Electrophysiologic effects of encainide on acutely ischemic rabbit myocardial cells

The proarrhythmic effect of hypoglycemia: evidence for increased risk from ischemia and bradycardia

Hypoglycemia increases the risk for both overall and sudden death. At a cellular level, hypoglycemia causes alterations in the physiology of myocardial tissue that are identical to proarrhythmic medications. Reduced serum glucose blocks the repolarizing K(+) channel HERG, which leads to action potential and QT prolongation and is uniformly associated with risk for torsades de pointes ventricular tachycardia. The sympathetic response induced by hypoglycemia also increases the risk of arrhythmias from Ca(2+) overload, which occur with sympathomimetic medications and excessive beta adrenergic stimulation. Thus, hypoglycemia can be considered a proarrhythmic event. This review focuses on emerging evidence for two other important changes induced by hypoglycemia that promote arrhythmias: ischemia and bradycardia. Studies of patients with "insulin shock" therapy from the early twentieth century and other more recent data strongly suggest that hypoglycemia can cause ischemia of myocardial tissue, both in association with coronary artery obstructions and by cellular mechanisms. Ischemia induces multiple proarrhythmic responses. Since ischemia itself reduces the possibility of using energy substrates other than glucose, hypoglycemia may generate positive feedback for electrophyisologic destabilization. Recent studies also show that hypoglycemia can cause bradycardia and heart block. Bradycardia is known to cause action potential prolongation and potentiate the development of torsades de pointes, particularly with low-serum K(+) which can be induced by hypoglycemic episodes. Thus, hypoglycemia-induced bradycardia may also create a dynamic, positive feedback for the development of arrhythmias and sudden death. These studies further support the hypothesis that hypoglycemia is a proarrhythmic event.

Selective depression of maximum rate of depolarization of guinea-pig ventricular action potentials by amiodarone and lignocaine in simulated ischaemia: comparison with encainide

1. Standard microelectrode techniques were used to record intracellular action potentials from guinea-pig ventricular myocardium superfused with either control physiological saline (pH 7.5; pO2 500 mmHg; [K+] 5.6 mmol/L) or 'simulated ischaemic' solution (pH 6.4; pO2 90 mmHg; [K+] 11.2 mmol/L). 2. The effects on action potential parameters of therapeutic concentrations of lignocaine, amiodarone and encainide were studied under both conditions. 3. Simulated ischaemia, in the absence of drugs, produced marked reductions in resting potential (-86.6 +/- 2.3 to -64.7 +/- 3.5 mV), maximum rate of depolarization (Vmax; 263 +/- 66 to 106 +/- 36 V/s) and action potential duration (164 +/- 24 to 97 +/- 26 ms). No drug produced any additional effect on resting potential. 4. All three drugs produced enhanced depression of Vmax in ischaemia compared to control conditions (class I effect). This was much more marked for lignocaine and amiodarone (inactivated channel blockers) than for encainide (open channel blocker). 5. In addition the prolongation of action potential duration seen with acute exposure to amiodarone (174 +/- 12 to 192 +/- 17 ms; class III effect) was abolished under simulated ischaemic conditions. 6. It is concluded that lignocaine and amiodarone exert greater selectivity for ischaemic tissue than does encainide and that amiodarone may function primarily as a class I agent under ischaemic conditions.

Encainide

Encainide is a class IC antiarrhythmic agent having little or no effect on action-potential duration or maximum diastolic potential but decreasing the maximum rate of phase O depolarization as well as increasing atrial and ventricular effective refractory periods. In intact animals or humans, encainide increases the AH, PR, QRS, and H-V intervals while not affecting the sinus node cycle length or JT interval. QT interval increases only by the concomitant increase in the QRS interval. Encainide is metabolized to O-demethyl encainide (ODE) and 3-methoxy-ODE (MODE), both of which are also antiarrhythmics with similar pharmacology to encainide. Encainide and its metabolites have little negative inotropic activity and ancillary pharmacology. Consequently, encainide has little or no effect on hemodynamic variables in patients with either normal or compromised cardiac function. The drug is well tolerated, with side effects being mainly those associated with its local anesthetic activity such as blurred vision and dizziness. Encainide is particularly effective in patients with excessive premature ventricular complexes (PVCs) and less so in patients with sustained ventricular tachycardia (VT). Like all antiarrhythmics, encainide may aggravate or precipitate new arrhythmias (proarrhythmia). The overall incidence of proarrhythmia is about 10%, with less occurring in patients with PVCs and more in those with sustained VT; also, the incidence of proarrhythmia is higher in patients with underlying heart disease. Encainide is also effective for the treatment of supra-ventricular arrhythmias, including atrial fibrillation, PSVT (both PAF as well as reentry of the nodal or W-P-W type), and ectopic atrial tachycardia. Its dosage and role in antiarrhythmic therapy are discussed.

Basic and clinical cardiac electrophysiology of encainide

Encainide is a potent sodium channel antagonist. It dissociates slowly from blocked, repolarized channels (time constant of recovery greater than 20 seconds). It markedly slows myocardial and His Purkinje conduction in vitro, in animal models and in humans. In vitro the parent compound and its major metabolites, O-demethyl and 3-methoxy-O-demethyl encainide, have variable effects on action potential duration and refractoriness. In man the parent compound has relatively little effect on refractoriness and QT interval, but its metabolites may increase refractoriness moderately. Encainide has no significant effect on the normal sinoatrial node, and only its metabolites significantly depress atrioventricular nodal conduction and refractoriness. In models of ischemia, all of encainide's actions are more pronounced in ischemic than in normal tissue. Encainide is similar in its basic and clinical electrophysiologic profile to flecainide and lorcainide although its constellation of electrophysiologic properties is unique. It differs from quinidine, procainamide and disopyramide by slowing conduction more and affecting refractoriness less, and by absence of anticholinergic side effects.

Electrophysiology, hemodynamic and arrhythmia efficacy model studies on encainide

Encainide is a class IC agent possessing a broad spectrum of antiarrhythmic actions in a variety of animal models. It increases the ventricular fibrillation threshold of the perfused rabbit heart and in situ dog myocardium. Encainide suppresses atrial fibrillation resulting from topical application of aconitine in the anesthetized dog and ventricular fibrillation induced by chloroform asphyxiation in the mouse. In these latter 2 models, encainide is approximately 7 to 11 and 16 to 18 times more potent, respectively, on a milligram basis than quinidine. In anesthetized dogs encainide converts ouabain-induced tachyarrhythmias to normal sinus rhythm at a mean intravenous dose of 0.67 mg/kg. Single doses of 0.5 mg/kg intravenously or 1 mg/kg orally significantly reduced ventricular ectopy in conscious dogs 18 to 22 hours after 2-stage ligation of the left coronary artery. At doses and plasma concentrations exceeding efficacious therapeutic levels, encainide has no major negative inotropic effects and does not compromise cardiac function or hemodynamics. It is devoid of peripheral autonomic or mediator-evoked responses and, in particular, lacks anticholinergic actions. Encainide is rapidly absorbed by all routes of administration and extensively metabolized by the liver. The major metabolites, O-demethyl encainide and 3-methoxy-O-demethyl encainide, have been shown to have quantitatively different, but qualitatively similar, profiles of pharmacodynamic effects. Subacute and chronic administration of encainide at doses representing 11 times an effective oral human dose have produced no distinct or consistent toxicologic findings. Carcinogenicity and mutagenicity studies were negative.(ABSTRACT TRUNCATED AT 250 WORDS)