Cerebral and peripheral haemodynamics at rest and in response to orthostasis during 3 days of simulated heatwave

Acute heat stress reduces cerebral haemodynamics and orthostatic tolerance. However, the haemodynamic response to orthostasis during prolonged heat stress typical of heatwaves is unknown. We hypothesised that thermoregulatory-induced elevations in peripheral haemodynamics during 3 days of simulated heatwave would exacerbate orthostatic-induced reductions in cerebral haemodynamics observed in temperate conditions. Seven males (22 ± 1 years) maintained a standard work/rest ratio during temperate (∼25.4°C) and heatwave (∼35.4°C) conditions. Gastrointestinal core, mean skin and body ( ${\overline{T}}_b$ ) temperatures, haemoglobin concentration, mean arterial pressure (MAP), cerebral and peripheral haemodynamics (middle cerebral artery blood velocity (MCA_v), internal carotid, vertebral, and superficial femoral artery blood flow (sFA_bf)) were assessed at rest. MAP, middle cerebral artery vascular conductance index (MCA_VCi), and superficial femoral artery vascular conductance (sFA_VC) were measured during orthostasis caused by 60° head-up tilt. Heatwave increased resting ${\overline{T}}_b$ (+0.76 ± 0.24°C, P < 0.001, d = 3.02 vs. temperate), haemoglobin concentration (P = 0.004), sFA_bf (P = 0.001) and sFA (+34.0 ± 14.0 mL O₂ min^-1, P < 0.001, d = 2.61) and global cerebral (+21.3 ± 16.8 mL O₂ min^-1, P = 0.009, d = 0.71) oxygen delivery, whereas MAP (P = 0.729) and MCA_v (P = 0.687) were unchanged. During tilt, MCA_VCi reductions were comparable between conditions (day × posture interaction, P = 0.959; posture main effect, -0.19 ± 0.06 cm s^-1 mmHg^-1, P < 0.001, d = 2.05) whilst the heatwave-induced elevation in sFA_VC was reduced (interaction, P = 0.008). In conclusion, orthostatic-induced reductions in cerebral haemodynamics were not exacerbated across heatwave days, whereas the heatwave-induced elevation in resting supine peripheral haemodynamics was attenuated during orthostasis. Additionally, resting global cerebral oxygen delivery was increased during the latter heatwave days. These divergent cerebral and peripheral haemodynamic responses in males suggest cerebral perfusion and oxygen delivery are prioritised during prolonged heat stress.