Skin cooling aids cerebrovascular function more effectively under severe than moderate heat stress

Skin surface cooling has been shown to improve orthostatic tolerance; however, the influence of severe heat stress on cardiovascular and cerebrovascular responses to skin cooling remains unknown. Nine healthy males, resting supine in a water-perfusion suit, were heated to +1.0 and +2.0°C elevation in body core temperature (T _c). Blood flow velocity in the middle cerebral artery (transcranial Doppler ultrasound), mean arterial pressure (MAP; photoplethysmography), stroke volume (SV; Modelflow), total peripheral resistance (TPR; Modelflow), heart rate (HR; ECG) and the partial pressure of end-tidal carbon dioxide (P_ETCO₂) were measured continuously during 1-min baseline and 3-min lower body negative pressure (LBNP, −15 mm Hg) when heated without and again with skin surface cooling. Nine participants tolerated +1°C and six participants reached +2°C. Skin cooling elevated (P = 0.004) MAP ~4% during baseline and LBNP at +1°C T _c. During LBNP, skin cooling increased SV (9%; P = 0.010) and TPR (0.9 mm Hg L⁻¹ min, P = 0.013) and lowered HR (13 b min⁻¹, P = 0.012) at +1°C T _c and +2°C T _c collectively. At +2°C T _c, skin cooling elevated P_ETCO₂ ~4.3 mm Hg (P = 0.011) and therefore reduced cerebral vascular resistance ~0.1 mm Hg cm⁻¹ s at baseline and LBNP (P = 0.012). In conclusion, skin cooling under severe heating and mild orthostatic stress maintained cerebral blood flow more effectively than it did under moderate heating, in conjunction with elevated carbon dioxide pressure, SV and arterial resistance.