Mean carbon dioxide tension in the brain after carbonic anhydrase inhibition

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Abstract
Measurements of the CO2 tensions in arterial blood, in blood from the superior sagittal sinus, in cisternal cerebrospinal fluid, and on the surface of the cerebral cortex were made in spontaneously breathing anesthetized cats and rats after inhibition of carbonic anhydrase with acetazolamide in doses of 50-150 mg/kg. After an acetazolamide dose of 50 mg/kg in cats there was a mean increase in the c.s.f. [cerebrospinal fluid] CO2 tension of 8 mm Hg; after a dose of 100 mg/kg the corresponding mean increase was 13 mm Hg. Since the CO2 tension measured in venous blood was only moderately influenced by the acetazolamide, the normal PCO2 difference between venous blood and c.s.f. was markedly reduced. Tne apparent arterial CO2 tension, i.e. the CO2 tension measured in vitro, always changed to the same extent as the c.s.f. CO2 tension. The findings were confirmed by measurements of the CO2 tension on the surface of the cerebral cortex, and by measurements of the blood and c.s.f. CO2 tensions in the rat. It is concluded that the mean tissue CO2 tension of the brain can be estimated from the measured arterial CO2 tension, even under conditions of carbonic anhydrase inhibition.