EFFECTS OF DIGOXIN AND ENALAPRIL ON HEART PERIOD VARIABILITY AND RESPONSE TO HEAD-UP TILT IN NORMAL SUBJECTS

To test the effects of digitalis and angiotensin-converting enzyme inh ibition on the RR interval variability in an electrocardiogram, 20 nor mal subjects were given digoxin 0.25 mg, enalapril 10 mg, and placebo twice daily in a randomized, double-blind, crossover study. Continuous 24-hour electrocardiographic recordings obtained on day 5 of each tre atment were analyzed and several time domain and power spectral measur es of heart period variability were calculated. Digoxin markedly incre ased (up to 51%) indexes of vagal modulation of heart period without c hanging mean RR interval. Enalapril did not change any measure of hear t period variability despite a modest hypotensive effect. To determine the effect of each treatment on the response to orthostatic stress, 1 0 subjects also underwent 15 minutes of 60 head-up tilt; power spectra were calculated for 15 minutes at 0-degrees and at 60-degrees of tilt . Neither active treatment affected the response to head-up tilt.