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 degrees 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 (PETCO2). The absolute increase in MAP during LBPP was lower at T-core +1 degrees 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 degrees C, 3 +/- 3 cm/s and +1 degrees C, 3 +/- 4 cm/s, p = .74). By design, PETCO2 was unchanged in all conditions from normothermic baseline (42 +/- 1, p = .81). LBPP-induced changes in HR were greater at +0.5 degrees C (-13 +/- 4 b/min) and +1 degrees 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.