Dynamic cerebral autoregulation during passive heat stress

Abstract

This study tested the hypothesis that passive heating impairs cerebral autoregulation. Transfer function analysis of resting arterial blood pressure (BP) and middle cerebral artery blood velocity (MCAVmean) were conducted on 9 healthy subjects, dressed in a water‐perfused suit, under normothermic (NT) and passive heat stress (HS, Δ core temperature 1.1 ± 0.2 °C, P<0.001) conditions. Passive heating reduced MCAVmean (8 ± 8 cm•s−1, P=0.01), whilst mean arterial BP was maintained (2 ± 5 mm Hg, P=0.36). In the very low frequency range (<0.07 Hz) coherence was lower during HS compared to NT conditions (0.26 ± 0.10 vs 0.57 ± 0.13, respectively, P<0.001). Coherence was >0.5 and similar between NT and HS in the low (0.07–0.20 Hz, 0.64 ± 0.19 vs 0.59 ± 0.10, respectively, P=0.40) and high (0.20–0.35 Hz, 0.63 ± 0.19 vs 0.72 ± 0.14, respectively, P=0.12) frequency ranges. Phase was higher during HS in the low frequency range (NT: 0.55 ± 0.31 vs HS: 0.82 ± 0.17°, P=0.01), whereas it was unchanged by HS in the high frequency range (NT: −0.10 ± 0.20 vs HS: 0.12 ± 0.71°, P=0.14). Transfer gain was higher during HS in the low (NT: 1.53 ± 0.41 vs HS: 1.73 ± 0.38 cm•s −1•mmHg−1, P=0.03) and high (NT: 1.55 ± 0.55 vs HS: 2.06 ± 0.37 cm•s−1•mmHg−1, P=0.03) frequency ranges. Higher transfer function estimates of gain suggest that dynamic cerebral autoregulation is impaired during HS.Supported by NIH Grant HL‐61388 & HL‐84072