Effects of oral clonidine on heart rate changes after neostigmine-atropine administration

Clonidine reduces heart rate (HR) responses to atropine, whereas neostigmine causes bradycardia. This study was designed to determine whether clonidine premedication would reduce tachycardia after neostigmine-atropine administration.

Fifty adult patients without cardiovascular disorders who were scheduled for elective surgeries were randomly assigned to receive approximately 5 microg/kg (oral clonidine clonidine group, n=25) or no clonidine (control group, n=25) 90 min before induction of general anesthesia. After tracheal intubation, anesthesia was maintained with N2O and 12% isoflurane in oxygen while patients were paralyzed with vecuronium and mechanically ventilated. When surgeries were completed, adequate spontaneous respiration, responses to verbal commands, and sustained tetanus by stimulating the ulnar nerve were confirmed, and patients' tracheas were extubated. Then a mixture of 0.05 mg/kg neostigmine and 0.02 mg/kg atropine was administered intravenously over 20 s under stable hemodynamic condition (systolic blood pressure and HR within +/-5% of preceding values), and blood pressure and HR were measured noninvasively at 1-min intervals for 10 min.

Increases in HR in the clonidine group were significantly less 1-4 min after neostigmine--atropine injections compared with HR values in the control group. A maximum increase in HR of the clonidine group was also significantly less than the control group (15+/-7 vs. 23+/-10 beats/min; means+/-SD), whereas absolute values of mean blood pressure were similar. Severe bradycardia (HR < 50 beats/min) developed in no patients in either group.

Premedication with 5 microg/kg oral clonidine attenuates the initial increases in HR without subsequent decreases in HR.