Author(s): Toivonen J, Kaukinen S
The effect of clonidine on the dose requirements of labetalol and isoflurane for hypotension was studied in 20 adult patients undergoing middle-ear surgery. Group I (10 patients) received as premedication only pethidine 1 mg.kg-1 i.m., and Group II (10 patients) clonidine 4-5 micrograms.kg-1 p.o. in addition to pethidine. Fentanyl was used for analgesia and d-tubocurarine for muscle relaxation. The dose of labetalol required to induce hypotension to the mean arterial pressure (MAP) 50 mmHg (6.7 kPa) was 0.85 +/- 0.08 mg.kg-1 in Group I, and 0.56 +/- 0.08 mg.kg-1 in Group II (P less than 0.05). The mean isoflurane concentration in inspiratory gas for the maintenance of hypotension was 0.8 +/- 0.1 vol\% in Group I, and 0.6 +/- 0.1 vol\% in Group II (P less than 0.05). Before hypotension, MAP decreased significantly in Group II, and the heart rate (HR) decreased in both groups. There were no significant differences in MAP and HR between the groups during any phase. Urine flow rates (UF) were lower in Group II (0.23 +/- 0.04 ml.min-1) than in Group I (0.68 +/- 0.16 ml.min-1) before hypotension (P less than 0.05) and during hypotension (0.08 +/- 0.02 ml.min-1 vs. 0.68 +/- 0.32 ml.min-1, P less than 0.05). After anaesthesia, there was no difference in UF between the groups. The results indicate that clonidine can be used for deliberate hypotension to decrease the dose requirements of labetalol and isoflurane.