[The effects of preanesthetic oral clonidine upon heart rate response to intravenous atropine in patients during general anesthesia].

In awake subjects the positive chronotropic effect of intravenously administered atropine 10 micrograms.kg-1 has been demonstrated to be blunted by preanesthetic medication of oral clonidine 5 micrograms.kg-1. The aim of the present study is to investigate whether general anesthesia could alter the clonidine-induced attenuation of positive chronotropic effect by atropine. Thirty-two patients were randomly assigned to one of the two groups; patients of the clonidine group received oral clonidine 5 micrograms.kg-1 (n = 12), whereas those of the control group received no clonidine. General anesthesia was induced with intravenous thiamylal 4-5 mg.kg-1, and was maintained with enflurane and nitrous oxide in oxygen after endotracheal intubation. Following the stable circulatory period of 10 min, hemodynamic measurements were made at 1 min intervals for 10 min after atropine 10 micrograms.kg-1 was administered intravenously as a bolus in both groups. A significant attenuation in heart rate response to intravenous atropine 10 micrograms.kg-1 was observed in patients receiving clonidine 5 micrograms.kg-1, as compared with that in the control group (P less than 0.01); maximal increases in heart rate were 15 +/- 8 and 22 +/- 6 beats.min-1 (mean +/- SD) in the clonidine and control groups, respectively. It is concluded that clonidine 5 micrograms.kg-1 blunts the heart rate response to intravenous atropine 10 micrograms.kg-1 in patients anesthetized with enflurane and nitrous oxide in oxygen.