Younger postnatal age is associated with a lower heart rate on Holter monitoring during the first week of life

Abstract

To evaluate heart rate (HR), the presence of extrasystoles and other Holter findings among healthy newborns, and to collect data for new normal limits for Holter parameters in newborns. For this cross-sectional study, 70 healthy term newborns were recruited to undergo 24-h Holter monitoring. Linear regression analysis was used in HR analyses. The age-specific limits for HRs were calculated using linear regression analysis coefficients and residuals. The mean (SD) age of the infants was 6.4 (1.7) days during the recording. Each consecutive day of age raised the minimum and mean HR by 3.8 beats per minute (bpm) (95% CI: 2.4, 5.2; P < .001) and 4.0 bpm (95% CI: 2.8, 5.2; P < .001), respectively. Age did not correlate with maximum HR. The lowest calculated limit for minimum HR ranged from 56 bpm (aged 3 days) to 78 bpm (aged 9 days). A small number of atrial extrasystoles and ventricular extrasystoles were observed in 54 (77%) and 28 (40%) recordings, respectively. Short supraventricular or ventricular tachycardias were found in 6 newborns (9%).Conclusion: The present study shows an increase of 20 bpm in both the minimum and mean HRs of healthy term newborns between the 3rd and 9th days of life. Daily reference values for HR could be adopted in the interpretation of HR monitoring results in newborns. A small number of extrasystoles are common in healthy newborns, and isolated short tachycardias may be normal in this age group. What is Known: • The current definition of bradycardia in newborns is 80 beats per minute. • This definition does not fit into the modern clinical setting of continuously monitored newborns, where benign bradycardias are commonly observed. What is New: • A linear and clinically significant increase in heart rate was observed in infants between the ages of 3 and 9 days. • It appears as though lower normal limits for heart rate could be applied to the youngest newborns.

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

“…Red dashed lines, ±2 standard deviation values for minimum HR from our previous publication. 4 Black dashed line, 80 bpm limit. bpm, beats per minute; HR, heart rate.…”

“…According to HR variability studies, the increase in HR during neonatal period is explained by postnatal autonomic nervous system maturation. 7 Using a HR threshold below 80 bpm, nearly 80% of newborns in the study exhibited bradycardia on Holter monitoring, while applying our new age-specific reference values 4 reduced the prevalence to 15%. In a previous study, bradycardias were reduced from 30% to 7% when the definition of bradycardia was based on the baseline HR during the monitoring instead of a HR below 80 bpm.…”

“…However, the incidence of short tachyarrhythmias was similar in healthy controls. 4 Blood test results were available in most of the newborns. The nearest plasma glucose value, but not the diagnosis of hypoglycaemia, was associated with the minimum HR.…”

“…Our recently published age-specific reference values for HR in newborns were used. 4 The definition for bradycardia was either a HR <80 bpm or a minimum HR < −2 standard deviations (SDs) of agespecific reference data.…”

“…This study aims to assess the definition and causes of bradycardias in term newborns, employing fixed criteria (HR below 80 bpm) along with our recently published age-specific reference values for HR in newborns. 4…”