Isoflurane in nitrous oxide and oxygen increases plasma concentrations of noradrenaline but attenuates the pressor response to intubation.

The haemodynamic response to laryngoscopy and intubation was evaluated in 52 ASA I patients anaesthetized with thiopentone 4-6 mg.kg-1. Vecuronium 0.1 mg.kg-1, followed by 3 min mask ventilation with nitrous oxide in oxygen with (isoflurane group) or without (control group) 3% inspired isoflurane preceded tracheal intubation. In 21 of 52 patients, concentrations of noradrenaline (NA), adrenaline and 3,4-dihydroxyphenylethyleneglycol (DHPG) were measured from central venous plasma. During the 3-min ventilation, in the isoflurane group, heart rate increased by 22% but remained stable in the control group. A marked pressor response to laryngoscopy and intubation was seen in the control but not in the isoflurane group. During the 3-min ventilation, the plasma concentration of NA was doubled in the isoflurane group and increased by 49% in the control group (P less than 0.05). The concentration of DHPG also increased in the isoflurane group. Peripheral skin temperature increased similarly after 3 min ventilation in both groups. After intubation, it was significantly higher in the isoflurane than in the control group (P less than 0.05). In conclusion, compared to nitrous oxide in oxygen, ventilation with isoflurane in nitrous oxide in oxygen causes a sympathetic activation combined with an increase in heart rate and peripheral temperature while the pressor response to laryngoscopy and intubation is attenuated.