Chronotropic effect of hydralazine and its mechanism in symptomatic sinus bradycardia

The positive chronotropic effect of hydralazine was studied in 9 patients with symptomatic sinus bradycardia. Hydralazine was given in an intravenous dose of 0.15 mg/kg and heart rate, blood pressure, sinoatrial conduction time (Narula method) and corrected sinus node recovery time were measured. The effect of hydralazine was also studied after total autonomic nervous system blockade using 0.04 mg/kg of atropine and 0.2 mg/kg of propranolol intravenously. In the control state hydralazine produced an increase of 28 ± 15% (mean ± standard deviation) in heart rate, and this was essentially due to a decrease in sinoatrial conduction time (by 32 ± 32%, p <0.05). Corrected sinus node recovery time also tended to shorten (decrease of 21 ± 34%, difference not significant). After total autonomic blockade intrinsic heart rate did not change or increased very little (9 ± 14%) after administration of hydralazine and there was no consistent change in sinoatrial conduction and corrected sinus node recovery times. The small residual effect of hydralazine on heart rate was related to incomplete autonomic blockade, since the effect of postural change (standing) on heart rate was also not totally abolished. The study showed that the positive chronotropic effect of hydralazine was mainly due to a change in sinoatrial conduction with a smaller change in corrected sinus node recovery time, and the major chronotropic effect of the drug was mediated by the autonomic nervous system.