Moderate hyperthermia impairs dynamic cerebral autoregulation but not its directional sensitivity

Dynamic cerebral autoregulation (dCA) maintains cerebral blood flow (CBF) relatively constant during rapid fluctuations in mean arterial pressure (MAP). This autoregulation exhibits directional sensitivity, characterized by greater buffering capacity of CBF changes when MAP increases than MAP decreases. Whole-body hyperthermia may alter autoregulatory function through physiological adjustments that elevate cerebrovascular resistance and reduce CBF. We hypothesized that moderate hyperthermia (+1°C core temperature) improves dCA without affecting its directional sensitivity. Twenty healthy young adults (9 males) underwent oscillatory lower body negative pressure (OLBNP, 0 to -90 Torr) at 0.05 Hz and 0.10 Hz under normothermic and hyperthermic conditions. Middle cerebral artery mean blood velocity (MCAvmean) and MAP were continuously recorded. A multi-metric approach was applied to quantify dCA using transfer function analysis (TFA) and directional sensitivity analysis time-adjusted changes in MCAvmean per alterations to MAP in absolute (∆MCAvmean T /∆MAP T ) and relative (%MCAvmeanT/%MAPT) terms. Hyperthermia increased TFA coherence and reduced TFA phase at both frequencies, while TFA gain and normalized gain were elevated at 0.10 Hz OLBNP (all P<0.05), indicating impaired dCA. Directional sensitivity, defined by higher ∆MCAvmean T /∆MAP T during MAP decreases than increases, was observed only at 0.10 Hz (P<0.0001), whereas %MCAvmean T /%MAP T revealed this pattern at both frequencies (all P<0.05) without thermal effects (all P>0.05). These novel findings demonstrate that moderate poikilocapnic hyperthermia attenuates the dynamic buffering capacity of the cerebrovasculature, particularly at higher frequency, while preserving the inherent directional sensitivity of the cerebral pressure-flow relationship. These findings suggest that distinct regulatory mechanisms may underlie the dynamic and directional components of cerebral autoregulation.