System for Selectively Reforming an ICD

The present invention is related to two co-pending applications. The first is a U.S. patent application filed on Oct. 27, 1995, entitled "AUTOMATIC CAPACITOR MAINTENANCE FOR AN ICD", Ser. No. 08/549,284. The second is a U.S. patent application filed on Mar. 20, 1996, entitled "AUTOMATIC CAPACITOR MAINTENANCE FOR AN ICD", Ser. No. 08/620,390. Both of these applications are assigned to the assignee of the present invention and the disclosure of each application is hereby incorporated by reference.

The present invention relates to implantable cardioverter defibrillator (ICD) devices. More particularly, the present invention pertains to a system for selectively reforming the high voltage output systems of an ICD based on the charge history and charge performance of the battery and capacitor systems so as to maintain charge performance while conserving battery power.

The implementation and use of high voltage output systems within implantable cardioverter defibrillator (ICD) devices is well known. In order to generate the high voltage output necessary for effective defibrillation countershocks, a low voltage, high current battery system is connected via a transformer to a high voltage capacitor system. When a cardiac arrhythmia is detected by the ICD, the battery system quickly charges the capacitor system so that a high voltage defibrillation countershock can be delivered by the device. An example of the high voltage output system for an existing ICD device is described in U.S. Pat. Nos. 5,404,363 and 5,372,605.

Generally, ICD devices have high voltage capacitor systems comprised of aluminum electrolytic capacitors and high current battery systems comprised of silver vanadium oxide (SVO) battery cells. Although aluminum electrolytic capacitors have a relatively high energy density per volume, these type of capacitors tend to degrade electrochemically over time thereby increasing the charge time required to fully charge the capacitor system. The SVO battery cells also have a tendency to degrade electrochemically over time due to the increased equivalent series resistance (ESR) within the battery which decreases the current output capabilities of the battery.

The conventional solution to both of these problems has been to conduct a periodic reforming of the high voltage output system of an ICD by charging the capacitor system to its full rated voltage and then allowing that voltage to slowly trickle off. In this way, both the high current battery system and the high voltage capacitor system are exercised so as to reform the electrochemistries of each system, thereby reducing the impact on charge performance and component life due to electrochemical degradation over time. Originally, this reforming process was accomplished manually by having a patient visit the physician every two to three months, at which time the physician would program the ICD to charge, but not deliver, a full voltage rated countershock. Presently, the reforming of the high voltage output system is accomplished automatically by the ICD based on a fixed time period (e.g., every month, every six months), at the end of which a full charge cycle of the capacitor system is automatically conducted. While this kind of simple periodic reform cycle was more than effective for early ICD devices where the life span of the device was typically less than three years and the battery budget could easily support the periodic reform cycles, newer ICD devices are smaller and have much longer life spans. An example of such an ICD which is used as a prophylactic device is described in U.S. Pat. No. 5,439,482. In these newer designs for an ICD, battery power is at more of a premium than in previous designs and the periodic reforming of the high voltage output system can represent a significant portion of the battery budget over the life of the device.

Two alternate techniques for accomplishing reforming of the battery system and the capacitor system are disclosed in the previously-identified co-pending applications. In the first application entitled "AUTOMATIC CAPACITOR MAINTENANCE FOR AN ICD", a technique is disclosed for measuring the leakage current of the capacitor system at a relatively low voltage and using this value to estimate whether the capacitor system needs to be reformed. By utilizing a low voltage test process, battery power is conserved and full voltage reforming is conducting only when it is determined that the capacitor is in need of reforming. In the second application entitled "AUTOMATIC BATTERY MAINTENANCE FOR AN ICD", a technique is disclosed for measuring an electrical parameter of the battery system and using this value to determine whether the battery system needs to be reformed. Again, battery power is conserved by only performing a full voltage reform when it is determined that the internal resistance of the battery system has increased to the point where charge performance is degraded. While each of these inventions represent a significant improvement over the existing periodic reform technique, both of these inventions suffer from the disadvantage of potentially requiring additional circuitry within the ICD in order to implement the invention.

Although existing techniques for reforming the high voltage output system of an ICD are adequate for current ICD systems, it would be advantageous to develop a more efficient reform system for the high voltage output system of an ICD. It would also be advantageous to develop a reform system that did not require significant additional circuitry within the ICD.

The present invention provides a system for selectively reforming the high voltage output systems of an ICD based on the charge history and charge performance of the battery and capacitor systems so as to maintain charge performance while conserving battery power. The system is preferably implemented in a microprocessor within the ICD which measures the charge history and charge performance of the high voltage output system of the ICD and periodically determines an estimated charge time based on these parameters. If the estimated charge time is greater than a predetermined value, a reform process is initiated. In a preferred embodiment, the reform process is a soft reform that utilizes lower voltage and frequency values so as to further conserve battery power during the reform process. In an alternate embodiment, the reform process includes circuitry to allow multiple capacitors within the capacitor system to be reformed individually, and can utilize the reform charge from one capacitor to aid in reforming another capacitor.

By monitoring the charge history and charge performance of the high voltage output system of the ICD, the present invention conserves valuable battery energy by only performing the reform process when actually needed, and further by performing only such reform as is actually needed. In a conventional ICD which utilizes a monthly reform, an energy equivalent to more than 80 countershocks is consumed simply in keeping the battery and capacitor systems reformed. By decreasing this energy requirement, the present invention optimizes the life and capacity of the device, without the necessity of additional circuitry.

FIG. 1 is a simplified representation of a circuit for a conventional ICD.

FIG. 2 is a circuit diagram shown the essential elements of the present invention.

FIG. 3 is a graphical representation showing increments in charge time, for a typical capacitor, as a function of time since the last full charge.

FIG. 4 is a graphical representation of the relationship between delay to mandatory reform time (D.sub.M) and last (most recent) charge time T.sub.L.

FIG. 5 is a diagram of a soft reform circuit.

FIG. 6 is a flow chart showing algorithmic logic steps of the present invention.

Referring now to the drawings, and in particular to FIG. 1, a simplified version of a typical ICD is presented. The charging circuit 10 charges through a flyback transformer secondary 12 to generate a high voltage output. Diodes 14 steer the high voltage and store it in capacitor system 16 and capacitor system 18. The capacitor systems 16 and 18 are discharged through shock delivery circuit 26. Preferably, shock delivery circuit 26 includes a pair of electrodes connected to the heart of a human patient to discharge high voltage to defibrillate the heart. For a more detailed description of the design and operation of an ICD, reference is made to U.S. Pat. No. 5,404,363 which is incorporated herein by reference.

The principle circuit elements of the present invention are shown in detail in FIG. 2. The circuit includes charge circuit 10, flyback transformer secondary 12 and diodes 14 with shock delivery circuit 26. Field Effect Transistor (FET) switches 28 and 28'control the distribution of charge energy, as will be discussed herein later. Silicon Controlled Rectifier (SCR) switches 30 and 32 isolate capacitor systems 16 and 18 from each other, respectively. Conventional diode 40 is a rectifier. Voltmeter 42 is connected across the terminals of shock delivery circuit 26 and is used to monitor the voltage across the terminals. Control system, CS, is operatively connected to the circuit as shown. The CS is preferably a microprocessor directed by an algorithmic software to implement the present invention.

FIG. 3 shows the increase in charge time, for a typical capacitor system, as a function of the time since its last full charge. This is an inverse exponential relationship and is shown on a logarithmic x-axis. The relationship shown in curve A is one of the parameters used in the algorithm of the present invention to estimate the performance of the capacitor system and the optimum remedial schedule to reform the capacitor system.

FIG. 4 shows a simplified linear relationship between reforming time and charge time. The charge time is used to determine the delay to mandatory reform time (D.sub.M). The D.sub.M is one of the important parameters of the present invention which indicates the capacitor system's conditions and also provides the combined capability status of the capacitor system and the battery. The (last) most recent charge time is designated T.sub.L.

FIG. 5 shows a circuit 43, a battery system 44, equivalent series resistance (ESR) 45, invertor 46 and transformer 47. This is an equivalent circuit which is used to explain the soft or a slow reform of the capacitor system to conserve energy. The ESR represents the internal resistance of a battery cell which causes its loaded voltage to be less than the open circuit voltage. A high battery ESR indicates a reduction in output current and thereby renders the ICD unreliable because the power supply will not sustain the provision of high current pulses. Periodic capacitor charging limits the development of ESR. Accordingly, the soft reform regimen provides the advantage of minimizing the energy waste in the ESR.

In the present invention, invertor 46 is operated in a manner (lower duty cyclethat limits the power consumption from the battery system 44 and enables the implementation of a soft reform regimen. The reforming cycle of the present invention is performed by the software, disclosed in FIG. 6, which directs control system "CS". Starting with logic step or block 50 the software program is initiated by calculating D.sub.M from T.sub.L. Consecutive steps and subroutines comprise the algorithm which performs intelligent calculations to automatically monitor the relevant perimeters in order to reform the capacitor system as needed.

As will be discussed hereinbelow, the present invention discloses a method and apparatus to reform capacitors based on intelligent analysis of the parameters relating to charge history and performance of a system of capacitors. Further, individual capacitors are reformed separately while battery energy is efficiently used in the reforming process. Thus, a real-time and continuous monitoring and reforming of the capacitor system in conjunction with conversation of battery energy is implemented to provide a reliable and ever-ready ICD system.

Referring now to FIGS. 1-5 in light of the algorithm disclosed in FIG. 6, the present invention is initiated by calculating the delay to mandatory reform time (D.sub.M) from the (last) most recent charge time (T.sub.L) under block 50 (See FIG. 6). The routine proceeds to decision block 52 wherein the delay from the last shock (D.sub.S) is compared against the calculated D.sub.M. If D.sub.S is greater than D.sub.M the subroutine proceeds to block 54 to perform a soft reform. From block 54, the subroutine reverts back to block 50 to execute calculations as needed. It is significant to note that the calculations performed under block 50 are particularly based on the disclosures of FIGS. 3 and 4. FIG. 3 shows the increase in charge time for a typical capacitor as a function of the time since the last full charge. The relationship between the variables (charge time and time since last full charge) is inverse exponentially and this relationship is shown based on a logarithmic x-axis. Curve A shows observations based over a two year period with the charge time increasing from 5 seconds to about 7.5 second. This increase will not be clinically significant for most patients, However, many capacitors exhibit significantly higher rates of charge time increase. Curve A is derived from bench testing a capacitor at 22 degrees Celsius. Curve B is obtained from data of characteristic depletion of capacitors in the human body at 37 degrees Celsius. It is clearly evident from curves A and B that capacitors in human implanted ICDs could have a higher rate of depletion and would need more frequent reforming.

Furthermore, the disclosure in FIG. 4 shows a simplified linear relationship for an algorithm to estimate the next time the capacitor systems 16 and 18 will need reforming. In the horizontal axis we observe a (last) most recent charge time (T.sub.L) of 5 seconds. This suggests that the capacitor and battery combination are both very well maintained. Based on this data a one year period would be allowed for the delay to mandatory reform time (D.sub.M). On the other hand, another most recent charge time (T.sub.L) is shown as 12 seconds. If the capacitors were to have a 12 second charge time, it would indicate that the capacitor systems 16 and 18 have depleted significantly since the last charge and that the capacitor systems 16 and 18, and or the battery system are in less than optimal state of charge conditions. Realizing this, the calculated delay to mandatory reform time (DM) would be one month. The interpolation for values between the 5 second and 12 seconds is based on a logarithmic scale for the vertical axis.

Referring now to FIG. 6, the calculation under block 50 could be based on a certain DM, for example three months, from the previous shock. The curve developed in FIG. 4 assumes a "hard charge" which is using a maximum charge rate. However, the algorithm of FIG. 6 reverts to a soft reform if delay form last shock (D.sub.S), is greater than D.sub.M. Further, under decision block 52 if D.sub.S is less than or equal to D.sub.M, the algorithm proceeds to decision block 56 to check it is time for a weekly charge time estimate. If it is time, the routine proceeds to block 58 where all or any one of the capacitor systems are charged to a minimum voltage, for example 100 v. If not, the routine proceeds back to block 50 to perform the calculations as indicated. Under block 58 the overall charge time is estimated as a test time (T.sub.TEST). Hereafter, when the maximum charge voltage is reached in block 58, the software program proceeds to decision block 64 where the estimated T.sub.TEST of block 58 is compared against a set time limit. Therefore, under decision block 64 if T.sub.TEST is greater than the set time limit, the subroutine precedes to perform a soft reform under block 66. On the other hand, if the estimated T.sub.TEST is less than or equal to the set limit, the routine precedes to block 50 to perform calculations as indicated.

Generally the operations of the present invention include, calculating D.sub.M from T.sub.L. On a predetermined time interval basis D.sub.M is compared with D.sub.S. In the preferred embodiment this comparison is made on a daily basis. If D.sub.S is greater than D.sub.M, then a `soft` reform is executed and a timer is reset to zero at the end of the reform. If D.sub.S is equal to or less than D.sub.M then the algorithm confirms if it is time to perform a weekly charge time estimate. If it is time, one or all of the capacitor systems are charged to a minimum voltage, for example 100 volts. The charge time is noted precisely. This charge time is then used to estimate the overall charge time as T TEST which would be required for a maximum energy shock. The empirical estimation formula is derived from the following equation: Full charge time=(V.sup.2.sub.MAX /V.sup.2.sub.TEST) T.sub.TEST where T.sub.TEST in this equation is the test voltage charge time. For example if the maximum voltage is 750 V and the test voltage is 100 V.sub.TEST, the ratio, i.e, 7.5 would be squared to yield a correction ratio of 56.25. This quantity will be multiplied by T.sub.TEST, the test voltage charge time required for the 100 V.sub.TEST, to come up with an estimate for the full charge time. Thus, if the test voltage charge time for the 100 V is 100 ms, the full charge time will be 5.625 seconds.

The advantages of using a low voltage full charge time such as the 5.625 seconds is that it incorporates the most important capacitor depletion parameters which include battery internal impedance and increase in capacitor leakage. The present invention enables to monitor these critical parameters by using a charge as low as 100 volts which requires only about one joule of energy in a typical ICD system. Accordingly, the tests performed by the method and device of the present invention may be scheduled to be performed on a weekly basis and consume energy of about 52 joules per year.

As discussed hereinabove, if T.sub.TEST is greater than the predetermined limit, then a soft reform is executed. Referring to FIG. 4, the method and device for soft reforming include the inverter 46 which is run at a very low rate such that less than one third of the power that is normally used to charge a capacitor for emergency shock delivery is consumed. This approach reduces the amount of energy consumed to about one half of what is normally required to totally reform the capacitor system. The prior art method of reforming capacitors which is referred to as a "hard charge" involves rapidly charging the capacitor system to deliver the shock. In order to accomplish this, the battery 44 and the invertor 46 are run at maximum power transfer. Accordingly, the battery voltage is drawn down to about half its normal voltage. While this results in a very rapid power transfer, it has the undesirable side effect of wasting about one half of the battery energy internally. This is because the equivalent series resistance (ESR) 45 of the battery is in the order of about one ohm. Thus, the losses can be fairly significant when a capacitor system is charged using the "hard charge" approach.

The present invention provides a unique circuit designed to reform the capacitor systems 16 and 18 using the most economical battery energy. Referring now to FIG. 2, FET switches 28 to 28' are used to control current flow into capacitor systems 16 and 18, respectively. The FET switches 28 and 28' operate in an enhanced mode which means that no current flows when zero gate voltage is applied and increasing the gate voltage increases the current. Therefore, to commence the reforming operation of capacitor system 16, for example, FET switch 28 is turned on and FET switch 28' is turned off. This allows the charging energy to go to only capacitor system 16 and prevents current from flowing to capacitor system 18. Similarly, if it is required to reform capacitor system 18, FET switch 28' is turned on and FET switch 28 is turned off thus preventing current from flowing to capacitor system 16.

Accordingly, the circuitry of the present invention provides the added advantage and option to charge a specific group of capacitor systems of an ICD. Allowing the charging energy to be directed to capacitor system 16, for example, enables to conserve battery energy by reforming only the capacitor system which needs reforming. Further, the present invention enables the energy from one capacitor system to jump start the other capacitor system. Specifically, for example, after capacitor system 16 is fully charged and reformed then SCR switches 30 and 32 are turned on and the charge from capacitor system 16 is thus shared with the capacitor system 18. It should be noted that SCR switches 30 and 32 are silicon diodes with an additional electrode or a gate. If a bias voltage is applied to the gate to keep it at or near the same potential as the cathode of the diode, the SCR behaves as if working with reverse voltage with both directions of applied voltage, a small leakage current will flow. If the gate is biased to be more positive than the cathode, the SCR behaves as a conventional diode. Therefore, the gate can be used to turn the SCR on thus enabling forward current to be controlled.

In the preferred embodiment, when a capacitor system is fully charged the excess charge can be directed to the next capacitor system. In the prior art, this excess energy was trickled off and wasted thus contributing to an ever increasing energy demand o the battery system. For illustration purposes, assuming capacitor system 16 to be fully charged and carrying an excess charge, then SCR switches 30 and 32 are turned on and the excess charge from capacitor system 16 is directed to capacitor system 18. Because of losses in the sharing process of the excess charge between the capacitor system, the charge of capacitor system 18 may not be brought to a full level. However, the amount of energy to then reform capacitor system 18 to its full capacity is reduced by the amount transferred from the excess charge and this in turn reduces the energy requirement on the battery therefore yielding longer battery life and enhancing the reliability of the ICD system.

Having thus described the preferred embodiments of the present invention, those skilled in the are will readily appreciate the many other embodiments which can be employed within the scope of the claims provided below.