We studied the dose-response relationships for atropine-induced heart rate (HR) changes in 61 patients during propofol anesthesia. The control group (n = 15) received no propofol. Group P-5 (n = 22) received IV propofol 1.25 mg/kg over 1 min followed by propofol at 5 mg. kg(-1). h(-1). After tracheal intubation, anesthesia was maintained with propofol 5 mg. kg(-1). h(-1) and 67% nitrous oxide in oxygen. Group P-10 (n = 24) received IV propofol 2.5 mg/kg over 1 min followed by propofol at 10 mg. kg(-1). h(-1). The P-10 protocol was otherwise identical. All patients received incremental doses of IV atropine 5 microg/kg over 5 s at 2-min intervals until HR increased >20 bpm from baseline values. Heart rate response to atropine 10 microg/kg was attenuated in Groups P-5 (12 +/- 7 bpm) and P-10 (9 +/- 6 bpm) compared with the control group (28 +/- 13 bpm, P<0.05). When atropine 20 microg/kg was administered, HR increased >20 bpm in all patients of the control group, but in only 43% and 13% of patients in Groups P-5 and P-10, respectively (P<0.05). These results indicate the decreased HR responsiveness to IV atropine in patients receiving propofol, which cannot be effectively overcome by a large dose of atropine, is possibly attributable to propofol-induced suppression of the sympathetic nervous system.
Implications: Heart rate response to IV atropine is attenuated during propofol anesthesia, and the decreased responsiveness to atropine cannot be effectively overcome by a large dose of atropine.