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

Dynamic cerebral autoregulation (dCA) reacts differently when mean arterial pressure (MAP) increases versus decreases (i.e., directional sensitivity). Although 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 yr; 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 increase (ΔMCAv_T/[Formula: see text]; %MCAv_T/[Formula: see text]) and decrease (ΔMCAv_T/[Formula: see text]; %MCAv_T/[Formula: see text]). Thereafter, we compared average absolute and relative ratios. There was no main effect of MAP direction on ΔMCAv_T/ΔMAP_T or %MCAv_T/%MAP_T during either 0.03 Hz (<i>P</i> = 0.291, <i>P</i> = 0.281) or 0.10 Hz (<i>P</i> = 0.295, <i>P</i> = 0.178) OLBNP. Regardless of MAP direction, ΔMCAv_T/[Formula: see text] (0.65 ± 0.17 vs. 0.84 ± 0.22 cm·s^-1·mmHg^-1), ΔMCAv_T/[Formula: see text] (0.70 ± 0.15 vs. 0.85 ± 0.18 cm·s^-1·mmHg^-1) (thermal state: <i>P</i> = 0.009), %MCAv_T/[Formula: see text] (0.92 ± 0.22 vs. 1.33 ± 0.60), and %MCAv_T/[Formula: see text] (1.01 ± 0.27 vs. 1.30 ± 0.51) (thermal state: <i>P</i> = 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.<b>NEW & NOTEWORTHY</b> Recent evidence highlights the importance of considering directional sensitivity in dynamic cerebral autoregulation. The current analysis found no directional sensitivity in the cerebral pressure-flow relationship during 0.03- and 0.10-Hz oscillatory lower body negative pressure in normothermia or moderate hyperthermia in healthy participants. However, reduced directional sensitivity metrics during moderate hyperthermia suggest that dynamic cerebral autoregulation may become more efficient under moderate heat stress.