The automatic implantable defibrillator is an electronic device designed to continuously monitor cardiac rhythm, identify ventricular fibrillation and deliver corrective defibrillatory discharges, when indicated. Physically similar to early pacemakers, it weights 250 grams and has a volume of 145 cc (Figure 1). All materials in contact with body tissue are biocompatible. The defibrillating electrodes are made from titanium and silicone rubber. One electrode, designed for placement in the superior vena cava near the right atrial junction, is located on the distal end of an intravascular catheter. The second electrode, in the form of a flexible rectangular patch, is placed extrapericardially over the apex of the heart. The outside surface of the apical electrode is insulated to achieve optimal current distribution.

The device is powered by lithium batteries having a projected monitoring life of approximately 3 years or a discharge capability of approximately 100 shocks. The sensing system detects ventricular fibrillation by monitoring a sampled probability density function of ventricular electrical activity.