Gradients of CO₂ Tension in the Brain

In cat experiments measurements were made of the CO 2 tensions in arterial and cerebral venous blood, in cisternal cerebrospinal fluid (CSF), and on the surface of the cerebral cortex. It was found that both the CSF CO 2 tension and the CO 2 tension measured on the surface of the brain exceeded the arithmetic mean of the arterial and the venous CO 2 tensions by 0.5–1 mm Hg. This means that the CSF and the cortical surface CO 2 tensions coincided with the mean tissue CO 2 tension, which can be calculated from the diffusion equations, and they were distinctly different from the cerebral venous CO 2 tension. The arterio‐venous PCO 2 difference was found to change linearly from 12 to 4 mm Hg when the arterial CO 2 tension increased from 30 to 90 mm Hg. The arterio‐venous difference was found to be very sensitive to the experimental conditions. Thus, it was influenced by the craniotomy and the other procedures connected with the tissue measurements, and also by the sampling of CSF. It is recommended that mean tissue CO 2 tensions in the brain are obtained by means of direct measurements on cisternal CSF, or by means of calculation from the arterial and the cerebral venous CO 2 tensions. In the latter case, a fairly exact value is obtained by adding 1 mm Hg to the arithmetic mean of the blood tensions. Tissue measurements are recommended for studies of CO 2 transients, or for conditions with a disturbed local circulation. If none of these measurements are feasible, the tissue CO 2 tension can be approximately calculated from the arterial CO 2 tension, as described in the text.