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

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Abstract
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 (V̇O2max) of 49 ± 1 (ml O2) kg−1min−1, and seven high fit (HF) individuals, with a V̇O2max 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.