Biomedical subjects
W R Macrae
Publications and source records attributed to W R Macrae.
Circulatory effects of labetalol during halothane anaesthesia.
Labetalol is a drug possessing both alpha and beta adrenergic receptor blocking properties. Its possible use in induced hypotension during halothane anaesthesia has been investigated. It causes a satisfactory decrease in arterial pressure unaccompanied by tachycardia. The circulatory effects of the drug during halothane anaesthesia, both with spontaneous and controlled respiration, have been measured and compared with those of halothane alone. In patients anaesthetised with 1% halothane, labetalol, with both spontaneous and controlled ventilation, was associated with a reduction in MAP from 71.5 mmHg to 54.0 mmHg (P less than 0.001) and 66.8 mmHg to 50.4 mmHg (P less than 0.001) respectively. This reduction was associated with decreases in Qt of 18% and 12% respectively. In the presence of labetalol, with 3% halothane and spontaneous respiration, the depressant effects of the anaesthetic on the heart became rapidly apparent: Qt was reduced by a further 28%. In patients not receiving labetalol, the depressant effects of 3% halothane were frequently countered by the positive inotropic effects of hypercarbia.
Cardiovascular effects of labetalol during halothane anaesthesia.
1 The circulatory effects of labetalol 25 mg intravenously in six patients during 1% halothane anaesthesia were studied. 2Labetalol caused a marked decrease in arterial pressure and a consistent but modest (20%) decrease in cardiac output. Heart rate was slowed and stroke volume did not change significantly. Central venous pressure increased and peripheral resistance decreased. 3 Increasing the halothane concentration of 3% led to marked myocardial depression as evidenced by reduced cardiac output and increased central venous pressure with increasing arterial hypotension. 4 Labetalol may be a suitable drug for controlling induced hypotension under general anaesthesia, although high concentrations of halothane should be used with care.