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Chest wall disorders range from cosmetic to life-threatening and are not uncommon in pediatric medicine. Disease states that lead to diffuse muscle weakness eventually involve the respiratory muscles and often lead to chest wall disorders secondarily. Respiratory “success” can be thought of as a balance between pump (respiratory muscles) and load (the resistance and compliance of the respiratory system).

Anatomy and Physiology

The physiology of the chest wall changes dramatically during the first year of life. The chest wall of the newborn is very compliant and cannot oppose the tendency of the lung to collapse it inward. Infants use a combination of a rapid respiratory rate, premature termination of the expiratory phase, and laryngeal braking to maintain an adequate volume of air (functional residual capacity) within the thorax. Over the course of the first year of life, there is significant decrease in chest wall compliance, leading to a respiratory rate that gradually slows. As the child ages, the relationship of the diaphragm to the chest wall changes as well, leading to a longer portion of the diaphragm involved in the “zone of apposition” (this refers to the portion of diaphragm in direct contact with the inner surface of the thoracic cavity). This creates a more efficient “piston” as the diaphragm muscle fibers shorten and increase the intrathoracic volume.

The main muscles of inspiration are the diaphragm and external intercostal muscles. The diaphragm acts as a piston and increases intrathoracic volume by displacing the abdominal contents downward. Contraction of the external intercostals increases the anteroposterior diameter of the thorax and aids in inspiration. In infants whose chest wall is overly compliant, the action of the intercostal muscles also opposes the tendency of the chest wall to buckle inward with the downward displacement of the diaphragm on inspiration. Accessory muscles of inspiration include the sternocleidomastoids, which lift the sternum and increase intrathoracic volume by a “bucket handle” mechanism. Other accessory muscles of inspiration include the scalenes, the shoulder girdle muscles, and serratus anterior.

During normal tidal breathing, expiration is passive, as the elastic recoil of the lung and chest wall return to their resting position (in which the two opposing forces are in balance). The accessory muscles of expiration are used for coughing, sneezing, and speech. Contraction of the internal intercostal muscles decreases the anteroposterior diameter of the thorax, aiding in expiration. The abdominal muscles aid in forceful expiration (such as coughing) by increasing intra-abdominal pressure, which is transmitted through the diaphragm to the pleural space.

Physiological Assessment

Physiological assessment of the respiration muscles can be performed by using a handheld manometer to measure maximum inspiratory and expiratory pressures. The sniff nasal inspiratory pressure (SNIP) has also been used to assess inspiratory muscle strength noninvasively. Twitch transdiaphragmatic pressure is another modality to measure diaphragm strength and requires magnetic stimulation of the phrenic nerve.

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