Unipolar electrode system for an implantable defibrillator: new experimental approaches.

Defibrillation thresholds are studied in normal dog hearts after induction of ventricular fibrillation by alternating current. Shocks of progressively increasing energies are tried after a period of 10 seconds of sustained ventricular fibrillation. The endocardial electrode system may be either unipolar or bipolar, the distal electrode being situated at the right ventricular apex. The results suggest that for an optimal capacitor in the range of 9-20 uF, unipolar shocks are more effective than bipolar shocks (3-10 versus 10-30 Joules). In addition, several cardiac arrhythmias, including rapid ventricular tachycardia, accelerated idioventricular rhythm, and transient AV block are frequently observed. We conclude that (1) an implantable unipolar endocardial defibrillator seems feasible; and (2) the design should include the appropriate circuits to treat the arrhythmias observed after the shocks.