EFFECTS OF HYDROPHILIC AND LIPOPHILIC BETA-BLOCKERS ON HEART-RATE-VARIABILITY AND BAROREFLEX SENSITIVITY IN NORMAL SUBJECTS

To evaluate the effect of a hydrophilic and a lipophilic beta-blocker on the autonomic nervous system, 20 normal subjects were studied under baseline conditions and 7 days after being randomly assigned to metop rolol (200 mg/day), nadolol (80 mg/day), and placebo. Under each condi tion, the time-domain parameters were analyzed by means of 24-hour ECG monitoring and the frequency-domain parameters by means of the autore gressive method using 10-minute ECGs during rest, controlled respirati on, and after a head-up tilt test. The alpha index (the gain in the re lationship between the RR period and systolic arterial pressure variab ility) was also calculated. Both nadolol and metoprolol significantly increased all of the time-domain parameters except the standard deviat ion of the RR intervals; they also modified the frequency-domain param eters. Both blunted the significant reduction in the high frequency (H F) component and alpha index during tilt. In normal subjects, hydrophi lic and lipophilic beta-blockers similarly modify the time-and frequen cy-domain parameters that are particularly evident when high sympathet ic tone is present (during daytime and tilt). The value of the alpha i ndex was increased by both beta-blockers in the HF, but not in the low frequency band; this difference might be due to the fact that the for mer is a measure of the vagal component of the baroreflex control and the latter a measure of the sympathethic component, The effects of hyd rophilic and lipophilic beta-blockers on the time- and frequency-domai n parameters of heart rate variability are similar.