[The effects of a small dose of atropine upon sinus bradycardia during spinal anesthesia]

Positive or negative chronotropic effects of atropine and their magnitude are known to be determined by its dose given, concomitant sympathetic tone, and patient's age. Thus, the aim of the present study is to investigate the small effective dose of atropine for acceleration of heart rate in sedated bradycardic patients under spinal anesthesia, and to evaluate the relationship between the heart rate response to atropine and level of analgesia or patient's age. Significant increases in heart rate were observed in patients receiving intravenous atropine 4 micrograms.kg-1 (n = 21) or 6 micrograms.kg-1 (n = 35) except 2 micrograms.kg-1 (n = 18). However, there were wide variations in cardiac responses to atropine among patients in each group. Negative chronotropic responses to atropine were elicited in 10 (56%), 2 (9%), and 1 (3%) of the patients receiving 2 micrograms.kg-1, 4 micrograms.kg-1 and 6 micrograms.kg-1, respectively. Although there was no correlation between the heart rate response to each dose of atropine and the level of spinal anesthesia, the magnitude of positive chronotropic response to atropine of 6 micrograms.kg-1 related inversely with age. The present results suggest that atropine above 4 micrograms.kg-1 is required to accelerate the heart rate in most bradycardic patients under spinal anesthesia irrespective of the level of analgesia. However, it seems appropriate to administer atropine intravenously in a cumulative fashion, because of individual's variability including age in the chronotropic response to atropine.