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 nodalfunction were assessed in 20 patients (age: 60±7 years)with sinus bradycardia and a prolonged A–H interval. Electrophysiologicalstudies were performed twice in each patient. In the first study,the measurements of sinus and A–V node function were evaluatedboth in the basal state and after autonomic blockade (propranolol0.2 mg kg^-1 and atropine 0.04 mg kg^-1). Oral quinidine was administeredfor 3–4 days (1200 mg day^-1) and the study was then repeatedusing the same methods. Comparison of data obtained in the twostudies in the basal state allowed us to evaluate the overalleffect of quinidine. Comparing the results obtained followingautonomic blockade, the direct action of the drug could be assessed.Inthe basal state quinidine did not significantly change the functionof either node. In contrast, after autonomic blockade, significantchanges were noted after quinidine. In 3 patients with sinusrate <50 beats min^-1 and an abnormal intrinsic heart rate,quinidine induced marked depression of sinus automaticity.Thesedata suggest that: (1) in patients with sinus bradycardia andprolongation of the A–H interval, oral quinidine has adirect 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 abnormalintrinsic heart rate, the drug can induce marked depressionof sinus automaticity.