Birth and Neonatal Cardiopulmonary Transition

At birth the newborn must switch within minutes from receiving oxygen through the placenta to breathing air, and the circulation reorganises to match. Aeration of the lungs, a sharp fall in pulmonary vascular resistance, and the functional closure of the fetal vascular shunts together convert the parallel fetal circulation into the series adult-type circulation. This topic covers the structural and physiological events of that cardiopulmonary transition.

Definition

The birth (neonatal) cardiopulmonary transition is the set of events by which the fetal circulation, organised around placental gas exchange and three vascular shunts, converts at birth into the postnatal circulation through lung aeration, a fall in pulmonary vascular resistance, and the closure of the ductus venosus, foramen ovale and ductus arteriosus.

Scope

The entry covers the layout of the fetal circulation and its shunts (the ductus venosus, foramen ovale and ductus arteriosus), the events of lung aeration and the fall in pulmonary vascular resistance at first breaths, the consequent functional and later anatomical closure of the shunts, and the fate of fetal vessels as adult ligaments. It is a developmental and physiological reference within embryology and does not provide clinical management instructions.

Key concepts

• Fetal circulation
• Ductus venosus
• Foramen ovale
• Ductus arteriosus
• Lung aeration and first breaths
• Fall in pulmonary vascular resistance
• Functional versus anatomical shunt closure

Mechanisms

Before birth, oxygenated blood returns from the placenta and is partly diverted past the liver through the ductus venosus and past the lungs through the foramen ovale (right-to-left across the atria) and the ductus arteriosus (aorta-pulmonary trunk), because pulmonary vascular resistance is high and the lungs are fluid-filled. At birth, lung aeration and the clearance of fetal lung liquid markedly lower pulmonary vascular resistance and increase pulmonary blood flow, raising left atrial pressure; this reverses or abolishes the pressure gradients that drove the shunts, leading to functional closure of the foramen ovale and constriction of the ductus arteriosus and ductus venosus (Hooper et al., 2019). Over the following days to months the shunts close anatomically and their remnants persist as the fossa ovalis, ligamentum arteriosum, ligamentum venosum and the medial umbilical ligaments (Moore et al., 2020; Sadler, 2018; Carlson, 2018).

Clinical relevance

Understanding the normal sequence of lung aeration, the pulmonary-resistance fall and shunt closure provides the developmental framework for conditions in which the transition is delayed or fails, such as persistent fetal shunts. This entry describes that physiology and developmental anatomy for reference and is not a basis for individual diagnostic or treatment decisions.

History

The anatomy of the fetal circulation and the fate of its shunts have long been described in embryology texts (Moore et al., 2020; Sadler, 2018; Carlson, 2018). Physiological study has reframed the transition as an integrated cardiopulmonary event triggered by lung aeration, clarifying the sequence and interdependence of resistance change and shunt closure (Hooper et al., 2019).

Key figures

• Stuart Hooper
• Arjan te Pas
• Keith L. Moore

Seminal works

• hooper-2019
• moore-2020
• sadler-2018

Frequently asked questions

What are the three fetal vascular shunts?

They are the ductus venosus (bypassing the liver), the foramen ovale (between the atria) and the ductus arteriosus (between the pulmonary trunk and aorta); all allow blood to bypass the non-functioning fetal lungs and liver.

What triggers the change in circulation at birth?

Aeration of the lungs with the first breaths lowers pulmonary vascular resistance and raises left atrial pressure; this reverses the gradients across the fetal shunts, leading to their functional closure and conversion to the postnatal circulation.