Lower body positive pressure affects systemic but not cerebral haemodynamics during incremental hyperthermia

Hyperthermia produces profound redistribution of blood and circulatory reflex function. We investigated the potential for lower body positive pressure (LBPP) to maintain or restore haemodynamics during graded hyperthermia. Eight healthy adults rested supine in a custom-made LBPP box, sealed distal to the iliac crest. Following 5 min of normothermic rest, 20 mmHg of LBPP was applied and repeated when core temperature (T_core ) had increased passively by +0.5 and +1°C. Primary dependent variables included mean middle cerebral artery blood velocity (MCAv_mean , transcranial Doppler), mean arterial blood pressure (MAP, finger photoplethysmography), heart rate (HR) and partial pressure of end-tidal carbon dioxide (P_ET CO₂ ). The absolute increase in MAP during LBPP was lower at T_core +1°C (2 ± 3 mmHg), compared with normothermia (7 ± 3 p = .01). The modest increase in MCAv_mean was unchanged by T_core (normothermia, 2 ± 3 cm/s; +0.5°C, 3 ± 3 cm/s and +1°C, 3 ± 4 cm/s, p = .74). By design, P_ET CO₂ was unchanged in all conditions from normothermic baseline (42 ± 1, p = .81). LBPP-induced changes in HR were greater at +0.5°C (-13 ± 4 b/min) and +1°C (-12 ± 6 b/min) compared with normothermia (-3 ± 3 b/min, p = .01 and p = .01, respectively). These data indicate that despite a significant attenuation in MAP to LBPP with moderate hyperthermia, MCAv_mean dynamics were unchanged among the thermal manipulations.