The purpose of this investigation was to examine the interaction between carotid baroreflex (CBR) responsiveness and maximal aerobic fitness during head-up tilt (HUT)-induced central hypovolaemia. Eight averagely fit (AF) men with a group mean bicycle ergometer maximal oxygen uptake (V_O2,peak) of 50 ± 1.0 ml O₂ kg^-1 min^-1 (mean ± S.E.M., range 45.4-55) and eight highly fit (HF) men with a V_O2,peak of 62 ± 1.0 ml O₂ kg^-1 min^-1 (range 57.2-65.9) volunteered as subjects in the investigation. All procedures were approved by the review board of the Fredricksberg Municipality. After a 30 min period of rest in the supine position each subject was tilted to a 30 deg HUT position for 5 min and then tilted further to 60 deg HUT for another 5 min. During the final 5 min of the supine rest and each position of HUT, transthoracic impedance (TI) and CBR responsiveness, using a rapid pulse (500 ms) train of neck pressure (NP) and neck suction (NS) ranging from +40 to -80 Torr, were measured. Throughout the experiment heart rate (HR) and directly measured brachial mean arterial blood pressure (MAP) were recorded on a beat-to-beat basis using a customized data acquisition system. CBR responsiveness of HR (G_max-HR) and MAP (G_max-MAP) were obtained by fitting the HR and MAP responses to the NP/NS stimuli to a logistic function curve. Statistical analysis of comparisons between HF and AF during the HUT conditions were performed using two-factor ANOVA with repeated measures across HUT conditions. During HUT the G_max-HR of the AF subjects measured (P < 0.08), while the G_max-HR of the HF subjects was unchanged. The G_max-MAP of the AF subjects increased (P < 0.03) during HUT, although the G_max-MAP of the HF subjects did not increase. Regression analysis identified a significant relationship between G_max-HR and G_max-MAP with ΔTI in the AF subjects. These relationships were not significant in the HF subjects (ΔTI, 30-60 deg, AF = 1.8 ± 0.8 V, P < 0.05; HF = 1.2 ± 0.5 V, P > 0.05). These data indicate that unloading of the cardiopulmonary (CP) baroreceptors by HUT increased the CBR responsiveness of the AF subjects because of a greater unloading of the CP baroreceptors. The CBR responsiveness of the HF subjects was unchanged from that observed in the supine position as a result of an insignificant decrease in central blood volume during the tilt.

This work was sponsored in part by NIH grant no. HL45547.