TRANSMEMBRANE VOLTAGE CHANGES PRODUCED BY REAL AND VIRTUAL ELECTRODESDURING MONOPHASIC DEFIBRILLATION SHOCK DELIVERED BY AN IMPLANTABLE ELECTRODE

Introduction: Epicardial point stimulation produces nonuniform changes in the transmembrane voltage of surrounding cells with simultaneous o ccurrence of areas of transient positive and negative polarization, Th is is the phenomenon of virtual electrode, We sought to characterize t he responses of epicardial ventricular tissue to the application of mo nophasic electric shocks from an internal transvenous implantable card ioverter defibrillator (ICD) lead, Methods and Results: Langendorff-pe rfused rabbit hearts (n = 12) were stained with di-4-ANEPPS, A 9-mm-lo ng distal electrode was placed in the right ventricle, A 6-cm proximal electrode was positioned horizontally 3 cm posteriorly and 1 cm super iorly with respect to the heart, Monophasic anodal and cathodal pulses were produced by discharging a 150-mu F capacitor, Shocks were applie d either during the plateau phase of an action potential (AP) or durin g ventricular fibrillation, Leading-edge voltage of the pulse was 50 t o 150 V, and the pulse duration was 10 msec, Transmembrane voltage was optically recorded during application of the shock, simultaneously fr om 256 sites on a 11 x 11 mm area of the anterior right ventricular ep icardium directly transmural to the distal electrode, The shock effect was evaluated by determining the difference between the AP affected b y the shock and the normal AP, During cathodal stimulation an area of depolarization near the electrode was observed, surrounded by areas of hyperpolarization. The amplitude of polarization gradually decreased in areas far from the electrode, Inverting shock polarity reversed thi s effect, Conclusion: ICD monophasic defibrillation shocks create larg e dynamically interacting areas of both negative and positive polariza tion.