PEDIATRIC AIRWAY & RESPIRATORY PHYSIOLOGY
S. Kache, MD
The respiratory mechanism of the pediatric patient varies from the adult in both
anatomy and physiology. As children grow, the airway enlarges and moves more
caudally as the c-spine elongates. The pediatric airway overall has poorly
developed cartilaginous integrity allowing for more laxity throughout the airway.
Another important distinction is the narrowest point in the airway in adults is at
the cords versus below the cords for children. Some of the important anatomic
differences are listed below.
Anatomy
PEDIATRIC
ADULT
Tongue
Large
Normal
Eiglottis Shape
Floppy, omega shaped
Firm, flatter
Epiglottis Level
Level of C3 - C4
Level of C5 - C6
Trachea
Smaller, shorter
Wider, longer
Larynx Shape
Funnel shaped
Column
Larynx Position
Narrowest Point
Angles posteriorly away from glottis Straight up and down
At level of Vocal
Sub-glottic region
cords
Lung Volume
250ml at birth
6000 ml as adult
An important aspect of the narrow airway in children is that resistance is
significantly increased. The formula to consider is
R ~ 8l / r4
R – resistance, l – length, r – radius
Small changes in the airway radius will therefore increase the resistance to the
fourth power. Therefore, a small amount of post-extubation sub-glottic edema
will significantly increase the work of breathing for an infant.
Children also have a smaller forced residual capacity (FRC) defined as the
residual volume plus the expiratory reserve volume. Physiologically, FRC occurs
when the outward pull of the chest wall equals the inward collapse of the lungs.
Pediatric Airway & Respiratory physiology
1
FRC essentially acts as a respiratory reserve. When patients begin to develop
respiratory distress, an increased FRC equates to a longer period of time prior to
respiratory failure. The reduced FRC is important in two particular
circumstances. First, it can be decreased by up to 30% in a supine patient as
compared to a sitting patient. As the abdominal contents push up on the
diaphragm in a supine patient, the FRC is affected. This situation is amplified in
pediatric patients because of a compliant chest wall, small thoracic cage, and
large abdominal contents. Second, while pre-oxygenating a patient prior to
intubation the reduced FRC decreases the amount of time allowed to establish an
endotracheal tube prior to desaturation.
There are also many physiologic differences in respiratory mechanisms between
children and adults. Children have a more complaint trachea, larynx, and
bronchi due to poor cartilaginous integrity. This in turn allows for dynamic
airway compression, i.e. a greater negative inspiratory force “sucks in” the floppy
airway and decreases airway diameter. This in turn increases the work of
breathing by increasing the negative inspiratory pressure generated. A vicious
cycle is created which may eventually lead to respiratory failure: subglottic
stenosis ⇒ ⇑ negative inspiratory force ⇒ airway collapse ⇒ ⇑ subglottic
stenosis ⇒ ⇑ negative inspiratory force ⇒ ⇑ work of breathing ⇒⇒ respiratory
failure. Pediatric patients also have more compliant chest walls also increasing
the work of breathing – i.e. the outward pull of the chest is greater.
Infants are dependent on functional diaphragms for adequate ventilation. The
accessory muscles contribute less to the overall work of breathing in infants as
compared to older children and adults. Therefore, a non-functional diaphragm
often leads to respiratory failure. Diaphragmatic fatigue is one amongst several
potential causes of respiratory failure and apnea in young patients with RSV
bronchilitis.
Finally, the respiratory muscles themselves have a significant oxygen and
metabolite requirement in children. In pediatric patients the work of breathing
can account for up to 40% of the cardiac output, particularly in stressed
conditions.
This cursory discussion of the pediatric respiratory anatomy and physiology
allows one to appreciate the significant differences between children and adults.
Therefore, the child with respiratory distress / failure should be approached and
treated with urgency, vigilance, and caution.
Pediatric Airway & Respiratory physiology
2