Initiation of Breathing

The initiation of breathing is the onset of continuous air breathing at birth, by which the fluid-filled fetal lung is aerated, the airspaces are cleared of liquid, and a stable functional residual capacity is established so that the lung can take over gas exchange from the placenta. It marks the change from the intermittent fetal breathing movements of intrauterine life to sustained pulmonary ventilation.

Definition

Initiation of breathing is the process by which the newborn clears liquid from its airways, aerates the lung, and establishes a stable functional residual capacity to begin continuous pulmonary gas exchange.

Scope

This topic covers the clearance of fetal lung fluid, the aeration of the distal airways, the establishment and maintenance of functional residual capacity, and the close coupling between lung aeration and the cardiovascular changeover. It treats these as physiological reference subjects; it does not provide resuscitation instructions or device settings.

Core questions

• How is liquid cleared from the airways so that air can enter the lung?
• How is a functional residual capacity created and then maintained between breaths?
• Why does lung aeration also drive the cardiovascular transition?

Key concepts

• Fetal breathing movements
• Fetal lung liquid
• Trans-airway pressure gradients during inspiration
• Airway liquid clearance
• Functional residual capacity
• Coupling of aeration to pulmonary blood flow
• Three-phase model of respiratory transition

Mechanisms

Before birth the lung is filled with secreted liquid and the fetus makes intermittent breathing movements without gas exchange. At birth the airways must be cleared of this liquid for air to enter; inspiratory efforts generate pressure gradients that move liquid distally across the airway wall into the surrounding tissue, after which it is gradually reabsorbed. As the distal airspaces aerate, a functional residual capacity is established that keeps the lung partly inflated between breaths and stabilises gas exchange. Imaging studies of the non-breathing and newly breathing lung describe this as a sequenced, three-phase process and show that liquid clearance and re-entry can occur breath by breath early in transition. Because aeration lowers pulmonary vascular resistance, the onset of breathing is tightly coupled to the rise in pulmonary blood flow that drives the cardiovascular transition.

Clinical relevance

Knowing how aeration and functional residual capacity are normally established provides the reference for understanding delayed clearance of lung fluid and impaired aeration, and it explains why support of the transition focuses on lung aeration. This is descriptive physiology for educational orientation and not a basis for managing an individual newborn's breathing.

Evidence & guidelines

The mechanism of lung aeration and functional residual capacity is drawn from integrative reviews and from phase-contrast imaging studies of the transitioning lung; delivery-room respiratory support practices are governed by resuscitation guidelines that fall outside this physiology entry.

History

Classic physiology established that the fetal lung is liquid-filled and makes breathing movements without gas exchange, and that this liquid must be removed for the lung to aerate. More recent phase-contrast imaging of newborn animals refined this picture into a sequenced view of airway liquid clearance and functional residual capacity establishment, and linked the onset of aeration to the cardiovascular changeover.

Debates

What is the dominant mechanism of fetal lung liquid clearance at birth?

Earlier accounts emphasised labour-related reabsorption of liquid through the airway epithelium, whereas imaging studies emphasise the role of inspiratory trans-airway pressure gradients in moving liquid across the airway wall during the first breaths; the relative contribution of each is still discussed.

Key figures

• Stuart Hooper
• Arjan te Pas
• Marcus Kitchen
• Alan Jobe

Seminal works

• hooper-2016-resp
• hooper-2013-frc
• hillman-2012

Frequently asked questions

What happens to the fluid that fills the fetal lung?

It must be cleared from the airways for air to enter; inspiratory pressure gradients move the liquid across the airway wall into the surrounding tissue, where it is reabsorbed as the lung aerates.

What is functional residual capacity and why does it matter at birth?

Functional residual capacity is the volume of air that remains in the lung at the end of a breath; establishing it keeps the airspaces partly inflated between breaths so that gas exchange can be continuous rather than collapsing with each expiration.