This invention relates to electrotherapy circuits and in particular to a defibrillator using multiple capacitors to provide for an impedance-compensated delivery of defibrillation pulses to the patient.
Electro-chemical activity within a human heart normally causes the heart muscle fibers to contract and relax in a synchronized manner that results in the effective pumping of blood from the ventricles to the body's vital organs. Sudden cardiac death is often caused by ventricular fibrillation (VF) in which abnormal electrical activity within the heart causes the individual muscle fibers to contract in an unsynchronized and chaotic way. The only effective treatment for VF is electrical defibrillation in which an electrical shock is applied to the heart to allow the heart's electro-chemical system to re-synchronize itself. Once organized electrical activity is restored, synchronized muscle contractions usually follow, leading to the restoration of cardiac rhythm.
The minimum amount of patient current and energy delivered that is required for effective defibrillation depends upon the particular shape of the defibrillation waveform, including its amplitude, duration, shape (such as sine, damped sine, square, exponential decay), and whether the current waveform has a single polarity (monophasic), both negative and positive polarities (biphasic) or multiple negative and positive polarities (multiphasic). At the same time, there exists a maximum value of current in the defibrillation pulse delivered to the patient above which will result in damage to tissue and decreased efficacy of the defibrillation pulse.
Peak current is the highest level of current that occurs during delivery of the defibrillation pulse. Limiting peak currents to less than the maximum value in the defibrillation pulse is desirable for both efficacy and patient safety. Because the transthoracic impedance ("patient impedance") of the human population may vary across a range spanning 20 to 200 ohms, it is desirable that an external defibrillator provide an impedance-compensated defibrillation pulse that delivers a desired amount of energy to any patient with the range of patient impedances and with peak currents limited to safe levels substantially less than the maximum value.
Most external defibrillators employ a single energy storage capacitor or a fixed bank of energy storage capacitors charged to a single voltage level. Controlling the amount of energy delivered to any given patient across the range of patient impedances is a problem commonly solved by controlling the "tilt" or difference between initial and final voltages of the energy storage capacitor as well as the discharge time of the defibrillation pulse. Most external defibrillators use a single energy storage capacitor charged to a fixed voltage level resulting in a broad range of possible discharge times and tilt values across the range of patient impedances. A method of shaping the waveform of the defibrillation pulse in terms of duration and tilt is discussed in U.S. Pat. No. 5,607,454, "Electrotherapy Method and Apparatus", issued Mar. 4, 1997 to Gliner et al. Using a single capacitor to provide the defibrillation pulse at adequate energy levels across the entire range of patient impedances can result in high peak currents being delivered to patients with relatively low impedances. At the same time, the charge voltage of the energy storage capacitor must be adequate to deliver a defibrillation pulse with the desired amount of energy to patients with high impedances.
Various prior art solutions to the problem of high peak currents exist. One method involves placing resistors in series with the energy storage capacitor to prevent excessive peak currents to low impedance patients. In U.S. Pat. No. 5,514,160, "Implantable Defibrillator For Producing A Rectangular-Shaped Defibrillation Waveform", issued May 7, 1996, to Kroll et al., an implantable defibrillator have a rectilinear-shaped first phase uses a MOSFET operating as a variable resistor in series with the energy storage capacitor to limit the peak current. In U.S. Pat. No. 5,733,310, "Electrotherapy Circuit and Method For Producing Therapeutic Discharge Waveform Immediately Following Sensing Pulse", issued Mar. 31, 1998, to Lopin et al., an electrotherapy circuit senses patient impedance and selects among a set of series resistors in series with the energy storage capacitor to create a sawtooth approximation to a rectilinear shape in the defibrillation pulse. Using current limiting resistors as taught by the prior art results in substantial amounts of power being dissipated in the resistors, which increases the energy requirements on the defibrillator battery.
Another approach to limiting peak currents involves using multiple truncated decaying exponential waveforms from multiple capacitors to form a sawtooth approximation of a rectilinear shape of the discharge waveform in an implantable defibrillator. In U.S. Pat. No. 5,199,429, "Implantable Defibrillation System Employing Capacitor Switching Networks", issued Apr. 6, 1993, to Kroll et al., a set of energy storage capacitors are charged and then successively discharged during the first phase to create the sawtooth pattern. Kroll et al. teach that multiple capacitors may be arbitrarily arranged in series, parallel, or series-parallel arrangements during the delivery of the defibrillation pulse in order to tailor the shape of the defibrillation waveform with a high degree of flexibility.
In U.S. Pat. No. 5,836,972, "Parallel Charging of Mixed Capacitors", issued Nov. 17, 1998, to Stendahl et al., a method for charging banks of energy storage capacitors in parallel is taught. The banks of energy storage capacitors may then be coupled in series in order to deliver a defibrillation pulse.
However, neither Kroll et al. nor Stendahl et al. address the issue of obtaining impedance-compensated defibrillation pulses which have peak currents less than the maximum value and with less variation of discharge times across the range of patient impedances. It would therefore be desirable to provide a defibrillator that selects among configurations of energy storage capacitors to deliver an impedance-compensated defibrillation pulse to the patient.