Anaesthesia and the QT interval in humans. The effects of isoflurane and halothane

Prolongation of the QT interval may cause potentially hazardous arrhythmias. The effects on the QT interval (QTc, corrected for heart rate) of isoflurane and halothane followed by vecuronium have been investigated during induction of anaesthesia in 51 patients. All patients were ASA 1 or 2, without cardiovascular problems or electrolyte abnormalities and were not receiving medication. Midazolam 0.08 mg.kg-1 was administered intramuscularly for premedication. Anaesthesia was induced with either isoflurane (n = 26) or halothane (n = 25), and the inspired concentration increased to reach an end-tidal concentration of 2.5% to 3%. Recordings of ECG, heart rate, systolic and diastolic arterial pressure were obtained at the following times: prior to induction of anaesthesia; 1 min and 3 min after a stable end-tidal concentration had been reached; 1 min and 3 min following vecuronium administration, at the time of tracheal intubation and 1 min and 3 min later. Halothane significantly shortened QTc (p < 0.05 to p < 0.001), in contrast to isoflurane which prolonged it (p < 0.01). The heart rate decreased (p < 0.01 to p < 0.001) after induction of anaesthesia with halothane and returned to pre-induction values after tracheal intubation. In contrast, heart rate increased after induction with isoflurane and increased further after laryngoscopy and tracheal intubation (p < 0.001). In the isoflurane group, ST depression was noticed in seven patients and nodal rhythm in two, while in the halothane group seven patients developed nodal rhythm and, in two patients, ventricular ectopics were recorded. There were no sequelae. In both groups, systolic and diastolic arterial pressure decreased after induction of anaesthesia (p < 0.01 to p < 0.001), increasing again after intubation.