[Change in impulse formation and impulse transmission of the heart in relation to high magnesium concentration]

The effects of magnesium ions on the cardiac conduction velocity and on the refractory periods of the single parts of Langendorff perfused guinea pig hearts were studied using a high amplifying ECG recording and a special stimulation technique. Concentrations of 2.3 mM or 3.45 mM Mg++ had only little inhibitory effects on the cardiac conduction velocity and on the sinus node activity. The elevation of the magnesium ion concentration to 4.6 mM caused a reduction of the spontaneous sinus rate and a prolongation of the AV nodal and His bundle conduction time. During stimulation with stepwise increasing pacing rate, a marked prolongation of the refractoriness of the His bundle conduction and of the ventricular myocardium was detectable. This frequency dependent inhibitory influence on the His bundle and the ventricular myocardium could be an explanation for the antiarrhythmic efficacy of magnesium ions during ventricular tachyarrhythmias.