Oral clonidine preanesthetic medication augments the pressor responses to intravenous ephedrine in awake or anesthetized patients.

To evaluate the possible interaction between clonidine and ephedrine, the authors studied hemodynamic responses to intravenous ephedrine in 80 patients who received either clonidine pre-anesthetic medication of approximately 5 micrograms.kg-1 orally (n = 40) or no medication (n = 40). The patients were studied while they were either awake (n = 40) or anesthetized with enflurane and nitrous oxide in oxygen (n = 40). Hemodynamic measurements were made at 1-min intervals for 10 min after ephedrine 0.1 mg.kg-1 was injected as a bolus. Although the responses to ephedrine were always greater in anesthetized patients, the magnitudes of mean blood pressure increases in patients who received clonidine (10 +/- 8% for awake and 27 +/- 11% for anesthetized subjects, mean +/- standard deviation [SD]) were significantly greater (P less than 0.05) than in patients not receiving clonidine (4 +/- 5% for awake and 17 +/- 11% for anesthetized subjects). The enhanced pressor responses to ephedrine observed in both awake and anesthetized patients in the presence of clonidine may be attributed to increased catecholamine storage at sympathetic nerve endings due to clonidine, enhanced sensitivity of tissue receptors to which ephedrine binds, potentiation of alpha-adrenoceptor mediated vasoconstriction of both agents, or all of these. It is concluded that oral clonidine preanesthetic medication of 5 micrograms.kg-1 does augment rather than attenuate the pressor responses to intravenous ephedrine in patients both prior to and during general anesthesia.