Apnea
Robert A. Herzlinger
Apnea of infancy is defined as an unexplained episode of cessation of breathing for 20 seconds or longer, or a shorter respiratory pause associated with bradycardia, cyanosis, pallor, and/or marked hypotonia. Apnea of infancy applies to infants at or beyond 37 weeks’ gestation at the onset of apnea. Apnea of prematurity is defined as a sudden cessation of breathing for 20 seconds or longer, or a shorter respiratory pause accompanied by bradycardia or oxygen desaturation in an infant younger than 37 weeks’ gestational age, without other identifiable causes. The incidence of apnea of prematurity increases
with decreasing gestational age. Apnea occurs in approximately 7% of infants born at 34 to 35 weeks’ gestation, 14% of infants born at 32 to 33 weeks’ gestation, 50% at 30 to 31 weeks’ gestation, and almost all infants of less than 28 weeks’ gestation. Apnea decreases with increasing postconceptional age, and usually resolves by 35 weeks’ postconceptional age; however, extremely premature infants born at gestational ages of between 24 to 28 weeks may have persistent apnea requiring prolonged hospitalization beyond 40 weeks of postconceptional age.
with decreasing gestational age. Apnea occurs in approximately 7% of infants born at 34 to 35 weeks’ gestation, 14% of infants born at 32 to 33 weeks’ gestation, 50% at 30 to 31 weeks’ gestation, and almost all infants of less than 28 weeks’ gestation. Apnea decreases with increasing postconceptional age, and usually resolves by 35 weeks’ postconceptional age; however, extremely premature infants born at gestational ages of between 24 to 28 weeks may have persistent apnea requiring prolonged hospitalization beyond 40 weeks of postconceptional age.
EPIDEMIOLOGY
Apnea is categorized into three types. Central apnea, which accounts for approximately 40% of episodes, is characterized by an absence of both chest wall movement and nasal air flow; obstructive apnea, accounting for 10% of spells, presents with chest wall movement without nasal air flow; and mixed apnea, accounting for 50% of spells, has both obstructive and central components. The location of the obstruction in obstructive and mixed apnea is usually at the level of the pharynx. Periodic breathing is another manifestation of immature ventilatory control, and is characterized by regular, recurring cycles of breathing of 10 to 15 seconds duration interrupted by pauses of at least 3 seconds in duration. The frequency of periodic breathing decreases with increasing postconceptional age. Periodic breathing is considered to be a normal breathing pattern in preterm and term infants.
PATHOPHYSIOLOGY
The most common cause of apnea during the newborn period is apnea of prematurity, which is attributed to the immaturity of the ventilatory control mechanism. Anatomic correlates of impaired control include decreased synaptic connections, dendritic arborization, and neural myelination. Delayed brainstem auditory evoked responses have been noted in premature infants with apnea, when compared with controls. Chemoreceptor function also is impaired, resulting in a blunted ventilatory response to hypercarbia and hypoxemia. The functional laryngeal obstruction associated with mixed and obstructive apnea has been attributed to discoordination of brainstem control of pharyngeal patency and diaphragmatic contractions. Inhibitory respiratory reflexes and inhibitory neurotransmitters may be more active at an earlier postconceptional age, which may also predispose to apnea. The causes of apnea are outlined in Box 49.1.
The physiologic consequences of severe apnea include hypoxemia, hypercarbia, reflex-induced bradycardia, hypotension, and a decrease in cerebral blood flow. In the face of an immature and unstable ventilatory control mechanism, a wide variety of conditions can induce apnea in premature infants. Full-term infants also may develop apnea as well, as a consequence of these underlying disorders. In contrast to premature infants, a specific etiology is more likely to be identified in the term or near-term newborn with apnea.
The relationship between apnea of prematurity and gastroesophageal reflux remains controversial. Studies have failed to demonstrate a causal relationship between reflux episodes and apnea. In addition, no clear evidence suggests that the pharmacologic treatment of reflux decreases apnea in premature infants. Therefore, it appears that gastroesophageal reflux is rarely the cause for most apneic spells in premature newborns; however, discoordination of suck, swallow, and breathing may present with feeding-associated apnea in premature infants. These episodes also resolve with increasing postconceptional age.
BOX 49.1 Causes of Apnea
Apnea of Prematurity
Central nervous system disorders
Intraventricular/periventricular hemorrhage
Subarachnoid hemorrhage
Infarction
Seizures
Structural anomalies
Central hypoventilation syndrome
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