Anatomy And Physiology

Airway

Figure 2.3 Summary of significant upper airway anatomy

In all young children the epiglottis is horseshoe-shaped, and projects posteriorly at 45° making tracheal intubation more difficult. This, together with the fact that the larynx is high and anterior (at the level of the second and third cervical vertebrae in the infant, compared with the fifth and sixth vertebrae in the adult), means that it is easier to intubate an infant using a straight-blade laryngoscope. The cricoid ring is the narrowest part of the upper airway (as opposed to the larynx in an adult). The narrow cross-sectional area at this point, together with the fact that the cricoid ring is lined by pseudostratified ciliated epithelium loosely bound to areolar tissue, makes it particularly susceptible to oedema. As tracheal tube cuffs tend to lie at this level, uncuffed tubes are preferred in pre-pubertal children.

The trachea is short and soft. Over-extension of the neck may therefore cause tracheal compression. The short trachea and the symmetry of the carinal angles mean that, not only is tube displacement more likely, but also a tube or a foreign body is just as likely to be displaced into the left as the right main-stem bronchus.

Breathing

The lungs are relatively immature at birth. The air tissue interface has a relatively small total surface area in the infant (less than 3 m2). In addition, there is a tenfold increase in the number of small airways from birth to adulthood.

Both the upper and lower airways are relatively small, and are consequently more easily obstructed. As resistance to flow is inversely proportional to the fourth power of the airway radius (halving the radius increases the resistance sixteenfold), seemingly small obstructions can have significant effects on air entry in children.

Infants rely mainly on diaphragmatic breathing. Their muscles are more likely to fatigue, as they have fewer type I (slow twitch, highly oxidative, fatigue-resistant) fibres compared with adults. Pre-term infants' muscles have even less type I fibres. These children are consequently more prone to respiratory failure.

The ribs lie more horizontally in infants, and therefore contribute less to chest expansion. In the injured child, the compliant chest wall may allow serious parenchymal injuries to occur without necessarily incurring rib fractures. For multiple rib fractures to occur the force must be very large; the parenchymal injury that results is consequently very severe and flail chest is tolerated badly.

Circulation

At birth the two cardiac ventricles are of similar weight; by 2 months of age the RV/LV weight ratio is 0-5.These changes are reflected in the infant's ECG. During the first months of life the right ventricle (RV) dominance is apparent, but by 4-6 months of age the left ventricle (LV) is dominant. As the heart develops during childhood, the sizes of the P wave and QRS complex increase, and the P-R interval and QRS duration become longer.

The child's circulating blood volume is higher per kilogram body weight (70-80 ml/kg) than that of an adult, but the actual volume is small. This means that in infants and small children relatively small absolute amounts of blood loss can be critically important.

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