Angiotensin converting enzyme inhibition, parasympathetic activity and exercise in essential hypertension

Reduced parasympathetic activity has been reported in essential hypertension. Converting enzyme inhibition seems to increase parasympathetic tone. In order to evaluate the effects of enalapril on parasympathetic control of heart rate, the authors studied ten mild-to-moderate essential hypertensive patients (7 F, 3 M), treated for 2 weeks with placebo and for 1 month with enalapril. Compared to placebo, enalapril significantly reduced blood pressure (p less than 0.005 at least, both systolic and diastolic), without any change in heart rate. Enalapril enhanced parasympathetic activity as judged by the increased variation of heart period (VHP) during regular breathing. VHP was derived during continuous ECG recording by the difference between the mean of all maximum and minimum R-R intervals, taken as a measure of respiratory sinus arrhythmia: the higher the VHP, the higher the parasympathetic cardiac influence and vice versa. The response to exercise, used as an index of sympathetic stimulation, was not modified by enalapril: the heart rate peak reached during either static (hand grip) or dynamic (bicycle ergometer) exercise and the slope of the increase in blood pressure were unchanged. Therefore, enalapril appears to increase parasympathetic tone in essential hypertension, without any interference with sympathetic adaptation to stress.