Considerations in the development of the automatic implantable defibrillator

A highly reliable ventricular fibrillation detector and a satisfactory electrode system for delivering defibrillating pulses to the heart play a central role in the development of an automatic implantable defibrillator suitable for clinical use. Among the four implanted electrode designs tested, the combination of an electrode placed in the superior vena cava with a conformal electrode on the apex of the heart provided satisfactory defibrillation thresholds with ease of implantation. A new sensing method is also described for which an electrogram derived from the defibrillating electrodes is used as input. A form of a density function is developed for a filtered version of the input, ventricular fibrillation being characterized by a density curve lacking a large peak occurring at a level corresponding to the baseline of the filtered signal. These ideas are being incorporated into the design of a prototype implantable defibrillator delivering pulses of 24 joules.