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

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 (V̇_O2max) of 49 ± 1 (ml O₂) kg⁻¹min⁻¹, and seven high fit (HF) individuals, with a V̇_O2max 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 (G_max‐HR) and carotid‐MAP (G_max‐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 G_max‐HR and G_max‐MAP of the AF subjects increased more than the G_max‐HR and G_max‐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.