Clonidine, which is known to have analgesic and sedative properties, has recently been shown to be an effective preanesthetic medication in humans. The drug may cause side effects, including bradycardia and hypotension. This study was conducted to evaluate the ability of intravenous atropine to increase the heart rate (HR) in awake adults receiving clonidine preanesthetic medication.

We studied HR responses to intravenous atropine in 45 patients assigned randomly to either a control group, who received no medication (group 1, n=15), or clonidine groups, who received oral clonidine of 2~2.5 mcg/kg (group 2, n=15), or 4.5~5 mcg/kg (group 3, n=15) 90 min before scheduled induction of anesthesia. When HR and blood pressure had been confirmed to be stable in operating room, all patients received incremental doses of atropine, 2.5, 2.5 and 5 mcg/kg at 2-min intervals. The HR and mean arterial pressure were recorded at 1-min intervals.

Before atropine injection, the HR decreased significantly (P<0.05) in group 3. The increases in HR in response to a cumulative dose of atropine 10 mcg/kg were 21±8, 17±7 and 7±5 beats/min (mean±SD) in group 1, 2 and 3, respectively (P<0.05). The positive chronotropic response to intravenous atropine was attenuated significantly only in group 3 (P<0.01).

It was concluded that oral clonidine of 4.5~5 mcg/kg decreased HR significantly, and blunted the increase in HR after intravenous atropine in awake adults although oral clonidine of 2~2.5 mcg/kg did not.