Cerebral oxygen extraction across different exercise intensities: Role of arterial P_CO2P_CO2

Stability in cerebral oxygen extraction fraction (OEF) is typically determined by alterations in cerebral blood flow (CBF). At rest, arterial partial pressure of carbon dioxide ((Formula presented.)) and OEF exhibit a strong inverse relationship owing to the powerful influence of (Formula presented.) on cerebral resistance, CBF and therefore oxygen delivery; however, it is unclear whether this relationship also exists during exercise, especially when supramaximal, during which marked hyperventilation-induced reductions in (Formula presented.) induce cerebrovascular vasoconstriction and lower CBF. We determined whether: (1) supramaximal exercise yields the largest change in OEF versus lower intensities, correlated with reductions in (Formula presented.); and (2) declines in (Formula presented.) (independent of exercise) determine changes in OEF. Blood was sampled from the brachial/radial artery and internal jugular vein during: (1) 60 min, 34% maximal O₂ uptake (SUB; n = six males, six females); (2) 4 min, 90% maximal O₂ uptake (MAX; n = six males, six females); (3) 1−2 min of high-intensity sprinting, ∼110% maximal O₂ uptake (HIS; n = six males, five females); and (4) resting hyperventilation-induced hypocapnia (HYPO; n = six males, five females). OEF was calculated as: [(Formula presented.) The ΔOEF was greatest during HIS [estimated marginal mean: 15.6% (95% confidence interval: 12.4, 18.8)] and HYPO [17.7% (14.5, 20.9)] compared with SUB [−0.9% (−4.0, 2.1); p < 0.0001 vs. HIS and vs. HYPO] and MAX [2.5% (−0.5, 5.6); p < 0.0001 vs. HIS and vs. HYPO]. Reductions in (Formula presented.) were greatest in HIS [−12.9 mmHg (−14.6, −11.2)] and HYPO [−9.2 mmHg (−10.9, −7.6)] compared with MAX [−6.2 mmHg (−7.8, −4.6)] and SUB [1.4 mmHg (−0.2, 2.9); all comparisons p < 0.0001]. The ΔOEF was inversely related to (Formula presented.) both in the pooled analysis [β = −1.65 (−2.23, −1.07); p < 0.0001] and within each of the conditions. In conclusion, probably owing to reductions in CBF, hypocapnia per se increased OEF in a similar manner to supramaximal sprinting, indicating that exercise is non-obligatory in this process.