Cerebral effects of fentanyl in dogs

We have studied the cerebral haemodynamic and metabolic effects of fentanyl 50 micrograms kg-1 and 100 micrograms kg-1 in dogs undergoing ventilation with oxygen and nitrogen at normocapnia. Cerebral blood flow was measured continuously by direct measurement of the outflow of the posterior sagittal sinus. Arterial and sagittal sinus blood-gas tensions were measured intermittently for calculation of cerebral metabolism. Systemic pressures were measured continuously with intermittent measurements of cardiac output. These doses of fentanyl had minimal effect on cerebral blood flow, cerebral oxygen consumption and the energy state of the brain reflected by the normal concentration of cerebral metabolites at the end of the study. There was also little effect on systemic haemodynamics. It is concluded that, in the absence of other anaesthetic agents which might influence cerebral haemodynamics and metabolism, large doses of fentanyl have little effect on the cerebral circulation.