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).