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).
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
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 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.