Heart rate during the first 24 hours in term-born infants

OBJECTIVE: Heart rate (HR) is an important clinical parameter in newborn infants, but normal ranges are poorly defined. Our aim was to establish normal reference ranges and individual variations in HR as obtained by auscultation in healthy term-born infants during the first 24 hours of life. DESIGN: Observational study. SETTING: Single hospital in Norway. METHODS: HR was assessed by auscultation for 30 s at 2, 4, 8, 16 and 24 hours of age. Auscultation was validated against ECG recordings. SUBJECTS: Healthy term-born infants who were asleep or awake in a quiet resting state. MAIN OUTCOME MEASURES: Construction of percentile curves for resting HR. RESULTS: The study included 953 infants. The 50th percentile was 126 beats per minute (bpm) at age 2 hours and thereafter 120-122 bpm. The respective 2nd and 98th percentiles were 102 (thereafter 96-100) bpm and 162 (thereafter 150-156) bpm. The mean HR was 5.6 bpm higher when awake than asleep, 4.9 bpm higher when on the mother's chest than in the cot, 1.6 bpm higher in girls than in boys, and increased by 0.5 bpm per 0.1°C increase in rectal temperature. Mode of delivery, meconium staining, birth weight and maternal smoking during pregnancy were of no significance. For each infant, HR varied considerably during the first 24 hours (intraclass correlation 0.21 (95% CI 0.18 to 0.24), coefficient of variation 9.2%). CONCLUSIONS: The HR percentiles allow for a scientifically based use of HR when assessing newborn infants born at term.