ELECTROPHYSIOLOGICAL EFFECTS OF LEFT-VENTRICULAR HYPERTROPHY - EFFECT OF CALCIUM AND POTASSIUM CHANNEL BLOCKADE

Background. To define the arrhythmogenic effects of left ventricular hypertrophy (LVH) in the intact heart, we carried out a detailed electrophysiological assessment in our previously validated feline aortic-banding model and then tested the effects of agents that blocked either the slow inward calcium or voltage-dependent potassium channel. Methods and Results. We measured intraventricular and interventricular conduction times, excitability thresholds, ventricular effective refractory periods, and monophasic action potential duration at several sites in cats with LVH as well as in concurrent control (sham-operated) cats. In addition, we assessed vulnerability to ventricular arrhythmia using direct measurement of ventricular fibrillation (VF) thresholds and by standard techniques of programmed stimulation. Despite finding no difference between LVH and sham-operated cats in mean values for several electrophysiological parameters, the former group was significantly more vulnerable to VF, with more spontaneous VF and lower VF thresholds. Compared with the sham controls, LVH cats also had a greater dispersion of effective refractory period (35 +/- 11 versus 12 +/- 4 msec, p < 0.01) and monophasic action potential duration at 90% repolarization (69 +/- 25 versus 39 +/- 7 msec, p < 0.02). Verapamil had no significant effect on these electrophysiological parameters, nor did it affect VF threshold. However, risotilide, an inhibitor of the voltage-dependent potassium channel, narrowed dispersion of the effective refractory period and monophasic action potential duration concomitant with a marked reduction in ventricular vulnerability. Conclusions. LVH has a pronounced effect on dispersion of refractoriness and repolarization and renders the ventricle more vulnerable to fibrillation. Blockade of the voltage-dependent potassium channel, but not the slow inward calcium channel, narrows the dispersion of recovery of excitability and protects against VF.