Directional sensitivity analysis of the cerebral pressure-flow relationship during normothermia and moderate hyperthermia.

Abstract

Dynamic cerebral autoregulation (dCA) reacts differently when mean arterial pressure (MAP) increases versus decreases (i.e. directional sensitivity). While heat stress alters dCA, its influence on directional sensitivity remains unclear. This analysis investigated the impact of moderate hyperthermia on the directional sensitivity in the cerebral pressure-flow relationship. Ten healthy participants (7 males; age: 37 ± 12 yrs.; body mass: 75 ± 9 kg) underwent 6 min of oscillatory lower body negative pressure (OLBNP) to induce large MAP fluctuations, at 0.03 and 0.10 Hz under normothermic and moderately hyperthermic conditions (+1.0°C increase in core temperature) induced via a water-perfused suit. We calculated changes in middle cerebral artery mean blood velocity (MCAv) per alterations to MAP to compute absolute and relative ratios adjusted for time intervals during each OLBNP-induced MAP increases (ΔMCAv_T/ΔMAP_T^INC; %MCAvT/%MAPT INC) and decreases (ΔMCAv_T/ΔMAP_T^DEC; %MCAvT/%MAPT DEC). Therea􀅌er, we compared average absolute and relative ratios. There was no main effect of MAP direction on ΔMCAv_T/ΔMAPT or %MCAvT/%MAPT during either 0.03 Hz (P=0.291, P=0.281) or 0.10 Hz (P=0.295, P=0.178) OLBNP. Regardless of MAP direction, ΔMCAv_T/ΔMAP_T^INC (0.65 ± 0.17 vs 0.84 ± 0.22 cm. s^-1/mmHg), ΔMCAv_T/ΔMAP_T^DEC (0.70 ± 0.15 vs 0.85 ± 0.18 cm. s^-1/mmHg) (thermal state: P=0.009), %MCAvT/%MAPT INC (0.92 ± 0.22 vs 1.33 ± 0.60), and %MCAvT/%MAPT DEC (1.01 ± 0.27 vs 1.30 ± 0.51) (thermal state: P=0.001) were lower in hyperthermia at 0.03 Hz OLBNP. Regardless of thermal states, these findings suggest an absence of dCA directional sensitivity. Reduced directional sensitivity metrics during hyperthermia may indicate more efficient dCA at very low frequency.