RESPIRATORY SYSTEM PHYSIOLOGY IN CHILDHOOD
In addition to the anatomical differences that we have described so far (Table 1), the respiratory system is also characterized by physiological peculiarities in childhood. First, since birth and up to school age, children have a higher metabolic rate, with a rate of oxygen consumption at rest that is more than double that of adults (7-9 mL/kg/min vs 3 mL/kg/min) (1). Moreover, in the case of hypoxia, oxygen release decreases even more due to bradycardia caused by activation of the parasympathetic nervous system, whose activity is dominant in newborns and infants. Because children have a mostly stable TV, the only way to guarantee the required oxygen supply at rest is through a higher respiratory rate (RR) (77): normal paediatric RR values during rest were described in evidence-based percentile charts by Fleming et al. in 2011, demonstrating how RR declines from birth to adolescence, with the steepest fall at 2 years of age (78). Considering that TV cannot be increased considerably and that minute ventilation (MV) is equal to RR multiplied by TV, when experiencing respiratory distress, infants must increase their RR to increase MV (79). However, when RR increases too much, TV starts to decrease (28). Notably, TV in infants is approximately 15 mL, while it is approximately 500 mL in adults (33), and MV is estimated to be 500 mL/min in infants and approximately 6 L/min in adults. All lung volumes are obviously reduced in children, and the difference is particularly noticeable in functional residual capacity (FRC), which is approximately 80 mL in newborns and 3,000 mL in adults (35,49). FRC is defined as the static passive balance between the inward collapse of the lungs and the outward pull of the chest wall: in children, this balance is achieved at a very small FRC because the outward pull of the chest is small due to the compliance of the rib cage (35). FRC can be functionally considered a respiratory reserve (70): in infants, a reduced FRC results in less air available in the lungs for gas exchange, especially during respiratory distress or when lying supine (i.e., when the lungs are compressed by ascended abdominal contents). In newborns and infants, FRC decreases during deep sleep and sedate states more than in adults because the chest wall is more compliant and muscle tone decreases more easily, so that during the period of apnoea, they undergo desaturation more rapidly. (28). However, in the first 6-12 months of life, infants can dynamically increase their FRC by actively modulating their expiration flow via post-inspiratory activity of the diaphragm (48,80) and laryngeal narrowing during expiration (81). Finally, it is important to remember that airway resistance is described by Poiselle’s Law: R = 8ηL/πr4: as shown in this equation, resistance (R) to flow is inversely proportional to the radius (r) of the airway raised to the fourth power (in the equation, η is the coefficient of viscosity of the air, L is the length of the airway, and V is the airflow volume). This means that a small narrowing (for example, due to oedema) results in significant obstruction in children with a large impact on airway resistance (Figure 6). Moreover, although the airway diameter decreases from the trachea to the terminal airways, there is a gradual increase in the cross-sectional area due to the increase in the number of airways, so peripheral resistance is notably less significant (47). Last but not least, even the cough reflex undergoes a process of maturation in the very first months of life; unsurprisingly, some studies have shown that newborns and infants respond to external stimuli in the pharynx, such as the introduction of water or saline, mainly with swallowing, apnoea and laryngeal closure (82).
CONCLUSIONS The paediatric airways are far from being just a smaller version of adult airways since they have many anatomical and physiological peculiarities that explain many symptoms and disease conditions in childhood. Some of these factors may contribute to the rapid development of respiratory distress in childhood (Table 2). These factors should be well known and kept in mind when managing acute and/or chronic airway disease and/or respiratory distress in infants and preschool-age children. Knowing the peculiarities of paediatric airways is helpful in the prevention, management and treatment of acute and chronic diseases of the respiratory tract.
ACKNOWLEDGEMENTS: None
CONFLICTS OF INTEREST: The authors have no conflicts of interest to declare.