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Objective To determine the heart rate response to atropine (<0.1 mg) in anaesthetised young infants. Design Prospective, observational and controlled. Setting Elective surgery. Patients Sixty unpremedicated healthy infants less than 15 kg were enrolled. Standard monitoring was applied. Anaesthesia was induced by mask with nitrous oxide (66%) and oxygen (33%) followed by sevoflurane (8%). Interventions Intravenous (IV) atropine (5 mg/kg) was flushed into a fast flowing IV. The ECG was recorded continuously from 30 s before the atropine until 5 min afterwards. Main outcome measures The incidence of bradycardia and arrhythmias was determined from the ECGs by a blinded observer. Results The median (IQR) age was 6.5 (4-12) months and the mean (95% CI) weight was 8.6 (8.1 to 9.1) kg. The mean (95% CI) dose of atropine was 40.9 (37.3 to 44) mg. Bradycardia did not occur. Two infants developed premature atrial contractions and one developed a premature ventricular contraction. When compared with baseline values, heart rate increased by 7% 30 s after atropine, 14% 1 min after atropine and 25% 5 min after atropine. Twenty-nine infants (48%) experienced tachycardia (>20% above baseline rate) after atropine lasting 222.7 s (range 27.9-286). The change in heart rate 5 min after atropine was inversely related to the baseline heart rate. Conclusions The upper 95% CI for the occurrence of bradycardia in the entire population of infants based on a zero incidence in this study is 5%. These results rebut the notion that atropine <0.1 mg IV causes bradycardia in young infants. Trial registration number ClinicalTrials.gov #NCT01819064. [ABSTRACT FROM AUTHOR]