Pneumothorax - Tension
Tension pneumothorax is the progressive
build-up of air within the pleural space, usually due
to a lung laceration which allows air to escape into the
pleural space but not to return. Positive pressure ventilation
may exacerbate this 'one-way-valve' effect.
Progressive build-up of pressure in
the pleural space pushes the mediastinum to the opposite
hemithorax, and obstructs venous return to the heart.
This leads to circulatory instability and may result in
traumatic arrest. The classic signs of a tension pneumothorax
are deviation of the trachea away from the side with the
tension, a hyper-expanded chest, an increased percussion
note and a hyper-expanded chest that moves little with
respiration. The central venous pressure is usually raised,
but will be normal or low in hypovolaemic states.
However these classic signs are usually
absent and more commonly the patient is tachycardic and
tachypnoeic, and may be hypoxic. These signs are followed
by circulatory collapse with hypotension and subsequent
traumatic arrest with pulseless electrical activity (PEA).
Breath sounds and percussion note may be very difficult
to appreciate and misleading in the trauma room.
Tension pneumothorax may develop insidiously,
especially in patients with positive pressure ventilation.
This may happen immediately or some hours down the line.
An unexplained tachycardia, hypotension and rise in airway
pressure are strongly suggestive of a developing tension.
The X-ray on the right is a post-mortem
film taken in a patient with severe blunt trauma to the
chest and a left tension pneumothorax. It illustrates
the classic features of a tension:
- Deviation of the trachea away from
the side of the tension.
- Shift of the mediastinum
- Depression of the hemi-diaphragm
With this degree of tension pneumothorax,
it is not difficult to appreciate how cardiovascular function
may be compromised by the tension, due to obstruction
of venous return to the heart. This massive tension pneumothorax
should indeed have been detectable clinically and, in
the face of haemodynamic collapse, been treated with emergent
thoracostomy - needle or otherwise.
A tension pneumothorax may develop while
the patient is undergoing investigations, such as CT scanning
(image at right) or operation. Whenever there is deterioration
in the patient's oxygenation or ventilatory status, the
chest should be re-examined and tension pneumothorax excluded.
The presence of chest tubes does not
mean a patient cannot develop a tension pneumothorax.
The patient below had a right sided tension despite the
presence of a chest tube. It is easy to appreciate how
this may happen on the CT image showing the chest tubes
in the oblique fissure. Chest tubes here, or placed posteriorly,
will be blocked as the overlying lung is compressed backwards.
Chest tubes in supine trauma patients should be placed
anteriorly to avoid this complication. Haemothoraces will
still be drained provided the lung expands fully.
The CT scan also shows why the tension
is not visible on the plain chest X-ray - the lung is
compressed posteriorly but extends out to the edge of
the chest wall, so lung markings are seen throughout the
lung fields. However there is midline shift compared to
the previous film.
Upper thorax showing
position of chest tubes
Tension pneumothorax may also persist
if there is an injury to a major airway, resulting in
a bronchopleural fistula. In this case a single chest
tube is cannot cope with the major air leak. Two, three
or occasionally more tubes may be needed to manage the
air leak. In these cases thoracotom is usually indicated
to repair the airway and resect damaged lung.