Effects of alfentanil on cerebral haemodynamics in an experimental model of traumatic brain injury.

Alfentanil is reported to increase intracranial pressure (ICP) after neurotrauma. A direct cerebral vasodilator effect has been postulated. We studied 17 Sprague-Dawley rats allocated to one of three groups. Animals were anaesthetized and their lungs ventilated, and arterial pressure, ICP and/or regional cerebral blood flow (CBF) measurements were undertaken. Group 1 (n = 6) received a severe closed head injury while group 2 (n = 5) received no injury. ICP and mean arterial pressure (MAP) were measured before, during and after rapid infusion of alfentanil 250, 500 and 750 micrograms kg-1. CBF was measured by hydrogen clearance before rapid infusions and at 30-min intervals after starting a subsequent slow infusion of alfentanil 500 micrograms kg-1 h-1. Group 3 (n = 6) underwent CBF measurement only, for comparison with those of groups 1 and 2. They received an injury but no alfentanil. ICP or MAP values did not differ significantly between groups 1 and 2. Rapid i.v. doses of alfentanil produced increases in ICP and reductions in MAP. ICP changes were consistent with a drug effect (P < 0.001) but were small. Reductions in MAP were significant (P < 0.05) and preceded changes in ICP. CBF values were similar and unaffected by slow alfentanil infusion in groups 1 and 2, and did not differ significantly between groups 1 and 3. We conclude that alfentanil did not appear to exert a direct effect on the cerebral circulation. Changes in ICP after rapid infusion were secondary to reductions in SAP. Slow infusion did not cause such changes.