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J Appl Physiol 91: 283-289, 2001;
8750-7587/01 $5.00
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Vol. 91, Issue 1, 283-289, July 2001

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

C. Keyl1, A. Schneider1, M. Dambacher2, and L. Bernardi3

1 Department of Anesthesiology, University Medical Center, 93042 Regensburg; 2 Fraunhofer Institut für Physikalische Messtechnik, 70110 Freiburg, Germany; and 3 Department of Internal Medicine, University of Pavia and Istituto di Ricovero e Cura a Carattere Scientifico, San Matteo, 27100 Pavia, Italy

To examine whether changes in autonomic activity have an effect on the latency of the vagally mediated cardiac baroreflex response in humans, we investigated 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° head-down tilt and 60° head-up tilt, and during vagotonic (2 µg/kg) and vagolytic (10 µg/kg) doses of atropine while 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 transfer function analysis and was used as a measure of the time delay between carotid baroreceptor stimulation and cardiac effector response. Cardiac baroreflex responsiveness increased significantly during low-dose atropine and decreased during head-up tilt or 10 µg/kg atropine. With increasing tilt angle, the time delay between cyclic baroreceptor stimulation and oscillations 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 head-down tilt on baroreflex latency, whereas 10 µg/kg atropine increased the time delay markedly to 1.24 ± 0.30 s. Our results demonstrate that changes 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 parasympathetic function may contribute to an unstable regulation of heart rate in patients with cardiac disease.

autonomic nervous system; heart rate; neck suction; spectral analysis


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