Sodium nitroprusside-produced hypotension during anesthesia and operation in the head-up position

Measurements before, during and after prolonged hypotension (148 +/- 40 min, mean +/- SE) produced by sodium nitroprusside (SNP, 2.4 +/- 1.1 microng/kg/min) were obtained from 5 anesthetized adult patients in the head-up position. Reduction of mean arterial pressure (MAP) from 81 to 54 torr (at the level of the circle of Willis) while maintaining similar concentrations of N2O and halothane was associated with an unchanged arterial oxygenation and pH, decreased systemic vascular resistance (p less than 0.05), increased heart rate (HR p less than 0.05) and an unchanged (2 patients) or increased (3 patients) cardiac output as compared with measurements during normotension. After SNP was discontinues, the MAP recovered within 3 to 5 minutes and the cardiac output and HR decreased. Two of these patients also received trimethaphan (TMP). In 1, TMP decreased MAP, but this was associated with a reduction in cardiac output. In the same patient, SNP produced a similar fall in MAP, but cardiac output was unchanged. In the 2nd patient, TMP did not reduce MAP, while SNP resulted in a prompt decrease. The authors conclude that circulatory changes during SNP administration for prolonged controlled hypotension in the semi-sitting position are similar to hemodynamic changes observed during short-term SNP administration to supine patients.