the biphasic defibrillation pulse. See, e.g., U.S. Pat. Nos. 5,199,429 (FIG. 7A) and 5,411,525. While such action produces a defibrillation waveform having a somewhat different shape, i.e., a waveform having a leading edge voltage of the second or negative phase which is approximately twice the trailing edge voltage of the first or positive phase, such action does little to achieve a maximum cell membrane response coincident with the trailing edge of the first or positive phase. 
     It is also known in the art to sequentially switch capacitors in an ICD device in order to allow waveform “tailoring”, e.g., prolong the positive phase duration by sequentially switching in a second charged capacitor as shown in FIG. 6A of U.S. Pat. No. 5,199,429, or by sequentially switching in second, third and fourth charged capacitors as shown in FIG. 6C of U.S. Pat. No. 5,199,429. However, such “tailoring” still does not address the main concern of achieving a maximum cell membrane response coincident with the trailing edge of the positive phase. 
     It is thus evident that what is needed is a capacitor switching scheme and/or method for use within an ICD device which achieves a maximum cell membrane response near or coincident with the trailing edge of the positive phase. 
     It is also desirable to provide an ICD that is as small as possible. The limiting factor on ICD thickness is the diameter of the high energy capacitors. As indicated above, current ICDs typically use two electrolytic capacitors. Current technology in electrolytic capacitors limits the stored voltage to about 370 V per capacitor. Therefore, the current approach is to use two large (≦180 μF) capacitors to achieve the stored energy of ≦25J required for defibrillation. Therefore, the thickness of the ICD is determined by the diameter of the large (≦180 μF) capacitors. There is thus a need for an ICD construction which would permit the needed energy for defibrillation to be stored in the ICD, while allowing a thinner ICD thickness. 
     The present invention advantageously addresses the above and other needs. 
     SUMMARY OF THE INVENTION 
     The present invention generates a highly efficient first phase (which is usually a positive phase) of a biphasic defibrillation pulse by switching at least two charged capacitors, preferably three capacitors, from a parallel connection to a series connection during the first or positive phase of the defibrillation pulse. Such mid-stream parallel-to-series switch advantageously steps up the voltage applied to the cardiac tissue during the first phase. A stepped-up voltage during the first phase, in turn, gives an extra boost to, and thereby forces additional charge (current) into, the cardiac tissue cells, and thereby transfers more charge into the membrane of the excitable cardiac cell than would be transferred if the capacitors were continuously discharged in series. Phase reversal, e.g., switching to a second or negative phase of the biphasic waveform) is timed to occur when the cell membrane voltage reaches its maximum value at the end of the first phase. 
     In accordance with one aspect of the invention, three capacitors are used within the ICD in order to provide a thinner ICD. These three capacitors store the same energy as a two-capacitor ICD. These smaller capacitors have a smaller diameter and therefore the ICD can be made thinner. 
     Disadvantageously, using three capacitors instead of two creates its own set of problems that must be overcome by the present invention. Using three capacitors discharged in series results in: (a) high peak voltages (generally the peak voltage can be three times 370 V or 1110 V); and (b) a small discharge time constant, since the effective capacitance is that of a single capacitor divided by three (or 40 μF if 120 μF capacitors are used), resulting in a mismatch between the discharge (τ=R*C, with R≈50Ω) and tissue (τ m≈ 3 ms) time constants. Advantageously, the present invention addresses both of these concerns. 
     In accordance with another aspect of the invention, the capacitors of the ICD are reconfigured from a parallel configuration to a series configuration during the defibrillation pulse. While this concept may be used effectively with a two-capacitor ICD, it is preferred for purposes of the present invention that at least three capacitors be used, thereby allowing the ICD to be somewhat thinner that it otherwise could be. 
     It is therefore a feature of the present invention to provide an ICD that generates a highly efficient stimulation waveform that transfers more charge to the membrane of an excitable cardiac cell than has heretofore been possible using conventional parallel-charge, series-discharge configurations. 
     It is a further feature of the invention to provide an ICD design that results in a thinner ICD than has heretofore been possible using a conventional two-capacitor ICDs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features, and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings, wherein: 
     FIG. 1A illustrates a preferred defibrillation biphasic pulse or waveform generated in accordance with a two-capacitor ICD in accordance with the present invention; 
     FIG. 1B depicts the excitable cardiac membrane response to the waveform of FIG. 1A; 
     FIG. 2 is a functional block diagram of a two-capacitor ICD device which generates the waveform of FIG. 1A; 
     FIG. 3 is a simplified schematic diagram of a three-capacitor ICD made in accordance with the invention; 
     FIG. 4A illustrates one type of defibrillation waveform that may be generated using the ICD of FIG. 3; 
     FIG. 4B depicts the excitable cardiac membrane response to the waveform of FIG. 4A; 
     FIG. 5A illustrates another type of defibrillation waveform that may be generated using the ICD of FIG. 3; 
     FIG. 5B depicts the excitable cardiac membrane response to the waveform of FIG. 5A; 
     FIG. 6A illustrates, for comparative purposes, the biphasic defibrillation waveform typically provided by a two-capacitor ICD of the prior art; and 
     FIG. 6B illustrates, again for comparative purposes, the membrane response to the waveform of FIG.  6 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode currently contemplated for practicing the invention. 
     The basic concept of the invention relating to forming an efficient defibrillation waveform can be practiced with two or more capacitors within the ICD. A preferred number of capacitors is three. However, the basic concept will first be explained in the context of a two-capacitor ICD. 
     In accordance with one aspect of the invention, then a biphasic pulse or waveform is generated by an ICD device having two capacitors that includes a positive phase of duration t 1 , ms and a negative phase of duration t 2  ms, as shown in FIG.  1 A. First and second capacitors, CA and CB, within the ICD device are initially charged to a voltage V1 and are connected in parallel. The biphasic defibrillation pulse begins by discharging the charged parallel capacitors through the cardiac tissue by way of defibrillation electrodes in contact with the cardiac tissue. Thus, a leading edge of the biphasic pulse starts at a first peak voltage of approximately V1 volts (the charge on the first and second capacitors when first connected to the electrodes). 
     During a first portion of the positive phase of the biphasic pulse, the amplitude of the biphasic pulse decays from the first peak voltage V1 to a voltage V2 in accordance with a first time constant τ1. The first time constant τ1 varies as a function of (CA+CB)R, where CA is the value of the first capacitor, CB is the value of the second capacitor, and R is an effective resistance associated with the discharge through the first and second electrodes. 
     A second portion of the positive phase begins by connecting the first and second capacitors in series. This sudden series connection increases the defibrillation pulse to a second peak voltage of approximately 2(V2) volts (the sum of the voltages on each of the first and second capacitors at the time the series connection is made), as illustrated in FIG.  1 A. The amplitude of the biphasic pulse decays during the second portion of the positive phase from the second peak voltage 2(V2) to a voltage V3 in accordance with a second time constant τ2. The second time constant τ2 varies as a function of (CACB/(CA+CB))R. Advantageously, the voltage at the trailing edge of the positive phase, V3, occurs at a time that is near the maximum cell membrane response. 
     The negative phase of the biphasic waveform begins by inverting the polarity of the series-connected first and second capacitors. Such negative phase thus commences at a third peak voltage of approximately −V3 volts, and decays thereafter towards zero in accordance with the second time constant τ2. After a prescribed time period t 2 , the negative phase ends. 
     The biphasic waveform produced in accordance with the two-capacitor ICD is illustrated in FIG.  1 A. The first portion of the positive phase may terminate when either: (1) the voltage decreases below a threshold voltage V3; or (2) a prescribed time period t a  has elapsed. 
     The tissue membrane voltage that results when the waveform of FIG. 1A is applied to excitable cardiac tissue membranes is as shown in FIG.  1 B. This membrane voltage is obtained by modeling the tissue membranes as taught in the Blair reference, previously cited. 
     A functional block diagram of the pulse generation circuitry used to generate the biphasic waveform of the two-capacitor ICD is shown in FIG.  2 . 
     As seen in FIG. 2, a cardiac tissue-stimulating device  10  includes a power source  12 , e.g., at least one battery, a timing and control circuit  14 , a charging circuit  16 , an isolation switch network SW 1 , a series parallel switch network SW 2 , at least two capacitors CA and CB, an output switch network SW 3 , and two electrodes  20  and  22 . The electrodes  20  and  22  are adapted to be positioned within or on the heart so as to be in contact with cardiac tissue  30 . The electrodes  20  and  22  are connected to the output switch SW 3  through conventional leads  21  and  23 , respectively. 
     A voltage sense amplifier  24  senses the voltage held on the capacitor CB (which will be the same voltage as capacitor CA when CA and CB are connected in parallel). In some embodiments of the invention, a current sense amplifier  26  may also be used to sense the current flowing to or returning from one of the electrodes  20  or  22 . In FIG. 2, such current is sensed by differentially measuring the voltage across a small current-sense resistor R s  connected in series with electrode  22 . The outputs of the voltage sense amplifier  24  and the current sense amplifier  26  are directed to the timing and control circuit  14 . 
     A suitable cardiac activity sensor  28  is also employed within the device  10  in order to detect cardiac activity. The function of the sensor  28  is to sense cardiac activity so that an assessment can be made by the timing and control circuitry whether a defibrillation pulse needs to be generated and delivered to the cardiac tissue. Such sensor  28  may take many forms, e.g, a simple R-wave sense amplifier of the type commonly employed in implantable pacemakers. The details of the sensor  28  are not important for purposes of the present invention. 
     The power source  12  is connected to provide operating power to all components and circuitry within the device  10 . The power source  12  also provides the energy needed to generate the biphasic defibrillation pulse. That is, energy stored within the power source  12  is used to charge capacitors CA and CB, through the charging circuit  18 , up to the desired initial defibrillation starting pulse voltage V1. Such charging is carried out under control of the timing and control circuit  14 . Typically, V1 may be a relatively high voltage, e.g., 350 volts, even though the power source  12  may only be able to provide a relatively low voltage, e.g., 3-6 volts. The charging circuit  16  takes the relatively low voltage from the power source  12  and steps it up to the desired high voltage V1, using conventional voltage step-up techniques as are known in the art. This stepped-up voltage V1 is then applied through the isolation switch SW 1  to both capacitors CA and CB at a time when CA and CB are connected in parallel, i.e., when SW 2  is in its “P” position, and at a time when the output switch is in its open, or OFF, position. As the capacitors CA and CB are being charged, the voltage sense amplifier  24  monitors the voltage level on the capacitors. When the desired voltage V1 has been reached, the timing and control circuitry  14  turns off the charging circuit  16  and opens the isolation switch SW 1 , thereby holding the voltage V1 on capacitors CA and CB until such time as a defibrillation pulse is needed. 
     When a defibrillation pulse is called for by the timing and control circuit  14 , the output switch SW 3  is placed in its positive phase position, POS, thereby connecting the parallel connected capacitors CA and CB (on which the starting voltage V1 resides) to the cardiac tissue through the electrodes  20  and  22 . Such connection starts the discharge of capacitors CA and CB through the cardiac tissue in accordance with the first time constant τ1 as described above in connection in FIG.  1 A. 
     After a period of time t a  or as soon as the voltage across the parallel-connected capacitors CA and CB has decreased to the threshold value V2 (as sensed by the voltage sense amplifier  24 ), the timing and control circuit switches SW 2  to its series-connected or “S” position, thereby connecting the capacitors CA and CB in series across the electrodes  20  and  22 . Such series connection doubles the voltage across the electrodes  20  and  22  to a value of 2(V2). Thereafter, the discharge of the series-connected capacitors CA and CB continues through the cardiac tissue in accordance with the second time constant τ2 as described above. This discharge continues until the end of the positive phase. 
     The positive or first phase ends at a time t 1 , from the beginning of the positive phase (as measured by timing circuits within the timing and control circuit  14 ), or when the voltage has decayed to a value V3 (as sensed by voltage sense amplifier  24 ). Alternatively, the positive phase may end as a function of the sensed current (as sensed by the current sense amplifier  26 ), e.g., at a time when the sensed current has decreased from a peak value by a prescribed amount or percentage. 
     As soon as the positive phase ends, the timing and control circuit  14  switches the output switch SW 3  to the negative phase position, NEG, thereby reversing the polarity of the discharge of the series-connected capacitors C 1  and C 2  through the cardiac tissue. The negative phase lasts thereafter for a time period t 2  determined by the timing and control circuitry. 
     The functions represented by the functional block diagram of FIG. 2 may be implemented by those of skill in the art using a wide variety of circuit elements and components. It is not intended that the present invention be directed to a specific circuit, device or method; but rather that any circuit, device or method which implements the functions described above in connection with FIG. 2 to produce a defibrillation waveform of the general type shown in FIG. 1 be covered by the invention. 
     Turning next to FIG. 3, there is shown a simplified schematic diagram of an ICD having three 120 μF capacitors C 1 , C 2  and C 3 . The manner of charging the capacitors while they are connected in parallel is the same or similar to that shown in FIG.  2 . When the capacitors C 1 , C 2  and C 3  have been charged to a high voltage, e.g., 370 V, a stored energy of approximately 25 Joules is realized. Once the capacitors have been charged by the ICD, the capacitors are configured for a parallel discharge. This is accomplished by closing switches S 1 , S 2 , S 3  and S 4 , while maintaining switches S 5  and S 6  open. The parallel discharge takes place from time t=0 until a time d 1 . Once d 1  elapses, one of two options may be used to discharge the remaining charge. 
     In accordance with a first option, or Option  1 , after d 1  has elapsed (i.e., after the capacitors are discharged in parallel until time d 1 ), all of the capacitors are discharged in series for the remainder of the pulse. This is accomplished by opening S 1 , S 2 , S 3  and S 4  and closing S 5  and S 6 . At a later time, d 2 , the “H Bridge” circuit  40  (FIG. 3) is used to reverse the polarity of the output. At yet a later time, d, the output pulse is truncated. 
     The waveform generated in accordance with Option  1  is illustrated in FIG.  4 A. The tissue membrane voltage associated with the waveform of FIG. 4A is modeled and computed, using the Blair model, as shown in FIG.  4 B. For the example shown in FIGS. 4A and 4B, the optimum value of d 1  is nominally about 3.5 ms. The optimum choice of d 2  is when the elapsed time at d 2  is about 1.5 times the elapsed time at d 1 , or when the elapsed time at d 2  (from t=0) is about 5.25 ms. 
     In accordance with a second option, or Option  2 , the capacitors C 1  and C 2  remain in parallel and are in series with C 3  until time d 2 . This is accomplished by opening S 3  and S 4  and closing S 6 . After d 2  all the capacitors are in series (S 1  and S 2  also open, S 5  closed) until C 3  runs out of charge at a time d 4 . After d 4 , the diode D 1  bypasses the depleted capacitor and the time constant of discharge is of C 1  and C 2  in series. At a time d 3 , where d 2 &lt;d 3 &lt;d 4 , the polarity of the output is reversed using the H Bridge 40. The pulse is truncated at time d. The resulting waveform is shown in FIG.  5 A. The resulting membrane voltage is modeled and computed and shown in FIG.  5 B. 
     For the example shown in FIGS. 5A and 5B, the optimum values of d 1  is 2.7 ms, d 2  is 1.5 times d 1  (or about 4 ms), d 3  is d 2 +1.25 ms. The value of d 4  is computed to be about 7.6 ms. The choice of d can be in the range of 1.5 to 2.0 times that of d 3 . 
     With either Option  1  or Option  2 , the choice of the values d 1 , d 2  and d 3  are primarily functions of the ICD&#39;s capacitance value, the discharge pathway impedance, and the tissue time constant (τ m ). 
     The advantage of Option  2  is that the peak waveform voltage is lower than Option  1  yet a minute increase in membrane voltage over Option  1  is achieved. However, Option  1  is simpler to implement and diode D 1  is not needed since all the capacitors are discharged equally. 
     The advantages of either Option  1  or Option  2  are better appreciated by comparing the results of such discharge, as presented in FIGS. 4A,  4 B,  5 A and  5 B, with the corresponding discharge achieved with a two-capacitor ICD series discharge, as is commonly used in a conventional ICD of the prior art. The discharge waveform achieved with a conventional two-capacitor ICD using series discharge, and the resulting membrane voltage, is shown in FIGS. 6A and 6B, respectively. Note, that to store equal energy to the three capacitor ICD, each capacitor of the two-capacitor ICD must have 1.5 times the capacitance value, or two capacitors each with C=180 μF. 
     As can be seen from a comparison of FIGS. 6A and 6B with FIGS. 4A and 4B (Option  1 ), and  5 A and  5 B (Option  2 ), for equal stored energy, the value of the peak membrane voltage for Option  2  is 1.18 times higher than the membrane voltage realized using the conventional waveform. Similarly, Option  1  yields a membrane voltage that is 1.17 times higher than is realized using the conventional waveform. In other words, a 25 Joule ICD with three 120μF capacitors and a switching network as in Option  2  performs equally to a 34.4 Joule conventional ICD with two 180μF capacitors. This represents a remarkable improvement in performance, while at the same time allowing a significantly thinner ICD to be made. An ICD made with three 120μF capacitors, for example, need only have a thickness of about 13 mm. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.