Quantitative cerebrospinal fluid acid-base balance in acute respiratory alkalosis

Data on canine cisternal cerebrospinal fluid (CSF) ions in acute respiratory alkalosis are limited and fragmentary. We hypothesized that with the fall in arterial PCO2 (PaCO2) and in the face of normal osmoregulation, CSF [Na+] remains relatively constant and CSF [Na+-Cl-] narrows to account in part for the fall in CSF [HCO3-]. We therefore measured blood and CSF acid-base variables and ions of two groups of pentobarbital-anesthetized, mechanically ventilated dogs (n = 10 in each group). In the control group, PaCO2 was kept constant and changes in serum and CSF ions were minimal. In Group II (acute respiratory alkalosis), both PaCO2 and cisternal CSF PCO2 decreased by 10 mm Hg. Five hours after induction of respiratory alkalosis, mean CSF [HCO3-] decreased significantly by 4.4 +/- 1.2 mEq/L (mean +/- SD). The fall in CSF [HCO3-] was similar to changes in CSF strong ion difference (SID = Na(+)+K(+)+Ca(2+)+Mg(2+)-CL(-)-lactate), which decreased 4.4 +/- 1.9 mEq/L. Concentrations of the four major CSF cations did not change significantly. Cisternal CSF lactate rose significantly by 1.2 +/- 0.9 mEq/L, accounting for 25% of the change in CSF [HCO3-]. The remaining (75%) change in CSF [HCO3-] was accounted for by changes in CSF [Cl-].