Abstract:
A system and method of power delivery to a main countershock electrical circuit in an implantable defibrillator comprises a staged energy configuration. Monitoring and control elements cooperate with staged batteries. The batteries comprise a first, non-rechargeable battery and a second, rechargeable battery for rapidly recharging a defibrillation capacitor.

Description:
FIELD OF THE INVENTION 
     This invention relates to the power supply for an implantable defibrillator, and more particularly to use of a rechargeable battery for rapidly recharging a defibrillation capacitor. 
     BACKGROUND OF THE INVENTION 
     Cardiac defibrillation in humans requires the delivery of an electrical pulse that is several milliseconds long with peak currents as high as 25 amperes. The total energy in such a pulse is about 30 Joules. In order to deliver such a high current, a capacitor is charged up to a voltage on the order of 750 volts. 
     Various special batteries have been developed to supply such a high current to charge the capacitor. However, the batteries in use comprise low energy density batteries which occupy a large volume within the defibrillator. In defibrillator devices, the combined volume of the battery and the capacitor(s) is greater than the electronics. Therefore, any reduction in battery size or weight provides significant enhancements to an implantable defibrillator. 
     SUMMARY OF THE INVENTION 
     An implantable defibrillator, and method of operation, is provided which comprises monitoring and control means and battery means for powering the monitoring and control means, and for providing defibrillation pulses. The battery means comprises a first, non-rechargeable battery and a second, rechargeable battery for rapidly recharging a defibrillation capacitor. 
     A staged energy delivery system for an implantable defibrillator is also provided. The staged energy delivery system comprises a first stage of energy concentration comprising a non-rechargeable battery, and a second stage of energy concentration comprising rechargeable battery means for rapidly charging storage and discharge capacitance means. 
     Another embodiment of an energy delivery system for an implantable defibrillator is provided. This embodiment comprises a first stage of energy concentration comprising a non-rechargeable battery, and a second stage of energy concentration comprising rechargeable battery means. The rechargeable battery means is configured for providing energy to a capacitor so that defibrillation pulses are readily available for use by the implantable defibrillator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified circuit diagram of a prior art implantable defibrillator circuit. 
     FIG. 2 is a simplified schematic circuit diagram of a staged energy concentration circuit. 
     FIG. 3 is a simplified schematic circuit diagram of an alternate embodiment staged energy concentration circuit. 
     FIG. 4 is a simplified schematic circuit diagram of an alternate embodiment staged energy concentration circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a simplified circuit diagram of a known implantable defibrillator circuit 10. Circuit 10 comprises a high current defibrillation battery 13, which is typically a lithium silver vanadium pentoxide (LiAgVO 5 ) battery. A high voltage transformer 15 comprises a transistor switch 18 which drives the primary 21. The oscillator driving switch 18 provides an alternating current through the primary of transformer 15. The secondary 25 of transformer 15 produces a significantly higher voltage which is rectified by diode 27 and stored in capacitor 30. When capacitor 30 is fully charged, the semiconductor switch 32 is activated to complete the circuit which delivers the charge of capacitor 30 to the cardiac electrodes 35 for defibrillation of the heart. A configuration which is similar to the above circuit comprises substitution of a H-bridge in place of switch 32. This permits delivery of the current from capacitor 30 in either polarity, which allows delivery of a biphasic pulse. 
     Circuit 10 works well in cardiac defibrillators. However, the LiAgVO 5  batteries have an energy storage density of only 500 Joules per gram (J/g). This is due to the tradeoff between energy storage capability and current delivery capability. In contrast, the battery chemistry of the well known Lithium Iodide (LiI) pacemaker battery has approximately twice the energy storage density of the LiAgVO 5  battery, or about 1000 J/g. This means that defibrillator devices using a LiAgVO 5  battery are utilizing a battery with a mass that is twice that which could be used if a LiI pacemaker battery were used. However, use of a LiI battery alone would result in delivery of only very small currents, on the order of milliamps. Therefore, as disclosed in copending U.S. patent application Ser. No. 670,188, it is possible to use an LiI battery to power monitoring and integration circuits/related systems, and a LiAgVO 5  to charge a capacitor sub-system. 
     FIG. 2 discloses a simplified schematic staged energy circuit 40. Circuit 40 comprises a first embodiment of an improved staged energy concentration means designed for replacing that portion of circuit 10 denoted as primary sub-circuit 42 in FIG. 1. Circuit 40 preferably comprises a first stage of energy concentration comprising a non-rechargeable battery, such as a high energy density pacing battery 45, configured for applying a small microampere current to the trickle charge control circuitry 48. This provides an optimum current to be supplied to a second stage of energy concentration, comprising at least rechargeable battery means. The rechargeable battery means preferably comprises a rechargeable defibrillator battery 50 and is maintained fully charged by the pacing battery 45. Rechargeable defibrillator battery 50 is used to drive primary 21 of the high voltage transformer, or similar power transfer means, through a switch 18 in a manner similar to conventional circuits, such as circuit 10. 
     The staged energy concentration configuration of circuit 40 permits use of high density pacing batteries to store energy in combination with a very small rechargeable defibrillator battery to deliver a high current for up to about 10 shocks. A typical defibrillator will deliver about 200 defibrillator shocks. Assuming each of the shocks is of 30 Joules, and with transformer losses of 25%, the energy system must store 200×40 J=8000 Joules. However, due to this staged energy arrangement, the rechargeable battery need only store enough energy for a typical cardiac defibrillation session of about 5 shocks. The battery means comprising the second stage of energy concentration must therefore only store about 5×40 J=200 Joules. Although this is very little energy, the second stage battery means must be able to deliver a fairly high current of about 1-2 amperes. Representative battery chemistries capable of meeting these specifications are shown in Table 1. 
     
                       TABLE 1______________________________________Second Stage BatteriesChemistry    Average Voltage______________________________________LiMoS.sub.2  1.85LiMnO.sub.2  3.0LiV.sub.2 O.sub.5        2.8LiTiS.sub.2  2.2LiV.sub.6 O.sub.13        2.3LiCuC.sub.12 3.2LiSO.sub.2   3.1______________________________________ 
    
     FIG. 3 discloses another embodiment of the staged energy concentration invention. Circuit 60 discloses a single cell pacing battery 63 which is used to power a voltage doubler circuit 67. This doubler circuit 67, which comprises numerous embodiments, may be configured to produce an output of approximately 6 volts for charging a rechargeable defibrillation battery, such as battery 70. 
     Another embodiment of a staged energy concentration defibrillator circuit is shown in FIG. 4, in which circuit 76 comprises first stage battery 80. Battery 80 is a low voltage, for example a 2.8 volt, LiI single cell battery which charges two second stage batteries 83 and 84. Batteries 83, 84 are preferably Lithium Titanium Disulfide (LiTiS 2 ) batteries. Preferably, battery 84 is charged through diode 86, battery 83 is charged through diode 87, and resistor 89 is used with a preferred value of 10K ohms. Field effect transistor switch 92 is off during this time. It is recognized that this schematic circuit is further simplified because there is optimal trickle charge current limiting between battery 80 and the two diodes, however, that detail is not considered important to this depiction of the invention. 
     When fibrillation is detected by related detection circuitry, it is then time to charge the defibrillation capacitor(s) and switch 92 is turned on. That places batteries 83 and 84 in series, providing a voltage of approximately 5 volts for the transformer primary 21. As above, oscillating switch 18 is used to cause a pulsating current to pass through primary 21 of the transformer. 
     Use of a multi-stage energy concentration defibrillator, as disclosed in FIGS. 2-4, provides great savings in both volume and weight of the defibrillator. For example, since the defibrillator battery chemistry has about half the density of the pacing battery, it is possible to reduce the total battery weight and volume by greater than about 50%. This provides dramatic improvement in the manufacture, implantation, and operation of the defibrillator, particularly in view of the restricted size of desired pectoral implant sites. 
     The invention further comprises a multi-stage energy concentration technique for a defibrillator in which the defibrillator capacitor means comprises either a third stage or a secondary sub-circuit of the second stage. In either configuration, it is advantageous to provide a rechargeable second stage or intermediate battery means as a fully charged high current output battery means. This permits rapid charging of the defibrillator capacitor means. Indeed, in certain configurations it is now possible to recharge at a rapid 3-5 second rate using this invention rather than at a slower rate, which is common in the industry. Therefore, yet another advantage of this invention derives from the use of the second stage energy concentration as a recharge rate accelerator. This also results in a defibrillator with reduced end of life charge degradation due to the constantly recharged second stage. This feature effectively provides a battery life extension capability before elective replacement, assuming certain accepted energy levels.