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The larynx plays a crucial role in respiratory control and lung development in neonates. Its functions, including expiratory resistance and airway protection, are vital for newborn survival, though anesthesia complicates research.
Area of Science:
- Neonatal physiology
- Respiratory system development
- Laryngeal function
Background:
- The larynx is integral to the respiratory system, functioning prenatally and postnatally.
- Laryngeal adduction influences lung development and maintains functional residual capacity (FRC) in neonates, particularly during respiratory distress.
Purpose of the Study:
- To investigate the role of the larynx in neonatal respiratory control and lung development.
- To explore the mechanisms of expiratory airflow regulation by the larynx and its sensory functions.
- To understand the implications of laryngeal stimulation and potential risks in neonates.
Main Methods:
- Analysis of laryngeal function in late fetal and neonatal periods.
- Examination of vagal control and pulmonary stretch receptors in expiratory resistance.
- Consideration of species differences and the impact of anesthetic agents on laryngeal adductor muscle activity.
Main Results:
- The larynx influences pulmonary liquid outflow, potentially aiding lung development.
- Elevated expiratory pressures, mediated by laryngeal adduction, support FRC in neonates, a mechanism also observed in quiet sleep.
- Laryngeal stimulation can induce respiratory arrest in neonates, with anesthesia complicating interpretation.
Conclusions:
- The larynx is vital for neonatal respiratory regulation, airway protection, and lung development.
- Further research using chronic instrumentation in non-anesthetized neonates is needed to clarify laryngeal mechanisms.
- While neonates possess defensive mechanisms, laryngeal entry of substances could, under specific conditions, trigger apnea or cardiac arrest.
Abstract:
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.