Heart rate in newborns is associated with age, sex and maternal levothyroxine therapy

Abstract

AbstractAimTo evaluate the definition and causes of neonatal bradycardias.MethodsThis retrospective study included 135 term‐born newborns referred for 24‐hour Holter monitoring due to bradycardia. Bradycardia was defined as either a heart rate below 80 beats per minute (standard definition) or a heart rate below our recently published age‐specific reference values for neonatal heart rate.ResultsThe mean (SD) age was 6.1 (1.3) days. With standard definition, 107 newborns (79%) had bradycardia, whereas only 20 (15%) had a minimum heart rate lower than the age‐specific reference. Younger newborns had lower heart rates. Each day increased the minimum, mean and maximum heart rate by 1.8 (95% CI: 1.0, 2.6), 4.2 (95% CI: 3.0, 5.3) and 2.1 beats per minute (95% CI: 0.3, 3.8), respectively. Male sex and maternal levothyroxine medication were negatively associated with the mean and maximum heart rate. None of the newborns had a cardiac cause for low heart rate.ConclusionAmong term newborns with bradycardias, younger age, male sex and maternal levothyroxine medication were associated with a lower heart rate on Holter monitoring. Given the age‐related increase in heart rate, the 80 beats per minute limit as a universal threshold for abnormal heart rate in newborns appears inappropriate.