RESUMO
We have studied the effect of labetalol on anaesthetic requirements, cardiac and humoral responses in 16 healthy women during elective abdominal surgery. The experimental group (n = 8) received increments of i.v. labetalol 0.15-0.3 mg kg-1 to reduce mean arterial pressure (MAP) by 15% from values obtained before anaesthesia. All patients received thiopentone 6 mg kg-1 and anaesthesia was maintained with isoflurane and nitrous oxide. In the placebo group, tracheal intubation provoked a 33% increase in heart rate (HR) and a 52% increase in MAP (P less than 0.001 vs baseline for both). In contrast, pretreatment with labetalol resulted in a 7.3% increase in HR and a 21.3% increase in MAP (P less than 0.05 vs baseline for MAP). Two patients treated with labetalol had a reduction in MAP to 50-60 mm Hg during surgery. After tracheal extubation, the labetalol group had a significantly slower HR than the placebo group (P less than 0.05). The cardiovascular response to tracheal intubation was associated with an increase in plasma noradrenaline concentration in both groups. Labetalol did not affect isoflurane requirements, plasma concentrations of adrenaline, cortisol and aldosterone or arterial PO2, but prevented the decrease in plasma concentration of potassium which occurred in placebo treated patients (P less than 0.05) in early recovery. We conclude that preanaesthetic administration of labetalol attenuated the perioperative hypertensive and tachycardic responses, but was accompanied by intraoperative hypotension when given in doses greater than 0.5 mg kg-1, despite an increase in plasma noradrenaline concentrations.
