Oral clonidine premedication does not alter the efficacy of epidural test doses in adult patients anesthetized with isoflurane.

Clonidine premedication has been increasingly used in clinical anesthesia. Though clonidine was found to alter pressor responses to various sympathomimetics, its effect on epidural test dose efficacy to detect intravascular injection has never been evaluated. Eighty healthy patients were randomly assigned to one of four groups, each of which was anesthetized with 1% end-tidal isoflurane and 67% nitrous oxide in oxygen after endotracheal intubation. The control-epinephrine group (n=20) given no clonidine premedication received 3 ml of 1.5% lidocain with 15 μg epinephrine (1:200000) intravenously to simulate an intravenously administered epidural test dose. The control-saline group (n=20) given no clonidine premedication received 3 ml of normal saline intravenously. The clonidine-epinephrine and clonidine-saline groups (n=20 each) were identical to the control groups, but were premedicated with oral clonidine, approximately 5 μg·kg(-1), 90 min before induction of general anesthesia. Heart rate (HR) and systolic blood pressure (SBP) were measured by a blinded observer at 20-s intervals for 4 min after intravenous injections of the test dose or saline. Following intravenous test dose injection, there were no significant diferences between the control-epinephrine and the clonidine-epinephrine groups in mean maximum increments of both HR (28±3vs 30±3 bpm, [mean±standard error], respectively) and SBP (46±6vs 45±4 mmHg, respectively). Six patients in the control-epinephrine and 4 in the clonidine-epinephrine group developed negative HR responses (HR increment <20 bpm). Since HR and SBP were essentially unchanged in the two groups receiving saline, sensitivities (negative predictive values) based on the HR criterion (positive if ≥20 bpm increase in HR) were 80% and 70% (83% and 77%) with and without clonidine premedication, respectively (P>0.05 between groups). However, when a modified HR criterion (positive if ≥10 bpm increase in HR) was used, sensitivities, specificities, and positive and negative predictive values were all 100% with or without clonidine. On the other hand, all of 20 patients in the control-epinephrine and the clonidine-epinephrine groups exhibited positive SBP responses (SBP increment ≥15 mmHg). Therefore, based on the SBP criterion, sensitivities, specificities, and positive and negative predictive values were all found to be 100% regardless of the presence of clonidine. We conclude that oral clonidine 5μg·kg(-1) premedication alters neither (a) hemodynamic responses to the intravenously administered epidural test dose containing 15 μg epinephrine, nor (b) the efficacy for detecting intravascular injection based on either criterion in adult patients under stable isoflurane anesthesia.