The Pulmonary System

The pulmonary system is not capable of sustaining life until both the pulmonary airways and the vascular system have matured sufficiently to allow exchange of oxygen from "air" to the bloodstream across the pulmonary alveolar/vascular bed. Independent life is not possible until gestational age is 24 to 26 weeks. Alveoli increase in number and size until the child is approximately 8 years old. ^[13] Further growth is manifested as an increase in size of the alveoli and airways. At term, full complete development of surface-active proteins helps maintain patency of the airways. If a child is born prematurely and these proteins are insufficient, respiratory failure (respiratory distress syndrome) may follow.

Several anatomic differences make respiration less efficient for infants. The small diameter of the airways increases resistance to airflow; resistance is inversely proportional to the radius raised to the fourth power. The airway of an infant is highly compliant and poorly supported by surrounding structures. The chest wall is also highly compliant, so the ribs provide little support for the lungs; that is, negative intrathoracic pressure is poorly maintained. Thus, each breath is accompanied by functional airway closure.^[14] Dead-space ventilation is proportionally similar to that in adults; however, oxygen consumption is two to three times as high as in adults. In premature infants, the work of breathing is approximately three times that of adults, and this work can be increased significantly by cold stress (increased metabolic demand for oxygen) or some degree of airway obstruction. These differences partially explain the high respiratory

Figure 60-2 The composition of the diaphragm and intercostal muscles changes markedly during the first 2 years of life. The number of type I muscle fibers is inversely related to age and may account in part for the ease of inducing respiratory fatigue as the work of breathing increases. (Data from Keens TG, Bryan AC, Levison H, et al: Developmental pattern of muscle fiber types in human ventilatory muscles. J Appl Physiol 44:909–913, 1978.)

Another important factor is the composition of diaphragmatic and intercostal muscles. These muscles do not achieve the adult configuration of type I muscle fibers until the child is approximately 2 years old ( Fig. 60-2 ).^[16] Because type I muscle fibers provide the ability to perform repeated exercise, any factor that increases the work of breathing contributes to early fatigue of the respiratory muscles of infants. Fatigue, in turn, leads to apnea or carbon dioxide retention and respiratory failure.

Differences in airway anatomy make the potential for technical airway difficulties greater in infants than in teenagers or adults. The airway of infants differs in five ways^{[17] [18]} : (1) the relatively large size of the infant's tongue in relation to the oropharynx increases the likelihood of airway obstruction and technical difficulties during laryngoscopy; (2) the larynx is located higher in the neck, thus making straight blades more useful than curved blades; (3) the epiglottis is shaped differently, being short and stubby, and is angled over the laryngeal inlet; control with the laryngoscope blade is therefore more difficult; (4) the vocal cords are angled, so a "blindly" passed endotracheal tube may easily lodge in the anterior commissure rather than slide into the trachea; and (5) the infant larynx is funnel shaped, the narrowest portion occurring at the cricoid cartilage ( Fig. 60-3 ). In adults, an endotracheal tube that passes the vocal cords will readily pass into the trachea because the glottic opening is the narrowest portion of the larynx. In infants or young children, an endotracheal tube that easily passes the vocal cords may be tight in the subglottic region because of narrowing at the cricoid cartilage. For this reason, uncuffed endotracheal tubes are usually preferred for patients younger than 6 years.

Infants have often been described as obligate nasal breathers; however, approximately 8% of premature

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Figure 60-3 The narrower part of the adult larynx (cylindric shape (A) occurs at the glottic opening, whereas the narrowest part of the infant larynx (funnel shaped) (B) occurs at the level of the cricoid cartilage. The normal adult configuration of the larynx is not achieved until the teenage years. This anatomic difference is one of the reasons why uncuffed endotracheal tubes are preferred for children younger than 6 years. A, anterior; Cricoid, cricoid cartilage; P, posterior. (Redrawn from Coté CJ, Todres ID: The pediatric airway. In Coté CJ, Ryan JF, Todres ID, et al [eds]: A Practice of Anesthesia for Infants and Children. Philadelphia, WB Saunders, 1992, p 55.)