The automatic implantable cardioverter-defibrillator: effect of patch polarity on defibrillation threshold. An automatic implantable cardioverter-defibrillator (AICD) was implanted in 40 patients with sudden cardiac arrest (n = 29), sustained monomorphic ventricular tachycardia (n = 10) or recurrent syncope (n = 1) who were unsuitable for direct ablative surgery or had had unsuccessful medical therapy. The effect of patch electrode polarity on the defibrillation threshold was prospectively evaluated. Two large epicardial patches were used. Initial polarity was selected at random. Ventricular fibrillation was induced by direct current and a preestablished defibrillation protocol employed to assess the minimal energy that would reproducibly defibrillate the heart. Nineteen patients had a lower defibrillation threshold with the inferior left ventricular patch as an anode and nine patients had a lower defibrillation threshold with this patch as a cathode. In general, the defibrillation threshold was lower when this patch was used as an anode than when it was used as a cathode (18 +/- 10 versus 22.6 +/- 12.2 J; p less than 0.01). No preoperative variable predicted optimal polarity. Therefore, the effect of patch polarity on defibrillation threshold should be assessed in each patient at the time of AICD implantation so that the safety margin for satisfactory device function can be maximized. 