Background: The transition from fetal to neonatal circulation is a complex physiological phenomenon, influenced by umbilical cord clamping and lung aeration, which triggers an increase in pulmonary blood flow and left ventricular preload. Although clinical evidence suggests that delayed cord clamping (DCC) prevents complications of haemodynamic instability, such as cerebrovascular injury, the cardiovascular consequences of DCC have not been investigated yet in humans. Methods: Echocardiography was performed in 46 term vigorous infants before DCC, immediately after DCC, and at 5 minutes of life. Pulsed-wave Doppler-derived cardiac output and the pulmonary artery acceleration time indexed to the right ventricle ejection time, as a proxy of right ventricular afterload, were obtained. As markers of pre- and afterload fluctuations, the myocardial performance indexes and the velocities of the tricuspid and mitral valve annuli were determined with tissue Doppler imaging. Heart rate was derived from Doppler imaging and obtained throughout the assessments. Echocardiographic parameters were compared across the three timepoints using repeated measures ANOVA or Friedman's test. Results: DCC occurred at a median of 65 seconds (interquartile range 60-70). Left ventricular output increased throughout the first minutes of life (mean(SD): 222.4(32.5) mL/Kg/min before CC vs. 239.7(33.6) mL/Kg/min at 5 minutes, P=0.01), while right ventricular output dropped after CC (306.5(48.2) mL/Kg/min before CC vs. 272.8(55.5) mL/Kg/min immediately after CC, P=0.001). Right ventricular afterload rose after CC, decreasing in the following minutes. The tissue Doppler measurements showed that the loading conditions of both ventricles were transiently impaired by CC, recovering at 5 minutes. The heart rate progressively decreased after birth, following a linear trend temporarily disrupted by CC. Forward stepwise regression indicated that the variation in left ventricular output across the CC was directly correlated to the fluctuation of left ventricular preload over the same period (P=0.03). Conclusion: This study unveils the cardiovascular consequences of DCC in term vigorous infants and offers insight into the haemodynamic transition from fetal to neonatal circulation in humans. Strategies that aim to enhance left ventricular preload before CC, such as initiating ventilation with an intact umbilical cord in apneic infants, may prevent complications of perinatal cardiovascular imbalance.

The authors have declared no competing interest.

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The Cork Teaching Hospitals' Research Ethics Committee approved this study [ECM 4 (z) 11/1/2022].

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