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Pediatric Airway & Respiratory physiology 1

PEDIATRIC AIRWAY & RESPIRATORY PHYSIOLOGYS. Kache, MD

The respiratory mechanism of the pediatric patient varies from the adult in bothanatomy and physiology. As children grow, the airway enlarges and moves more

caudally as the c-spine elongates. The pediatric airway overall has poorlydeveloped cartilaginous integrity allowing for more laxity throughout the airway.Another important distinction is the narrowest point in the airway in adults is atthe cords versus below the cords for children. Some of the important anatomicdifferences 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 Angles posteriorly away from glottis Straight up and down

Narrowest Point Sub-glottic regionAt level of Vocalcords

Lung Volume 250ml at birth 6000 ml as adult

An important aspect of the narrow airway in children is that resistance issignificantly 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 thefourth power. Therefore, a small amount of post-extubation sub-glottic edemawill significantly increase the work of breathing for an infant.

Children also have a smaller forced residual capacity (FRC) defined as theresidual volume plus the expiratory reserve volume. Physiologically, FRC occurswhen the outward pull of the chest wall equals the inward collapse of the lungs.

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Pediatric Airway & Respiratory physiology 2

FRC essentially acts as a respiratory reserve. When patients begin to developrespiratory distress, an increased FRC equates to a longer period of time prior torespiratory failure. The reduced FRC is important in two particularcircumstances. First, it can be decreased by up to 30% in a supine patient ascompared 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 inpediatric patients because of a compliant chest wall, small thoracic cage, andlarge abdominal contents. Second, while pre-oxygenating a patient prior tointubation the reduced FRC decreases the amount of time allowed to establish anendotracheal tube prior to desaturation.

There are also many physiologic differences in respiratory mechanisms betweenchildren and adults. Children have a more complaint trachea, larynx, andbronchi due to poor cartilaginous integrity. This in turn allows for dynamicairway compression, i.e. a greater negative inspiratory force sucks in the floppyairway and decreases airway diameter. This in turn increases the work ofbreathing by increasing the negative inspiratory pressure generated. A viciouscycle 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 increasingthe work of breathing i.e. the outward pull of the chest is greater.

Infants are dependent on functional diaphragms for adequate ventilation. Theaccessory muscles contribute less to the overall work of breathing in infants ascompared to older children and adults. Therefore, a non-functional diaphragm

often leads to respiratory failure. Diaphragmatic fatigue is one amongst severalpotential causes of respiratory failure and apnea in young patients with RSVbronchilitis.

Finally, the respiratory muscles themselves have a significant oxygen andmetabolite requirement in children. In pediatric patients the work of breathingcan account for up to 40% of the cardiac output, particularly in stressedconditions.

This cursory discussion of the pediatric respiratory anatomy and physiologyallows one to appreciate the significant differences between children and adults.

Therefore, the child with respiratory distress / failure should be approached andtreated with urgency, vigilance, and caution.