This introduction will highlight the key physiological, anatomical, and pharmacological concepts that novices in pediatric anesthesiology will find helpful for understanding current practice in this field.
Seven anatomical features distinguish the infant airway from the adult.
The tongue is large in relation to the oral cavity, predisposing infants to airway obstruction and challenging intubation. Infants are obligate nasal breathers until 3-5 months of life. Obstruction of the anterior and/or posterior nares (secondary to nasal congestion, stenosis, or choanal atresia) may cause asphyxia.
The larynx is positioned higher in the neck (C3-C4) than in adults (C5-C6), allowing for simultaneous nasal breathing and swallowing.
The larynx creates an acute angulation at the base of the tongue, creating the impression of an anterior larynx. Use of a straight laryngoscope blade to lift the base of the tongue and epiglottis, along with external laryngeal pressure, can aid in viewing the larynx during intubation.
The epiglottis is Ω-shaped and protrudes posteriorly over the larynx at a 45° angle; it may be difficult to lift during laryngoscopy.
The vocal cords attach anteriorly, which is more caudal and predisposes to catching the tip of the endotracheal tube in the anterior commissure during intubation.
The cricoid cartilage is conically shaped and is the narrowest portion of the upper airway (true for the first decade of life) (Fig. 1-1).
Precise endotracheal tube sizing is critical to avoid cricoid edema and postintubation croup. A pressure leak should be no greater than 18-20 cm H2O. Newer high-volume–low-pressure cuffed endotracheal tubes for infants avoid repeated laryngoscopies to determine the most appropriate endotracheal tube size.
Given that resistance to airflow is inversely proportional to radius to the fourth power, a 1-mm reduction in airway diameter increases resistance to airflow by 16-fold in the infant airway.
The tonsils and adenoids are small in the neonate but reach maximal size in the first 4-5 years of age. Use of continuous positive pressure and/or an oral airway will commonly overcome this obstruction.
The occiput is large. When the infant is placed on a flat surface, extreme neck flexion will cause airway obstruction. A small roll placed behind the baby’s shoulders will reduce neck flexion and aid in maintaining the airway.
Schematic of an adult (a) and infant (b) airway. A, Anterior; P, Posterior. [Reprinted from Cote CJ, Todres ID. The pediatric airway. In: Ryan JF, Todres ID, Cote CJ, et al, eds. A Practice of Anesthesia for Infants and Children. Philadelphia, PA: WB Saunders; 1986:35-58, with permission from Elsevier.]
PEDIATRIC RESPIRATORY PHYSIOLOGY
The alveolar bed is incompletely developed at birth; mature alveoli are seen at 5 weeks of age, with alveolar multiplication with adult morphology being reached by 8 years ...