Cardiovascular and thermal responses to repeated head-up tilts following exercise-induced heat stress

Background: We evaluated the acute cardiovascular and thermoregulatory responses to repeated 70° head-up tilts (HUT) performed following exercise-induced hyperthermia. Methods: Eight male subjects underwent intermittent episodes of 70° HUT in either a non-exercise/ non-heat stress state (NH) or an exercise-induced hyperthermic state (EIH). Subjects remained supine for 30 min in a thermoneutral environment (22°C) and were subsequently exposed to three successive 6-min 70° head-up tilts (HUT1, HUT2, HUT3), each separated by 10 min of supine resting. During EIH, subjects were rendered hyperthermic by exercise in the heat (core temperature of ∼40.0°C) and were then transferred to an ambient temperature of 22°C. We evaluated the relative change in hemodynamic and thermal responses from the last minute in the supine position preceding the HUT to the final minute in HUT. Results: While we measured a difference in the relative change in heart rate between conditions for all HUTs, no differences were observed in mean arterial pressure (MAP), total peripheral resistance, or cardiac output. A reduced change in baroreceptor sensitivity was measured in EIH for HUT1 only (-2 ± 1 ms·mmHg^-1 following EIH compared to -13 ± 3 ms·mmHg^-1 during NH). A significant transient reduction in cutaneous vascular conductance (CVC) occurred during HUT1 and HUT2 following EIH (-20 ± 5 %CVC_max and -9 ± 3 %CVC_max, respectively), despite significant elevations in core temperature above resting levels (i.e., 1.4°C and 0.9°C for HUT1 and HUT2). Conclusion: We conclude that the maintenance of MAP following exercise in the heat is mitigated by reductions in skin perfusion despite significant elevations in core temperature. Reprint &