Cerebral vasoreactivity during hypercapnia is reset by augmented sympathetic influence

Sympathetic nerve activity influences cerebral blood flow, but it is unknown whether augmented sympathetic nerve activity resets cerebral vasoreactivity to hypercapnia. This study tested the hypothesis that cerebral vasodilation during hypercapnia is restrained by lower-body negative pressure (LBNP)-stimulated sympathoexcitation. Cerebral hemodynamic responses were assessed in nine healthy volunteers [age 25 yr (SD 3)] during rebreathing-induced increases in partial pressure of end-tidal CO₂ (PETCO₂) at rest and during LBNP. Cerebral hemodynamic responses were determined by changes in flow velocity of middle cerebral artery (MCAV) using transcranial Doppler sonography and in regional cerebral tissue oxygenation (ScO₂) using near-infrared spectroscopy. PETCO₂ values during rebreathing were similarly increased from 41.9 to 56.5 mmHg at rest and from 40.7 to 56.0 mmHg during LBNP of - 15 Torr. However, the rates of increases in MCAV and in ScO₂ per unit increase in PETCO₂ (i.e., the slopes of MCAV/PETCO₂ and ScO₂/PETCO₂) were significantly (P ≤0.05) decreased from 2.62 ± 0.16 cm·s ^-1·mmHg^-1 and 0.89 ± 0.10%/mmHg at rest to 1.68 ± 0.18 cm·s^-1·mmHg^-1 and 0.63 ± 0.07%/mmHg during LBNP. In conclusion, the sensitivity of cerebral vasoreactivity to hypercapnia, in terms of the rate of increases in MCAV and in ScO₂, is diminished by LBNP-stimulated sympathoexcitation.