Carotid baroreflex function during prolonged exercise

Carotid baroreflex function during prolonged exercise. J. Appl. Physiol. 87 (1). 339-347, 1999.-The present investigation was designed to uncouple the hemodynamic physiological effects of thermoregulation from the effects of a progressively increasing central command activation during prolonged exerc ise. Subjects performed two l-h bouts of leg cycling exercise with 1) no in tervention and 2) continuous infusion of a dextran solution to maintain cen tral venous pressure constant at the 10-min pressure. Volume infusion resul ted in a significant reduction in the decrement in mean arterial pressure s een in the control exercise it,out (6.7 +/- 1.8 vs. 11.6 +/- 1.3 mmHg, resp ectively). However, indexes of central command such as heart rate and ratin gs of perceived exertion rose to a similar extent during both exercise cond itions. In addition, the carotid-cardiac baroreflex stimulus-response relat ionship, as measured by using the neck pressure-neck suction technique, was reset from rest to 10 min of exercise and was further reset from 10 to 50 min of exercise in both exercise conditions, with the operating point being shifted toward the reflex threshold. We conclude that the progressive rese tting of the carotid baroreflex and the shift of the reflex operating point render the carotid-cardiac reflex ineffectual in counteracting the continu ed decrement in mean arterial pressure that occurs during the prolonged exe rcise.