Intracranial volume/pressure relationship during desflurane anesthesia in dogs: comparison with isoflurane and thiopental/halothane.

Desflurane-induced increase of intracranial pressure (ICP) does not appear to be completely explained by desflurane-induced changes in cerebral blood flow, cerebrospinal fluid (CSF) formation and reabsorption, or brain tissue water content. The present study was designed to determine whether desflurane alters intracranial volume/pressure relationships sufficiently to account for desflurane-induced increase of ICP. In 24 dogs, infusions of mock CSF were used to determine the CSF pressure increase due to increase of CSF volume, and the capacity of CSF pressure to return to baseline after a CSF pressure increase (CSF pressure normalization). CSF pressure increase was assessed by determining (a) CSF pressure prior to volume infusion (P(o)), (b) peak CSF pressure (Pp) caused by volume injection, (c) intracranial compliance (C, calculated as the ratio of change of intracranial volume [delta V] to change of CSF pressure [delta P]), (d) the volume pressure response (VPR, a measure of elastance, calculated as the ratio of delta P to delta V, (e) the pressure/volume index (PVI, calculated as the ratio of delta V to log Pp/P(o)), and (f) estimated intracranial compliance (C(e), calculated from PVI as 0.4343PVI/P(o)). CSF pressure normalization was assessed by determining the first (S1) and second (S2) phase slopes of decrease of CSF pressure from Pp, the duration of S1 (DS1), and the intersection of extrapolated S2 with the P(o) to Pp slope (delta Ps). During normocapnia (Group 1, n = 6) and hypocapnia (Group 2, n = 6), two infusions were made at each of four experimental conditions: 0.5 minimum alveolar anesthetic concentration (MAC) desflurane and normal or increased CSF pressure, and 1.0 MAC desflurane and normal or increased CSF pressure. For comparison to these two desflurane-anesthetized groups, intracranial volume/pressure values were determined at the same experimental conditions during 0.5 MAC isoflurane (Group 3, n = 6) and in controls (Group 4, n = 6) anesthetized with thiopental 12 mg/kg then 12 mg.kg-1.h-1 intravenously combined with halothane 0.5% inspired. In comparison to controls, both desflurane and isoflurane decreased Ce at normal CSF pressure but not at increased CSF pressure. However, desflurane and isoflurane had no consistent effect on the other measures of CSF pressure increase caused by increase of CSF volume. Isoflurane also decreased the capacity for CSF pressure normalization at normal CSF pressure as indicated by decreased S1 and increased delta Ps. It is concluded that, under conditions of normal ICP, desflurane may decrease Ce, favoring an increase of ICP.