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Moderate hyperthermia impairs dynamic cerebral autoregulation but not its directional sensitivity | Journal of Applied Physiology | American Physiological Society
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 when 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 multimetric 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 (ΔMCAvmeanT/ΔMAPT) 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 ΔMCAvmeanT/ΔMAPT during MAP decreases than increases, was observed only at 0.10 Hz (P < 0.0001), whereas %MCAvmeanT/%MAPT 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 frequencies, 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. NEW & NOTEWORTHY Using a multimetric approach combining transfer function analysis with absolute and normalized directional sensitivity metrics, we show that moderate hyperthermia impairs dynamic cerebral autoregulation while preserving its directional sensitivity. Notably, asymmetry in the cerebral pressure-flow relationship was evident at both 0.05 and 0.10 Hz when using the normalized metric, despite heat-induced reductions in dynamic buffering. These findings suggest that distinct mechanisms govern the dynamic and directional components of cerebral autoregulation.
