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 respon se to intravenous ephedrine. If this effect is partly due to clondine- induced potentiation of alpha(1)-adrenoceptor-mediated vasoconstrictio n, it is also assumed that clonidine would enhance the pressor effect of phenylephrine as an alpha(1)-adrenergic agonist. The authors studie d haemodynamic responses to intravenous phenylephrine in 80 patients w ho received either preanaesthetic medication with clonidine approximat ely 5 mu g . kg(-1) po (clonidine group, n = 40), or no medication (co ntrol group, n = 40). Each group was further divided into either awake subjects (n = 20) or subjects anaesthetized with enflurane and nitrou s 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 press ure increases in the clonidine group (26 +/- 7% (mean +/- SD) for awak e and 32 +/- 15% for anaesthetized subjects) were greater (P < 0.05) t han in the control group (13 +/- 7% for awake and 18 +/- 7% anaestheti zed subjects). However, there war no difference in pressor effect of p henylephrine between awake and anaesthetized patients in both groups. Oral clonidine preanaesthetic medication, 5 mu g . kg(-1), augments th e presser responses to phenylephrine 2 mu g . kg(-1) iv in awake and a naesthetized patients. These results suggest that the enhancement of t he presser responses to phenylephrine following oral clonidine may be due to clonidine-induced potentiation of alpha(1)-adrenoceptor-mediate d vasoconstriction. This implies that restoration of blood pressure ca n be achieved effectively by phenylephrine in hypotensive patients wit h clonidine premedication.