Effect of direct current countershock on atrial and ventricular electrophysiological properties and myocardial potassium efflux in the thoracotomised dog

Abstract

We reinvestigated the effect of direct current countershock (DCS) on atrial and ventricular electrophysiological properties and myocardial K⁺ release in the open-chested dog. This model allowed direct application of the electroshock paddles to the heart permitting reproducible energy input, simultaneous and comparative epicardial recordings for the measurement of conduction times (CT) and current threshold in different tissues, the determination of monophasic action potential duration (MAPD) with suction electrodes, and the measurement of coronary sinus K⁺ concentration [K⁺]_cs. DCS produced changes in atrial and ventricular CT, in excitability, and in atrioventricular conduction (AVCT) even in the presence of cholinergic and (β-adrenergic receptor blockade. The type of electrophysiological response and its temporal relationship to [K⁺]_cs after DCS suggest the DCS-induced release of myocardial K⁺ to play a role both in the antidysrhythmic effect of DCS and in the dysrhythmias following DCS. These effects were quite distinguishable both electrophysiologically and temporally from those of acetylcholine and catecholamines. More specifically, acetylcholine's effect on sinus rate, atrial APD, AVCT, and perhaps atrial excitability persisted for less than 10 s. The effect of catecholamines on automaticity and ectopic activity had a similar time course. The alterations in CT and excitability (often biphasic) attributable to the transient K⁺ elevation induced by DCS persisted for a half minute or longer. It seems DCS may have 2 antidysrhythmic phases: the first being immediate involving synchronisation of the non-refractory myocardium and the suppression of automaticity; the latter being delayed and dependent on transient local hyperkalemia.