The respiratory system functions primarily to oxygenate the tissues and eliminate carbon dioxide and secondarily to provide immunologic defense and acid–base balance. Control of gas exchange is maintained through a well-coordinated interaction of the respiratory system, the central and peripheral nervous systems, the diaphragm, the chest wall, and the circulatory system.
The respiratory system can be divided into the upper airway which includes the nose, nasopharynx, oropharynx, larynx, trachea, and bronchi and the lower airway consisting of bronchioles, alveoli, and interstitium. Pathology anywhere along this anatomic pathway can produce respiratory distress. For instance, airway obstruction secondary to croup or a foreign body in the larynx produces respiratory distress originating from the upper airway, whereas pulmonary edema, fibrosis, or pneumonia produces respiratory distress originating from the lower airway.
Central nervous system (CNS) control of respiration lies in the respiratory centers of the medulla oblongata. Central chemoreceptors in the medulla respond to CSF pH changes. Peripheral chemoreceptors, located in the aortic and carotid bodies, send afferent signals via the vagus and the glossopharyngeal nerves regarding changes in oxygen, carbon dioxide, and pH in the arterial blood. Disruption of the CNS control of respiration, such as in hydrocephalus or CNS immaturity in the case of premature infants, can produce respiratory distress. The peripheral nervous system provides innervation to the muscles of respiration and can be disrupted in diseases of the peripheral motor nerve, neuromuscular junction, or the muscle itself.
The diaphragm is the principal muscle of inspiration, whereas the intercostal muscles help to lower the ribs. The accessory muscles, such as the sternocleidomastoid, come into play when respiratory effort is increased. In infants, the chest wall is more compliant than in adults so that during inspiration the lower ribs descend rather than elevate. This provides for less-efficient expansion of the lungs, meaning that the diaphragm needs to do more work in children than in adults.1 This predisposes infants to more rapidly progressive and severe respiratory distress.
Oxygen and carbon dioxide exchange at the alveolo-capillary membrane depends on adequate ventilation and perfusion (V/Q) matching. Any process that compromises the delivery of oxygen to the alveoli or blood to the capillaries will cause a V/Q mismatch and lead to respiratory distress. Increased metabolic demands, such as in exercise or illness, can also produce respiratory distress, as can states that affect the blood's ability to deliver oxygen to the tissues, such as anemia or abnormal hemoglobin states (methemoglobinemia or carboxyhemoglobinemia). Decreased blood flow to the lungs secondary to poor cardiac output or shock can also cause respiratory distress.