ENHANCEMENT OF PRESSOR-RESPONSE TO INTRAVENOUS PHENYLEPHRINE FOLLOWING ORAL CLONIDINE MEDICATION IN AWAKE AND ANESTHETIZED PATIENTS

Clonidine, an alpha(2)-adrenergic agonist, augments the pressor response to intravenous ephedrine. If this effect is partly due to clondine-induced potentiation of alpha(1)-adrenoceptor-mediated vasoconstriction, it is also assumed that clonidine would enhance the pressor effect of phenylephrine as an alpha(1)-adrenergic agonist. The authors studied haemodynamic responses to intravenous phenylephrine in 80 patients who received either preanaesthetic medication with clonidine approximately 5 mu g . kg(-1) po (clonidine group, n = 40), or no medication (control group, n = 40). Each group was further divided into either awake subjects (n = 20) or subjects anaesthetized with enflurane and nitrous oxide in oxygen (n = 20). Haemodynamic measurements were made at one-minute intervals for ten minutes after phenylephrine 2 mu g . kg(-1) iv was injected as a bolus. The magnitudes of maximal mean blood pressure increases in the clonidine group (26 +/- 7% (mean +/- SD) for awake and 32 +/- 15% for anaesthetized subjects) were greater (P < 0.05) than in the control group (13 +/- 7% for awake and 18 +/- 7% anaesthetized subjects). However, there war no difference in pressor effect of phenylephrine between awake and anaesthetized patients in both groups. Oral clonidine preanaesthetic medication, 5 mu g . kg(-1), augments the presser responses to phenylephrine 2 mu g . kg(-1) iv in awake and anaesthetized patients. These results suggest that the enhancement of the presser responses to phenylephrine following oral clonidine may be due to clonidine-induced potentiation of alpha(1)-adrenoceptor-mediated vasoconstriction. This implies that restoration of blood pressure can be achieved effectively by phenylephrine in hypotensive patients with clonidine premedication.