title: Enhancement of pressor response to intravenous phenylephrine following oral clonidine medication in awake and anaesthetized patients.
abstract: 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 < 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.
authors.affiliation: Department of Anaesthesia, Iwaki Kyoritsu General Hospital, Fukushima, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Inomata
authors.forename: T
authors.initials: T
authors.lastname: Nishikawa
authors.forename: S
authors.initials: S
authors.lastname: Kihara
authors.forename: Y
authors.initials: Y
authors.lastname: Akiyoshi
mesh_terms: D000284:Administration, Oral; D000328:Adult; D000769:Anesthesia, Inhalation; D001794:Blood Pressure / Q000187:drug effects; D003000:Clonidine / Q000008:administration & dosage / Q000494:pharmacology*; D004357:Drug Synergism; D004737:Enflurane / Q000008:administration & dosage; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D007275:Injections, Intravenous; D008297:Male; D008875:Middle Aged; D009609:Nitrous Oxide / Q000008:administration & dosage; D010656:Phenylephrine / Q000008:administration & dosage / Q000494:pharmacology*; D011229:Preanesthetic Medication; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects; D014662:Vasoconstrictor Agents / Q000494:pharmacology*; D014851:Wakefulness
chemical_list: D018340:Receptors, Adrenergic, alpha-1; D014662:Vasoconstrictor Agents; D010656:Phenylephrine; D004737:Enflurane; D009609:Nitrous Oxide; D003000:Clonidine