The Response of Cerebrospinal Fluid Composition to Sustained Hypercapnia*

Abstract

That induction of hypercapnia promptly in-creases the partial pressure of carbon dioxide in the spinal fluid is well recognized (1). The se-verity of the resulting acidosis depends, therefore, upon the extent to which the concentration of bi-carbonate rises. Previous studies of spinal fluid bicarbonate concentration during adaptation to hypercapnia have been so brief that determining if a new steady state was achieved has been impossible (2-4). In addition, most observa-tions have been carried out under unphysiologi-cal conditions which complicated interpretation of the experimental results (2, 3). There has been, consequently, no satisfactory delineation of the sequential changes in the bicarbonate concen-tration of the cerebrospinal fluid, nor determina-tion of the relationship between spinal fluid and plasma electrolytes under steady-state conditions. Our study was undertaken to evaluate the acute and chronic changes in cerebrospinal fluid com-position in dogs exposed to 12 % carbon dioxide in an environmental chamber. The data demon-strate that under these conditions there is initially a marked dissociation between the rise in the bi-carbonate concentration of plasma water and of spinal fluid, but that a new steady state is ulti-mately achieved that restores the normal ratios between the cerebrospinal fluid and plasma water concentrations of each electrolyte. Methods Experiments were performed on 35 dogs weighing between 7.7 and 22.7 kg. Each animal was lightly anes-thetized with iv thiopental sodium (average dose, 20 mg per kg). Promptly thereafter 6 to 12 ml of cerebro-spinal fluid was gently aspirated from the cisterna