Membrane time constant during internal defibrillation strength shocks in intact heart: Effects of Na⁺ and Ca²⁺ channel blockers

Na⁺ and Ca²⁺ Channel Blockers on Kinetics of Shock-Induced Response. Introduction: We assessed defibrillation strength shock-induced changes of the membrane time constant (τ) and membrane potential (ΔVm) in intact rabbit hearts after administration of lidocaine, a sodium (Na⁺) channel blocker, or nifedipine, a L-type calcium (Ca²⁺) channel blocker. Methods and Results: We optically mapped anterior, epicardial, electrical activity during monophasic shocks (±100, ±130, ±160, ±190, and ±220 V; 150 μF; 8 ms) applied at 25%, 50%, and 75% of the action potential duration via a shock lead system in Langendorff-perfused hearts. The protocol was run twice for each heart under control and after lidocaine (15 μM, n = 6) or nifedipine (2μM, n = 6) addition. τ in the virtual electrode area away from the shock lead was approximated with single-exponential fits from a total of 121,125 recordings. The same data set was used to calculate ΔVm. We found (1) Under all conditions, there is inverse relationship between τ and ΔVm with respect to changes of shock strength, regardless of shock polarity and phase of application: a stronger shock resulted in a larger ΔVm, which corresponded to a smaller τ (faster cellular response); (2) Lidocaine did not cause appreciable changes in either τ or ΔVm versus control, and (3) Nifedipine significantly increased both τ and ΔVm in the virtual cathode area; in contrast, in the virtual anode area, this effect depended on the phase of shock application. Conclusion: τ and ΔVm are inversely related. Na⁺ channel blocker has minimal impact on either τ or ΔVm. Ca²⁺ blocker caused polarity and phase-dependent significant changes in τ and ΔVm.