The diaphragm, a dome-shaped muscle located beneath the lungs, is the primary muscle responsible for respiration. Its contraction and relaxation drive breathing, allowing air to enter and exit the lungs. This muscular partition separates the chest cavity from the abdomen. For infants, the diaphragm’s function is particularly important, as their respiratory system is still developing, making them highly reliant on this single muscle for efficient breathing.
The Mechanics of Diaphragmatic Breathing
Breathing begins with the diaphragm contracting, causing it to flatten and move downward. This movement increases the chest cavity’s volume. As the chest cavity expands, pressure inside the lungs drops below atmospheric pressure, pulling air inward into the lungs. This process, known as inspiration, fills the lungs with oxygen-rich air for gas exchange.
Following inspiration, the diaphragm relaxes and returns to its original dome shape, moving upward into the chest cavity. This upward movement reduces the chest cavity’s volume, increasing pressure within the lungs above atmospheric pressure. The increased pressure then forces air out of the lungs, expelling carbon dioxide. This cycle of contraction and relaxation occurs rhythmically and often involuntarily.
Distinctive Features of the Infant Diaphragm
The infant diaphragm and chest wall possess unique characteristics. The infant rib cage exhibits higher compliance, meaning it is more flexible and less rigid compared to an adult’s. This increased flexibility makes it less effective at stabilizing lung volume, as a significant portion of the diaphragm’s generated mechanical energy can be dissipated in distorting the rib cage. Consequently, an infant’s chest wall may move inward paradoxically during inspiration as the diaphragm descends, reducing the efficiency of air intake.
The infant diaphragm has a more horizontal orientation compared to the adult diaphragm’s more dome-like shape. This flattened configuration can limit its mechanical advantage and the effectiveness of its contraction. The muscle fiber composition also differs; infants have fewer fatigue-resistant Type I muscle fibers. This higher proportion of fatigable muscle fibers makes the infant diaphragm more susceptible to fatigue, especially under increased respiratory demand.
Infants are highly reliant on their diaphragms as the primary respiratory muscle because their intercostal and other accessory muscles are less developed. The diaphragm also plays a role in maintaining functional residual capacity (FRC), the volume of air remaining in the lungs after a normal exhalation. Due to the highly compliant chest wall, infants often employ “expiratory braking” maneuvers to extend expiratory flow time and increase airway pressure, thereby helping to preserve FRC. This mechanism is important as the soft chest wall struggles to maintain a large intrathoracic volume on its own, tending to collapse to a lower volume.
Compensatory Breathing in Infants
When an infant’s diaphragm function is compromised or respiratory demand increases, their body attempts to compensate by recruiting accessory respiratory muscles. These muscles, including those in the neck and intercostal muscles between the ribs, are typically not used for quiet breathing but are activated to assist in drawing air into the lungs or expelling it. The use of these muscles indicates increased work of breathing and can be observed through specific signs.
Signs of compensatory breathing include retractions, where the skin pulls inward between the ribs (intercostal), under the rib cage (subcostal), or below the neck (suprasternal) with each breath. Nasal flaring, the widening of the nostrils during inhalation, is another common sign as the infant tries to reduce airway resistance and increase airflow.
Grunting, a short, audible sound during exhalation, often results from the infant attempting to keep air in the lungs by briefly closing the glottis, which helps maintain lung volume and prevent alveolar collapse. Head bobbing, where the head bobs forward with each breath, signifies the use of neck muscles to lift the chest and pull in more air. While these mechanisms temporarily aid breathing, they are less efficient than diaphragmatic breathing and can quickly lead to respiratory fatigue in infants.