Cerebral oxygen extraction across different exercise intensities: Role of arterial PCO2${P_{{\mathrm{C}}{{\mathrm{O}}_2}}}$

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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>P</mml:mi> <mml:mrow><mml:mi>aC</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${P_{{\mathrm{aC}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> ) and OEF exhibit a strong inverse relationship owing to the powerful influence of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>P</mml:mi> <mml:mrow><mml:mi>aC</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${P_{{\mathrm{aC}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>P</mml:mi> <mml:mrow><mml:mi>aC</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${P_{{\mathrm{aC}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>P</mml:mi> <mml:mrow><mml:mi>aC</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${P_{{\mathrm{aC}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> ; and (2) declines in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>P</mml:mi> <mml:mrow><mml:mi>aC</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${P_{{\mathrm{aC}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> (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: [ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:mrow><mml:mo>(</mml:mo> <mml:mrow><mml:mrow><mml:mi>arterial</mml:mi> <mml:mspace></mml:mspace></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mrow><mml:mspace></mml:mspace> <mml:mi>content</mml:mi></mml:mrow> <mml:mo>-</mml:mo> <mml:mrow><mml:mi>jugular</mml:mi> <mml:mspace></mml:mspace> <mml:mi>venous</mml:mi> <mml:mspace></mml:mspace></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mrow><mml:mspace></mml:mspace> <mml:mi>content</mml:mi></mml:mrow> </mml:mrow> <mml:mo>)</mml:mo></mml:mrow> <mml:mo>/</mml:mo> <mml:mrow><mml:mi>arterial</mml:mi> <mml:mspace></mml:mspace></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mrow><mml:mrow><mml:mspace></mml:mspace> <mml:mi>content</mml:mi></mml:mrow> <mml:mo>]</mml:mo> <mml:mspace></mml:mspace> <mml:mo>×</mml:mo> <mml:mspace></mml:mspace> <mml:mn>100</mml:mn> <mml:mo>.</mml:mo></mml:mrow> </mml:mrow> <mml:annotation>$( {{\mathrm{arterial\;}}{{\mathrm{O}}_2}{\mathrm{\;content}} - {\mathrm{jugular\;venous\;}}{{\mathrm{O}}_2}{\mathrm{\;content}}} )/{\mathrm{arterial\;}}{{\mathrm{O}}_2}{\mathrm{\;content}}]\; \times \;100.$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>Δ</mml:mi> <mml:msub><mml:mi>P</mml:mi> <mml:mrow><mml:mi>aC</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$\Delta {P_{{\mathrm{aC}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>Δ</mml:mi> <mml:msub><mml:mi>P</mml:mi> <mml:mrow><mml:mi>aC</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$\Delta {P_{{\mathrm{aC}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> 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.