The Response of Cerebrospinal Fluid Composition to Sustained Hypercapnia*

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Abstract
Studies have been carried out in 35 dogs exposed to an atmosphere of 12% carbon dioxide in an environmental chamber for periods ranging from one-half hour to 5 days. During the first 24 hours of hypercapnia, the bicarbonate concentration in the spinal fluid gradually rose to a value approximately 13 mEq/1 higher than that of the control state. No further significant change occurred during the subsequent 4-day period of observation. In plasma water the concentrationof bicarbonate also rose by approximately 13 mEq/l, but the new steady state was not achieved as quickly as in spinal fluid. The hydrogen ion concentration in both cerebrospinal fluid and plasma was increased by approximately 15 n[image]/l. After 5 days of hypercapnia, the chloride concentration in both compartments had decreased by an amount nearly identical to the rise in the concentration of bicarbonate. The concentrations of sodium and potassium were not significantly different from the values present in the control state. Despite the changes in the concentrations of bicarbonate, chloride, and hydrogen ion, the cerebrospinal fluid to plasma ratios of all the measured electrolytes in the new steady state were virtually identical to those present in the control state. This finding would suggest, according to the Nernst equation, that if some potential difference governs the partition of any of the measured ions then this potential at 5 days was virtually identical to that in the control state.