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Clonidine, an alpha 2-adrenergic agonist, augments the pressor response to intravenous ephedrine. If this effect is partly due to clonidine-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 micrograms.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 micrograms.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 &lt; 0.05) than in the control group (13 +/- 7% for awake and 18 +/- 7% for anaesthetized subjects). However, there was no difference in the pressor effect of phenylephrine between awake and anaesthetized patients in both groups. Oral clonidine preanaesthetic medication, 5 micrograms.kg-1, augments the pressor responses to phenylephrine 2 micrograms.kg-1 iv in awake and anaesthetized patients. These results suggest that the enhancement of the pressor 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.

Enhancement of pressor response to intravenous phenylephrine following oral clonidine medication in awake and anaesthetized patients