Effect of dynamic exercise on human carotid-cardiac baroreflex latency
Potts, J.T.; Raven, P.B.; Raven, P.B.
American Journal of Physiology 268(3 Pt 2): H1208-H1214
1995
ISSN/ISBN: 0002-9513 PMID: 7900874 Document Number: 441500
We compared the beat-to-beat responses of heart rate (HR) after brief activation of carotid baroreceptors in resting humans with the responses obtained during mild-to-moderate levels of dynamic exercise (25 and 50% of peak O-2 uptake ( ovrhdot VO-2peak)) to investigate the effect of exercise on baroreflex latency. Carotid baroreceptors were activated by a pressure pulse (5 s) of neck suction (NS, -80 Torr) and neck pressure (NP, +40 Torr) during held expiration. At rest the peak change in HR to NS/NP occurred during the first several heartbeats (1st-3rd beat), whereas during mild and moderate exercise peak HR responses occurred near the end of the NS./NP pulse (6th-8th beat). In contrast, time (s) to the peak change in HR was not different between rest and exercise (P gt 0.05). Reflex tachycardia to NP progressively decreased during exercise (17 +- 3, 10 +- 1, and 4 +- 1% of control, rest vs. 25% ovrhdot VO-2peak, vs. 50% VO-2peak, respectively, P lt 0.05), and a strong positive correlation was found between the magnitude of the reflex tachycardia and a measure of HR variability (cardiac vagal tone index, r = 0.74, P lt 0.0001). Reflex bradycardia to NS gradually increased during exercise (13 +- 2, 17 +- 2, and 18 +- 2% of control, rest vs. 25% ovrhdot VO-2peak, vs. 50% ovrhdot VO-2peak, respectively, P = 0.10) and was negatively correlated with cardiac vagal tone (r = 0.42, P lt 0.06). These results suggested that 1) mild-to-moderate levels of dynamic leg exercise failed to alter carotid baroreflex latency and 2) the magnitude of carotid baroreflex tachycardia and bradycardia may depend on the functional level of cardiac vagal tone. Additionally, these findings suggest important implications regarding the use of pulse-synchronous changes in neck chamber pressure to assess maximal baroreflex sensitivity during exercise.