Electrophysiological effects and mechanism of action of oral quinidine in patients with sinus bradycardia and first degree A-V nodal block.

The effects of quinidine on sinus nodal and A-V nodal function were assessed in 20 patients (age: 60 +/- 7 years) with sinus bradycardia and a prolonged A-H interval. Electrophysiological studies were performed twice in each patient. In the first study, the measurements of sinus and A-V node function were evaluated both in the basal state and after autonomic blockade (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 study was then repeated using the same methods. Comparison of data obtained in the two studies in the basal state allowed us to evaluate the overall effect of quinidine. Comparing the results obtained following autonomic blockade, the direct action of the drug could be assessed. In the basal state quinidine did not significantly change the function of either node. In contrast, after autonomic blockade, significant changes were noted after quinidine. In 3 patients with sinus rate less than 50 beats min-1 and an abnormal intrinsic heart rate, quinidine induced marked depression of sinus automaticity. These data suggest that: (1) in patients with sinus bradycardia and prolongation of the A-H interval, oral quinidine has a direct depressant effect on sinus and A-V nodal function, but this effect is counteracted by autonomically mediated actions; (2) in patients with moderate or severe bradycardia and an abnormal intrinsic heart rate, the drug can induce marked depression of sinus automaticity.