CHEST WALL AND RESPIRATORY MUSCLES
The paediatric chest wall has unique features and undergoes significant
developmental changes during growth in terms of shape, compliance, and
deformability (48). In newborns and infants, the ribs have a typical
horizontal orientation, and the transverse chest section is almost
circular, rather than oval: when they adopt an upright posture,
gravitational forces gradually change the orientation of the ribs and
chest section, as clearly noticeable in chest X-rays and CT scans
(Figure 5) (70, 71). The horizontal orientation of the ribs makes it
challenging for younger children to elevate them as adults do with the
normal “bucket handle” effect to enlarge the rib cage and the thoracic
volume: this unique feature contributes to the fixity of tidal volume
(TV) during childhood so that ventilation is primarily diaphragmatic and
respiratory dynamics are less efficient (72, 73). Moreover, in children,
costae consist mainly of cartilage, which makes the rib cage highly
compliant, further reducing respiratory pump efficiency (70, 71). The
diaphragm also has a more horizontal position, being flatter than in
adults; therefore, its ability to contract is limited. External and
internal intercostal muscles are not well developed in children,
especially in infants. As a result, contraction of these muscles cannot
contribute to enlargement of the chest wall as in adults, and their
contribution to respiratory effort and tidal volume is minimal (73).
Instead, these muscles act primarily to stabilize the more compliant
chest wall, minimizing the inward displacement of the rib cage caused by
negative intrathoracic pressure produced by downward diaphragmatic
contraction (70). Moreover, in infancy, respiratory muscles are mainly
composed of type II fibres (fatigue-susceptible, due to lower stores of
glycogen and fat) since type I fibres (fatigue-resistant) develop later
in life (73, 74). Consequently, children have greater susceptibility to
ventilator muscle fatigue when the respiratory rate (RR) is increased,
and this is particularly true in preterm infants who have the lowest
percentage of type I fibres (74, 75). At the age of 2 years, the
diaphragm is composed of 55% type I fibres (74). Smooth muscle begins
to appear in the airways at 6-8 weeks of gestation, and its amount
continues to grow during childhood, with a progressive increase in
beta-adrenoreceptors, particularly noticeable in the first year of life
(34, 76).