Cardiovascular and Thermal Responses to Repeated Head-Up Tilts Following Exercise-Induced Heat Stress

Abstract

We evaluated the acute cardiovascular and thermoregulatory responses to repeated 70 degrees head-up tilts (HUT) performed following exercise-induced hyperthermia. Eight male subjects underwent intermittent episodes of 70 degrees 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 degrees C) and were subsequently exposed to three successive 6-min 70 degrees 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 approximately 40.0 degrees C) and were then transferred to an ambient temperature of 22 degrees 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. 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 x mmHg(-1) following EIH compared to -13 +/- 3 ms x mmHg(-1) during NH). A significant transient reduction in cutaneous vascular conductance (CVC) occurred during HUT1 and HUT2 following EIH (-20 +/- 5% CVCmax and -9 +/- 3 %CVCmax, respectively), despite significant elevations in core temperature above resting levels (i.e., 1.4 degrees C and 0.9 degrees C for HUT1 and HUT2). 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.