Porth's Essentials of Pathophysiology, 4e

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Respiratory Function

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intercostal muscles of the chest wall. In the infant, the diaphragm inserts more horizontally than in the adult. As a result, contraction of the diaphragm tends to draw the lower ribs inward, especially if the infant is placed in the horizontal position. 54 The intercostal muscles, which normally lift the ribs during inspiration, are not fully developed in the infant. Instead, they function largely to stabilize the chest. Under circumstances such as crying, the intercostal muscles of the neonate function together with the diaphragm to splint the chest wall and prevent its collapse. The chest wall of the neonate is highly compliant. 54 A striking characteristic of neonatal breathing is the paradoxical inward movement of the upper chest during inspiration, especially during active sleep. Normally, the infant’s lungs also are compliant, which is advantageous to the infant with its compliant chest cage because it takes only small changes in inspiratory pressure to inflate a compliant lung. However, with respiratory disorders that decrease lung compliance, the diaphragm must generate more negative pressure; as a result, the compli- ant chest wall structures are sucked inward. Inspiratory retractions are abnormal inward movements of the chest wall during inspiration; they may occur intercostally (between the ribs), in the substernal or epigastric area, and in the supraclavicular spaces (Fig. 22-11). Airway Resistance Normal lung inflation requires uninterrupted movement of air through the extrathoracic airways (i.e., nose, pharynx, larynx, and upper trachea) and intrathoracic airways (i.e., bronchi and bronchioles). The neonate (0 to 4 weeks of age) breathes predominantly through the nose and does not adapt well to mouth breath- ing. Any obstruction of the nose or nasopharynx may increase upper airway resistance and increase the work of breathing. The airways of the infant and small child are much smaller than those of the adult. Because the resistance to airflow is inversely related to the fourth power of the radius (resistance = 1/radius 4 ), relatively small amounts of mucus secretion, edema, or airway constriction can produce marked changes in airway resistance and

airflow. Nasal flaring (enlargement of the nares) is a method that infants use to take in more air. This method of breathing increases the size of the nares and decreases the resistance of the small airways. Normally, the extrathoracic airways (i.e., those extend- ing from the nose to the thoracic inlet) in the infant nar- row during inspiration and widen during expiration, and the intrathoracic airways (i.e., those located within the thorax) widen during inspiration and narrow during expiration. 54 This occurs because the pressure inside the extrathoracic airways reflects the intrapleural pressures that are generated during breathing, whereas the pressure outside the airways is similar to atmospheric pressure. Thus, during inspiration, the pressure inside becomes more negative, causing the airways to narrow, and dur- ing expiration it becomes more positive, causing them to widen. In contrast to the extrathoracic airways, the pressure outside the intrathoracic airways is equal to the intrapleural pressure. These airways widen during inspi- ration as the surrounding intrapleural pressure becomes more negative and pulls them open, and they narrow dur- ing expiration as the surrounding pressure becomes more positive. 54,55 These changes are exaggerated in conditions that cause airway obstruction, particularly in infants with their softer and more compliant airways. LungVolumes and Gas Exchange The functional residual capacity, which is the air left in the lungs at the end of normal expiration, plays an important role in the infant’s gas exchange. In the infant, the functional residual capacity occurs at a higher lung volume than in the older child or adult. 53,55 This higher end-expiratory volume results from a more rapid respiratory rate, which leaves less time for expiration. However, the increased residual volume is important to the neonate for several reasons: (1) it holds the airways open throughout all phases of respiration, (2) it favors the reabsorption of intrapulmonary fluids, and (3) it maintains more uniform lung expansion and enhances gas exchange. During active sleep, the tone of the upper airway muscles is reduced, so that the time spent in expiration is shorter and the intercostal activity that stabilizes the chest wall is less. This results in a lower end-expiratory volume and less optimal gas exchange during active sleep. Control of Ventilation Fetal arterial oxygen pressures (PO 2 ) normally range from 25 to 30 mm Hg, and carbon dioxide pressures (PCO 2 ) range from 45 to 50 mm Hg, independent of any respiratory movements. Any decrease in oxygen levels induces quiet sleep in the fetus with subsequent cessa- tion of breathing movements, both of which lead to a decrease in oxygen consumption. At birth, switching to oxygen derived from the aerated lung causes an imme- diate increase in arterial PO 2 to approximately 50 mm Hg; within a few hours, it increases to approximately 70 mm Hg. 55 These levels, which greatly exceed fetal levels, cause the chemoreceptors that sense arterial PO 2 levels to become silent for several days. Although the

A B FIGURE 22-11. (A) Normal inspiratory appearance of the chest during unobstructed breathing in the neonate. (B) Sternal and intercostal retractions during obstructed breathing in the neonate.