Background: In recent years, there has been an increasing use of antiarrhythmic drugs that act predominantly by prolonging myocardial repolarization. An inevitable electrophysiologic con sequence of these drugs is the development of torsade de pointes as a proarrhythmic reaction. Both intravenous lidocaine and magnesium sulphate have been used in the acute control of such a proarrhythmia. Their electrophysiologic mechanisms in this setting are not well defined.

Methods and Results: Using the standard microelectrode techniques, the effects of magne sium (Mg) and lidocaine on action potential duration (APD), and on barium-induced sponta neous action potentials, were studied in canine Purkinje fiber preparations. The objective was to clarify the direct and indirect effects of magnesium on triggered activities due to early afterdepolarizations. Superfusion in media with 0.1 mM Mg and 2.5 mM K produced more pronounced increases in APD measured at -20mV repolarization time [APD₂₀] than those in a solution with 5 mM K. This effect was further enhanced at lower stimulation frequencies. The striking prolongation of APD₂₀ by solutions with low potassium concentrations dimin ished as the Mg concentration was increased. In solutions with 2.5 mM K, Mg produced con centration-dependent decreases in APD₂₀. This effect was greater at lower stimulation fre quencies. Lidocaine at 4.0 × 10-5 M produced a marked shortening of the APD in the entire voltage range. On barium-induced automaticity, Mg markedly suppressed the spontaneous firing frequency of the abnormal automaticity in a concentration-dependent manner. With 10 mM Mg, such action potentials appeared only sporadically. Magnesium also decreased the amplitude and the maximum upstroke velocity of these action potentials. In contrast, lido caine at 4.0 × 10-5 M exhibited no significant effects on action potentials due to barium- induced abnormal automaticity, or on additional depolarizations developing from the repolar ization phase of these action potentials.

Conclusions: The data indicate that (i) hypomagnesemia may be arrhythmogenic when com bined with hypokalemia and bradycardia leading to a prolongation of the plateau phase of the action potential, (ii) magnesium administration may suppress triggered activities mainly by a direct inhibition of the development of triggered potentials, and (iii) lidocaine may suppress triggered potentials only indirectly by preventing the development of early afterdepolariza tions due to the shortening effect on the APD. These findings are consistent with the clinical observation of a high incidence of torsade de pointes in the setting of hypokalemia and hypo magnesemia induced by a chronic diuretic therapy. They are also consistent with the marked effectiveness of intravenous Mg relative to the inconsistent clinical effects of lidocaine in controlling torsade de pointes.

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Antiarrhythmic and Arrhythmogenic Actions of Varying Levels of Extracellular Magnesium: Possible Cellular Basis for the Differences in the Efficacy of Magnesium and Lidocaine in Torsade de Pointes