CEREBROSPINAL-FLUID PH AND PCO2 RAPIDLY FOLLOW ARTERIAL BLOOD-PH AND PCO2 WITH CHANGES IN VENTILATION

CHANGES IN VENTILATORY rate affect arterial blood pH and PCO2 within s econds to minutes, but the corresponding acute changes for cerebrospin al fluid (CSF) pH and PCO2 have not been as well documented. Using our previously-described swine model of brain retraction ischemia, we exa mined changes in arterial and CSF pH and PCO2 with acute changes in ve ntilation in four animals. Newly developed fluorescent dye technology permitted near-instantaneous recording of CSF pH and PCO2 during acute hyperventilation (end-tidal PCO2 of 20 mm Hg) and acute hypoventilati on (end-tidal PCO2 of 50 mm Hg). The Puritan-Bennett 3300 Intra -Arter ial Blood Gas Monitor (PB3300) was used with the sensor placed in the CSF in the interhemispheric fissure posterior to the corpus callosum. The following data were gathered at 5, 15, 30, and 60 minutes after th e ventilatory change: arterial pH and PCO2, end-tidal CO2, laser-Doppl er cerebral blood flow, and CSF pH and PCO2. The baseline (normoventil ation) values for arterial and CSF pH and PCO2 in swine were comparabl e to those in humans: arterial pH 7.44 and PCO2 43 mm Hg; CSF pH 7.31 and PCO2 55 mm Hg. Changes in pH and PCO2 With hyperventilation and hy poventilation occurred rapidly in both arterial blood and CSF. Steady- state values were reached within 15 minutes for hypoventilation, and 3 0 minutes for hyperventilation. The correlation between arterial and C SF values for both pH and PCO2 at 5, 15, 30, and 60 minutes were all v ery highly significant (P < 0.001) except for arterial and CSF PCO2 at 5 minutes (P < 0.01). CSF pH and PCO2 follow arterial blood gas pH an d PCO2 rapidly with acute changes in ventilation. These results are co nsistent with previous research, and indicate the PB3300 may be useful in monitoring CSF pH and PCO2.