The Effects of Clonidine on Sensitivity to Phenylephrine and Nitroprusside in Patients with Essential Hypertension Recovering from Surgery

Clonidine reduces postoperative circulatory instability in patients with essential hypertension.It also increases the sensitivity to vasopressors before and during anesthesia. We investigated blood pressure responses to phenylephrine and nitroprusside pre- vs postoperatively and the effect of clonidine on these responses in patients with essential hypertension. Twenty patients received clonidine 6 [micro sign]g/kg orally 120 min before anesthesia and 3 [micro sign]g/kg IV over the final hour of surgery or an identical placebo. During increasing bolus doses of phenylephrine and nitroprusside (30-300 [micro sign]g), the maximal systolic pressure responses were recorded at baseline on the day before surgery, before the induction of anesthesia, and 1 and 3 h postoperatively. Sensitivity to phenylephrine and nitroprusside was interpolated from linear regression of the data. There was no difference between preoperative and postoperative sensitivity to phenylephrine or nitroprusside in either group. Clonidine increased sensitivity to phenylephrine versus placebo before and after surgery (response to dose of 1.5 [micro sign]g/kg: 42 +/- 14 vs 27 +/- 8 mm Hg preinduction, 37 +/- 10 vs 26 +/- 8 mm Hg 3 h postoperatively; both P < 0.01), but not to nitroprusside (38 +/- 6 vs 37 +/- 10 mm Hg preinduction and 40 +/- 6 vs 39 +/- 8 mm Hg postoperatively). Clonidine increases the sensitivity to phenylephrine but not nitroprusside at baseline and postoperatively in hypertensive patients. Implications: Clonidine increases the sensitivity to bolus injections of the vasoconstrictor phenylephrine, but not the vasodilator sodium nitroprusside, before and after surgery in patients with preexisting hypertension. The doses of vasopressors should be reduced accordingly in hypertensive patients receiving perioperative clonidine. (Anesth Analg 1999;88:1239-43)