Cerebral blood flow responses to changes in oxygen and carbon dioxide in humans

This study characterized cerebral blood flow (CBF) responses in the middle cerebral artery to PCO2 ranging from 30 to 60 mmHg (1 mmHg = 133.322 Pa) during hypoxia (50 mmHg) and hyperoxia (200 mmHg). Eight subjects (25 +/- 3 years) underwent modified Read rebreathing tests in a background of constant hypoxia or hyperoxia. Mean cerebral blood velocity was measured using a transcranial Doppler ultrasound. Ventilation (VE), end-tidal PCO2 (PETCO2), and mean arterial blood pressure (MAP) data were also collected. CBF increased with rising PETCO2 at two rates, 1.63 +/- 0.21 and 2.75 +/- 0.27 cm x s(-1) x mmHg(-1) (p < 0.05) during hypoxic and 1.69 +/- 0.17 and 2.80 +/- 0.14 cm x s(-1) x mmHg(-1) (p < 0.05) during hyperoxic rebreathing. VE also increased at two rates (5.08 +/- 0.67 and 10.89 +/- 2.55 L min(-1) m mHg(-1) and 3.31 +/- 0.50 and 7.86 +/- 1.43 L x min(-1) x mmHg(-1)) during hypoxic and hyperoxic rebreathing. MAP and PETCO2 increased linearly during both hypoxic and hyperoxic rebreathing. The breakpoint separating the two-component rise in CBF (42.92 +/- 1.29 and 49.00 +/- 1.56 mmHg CO2 during hypoxic and hyperoxic rebreathing) was likely not due to PCO2 or perfusion pressure, since PETCO2 and MAP increased linearly, but it may be related to VE, since both CBF and VE exhibited similar responses, suggesting that the two responses may be regulated by a common neural linkage.