FETAL TO NEONATAL TRANSITION

First breath after birth is a watershed event, when circulation changes from fetal to postnatal physiology. Two critical events at birth—sudden cessation of umbilical flow and expansion of lungs with first breath, trigger many hemodynamic changes to adopt from fetal to postnatal physiology, as follows:

• Sudden drop in pulmonary vascular resistance due to expansion of lungs and vasodilatory effects of increasing PaO₂.

• Sudden rise in systemic vascular resistance due to clamping of umbilical cord and obliteration of low resistance placental circulation.

• Closure of ductus arteriosus following equalization of pressures between pulmonary artery and aorta (due to a and b above). Rising PaO₂ of shunted blood

#9632; ^l^= #953; #953;#960;d#954;3le De-IcentsgE- oxygen MhiraBcn oi bftnd Fig. 17.3: Hemodynamics: Adult circulation.

For abbreviations see Fig. 17.2.

Fig. 17.4: Cardiac cycle.

through ductus arteriosus after lung expansion also contributes to its closure. While functional closure of ductus is complete within 10-12 hours of birth, anatomical obliteration (ligamentum arteriosum) may take 4-6 weeks.

• Closure of foramen ovale due to loss of pressure gradient between two atria by—(a) drop in RA pressure after 'opened-up' pulmonary circulation and reduced systemic venous return after cord clamping, and (b) rise in LA pressure due to increasing pulmonary venous return. While functional closure of foramen ovale is complete within 4-6 hours of birth, anatomical closure takes many months or it may remain probe-patent throughout life.

• Closure of ductus venosus by 7^th day of life, changing into Ligamentum venosum.

Above mentioned changes transform the 'parallel' fetal circulation into 'serial' postnatal circulation at birth (Fig. 17.3). Over the next few weeks, pulmonary vascular resistance continues to decline, leading to gradual shift from RV dominance in fetus to LV dominance in later life, with consequent changes in cardiac position, chamber pressures and axis.

17.1.4