Impact of moderate isocapnic hyperthermia on dynamic cerebral autoregulation and its directional sensitivity

The effects of heat exposure on dynamic cerebral autoregulation (dCA), the capacity of the cerebrovasculature to buffer rapid changes in arterial pressure, and its directional sensitivity, defined as the asymmetric cerebrovascular response to increases versus decreases in mean arterial pressure (MAP), remain incompletely understood. This uncertainty is largely attributable to concomitant heat-induced reductions in arterial carbon dioxide. We hypothesized that moderate isocapnic hyperthermia would impair dCA, particularly at higher frequencies of MAP oscillations, while preserving directional sensitivity across thermal conditions. Twenty healthy young participants (9 females, age: 24 ± 5 yrs) completed oscillatory lower body negative pressure trials at 0.05 and 0.10 Hz under normothermic and hyperthermic (core temperature +1.0°C) conditions. End-tidal carbon dioxide partial pressure (P _ET CO ₂ ) was clamped at baseline using a computer-controlled gas delivery system. Middle cerebral artery mean blood velocity (MCAvmean), MAP, P _ET CO ₂ , heart rate, and core temperature were continuously recorded. dCA was assessed using transfer function analysis (TFA), and directional sensitivity was quantified using time-adjusted absolute (∆MCAvmean _T /∆MAP _T ) and relative (RelMCAvmean _T /RelMAP _T ) metrics. Moderate isocapnic hyperthermia increased TFA coherence at both frequencies and selectively impaired dCA at 0.10 Hz, as evidenced by increased TFA gain and normalized gain. Directional sensitivity was present at 0.05 Hz, indicated by higher ∆MCAvmean _T /∆MAP _T and RelMCAvmean _T /RelMAP _T during MAP decreases compared with increases, but was absent at 0.10 Hz. These findings demonstrate that isocapnic hyperthermia impairs dCA at higher frequency while preserving directional sensitivity at lower frequency, suggesting that distinct physiological mechanisms govern these components of cerebrovascular regulation beyond the influence of carbon dioxide.