PH of cerebrospinal fluid in the cisterna Magna and on the surface of the choroid plexus of the 4th ventricle and its effect on ventilation in experimental disturbances of acid base balance. Transients and steady states.

Abstract

In cats anesthetized with chloralose urethane, the arterial pH and the pH either in the cisterna magna or on the surface of the choroid plexus of the 4th ventricle as well as the tidal volume, expiratory CO2 partial pressure and arterial pressure were continuously recorded. Transients and steady states were observed after the inhalation of CO2, and the i.v. injection of HCl, NaHCO3, NH4Cl, and (NH4)2CO3. 1. pH on the surface of the choroid plexus follows changes in arterialpco2 almost instantaneously. 2. Injection of HCl into the blood is followed by a transient increase in endtidalpco2 and at the same time by a transient decrease of pH on the surface of the plexus. 3. After i.v. injection of NH4Cl and (NH4)2CO3 an abrupt increase of pH on the surface of the choroid plexus is observed indicating instantaneous permeation of NH3 from the blood into the CSF. This is in agreement with the distribution theory of the early phase of NH4Cl acidosis proposed by the authors in an earlier paper. 4. If the pH is measured in the cisterna magna all reactions are much slower but are in the same direction. 5. In the steady state of respiratory acidosis, the change of pH on the surface of the choroid plexus is less than that of the blood. 6. In metabolic acidosis and alkalosis only minimal—if any—changes of pH on the surface of the choroid plexus can be demonstrated, indicating that the regulation of plexus pH in these instances is very accurate. 7. In respiratory as well as in metabolic disturbances it would seem from the observed data that even in the first few minutes some bicarbonate exchange between the blood and the CSF occurs. 8. The increase of bicarbonate concentration in the CSF after i.v. injection of ammonium salts is mainly the result of the NH3 entering the CSF and reacting with H2CO3.