Clonidine premedication for isoflurane-induced hypotension. Sympathoadrenal responses and a computer-controlled assessment of the vapour requirement

The effect of single-dose clonidine premedication on the vapour requirement for isoflurane-induced hypotension in patients undergoing middle ear or nasal surgery was evaluated in an open, controlled, randomized study. Inspired isoflurane concentration was regulated by a microcomputer-based, self-tuning control program when hypotension was required. Patients given clonidine 0.6 mg by mouth 2 h before operation required a mean inspired isoflurane concentration of 2.0% to induce hypotension (mean intra-arterial pressure 50 mm Hg) compared with 3.01% in the control group (P less than 0.05). Five out of 10 patients in the control group required a supplementary dose of labetalol 5 mg i.v. to achieve satisfactory hypotension, compared with one of 10 patients given clonidine premedication (Fisher's exact probability, 0.07). A mean concentration of 1.4% isoflurane was required to maintain hypotension in the clonidine group, compared with 2.3% in the control group (P less than 0.01). Plasma adrenaline and noradrenaline concentrations did not increase during induced hypotension in each group.