Changes of Gas Values in the CSF and Blood during Sustained Hypoventilation

Choi, Kyu Taek; Cheun, Jae Kyu

Abstract

To evaluate the regulation of and in the CSF during respiratory acidosis, the changes in cisternal CSF and arterial plasma acid-base status were asaessed in anesthetized, paralyzed, mechanically ventilated dogs rendered hypercapneic (PaCO2 of 60~70 mmHg) by hypoventilation. The electrochemical potential difference (u) between the CSF and blood for H+ and HCO2- was calculated from values for and in CSF and arterial plasma, and CSF/plasma DC potential difference was calculated. Simultaneously arterial blood and the CSF samples were drawn at 0, 1, 2, 3, 4, 6, 8 hours after hypercapnia. After 8 hours of respiratory acidosis, the arterial pH declined 0.09 units to 7.26 whereas the CSF pH fell by 0.07 units to 7.29. The CSF rose to 34 mEq/L whereas arterial plasma increased to 29 mEq/L. Therefore, increase of the CSF was almost 11 mEq/L, while the arterial plasma HCO, had increased 7 mE/L. So, much increase in CSF bicarbonate maintained the spinal fluid significantly less acidic than the blood during sustained hypercapnia. CSF/plasma PD was not only increased during acute respiratory acidosis, but also remained high as long as the plasma pH was acid. Values of p for H+ and HCO at 8 hours had returned to +1.1 and -0.1 mV of the control value. The attainment of steady-state values for u close to the control value may be compatible with passive distribution of these ions between the CSF and blood. But active mechanism can not be ruled out, because CSF/plasma PD remained high during the study.