Abstract: A DC/DC converter circuit to convert input source energy to output load energy, for example, a flyback converter to charge a defibrillation storage capacitor from a low-voltage battery in an implantable cardiac defibrillator. The converter allows input current to flow in the flyback inductor for a fixed time, and it allows output current to flow for a time directly proportional to average battery voltage and inversely proportional to the voltage on the capacitor being charged. When used near an electromagnetic communication link, to avoid interference, a control circuit enables and disables the converter at a maximum frequency that is substantially below the operating frequency of the communication link. In addition, the control circuit restricts the converter to transfer energy at a minimum fundamental frequency substantially above the frequency of the communication link.

Abstract: A defibrillator output circuit utilizes an optically coupled signal for controlling an isolated electronic switch. Variants of the output circuit include coupling via phototransistors or photodiodes for the control of at least one electronic switch. An H-bridge circuit configuration with four switches is connected to a single energy storage capacitor for generating multiphasic shocks across a load. The polarity of the shocks is selectable. Optical coupling methods are employed for driving the high side switches in the H-bridge.

Abstract: The invention protects circuits connected to low energy stimulating (pacing) or detection (sensing) electrodes of an implantable medical device from high energy pulses generated by the device itself or by external devices such as external defibrillators. It also protects cardiac tissue near the low energy electrodes. The invention provides for an automatic and/or foldback-type unidirectional current limiter in each low energy lead. The invention, when applied to an implantable defibrillator/pacemaker, also provides a shock generator presenting pulses to the pacing electrodes only in the direction blocked by the current limiter.

Abstract: An improved automatic sensing threshold control for cardiac rhythm detection in an implantable device. The improvement includes setting the sensing threshold according to the amplitude of both sensed signals corresponding to cardiac activity and sensed noise, for setting the threshold substantially below the sensed signal amplitude, but above the noise amplitude. The invention provides absolute refractory periods after pacing or sensing, followed by a retriggerable noise refractory period. Following sensing the device sets the sensing threshold to approximately one-fourth of the measured amplitude, and then increases the threshold when it detects noise.

Abstract: A method and apparatus for optimizing the sensing circuit of a tachyarrhythmia detector for use with direct heart (cardiac) electrodes. The method is applied to detection of ventricular fibrillation and multifocal ventricular tachyarrhythmia, with rejection of T wave and pacing artifact. The sensing circuit optimized with this method includes a third-order high-pass filter followed by at least one comparator, with the filter corner frequency near 16 Hz and the comparator threshold near 0.40 mV referred to the input, in-band.

Abstract: A configuration for generating single capacitor multiphasic shocks is disclosed, using an H-bridge with four switches to connect an energy storage capacitor to a load with selectable polarity. The improvement concerns referring the negative side of the shock generator to a supply voltage which is negative with respect to pacing and sensing ground, and to connect the battery positive terminal to this same ground. This configuration permits operating the low side switches in the H-bridge, the pace/sense circuits, and the control circuitry from negative supplies derived without inversion from the negative battery terminal.

Abstract: This invention provides an improved Holter function in an implantable device for arrhythrnia monitoring or therapy. One improvement consists of a zoom feature which stores information about an arrhythmia episode at several levels of detail, with increasing levels of detail corresponding to shorter recording periods, and with a way to display the relation in time of the recordings at different levels of detail. The invention also introduces a novel implantable Holter function called an event log, which records time-stamped events once each minute, on changes in detected rhythm, or on application of antiarrhythmia therapy. The invention also provides logic for arbitrating which arrhythmia episodes the device saves in its memory.

Abstract: Methods and apparatus for classifying cardiac rhythm by analyzing sensed atrial and ventricular heartbeats, and determining if the rhythm might be pace-terminable in a given chamber. The method maintains a histogram of the sensed beat interval in the chamber of interest and a second histogram of conduction time from the other chamber. The method classifies a rhythm as pace terminable in the given chamber when the intervals in that chamber are stable, and the conduction times to that chamber are unstable. It applies further criteria when the intervals in the given chamber are stable, and the conduction times are also stable. Antitachycardia pacing is deliverable to terminate tachyarrhythmias determined to be pace terminable.

Abstract: The difference between the divergences between the determined PR and RR intervals (dPR-dRR) is measured, cycle by cycle, and compared to two selected thresholds S1 and S2. If the difference exceeds threshold S1, a signal corresponding to a ventricular tachycardia is triggered. If the difference is below threshold S2, a signal corresponding to a supraventricular tachycardia related signal is triggered. The trigger signals may be used to control the therapeutic treatment of an implantable tachycardia treatment device accordingly.