Use of opposing reflex stimuli and heart rate variability to examine the effects of lipophilic and hydrophilic β-blockers on human cardiac vagal control

Evidence from animal studies suggests that β-blockers can act within the central nervous system to increase cardiac vagal motoneuron activity. We have attempted to determine whether such an effect is evident in healthy humans, by examining the effects of lipophilic and hydrophilic agents on heart rate variability and cardiac vagal reflexes. A total of 20 healthy volunteers took part in the study. Autonomic studies were performed after 72 h of treatment with placebo, atenolol or metoprolol in a blinded cross-over design. ECG recordings were taken at rest and during mental and orthostatic stress. Heart rate variability was measured in the time and frequency domains. The effects on heart rate of two opposing cardiac vagal reflexes were examined. Trigeminal stimulation causing vagal stimulation, and isometric forearm muscle contraction (‘muscle heart reflex’) causing vagal inhibition, were performed alone and simultaneously. At rest, during mental stress and during trigeminal stimulation, β-blocker therapy was associated with significantly increased high-frequency beat-to-beat heart rate variability when compared with placebo. There were no significant differences in effects on heart rate or heart rate variability between atenolol and metoprolol. Analysis of the muscle heart reflex, alone and with simultaneous trigeminal stimulation, showed that the magnitude of the R–R interval response was significantly greater after β-blocker therapy compared with placebo, but the effects of atenolol and metoprolol were equivalent. β-Blocker therapy increased cardiac vagal activity, as shown by measures of high-frequency heart rate variability and reflex studies. Lipophilic and hydrophilic β-blockers appeared to be equally efficacious in increasing the cardiac vagal modulation of heart rate.