Carotid baroreflex responsiveness to head-up tilt-induced central hypovolaemia: effect of aerobic fitness.

This investigation examined the interaction between carotid baroreflex (CBR) responsiveness during head-up tilt (HUT)-induced central hypovolaemia and aerobic fitness. Seven average fit (AF) individuals, with a mean maximal oxygen uptake (VO2max) of 49 +/- 1 (ml O2) kg-1 min-1, and seven high fit (HF) individuals, with a VO2max of 61 +/- 1 (ml O2) kg-1 min-1, voluntarily participated in the investigation. After 10-15 min supine, each subject was exposed to nine levels of progressively increasing HUT by 10 deg increments from -20 deg to +60 deg. During the final 3 min of each stage of HUT, the CBR responsiveness was measured using a rapid pulse (500 ms) train of neck pressure (NP) and neck suction (NS) ranging from +40 to -80 Torr. The maximal gain of the carotid-HR (Gmax-HR) and carotid-MAP (Gmax-MAP) baroreflex function curves was identified as measures of CBR responsiveness. During HUT-induced decreases in thoracic admittance, an index of central blood volume (CBV), the Gmax-HR and Gmax-MAP of the AF subjects increased more than the Gmax-HR and Gmax-MAP of the HF subjects (P < 0.05). The data demonstrate that the increase in the CBR responsiveness during a tilt-induced progressive unloading of the cardiopulmonary baroreceptors was attenuated in endurance-trained subjects. These findings provide an explanation for the predisposition to orthostatic hypotension and intolerance in well-trained athletes.