The influence of halothane, isoflurane, and pentobarbital on cerebral plasma volume in hypocapnic and normocapnic rats.

Cerebral blood volume (CBV = cerebral plasma volume [CPV] + cerebral red cell volume [CRBCV]) is one determinant of intracranial pressure. In an effort to quantitate the effects of anesthetics and PaCO2 on CBV, the authors measured cerebral plasma volume (CPV) in normocapnic (PaCO2 approximately 42 mmHg) and hypocapnic (PaCO2 approximately 22 mmHg) rats receiving 1 MAC doses of isoflurane or halothane, or given an approximately equivalent dose of pentobarbital. All animals were paralyzed, their lungs mechanically ventilated, and body temperature kept normal throughout the study. CPV was measured using 14C-labeled dextran, a large (70,000 molecular weight [M.W.]), nondiffusible compound that was given intravenously and allowed to circulate for approximately 5 min. The experiments then were terminated by freezing the brains in situ with liquid N2 poured into a funnel affixed to the exposed calvarium. Isotope concentrations in solubilized brain and in plasma were determined by scintillation counting, and CPV was calculated as the ratio between these values. CPV during both hypocapnic and normocapnic pentobarbital anesthesia was less than with either volatile agent. During normocapnia, CPV for pentobarbital = 2.1 +/- 0.26 ml/100 g (mean +/- SD, n = 8), compared with 2.96 +/- 0.44 ml/100 g (n = 9) and 3.06 +/- 0.44 ml/100 g (n = 9) for halothane and isoflurane, respectively. There were no differences in CPV between the two volatile agents during normocapnia.(ABSTRACT TRUNCATED AT 250 WORDS)