Effects of clonidine premedication on the pressor response to alpha-adrenergic agonists.

It has been suggested that postjunctional alpha 1-adrenoceptor mediated vasoconstriction is enhanced by clonidine. We have examined in humans if the pressor responses to noradrenaline and phenylephrine are enhanced by clonidine premedication. Seventy-seven patients were allocated randomly to either clonidine (n = 38) or control (n = 39) groups. Patients in the clonidine group received approximately 5 micrograms kg-1 with famotidine 20 mg, while the control group received famotidine 20 mg alone orally, 90 min before induction of general anaesthesia with thiamylal. In all patients the lungs were ventilated mechanically via tracheal tubes and anaesthesia maintained with 1% end-tidal enflurane and 67% nitrous oxide in oxygen. When a stable haemodynamic state was obtained, either noradrenaline 0.5 microgram kg-1 (n = 40) or phenylephrine 2 micrograms kg-1 (n = 37) was administered randomly i.v. as a bolus, while arterial pressure and heart rate were measured noninvasively at 1-min intervals for 10 min. Although noradrenaline caused significantly greater increases in mean arterial pressure (MAP) in the clonidine group (from 2 to 4 min after i.v. injection) compared with the control group, there were no significant differences in the mean maximal increment in MAP or area under the MAP curve between the two groups. However, i.v. phenylephrine produced a significantly greater increase in MAP from 2 to 7 min (P < 0.05), and greater mean maximal increase in MAP from the baseline value (21 (9) vs 14 (7) mm Hg; P < 0.05) in the clonidine than in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)