Page 169 - BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery, 2nd Edition

BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery

An underlying cause for the pneumothorax (e.g. a
pulmonary mass) is most easily seen on radiographs after
VetBooks.ir Other imaging modalities, such as computed tomography
drainage of the pleural air and re-expansion of the lungs.
(CT) or magnetic resonance imaging (MRI), may be useful
for identifying an underlying cause that is not apparent on
radiographs. CT or MRI can help construct a surgical plan
by demonstrating the location and extent of a lesion, as
well as the involvement of other important structures.
However, a retrospective study in which dogs with spon-
taneous pneumothorax were investigated by CT found
that the sensitivity and positive predictive value of CT for
bulla detection were low, and CT was of questionable
utility (Reetz et al., 2013).

Pleural effusions
Pneumothorax in a dog. This lateral radiograph demonstrates Pleural effusions develop if the rate of pleural fluid pro-
12.2
elevation of the heart from the sternum. The lungs are duction increases and/or the rate of pleural drainage
retracted away from the thoracic wall and partially collapsed. decreases or is overwhelmed. The fluid that accumulates
is not necessarily of pleural origin, for example, haemor-
rhage, chylothorax or an effusion across the capsule of an
incarcerated or twisted organ. Regardless of underlying
aetiology, pleural effusion inhibits lung expansion.

Diagnostic imaging
Thoracic radiography is a useful method of visualizing
pleural fluid and can demonstrate smaller amounts of
pleural fluid than can be detected clinically. A DV view is
appropriate for initial ‘screening’ of the dyspnoeic patient;
this view can be supplemented with lateral views as nec-
essary. Both right and left lateral views may be useful.
Although a VD radiograph is more sensitive than a DV
radiograph for small volumes of pleural fluid, positioning
an animal that has a large amount of pleural fluid for this
view can cause decompensation and respiratory arrest.
Free pleural fluid is seen surrounding lung lobes that
have retracted from the chest wall, outlining the ventral
borders of the lungs and producing a ‘scalloped’ appear-
ance on a lateral radiograph (Figure 12.4). Pleural fluid may
also obscure the cardiac silhouette and the diaphragmatic
outline. The cause of the pleural effusion (e.g. mass, lung
lobe torsion, diaphragmatic rupture, congestive heart fail-
Unilateral left tension pneumothorax in a dog. This transverse ure) may be apparent on initial radiography, but such
12.3 computed tomographic image demonstrates a large volume
of air in the left pleural cavity causing almost complete collapse of the lesions are more likely to be appreciated on radiographs
taken following removal of the pleural fluid.
left lung lobes, a right mediastinal shift and caudal displacement of the
diaphragm: 1.5 litres of air was subsequently drained by thoracocentesis Ultrasound imaging can detect small amounts of pleural
from the left pleural cavity. fluid and is useful for evaluating underlying heart disease
and mediastinal masses. Pleural fluid acts as an acoustic
It is important to be able to differentiate quickly between
varying degrees of simple pneumothorax and tension
pneumothorax because the latter is rapidly fatal if left
untreated and warrants immediate thoracocentesis.
Recently, the use of thoracic ultrasonography for the
diagnosis of pneumothorax has become more common.
The tFAST (thoracic focused assessment with sonography
for trauma) technique can be used to assess the presence
of either pleural effusion or a pneumothorax. Pneumothorax
is diagnosed by the absence of the ‘glide sign’, defined as
the lack of the normal dynamic interface between lung
margins gliding along the thoracic wall during respiration.
Concurrent thoracic trauma can be diagnosed by the pres-
ence of pleural or pericardial fluid or the presence of a ‘step
sign’, defined as an abnormal glide sign. A step sign is a
glide sign that has deviated from the normal linear continu- leural effusion in a cat. This lateral radiograph demonstrates
ity of the pulmonary–pleural interface, and is assumed to 12.4 outlining of the ventral lung borders by fluid. The cardiac
represent concurrent thoracic injury (Lisciandro et al., 2008). silhouette is also partially obscured.

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