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The purpose of this study was to evaluate the effects of oral quinidine on the normal sinus node (SN) and A-V node and to determine if the drug exerts in man the same effects observed in cardiac tissue preparations (i.e. both direct and vagolytic action). Electrophysiological studies were performed twice in each of 16 patients (mean age: 57.7 +/- 12 years) with normal resting and intrinsic heart rates and normal A-H intervals. In the first study, the parameters of SN and A-V node were evaluated both in the basal state and following pharmacological autonomic blockade (AB), (propranolol 0.2 mg kg-1 and atropine 0.04 mg kg-1), Oral quinidine was administered for 3-4 days (1200 mg day-1) and the electrophysiological study was then repeated using the same methods. From the comparison of data obtained in the two studies in the basal state the overall effect of quinidine was evaluated, and by comparing those obtained following AB the direct action of the drug was assessed. The overall effect of quinidine on SN and A-V nodal functions was very slight since sinus cycle length, corrected SN recovery time, sino-atrial conduction time, A-H interval, A1-H1 interval at a cycle length of 600 ms and Wenckebach periods did not change significantly after the drug. On the contrary, following AB these measures increased significantly (P less than or equal to 0.01). These results provide evidence of dual effects of oral quinidine in man: a direct depressant action and an autonomically mediated opposing action, very probably vagolytic. The overall effect of the drug is very slight.

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