Heart rate and systolic time intervals in healthy newborn infants: longitudinal study.

To determine the influence of heart rate (HR) on systolic time intervals (STI) in neonates, serial measurements of right ventricular (RVSTI) and left ventricular systolic time intervals (LVSTI) were made on 30 healthy term newborn infants at age 4-8 h, 24-30 h, eight days, and four weeks. STI was related to HR and age. Age-related changes were similar to previously reported results. The preejection periods (RPEP and LPEP) significantly shortened with increasing age, whereas the right and left ventricular ejection times (RVET and LVET) were unrelated to age. RPEP was unrelated to HR, but tended to be prolonged in restless infants. With increasing HR, RVET decreased and RPEP/RVET increased in all age groups, but less at four weeks. A rise in HR of 50/min resulted in an increase of RPEP/RVET by 26% of the mean value at age 4-8 h and by 20% at four weeks. In 14 infants, RVSTI was recorded during a change in HR. In all these infants, RPEP and RPEP/RVET increased with increasing HR. We conclude that HR-related changes of RVSTI in neonates are different from those in older subjects. It should be considered that in neonates elevated values of RPEP/RVET, suggesting increased pulmonary vascular resistance, may be caused by high HR and unrest.