Effects of halothane and sodium thiopentone on surface brain pressure and brain electrical impedance in dogs with normal intracranial tension.

The effects of the inhalation of halothane and the i.v. administration of thiopentone on surface brain pressure and brain electrical impedance, at a frequency of 1 kHz, were investigated in 14 chronically implanted dogs. In dogs with normal PaCO2, halothane, at inspired concentrations of 0.8, 1.2, 1.5 and 2.0%, produced increases in brain pressure and impedance. Thiopentone i.v. and hyperventilation preceding the administration of halothane lessened, but did not prevent, the increases in brain pressure and impedance. Thiopentone 35 mg kg-1 decreased both pressure and impedance. The dose-related (P less than 0.01) increase in surface brain pressure was attributed to cerebral vascular expansion; the increase in impedance was not dose related and was interpreted as a loss of conductivity in brain extracellular space (e.c.s.). This loss could be a result of a decrease in both brain e.c.s. volume and its electrolyte concentration resulting from intracellular translocation and electrical inactivation of available ions in brain e.c.s. caused by increased cation binding. These phenomena might be related to the interaction between the anaesthetic molecule and cell proteins. The present study suggests that, in contrast to thiopentone, halothane induced a reversible ionic imbalance in the central nervous system.