Biphasic pulse generator for an implantable defibrillator

Abstract: A circuit for generating a biphasic voltage pulse to restore rhythm to a fibrillating heart, the circuit utilizing a capacitor (40) for providing the voltage pulse. The circuit further comprises first and second thyristors (34,36) to regulate the voltage and first and second electrodes (10,12) to apply the regulated voltage to the fibrillating heart. The circuit further comprises an output sensing circuit (14) that senses the exponential decay of the capacitor (40) and signals a control circuit (16) to switch the thyristors (34,36) such that after one thyristor applies the voltage pulse to the heart in a first polarity, the other thyristor applies the voltage pulse in the opposite polarity. Therefore, substantially all energy in the capacitor (40) is provided to the heart.

Description: BACKGROUND OF THE INVENTION The present invention relates, generally, to the field of automatic implantable defibrillators. In such implantable defibrillators, the defibrillation pulse is stored and delivered by a high voltage capacitor; this capacitor is short circuited in order to terminate the defibrillation pulse. A majority of defibrillators, found in the prior art, are short circuiting the charged capacitor which is a waste of energy. This energy could easily be applied to the fibrillating heart. Also, by short circuiting the charged capacitor, a significant amount of electrical stress is absorbed through the electronic parts of the defibrillator circuit. This electrical stress shortens the life of these parts. One attempt to use the wasted energy of the capacitor is by using triphasic wave defibrillation. U.S. Patent No. 4,637,397 to Jones et al discloses a defibrillator that uses a first conditioning pulse, followed by a second pulse of opposite polarity used for defibrillating, and finally a third pulse having the same polarization as the first pulse. The circuit of this defibrillator comprises: Capacitor means for providing a voltage to restore rhythm to a fibrillating heart, defibrillation electrode means for receiving and applying said voltage to said heart, and control circuit means for controlling charge and discharge of said capacitor means. The EP-A-0 281 219, a document which meets the provisions of Art. 54 (3) EPC with respect to the Designated Contracting States DE FR IT NL, discloses a biphasic pulse generator for a cardiac defibrillator, delivering an asymmetric biphasic current pulse to three separate electrodes. The circuit generating those biphasic current pulses comprises thyristors connected between a charge capacitor and said electrodes. The circuit of this defibrillator comprises: Capacitor means for providing a voltage to restore rhythm to a fibrillating heart, defibrillation electrode means for receiving and applying said voltage to said heart, control circuit means for controlling charge and discharge of said capacitor means, and thyristor means connected between said capacitor means and said electrode means. The following description discloses a biphasic pulse generator for an implantable defibrillator which uses only two pulses to deliver more energy stored in the charged capacitor to the fibrillating heart and thus, lowering defibrillation energy requirements. SUMMARY OF THE INVENTION The present invention eliminates the need to short circuit the high voltage capacitor. Thus, the stress on electronic parts is reduced. Additionally, the energy in the high voltage capacitor that was previously wasted is provided to the heart in the reverse direction, thereby reducing overal defibrillation energy requirements. An object of the present invention is to provide a pulse generation in which a silicon controlled rectifier thyristor is turned off by means of an insulated gate transistor. Another object of the invention is to allow a "steering" of the high voltage side of a charged capacitor to one of two electrodes while steering the low voltage side to the other electrode. A further object of the invention is to utilize a low "on" impedance electronic switch which in the preferred embodiment is an insulated gate transistor. Another object of the invention is to protect the limited reverse voltage handling capability of an insulated gate transistor with a diode. A feature of the invention is that the low "on" impedance high voltage electronic switch is positioned in a low potential gate drive circuit which obviates the use of large transformers or optical isolators in the circuit. An advantage of the invention is that since the shorting of the high voltage capacitor has been eliminated, the stress on the electronic parts is reduced. These, together with other objects and advantages which will become subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numbers refer to like parts throughout. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a biphasic defibrillation pulse from a single capacitor as delivered from a circuit forming a part of the present invention; Fig. 2 shows a block diagram of the circuit of the present invention; and Fig. 3 shows a schematic drawing of the circuit of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the figures, the circuit of the present invention is designed to deliver a biphasic defibrillation pulse from a single capacitor as shown in Fig. 1. The voltage at peak 1, or Vpk1, is equal to 25 to 1000 volts delivered to the heart. After reaching Vpk1, the voltage will exponentially decay until an SCR thyristor is turned off and the high voltage side of the charged capacitor is steered to the other electrode. Now, Vpk2 is delivered to the heart in the reverse direction and is equal to 50 to 95% of Vpk1. Thus, the short circuiting of the high voltage capacitor is eliminated. Fig. 2 shows the biphasic pulse generator block diagram. Control circuit 16 is a simple Four State Sequencer specifically designed to provide wait 1, phase 1, wait 2, and phase 2 states by monitoring the capacitor voltage and providing timed stimulation to the electronic switches via circuits 20 and 22. When drive circuit 20 is set high, the electronic switch 30 is allowed to conduct and a thyristor means or SCR (silicon controlled rectifier) 34 is turned on from drive circuit 20. At this point, the charge stored across capacitor 40 is delivered to the heart across electrodes 10 and 12 in a first polarity. After a short period of time, sensed by circuit 14, drive circuit 20 is forced low turning off electronic switch 30 and thyristor means 34. After a short delay, less than 500 microseconds generally indicated by numeral 15, drive circuit 22 is set high turning on electronic switch 32 and a thyristor means or SCR 36 thus, providing an opposite current through the heart. After another period of time sensed by circuit 14, the electronic switch 32 is turned off which turns off thyristor means 36. In other words, the electronic switch 30 conducts to steer the low voltage side of the main storage capacitor 40 to electrode 12 while the high side is connected to electrode 10. Electronic switch 32 conducts to steer the low voltage side of the main storage capacitor 40 to electrode 10 while the high side is connected to electrode 12. The thyristor means 34, when switched "on", provides the high voltage to electrode 10. The thyristor means 36, when switched "on", provides the high voltage to electrode 12. Output sense circuit 14 monitors the output to electrode 10. When the output voltage at electrode 10 falls to a suitable low level, the output sense circuit 14 will signal the control circuit 16, which then forces drive circuit 20 to a low. This shuts off electronic switch 30 and, therefore, thyristor means 34. When the output voltage to electrode 10 falls still further, the output sense circuit 14 signals the control circuit 16. This forces drive circuit 22 to be switched to a low, which shuts off electronic switch 32 and, therefore, the thyristor means 36. The high voltage isolation transformers 24 and 26 are used to isolate the thyristor means drive circuits and prevent the transmission of undesired currents to them. Also, the high voltage isolation transformers are used to separate one section of the system from undesired influences of the other sections. Referring to the detailed schematic shown in Fig. 3, the operation of the system is as follows. The drive circuit 20 is set high (about 10 volts) which allows the electronic switch 30 including an insulated gate transistor Q4 to conduct when the thyristor means 34 including a thyristor Q5 is turned on from the drive circuit 20. There is a 20 microsecond delay through the network preceding the Schmidt trigger circuits IC1 and IC2 to allow switch 30 to be completely on before thyristor Q5 is fired. This results in the high voltage of first side of the main storage capacitor means 40 being switched to first electrode 10 while the low second side is connected to second electrode 12. Now the high voltage Vpk1 is delivered to the heart. When the output voltage falls to a suitable low level the drive circuit 20 is set low, turning off electronic switch 30 as shown as an insulated gate transistor Q4 and, therefore, the thyristor Q5. Next, after a 500 microsecond delay and drive circuit 20 has gone low, drive circuit 22 is switched to a high (i.e., approximately 10 volts). This allows electronic switch 32 including an insulated gate transistor Q3 to be operational and thyristor means 36 including a thyristor Q6 is fired by its nominal 20 microsecond delay circuit (IC3-4 and network). The delay is again necessary to allow switch 32 to be fully on before thyristor Q6 is fired. The Schmidt trigger circuits IC3 and IC4 are for squaring the pulses for good logic operations. The process steers the high voltage of first side of capacitor means 40 to second electrode 12 and the low voltage of second side of capacitor means 40 to first electrode 10. Therefore, the voltage to the load is reversed and the high voltage Vpk2 is delivered to the heart. As soon as the voltage falls to another low level, drive circuit 22 will be forced to a low and shut off electronic switch 32 shown as an insulated gate transistor Q3. Therefore, the thyristor Q6 will also be off. Drive circuit 20 and drive circuit 22 are derived from a simple electronic sequence circuit which incorporates a chip such as the 14017B manufactured by Motorola. These circuits are also used to monitor the main capacitor voltage. A possible modification is that the electronic switches 32 and 30 shown in Fig. 3 to include insulated gate transistors, could also include power field effect transistors or high voltage bipolar transistors.

Claims: A circuit for generating a biphasic pulse to restore rhythm to a fibrillating heart, said circuit comprising: capacitor means (40) for providing a voltage to restore rhythm to the fibrillating heart; defibrillation electrode means (10, 12) for receiving and applying said voltage to said heart, and control circuit means (16) for controlling charge and discharge of said capacitor means (40), wherein first and second thyristor means (34, 36) are provided for regulating said voltage; said defibrillation electrode means comprises first electrode means (10), and second electrode means (12), said first thyristor means (34) being connected between the first side of said capacitor means (40) and said first electrode means (10), said second thyristor means (36) being connected between the first side of said capacitor means (40) and said second electrode means (12); a first electronic switch (30) is connected between the second side of said capacitor means (40) and said second electrode means (12); a second electronic switch (32) is connected between the second side of said capacitor means (40) and said first electrode means (10); an output sensing means (14) is connected between said first electrode means (10) and said control circuit means (16) for sensing an exponential decay of said regulated voltage and providing a sensed signal corresponding to said sensed exponential decay; and said control circuit means (16) is connected to said output sensing means (14) for controlling which of said first and second thyristor means (34, 36) are on to receive said voltage to restore rhythm to said fibrillating heart. The circuit of claim 1, characterized by first and second drive circuits (20, 22) connected to said control circuit means (16), said first drive circuit (20) driving said first thyristor means (34) and said second drive circuit (22), driving said second thyristor means (36). The circuit of claim 2, characterized in that said first drive circuit (20) is connected to said first electronic switch (30) and said first thyristor means (34) for forming a first circuit such that said first circuit delivers said voltage to said fibrillating heart; and said second drive circuit (22) is connected to said second electronic switch (32) and said second thyristor means (36) for forming a second circuit such that said second circuit delivers said voltage to said fibrillating heart. The circuit of anyone of the claims 1 to 3, characterized in that said capacitor means (40) charges to 25 to 1000 Volts and then discharges to 50 % to 95 % of said voltage. The circuit of anyone of the claims 1 to 4, characterized in that said first and second thyristor means (34, 36) further comprise a first and second high voltage isolation transformer (24, 26) for isolating said first and second thyristor means (34, 36).

Inventors: BACH STANLEY M JR, BACH, STANLEY M., JR.
Applicants: MIROWSKI MIECZYSLAW, MIROWSKI, MIECZYSLAW
Application year: 1989
Main classification: A61N1
IPC codes: A61N1