Function of the larynx in the fetus and newborn.

The muscles of the larynx function as a part of the respiratory system before birth, and like other respiratory muscles, have experienced considerable use by the moment of birth. In late fetal life the larynx appears to influence the outward flow of pulmonary liquid and thus may play a role in lung development. Immediately after birth and in cases of neonatal lung disease, elevated pressures within the airways during expiration, probably a result of laryngeal adduction, are involved in the maintenance of FRC. This mechanism is also present, to a lesser degree, in normal ovine (and probably human) neonates during quiet sleep. Whether it exists in other species remains to be established. Expiratory resistance of the larynx is under vagal control, and pulmonary stretch receptors are the likely sensors. Species differences apparently exist in the means by which expiratory airflow is retarded. These may be due in part, however, to the widespread use of anesthetic agents that selectively depress the activity of laryngeal adductor muscles. There is clearly a need for wider use of techniques involving chronic instrumentation, particularly in the neonatal period. Because the upper airway is involved in the regulation of tidal airflow, it also seems vital that the airway remains intact wherever possible. In addition to controlling airflow, the larynx is an important sensory organ, protecting the lower airways from invasion by potentially harmful substances, e.g. during suckle feeding and regurgitation. In the neonate, laryngeal stimulation may result in prolonged respiratory arrest. Although there is some evidence that longer apnea can be elicited in the neonate than in the adult, the use of anesthesia, which may more strongly depress respiration in the young, complicates the issue. As yet, there are no firm grounds for explaining these findings, at either a peripheral or central level. Defensive mechanisms, including arousal, swallowing, and circulatory changes to cope with hypoxemia, are well established at birth. The healthy neonate would seem well equipped to survive entry of liquids into the larynx. However, it is not inconceivable that, under certain circumstances and in the absence of anesthesia, substances entering the larynx could trigger prolonged apnea or cardiac arrest.