Clinical overview
The first breath is the most dramatic physiological event of a human life. In utero the fetal lungs are not respiratory organs at all — they are fluid-filled, secretory, and bypassed by a circulation designed around the placenta. Within seconds to minutes of birth the newborn must clear that lung fluid, aerate hundreds of millions of alveoli for the first time, establish a functional residual capacity, drop pulmonary vascular resistance, and switch the entire circulation from a parallel (placental) to a series (pulmonary) arrangement. Most babies accomplish this themselves, driven by a tightly choreographed set of mechanical, chemical, thermal and sensory stimuli. The registrar's job is to understand which factors normally drive that transition, so that when a baby fails to breathe you can reason about why and intervene logically rather than by rote.
This is a high-yield, frequently-examined topic precisely because it underpins neonatal resuscitation. Roughly 5–10% of newborns need some help to start breathing and around 3–6% need positive-pressure ventilation; a much smaller fraction need chest compressions or adrenaline (standard teaching, ILCOR). Establishing aeration of the lungs is the single most important step in newborn resuscitation — and you cannot prioritise it correctly unless you know the normal physiology it is trying to recapitulate. This chapter lists and explains those factors; the practical sequence is developed in Neonatal transition and the formal drill in Neonatal resuscitation.
Core knowledge
The intrauterine baseline
Before birth the alveoli are filled with lung liquid actively secreted by the pulmonary epithelium (chloride-driven, ~20–30 mL/kg). Fetal breathing movements occur episodically — they are essential for lung growth and for conditioning the diaphragm and respiratory muscles — but they do not exchange gas; oxygenation is entirely placental (see Contractions fetal oxygenation). Pulmonary blood flow is minimal because pulmonary vascular resistance (PVR) is very high in the hypoxic, fluid-filled, unexpanded lung, so most right-ventricular output is diverted across the ductus arteriosus to the systemic circulation, and oxygenated placental blood is shunted right-to-left across the foramen ovale.
For air breathing to begin, four things must happen, broadly in parallel: lung liquid must be cleared, the lungs must be inflated and a functional residual capacity (FRC) established, PVR must fall so pulmonary blood flow rises, and respiratory drive must switch from episodic to continuous.
Factors that trigger and sustain the first breaths
Figure L2.1 — Chemical, thermal, tactile, mechanical and cord-related stimuli acting on the substrates that let fluid-filled fetal lungs establish FRC and continuous breathing.
A useful way to "list the factors" for the exam is to group them into chemical, thermal, mechanical/tactile, and clamping-related stimuli, then add the structural/biochemical prerequisites that make breathing possible.
- Chemical (the dominant central drivers). The transient asphyxia of normal birth — interruption of placental gas exchange during labour and at cord clamping — produces a rise in PaCO₂ (hypercapnia), a fall in PaO₂ (hypoxia/relative hypoxaemia) and a fall in pH (acidaemia). These stimulate central and peripheral chemoreceptors and are classically regarded as the principal stimulus to the first breath. Importantly, profound hypoxia does the opposite — it depresses the respiratory centre (primary then secondary/terminal apnoea), which is why a severely asphyxiated baby is floppy and apnoeic rather than gasping.
- Thermal. The abrupt fall in ambient temperature on delivery (from ~37 °C to a much cooler delivery room) stimulates skin thermoreceptors, particularly on the face, providing a powerful afferent stimulus to breathe. This is one reason newborns gasp on exposure — and a reason hypothermia and overheating both blunt the response.
- Tactile and sensory. Handling, drying and the general somatosensory bombardment of being born (touch, light, sound, gravity, proprioception) stimulate breathing. Drying and gentle stimulation of the trunk/back/soles is the deliberate clinical use of this factor in resuscitation.
- Mechanical — chest recoil and the establishment of FRC. As the chest is delivered, recoil of the thorax after passage through the birth canal (and the elastic recoil after the first breaths) helps draw air in and expel some liquid. The first active inspirations generate large transpulmonary pressures (transiently very high, of the order of tens of cmH₂O) to overcome surface tension and the viscosity of residual liquid; subsequent breaths need far less pressure once an FRC is established.
- Cord clamping / loss of placental circulation. Removing the low-resistance placenta and the interruption of umbilical venous return raise systemic vascular resistance and contribute to the chemical stimulus (rising CO₂). The timing of clamping matters for the smoothness of the cardiovascular transition (see Management).