Time delay of vagally mediated cardiac baroreflex response varies with autonomic cardiovascular control

To examine whether changes in autonomic activity have an effect on the late ncy of the vagally mediated cardiac baroreflex response in humans, we inves tigated the effects of neck suction fluctuating sinusoidally at 0.2 Hz on R -R intervals (known to be mediated mainly by vagal activity) in the supine position, during 15 degrees head-down tilt and 60 degrees head-up tilt, and during vagotonic (2 mug/kg) and vagolytic (10 mug/kg) doses of atropine wh ile the subjects breathed at 0.25 Hz. The phase shift between fluctuations in neck chamber pressure and in R-R interval was calculated by complex tran sfer function analysis and was used as a measure of the time delay between carotid baroreceptor stimulation and cardiac effector response. Cardiac bar oreflex responsiveness increased significantly during low-dose atropine and decreased during head-up tilt or 10 mug/kg atropine. With increasing tilt angle, the time delay between cyclic baroreceptor stimulation and oscillati ons in R-R interval increased from 0.32 +/- 0.27 s (head down), to 0.59 +/- 0.25 s (supine position, P < 0.05 vs. head down), and to 0.86 +/- 0.27 s ( head up, P < 0.01 vs. supine). Low-dose atropine had a similar effect to he ad-down tilt on baroreflex latency, whereas 10 mug/kg atropine increased th e time delay markedly to 1.24 +/- 0.30 s. Our results demonstrate that chan ges in autonomic activity, generated either by gravitational stimulus or by atropine, not only affect baroreflex responsiveness but also have a major influence on the latency of the vagally mediated carotid baroreceptor-heart rate reflex. The prolonged baroreflex latency during decreased parasympath etic function may contribute to an unstable regulation of heart rate in pat ients with cardiac disease.