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.