To evaluate the effect of converting enzyme inhibition induced by enalapril on parasympathetic activity, we studied ten essential hypertensive patients, age range 38-58 years, WHO I-II. Parasympathetic evaluation was obtained by measuring the variation of heart period (VHP) during at least 1 minute of steady-state, regular respiration. VHP was derived from the difference between the mean of all maximum and the mean of all minimum heart periods. The higher the VHP, the higher the parasympathetic control of heart rate and vice versa. VHP was measured supine and with tilting (30 degrees, 60 degrees, 85 degrees). Blood pressure was reduced after 1 month of enalapril treatment, while the heart rate did not change. VHP increased at the end of enalapril treatment compared with placebo: in the supine position it increased from 36 +/- 3.2 ms to 44 +/- 3.5 ms, p less than 0.01. VHP was also increased by enalapril at 30 degrees (p less than 0.05) and 60 degrees (p less than 0.05), while no difference was observed at 85 degrees between placebo and enalapril. A positive correlation was found between supine enalapril changes of VHP and those of systolic and diastolic BP. In conclusion, enalapril seems to increase parasympathetic cardiovascular control in essential hypertensive patients. This result might explain the lack of increase in heart rate that would be expected as a result of the vasodilating effect of enalapril.
Vasodilator drugs reduce peripheral vascular resistance but lead to a secondary baroreflex-mediated chronotropic effect. After angiotensin-converting enzyme inhibition, blood pressure falls without associated tachycardia. In a previous study it was observed that enalapril increased vagal tone in essential hypertensive patients. In order to evaluate the effect of enalapril on sympathetic stimulation 10 mild to moderate hypertensive patients were studied during static (hand grip) and dynamic exercise (bicycle ergometer), after 2 weeks of placebo and after 1 month of treatment with 20-40 mg enalapril once daily. Enalapril significantly reduced blood pressure and the rate-pressure product at rest and at peak dynamic exercise. There was no effect on supine and maximal heart rate. Enalapril also significantly reduced blood pressure during hand grip, but did not interfere with the rate of the increase. Thus, enalapril does not seem to interfere with sympathetic adaptation to stress.
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