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The effects of propranolol on the electrical behavior of ventricular muscle fibers were investigated in the anesthetized dog by simultaneously recording surface and intracellular electrograms. After administration of propranolol during sinus rhythm, the RR, PQ and QT intervals were significantly prolonged, while the QRS duration remained unchanged. During electrically driven rhythm, propranolol caused a slight but insignificant prolongation of the QT interval. There were no changes in the QRS duration. Significant delay of the interatrial and atrioventricular conduction times was observed. There were no significant changes in the intraventricular conduction time. Propranolol principally affects the sinoatrial node, the atrioventricular node and the conduction time of the atria. Effects of propranolol on the intracellular electrograms from the ventricle were as follows. The duration of the membrane action potential was significantly prolonged. No changes were observed in the amplitudes of the membrane action potential, membrane resting potential and overshoot. The maximum upstroke velocity of the membrane action potential did not change. Propranolol has a quinidine-like action as well as a beta adrenergic blocking action. Both actions contribute to the effectiveness of the drug in the treatment of cardiac arrhythmias.

The Effects of Propranolol on the Electrophysiology of the Heart