Heart Rate and Rhythm in Newborns During 24-hour Electrocardiographic Monitoring

During early postnatal life, both major changes in the cardiovascular system and progressive maturation of cardiac autonomic nervous control occur. These changes affect the heart rate (HR) and heart rhythm of newborns. Cardiac arrhythmias are common in the neonatal period and generally benign. However, clinically significant neonatal arrhythmias need to be diagnosed early for optimal management.
In the era of continuous electrocardiographic (ECG) monitoring, both bradycardia and extrasystoles are common findings among admitted newborns. It may be challenging to distinguish between benign and non-benign phenomenon when evaluating bradycardia and extrasystoles according to current thresholds, which are mainly based on studies from the 1980s.

The aim of this study was to utilise modern 24-hour ECG (Holter) analytics to evaluate HR and rhythm in newborns and collect data on normal HR limits for Holter monitoring in newborns. The further aims were to evaluate the need for 24- hour Holter monitoring in newborns with bradycardia or extrasystoles, to assess the definition of sinus bradycardia in newborns, and to assess the natural course of extrasystoles during the first year of life.

The study population was comprised of 352 infants between 0 and 2 months of age who were referred for 24-hour Holter monitoring at Tampere University Hospital from 2011 to 2017 as well as 70 healthy newborns recruited to undergo 24- hour Holter monitoring at Tampere University Hospital between December 2018 and April 2021. The study included only term-born infants.

The results show that both minimum and mean HR increased by 20 beats per minute (bpm) between the third and ninth days of life. Based on this finding, we calculated age-specific reference limits for HR in newborns. Sinus bradycardia, defined as an HR below 80 bpm, was observed in up to 79% of newborns under 10 days old. However, when bradycardia was assessed using age-specific thresholds, abnormal results were found in only 15% of the same infants. In addition to younger age, lower HR was associated with male sex, older gestational age and maternal levothyroxine therapy during pregnancy.

We found atrial and ventricular extrasystoles in 77% and 40%, respectively, of the healthy newborns. The rates were higher than those reported in previous studies. In up to 92% of the infants, the tendency for extrasystoles ceased during the first year of life. However, infants with a burden of atrial and ventricular extrasystoles exceeding 5% and 30%, respectively, on Holter monitoring were at risk for tachyarrhythmias. A peculiar finding was that short isolated tachyarrhythmias were also observed among 9% of healthy newborns. Among newborns with bradycardia or extrasystoles, Holter monitoring was abnormal in less than 10% of cases with a normal 12-lead ECG and no clinical signs of heart disease present.

In conclusion, sinus bradycardia below 80 bpm and extrasystoles were common in newborns, and performing Holter monitoring on infants with these findings resulted in low diagnostic yield when the resting 12-lead ECG and evaluation for heart disease were normal. However, a higher burden of atrial or ventricular extrasystoles was associated with tachyarrhythmias. Further research is warranted to determine the threshold values for frequent extrasystoles that require follow-up in newborns. Since HR increased significantly during the first week of life in healthy newborns, it is appropriate to use daily HR reference values in the interpretation of HR monitoring results in newborns.