Cerebrospinal fluid pressure in patients with brain tumors: Impact of fentanyl versus alfentanil during nitrous oxide-oxygen anesthesia

The effects on the cerebrospinal fluid pressure (CSFP) of alfentanil and fentanyl were compared during nitrous oxide-oxygen (N₂O-O₂) anesthesia in 24 patients who had brain tumors. Monitored variables included CSFP (lumbar subarachnoid catheter), heart rate from electrocardiographic lead II, mean radial arterial blood pressure, and arterial blood gas tensions. General anesthesia was induced with thiopental, 5 mg/kg IV in divided doses, and maintained with 70% N₂O in O₂; ventilation was held constant (PaCO₂ = 37.4 ± 1.6 mm Hg [mean ± SEM]). After baseline data were recorded, 16 subjects were randomly assigned to receive either 5 μg/kg fentanyl as an intravenous bolus or 50 μg/kg alfentanil as an intravenous bolus, followed by an infusion of alfentanil at 1 μg·kg^-1·min^-1. Monitored variables were continuously recorded for 15 min after opioid injection. A third group of 8 patients was studied subsequently; they received only N₂O-O₂ during a 15-min observation period and served as controls. Blood pressure was held constant with an intravenous infusion of 0.1% phenylephrine, as needed; noxious stimulation was carefully avoided. Cerebrospinal fluid pressure remained unchanged both in patients who received N₂O-O₂ alone and in those who received fentanyl-N₂O-O₂. By contrast, those who received alfentanil-N₂O-O₂ had a gradual increase in CSFP, reaching 30% above baseline values after 10 min and stabilizing thereafter. Although the absolute increase in CSFP during normocarbic alfentanil-N₂O anesthesia was relatively small (9.5 ± 1.3 mm Hg to 13.0 ± 1.3 mm Hg [mean ± SE], P < 0.05), the absence of a similar effect after fentanyl administration suggests that precautionary measures such as hyperventilation are advisable if alfentanil is used for potentiating normocarbic N₂O-O₂ anesthesia in neurosurgical patients with intracranial mass lesions.