Prolonged membrane depolarization induced by an electric shock in the heart may produce propagation block leading to repetitive beats. We studied prolonged depolarization and its role in repetitive beats in a thin epicardial layer of endocardially prefrozen arterially perfused rabbit heart.Methods and Results
A laser scanner recorded optical action potentials at 63 sites within a 1-cm2 area on the left ventricle of hearts stained with potentiometric fluorescent dye. Pacing (S1) produced propagation across the myofibers; then, a 3-millisecond shock (S2) given in the S1 refractory period produced an electric field that decreased in strength with distance along the fibers. The S2 strengths at the center of the scanned region (C) were 2.1±0.2 or 5.6±03 V/cm (mean±SD, n=4). Repetitive beats occurred in 50% of hearts when C was 2.1 V/cm and in 100% of hearts when C was 5.6 V/cm. With each occurrence of repetitive beats, prolonged depolarization of the shocked action potential occurred within 1 mm of the S2 electrode when C was 2.1 V/cm and within 3 mm when C was 5.6 V/cm. Transient block immediately after S2 occurred between tissue with prolonged depolarization (S2 strength, 6 to 9 V/cm) and tissue without prolonged depolarization (S2 strength, 1 to 3 V/cm). Propagation in the scanned region after S2 occurred first on the side of the block distal to the S2 electrode, propagated from the most recovered to the least recovered tissue, and then turned toward the S2 electrode. When C was 5.6 V/cm, reentry by retrograde propagation near the S2 electrode produced repetitive beats. The center of the reentrant circuit exhibited further transient block and small depolarizations associated with the circulating activation.Conclusions
Prolonged depolarization occurs where the S2 strength is more than 6 V/cm, block occurs between regions of prolonged depolarization and no prolonged depolarization, and reentry occurs around the block. Shock-induced prolonged depolarization can be proarrhythmic and may account for electrically induced arrhythmias.