Regulation of cerebral blood flow by arterial PCO2 independent of metabolic acidosis at 5050 m

Abstract

Key points: We investigated the influence of arterial PCO₂ (PaCO₂ ) with and without experimentally altered pH on cerebral blood flow (CBF) regulation at sea level and with acclimatization to 5050 m. At sea level and high altitude, we assessed stepwise alterations in PaCO₂ following metabolic acidosis (via 2 days of oral acetazolamide; ACZ) with and without acute restoration of pH (via intravenous sodium bicarbonate; ACZ+HCO₃^- ). Total resting CBF was unchanged between trials at each altitude even though arterial pH and [HCO₃^- ] (i.e. buffering capacity) were effectively altered. The cerebrovascular responses to changes in arterial [H⁺ ]/pH were consistent with the altered relationship between PaCO₂ and [H⁺ ]/pH following ACZ at high altitude (i.e. leftward x-intercept shifts). Absolute cerebral blood velocity (CBV) and the sensitivity of CBV to PaCO₂ was unchanged between trials at high altitude, indicating that CBF is acutely regulated by PaCO₂ rather than arterial pH.

Abstract: Alterations in acid-base balance with progressive acclimatization to high altitude have been well-established. However, how respiratory alkalosis and the resultant metabolic compensation interact to regulate cerebral blood flow (CBF) is uncertain. We addressed this via three separate experimental trials at sea level and following partial acclimatization (14 to 20 days) at 5050 m; involving: (1) resting acid-base balance (control); (2) following metabolic acidosis via 2 days of oral acetazolamide at 250 mg every 8 h (ACZ; pH: Δ -0.07 ± 0.04 and base excess: Δ -5.7 ± 1.9 mEq⋅l^-1 , trial effects: P < 0.001 and P < 0.001, respectively); and (3) after acute normalization of arterial acidosis via intravenous sodium bicarbonate (ACZ + HCO₃^- ; pH: Δ -0.01 ± 0.04 and base excess: Δ -1.5 ± 2.1 mEq⋅l^-1 , trial effects: P = 1.000 and P = 0.052, respectively). Within each trial, we utilized transcranial Doppler ultrasound to assess the cerebral blood velocity (CBV) response to stepwise alterations in arterial PCO₂ (PaCO₂ ), i.e. cerebrovascular CO₂ reactivity. Resting CBF (via Duplex ultrasound) was unaltered between trials within each altitude, indicating that respiratory compensation (i.e. Δ -3.4 ± 2.3 mmHg PaCO₂ , trial effect: P < 0.001) was sufficient to offset any elevations in CBF induced via the ACZ-mediated metabolic acidosis. Between trials at high altitude, we observed consistent leftward shifts in both the PaCO₂ -pH and CBV-pH responses across the CO₂ reactivity tests with experimentally reduced arterial pH via ACZ. When indexed against PaCO₂ - rather than pH - the absolute CBV and sensitivity of CBV-PaCO₂ was unchanged between trials at high altitude. Taken together, following acclimatization, CO₂ -mediated changes in cerebrovascular tone rather than arterial [H⁺ ]/pH is integral to CBF regulation at high altitude.

Keywords: CO2 reactivity; acetazolamide; acid-base balance; cerebral blood flow; high altitude; metabolic acidosis; sodium bicarbonate.