PNEUMOTHORAX
Pneumothorax is a collection of air within the pleural space; after trauma, pneumothorax is most commonly caused by a rib fracture tearing the visceral pleura of the lung, allowing air to escape during inspiration. Penetrating injuries (e.g., stab wounds, gunshot wounds) also frequently produce a pneumothorax via this mechanism. In these cases of penetrating trauma, 80% of patients will also have blood in the pleural space. Pneumothorax is usually identified on chest radiographs, although it may also be seen during chest or abdominal computed tomography scanning or during ultrasound examination of the abdomen after trauma (focal assessment with sonography for trauma [FAST] examination).
Other causes of traumatic pneumothorax include
inadvertant puncture of the lung during central venous access or thoracentesis.
The lung can also be ruptured by excessive positive airway pressure during
mechanical ventilation, termed barotrauma. Spontaneous pneumo- thorax is
usually caused by a ruptured bleb that is often precipitated by coughing.
Irrespective of the cause, when the pleural pressure exceeds the normal
subatmospheric pressure, the elastic recoil of the lung results in partial
collapse. If air continues to flow into the pleural space, the lung collapses
entirely and can no longer serve to exchange oxygen (O2) and carbon
dioxide (CO2). A one-way valve typically occurs on the lung surface,
and air is forced into the pleural space with each breath, which progressively
increases the intrapleural pressure and may result in escape of air into the
subcutaneous tissues, manifesting as diffuse upper torso swelling and palpable
crepitus. Ultimately, if the intrapleural pressure continues to increase, a tension
pneumothorax develops. This condition may occur rapidly when the patient is
ventilated mechanically, increasing the airway pressure. Eventually, the
pressure within the pleural cavity can shift the mediastinum and impede blood
return to the right heart. Thus, clinical manifestations of tension pneumothorax
reflect progressive impairment of pulmonary and myocardial function.
Patients with a tension pneumothorax become dyspneic
or hypoxic if ventilated mechanically, with cyanosis and distended neck veins.
Hyperresonance and lack of breath sounds on the involved side of the thorax
cement the diagnosis without the need for radiographic confirmation.
Electrocardiographic changes include (1) rightward shift in the QRS axis, (2)
diminution in the QRS amplitude, and (3) inversion of precordial T waves. Tension
pneumothorax is a life-threatening emergency, and the air must be urgently released from
the pleural cavity. If it is clinically suspected in a patient who is unstable,
immediate treatment is indicated without any further diagnostic tests. In an
intubated patient in the prehospital setting, air can be vented with a
large-bore needle via the anterior second intercostal space in the
midclavicular line. Subsequent definitive treatment with tube thoracostomy
should follow. In the hospital, a tube thoracostomy is usually done via the
fifth intercostal space at the anterior axillary line. Under these dire
circumstances, the tube should be placed expeditiously using primarily a
scalpel and scissors. After a limited chest wall preparation and local
anesthesia, a 2-cm incision should be made into the intercostal space and the
chest entered directly using heavy scissors. The tube should then be directed
into the posterior sulcus to optimize subsequent drainage of blood or other
pleural fluid.
Alternatively, air can accumulate within the pleural
space because of an external wound that violates the parietal pleural, exposing
it to the atmosphere. This form of pneumothorax is usually self-limited because
the skin edges and adjacent chest wall soft tissue seal the opening. The
notable exception is open chest wounds, in which the chest wall defect is
sufficiently large to remain open, permitting air to move freely in both
directions. Open pneumothoraces are usually caused by high-energy gunshot
wounds (e.g., close-range shotgun wounds) or impalement during motor vehicle
crashes. An open pneumothorax is often referred to as a sucking chest wound because
of the sound made as a relatively large volume of air moves through the defect
with respiratory effort. The lung on the involved side collapses upon exposure
to atmospheric pressure, rendering it nonfunctional. Additionally, because air
passes more easily into the chest on inspiration than it exits during
expiration, an element of tension pneumothorax with mediastinal shift occurs. Ultimately,
this impedes blood return to the heart, leading to clinical signs of cardiac as
well as pulmonary dysfunction.
Prehospital management of an open pneumothorax is a
partially occlusive dressing in which one corner of the bandage is free to
permit escape of pleural air under pressure. In the hospital, treatment
consists of applying a completely occlusive dressing, usually of petroleum gauze, followed by
standard tube thoracostomy. Although a slash wound may occasionally be managed
definitively in the emergency department, most patients with an open
pneumothorax warrant prompt operative care for associated visceral injury as
well as chest wall reconstruction. One approach to extension chest wall defects
is cephalad transposition of the
diaphragm.
