Diastolic Intracellular Calcium-Membrane Voltage Coupling Gain and Postshock Arrhythmias

Abstract

Rationale: Recurrent ventricular arrhythmias after initial successful defibrillation are associated with poor clinical outcome. Objective: We tested the hypothesis that postshock arrhythmias occur because of spontaneous sarcoplasmic reticulum Ca release, delayed afterdepolarization (DAD), and triggered activity (TA) from tissues with high sensitivity of resting membrane voltage (V_m) to elevated intracellular calcium (Ca_i) (high diastolic Ca_i–voltage coupling gains). Methods and Results: We simultaneously mapped Ca_i and V_m on epicardial (n=14) or endocardial (n=14) surfaces of Langendorff-perfused rabbit ventricles. Spontaneous Ca_i elevation (SCaE) was noted after defibrillation in 32% of ventricular tachycardia/ventricular fibrillation at baseline and in 81% during isoproterenol infusion (0.01 to 1 μmol/L). SCaE was reproducibly induced by rapid ventricular pacing and inhibited by 3 μmol/L of ryanodine. The SCaE amplitude and slope increased with increasing pacing rate, duration, and dose of isoproterenol. We found TAs originating from 6 of 14 endocardial surfaces but none from epicardial surfaces, despite similar amplitudes and slopes of SCaEs between epicardial and endocardial surfaces. This was because DADs were larger on endocardial surfaces as a result of higher diastolic Ca_i–voltage coupling gain, compared to those of epicardial surfaces. Purkinje-like potentials preceded TAs in all hearts studied (n=7). I_K1 suppression with CsCl (5 mmol/L, n=3), BaCl₂ (3 μmol/L, n=3), and low extracellular potassium (1 mmol/L, n=2) enhanced diastolic Ca_i–voltage coupling gain and enabled epicardium to also generate TAs. Conclusions: Higher diastolic Ca_i–voltage coupling gain is essential for genesis of TAs and may underlie postshock arrhythmias arising from Purkinje fibers. I_K1 is a major factor that determines the diastolic Ca_i–voltage coupling gain.