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thoracocentesis should be performed to remove as much cytologic characteristics. Samples should also be saved
VetBooks.ir fluid as possible to enhance visualization of intrathoracic in ethylenediaminetetraacetic acid (EDTA, lavender
top), serum (red top) tubes, sterile tubes for culture, and/
and extrathoracic structures. Patients are maintained in
dorsal or ventral recumbency, manually hyperventilated
merase chain reaction (PCR). Information gained from
immediately preceding the helical scan to decrease res- or other tubes for effusion‐specific tests such as poly-
piratory drive during a forced breath hold, and the entire protein content and cellularity may help guide future
thorax is acquired using a slice collimation of 3–7 mm, diagnostics or treatment. Gram stain should be per-
depending on patient size. During a single inspiratory formed if pyothorax is suspected on preliminary diag-
hold not exceeding one minute, the entire thorax is nostics to guide antimicrobial therapy while cultures are
scanned which determines the slice thickness of the pending. Sterile samples can be stored for bacterial,
images, reserving the ability to make thinner slices of fungal, and/or Mycoplasma culture followed by antimi-
areas of pathologic importance if needed. Both ultra- crobial susceptibility. Fluids are typically divided into
sound and conventional radiographs are poorly sensitive three categories based on protein content and nucleated
to determine vascular invasion, displacement of normal cell count: transudate, modified transudate, and exudate.
anatomic structures, or invasion of adjacent structures Subcategories of exudative effusions include hemor-
by mediastinal masses; however, CT enhances the ability rhage, bile, chyle, septic and aseptic (Table 34.4).
to determine the aggressiveness of the tumor prior to Pure transudates are low in protein (<2.5 g/dL) and
surgical exploration. Regarding chylothorax, CT may be total nucleated cell count (<1500 cells/μL) with a clear,
superior to traditional radiographic lymphangiography colorless, or straw‐colored appearance. Cytologic exami-
to show contrast enhancement of the thoracic duct. nation shows predominantly mononuclear cells such as
Thoracic CT may also be indicated in dogs and cats with lymphocytes, macrophages, and mesothelial cells with
pyothorax not responding to conventional management low to absent numbers of nondegenerate neutrophils.
or if a pleural or pulmonary foreign body or abscess is Transudative pleural effusion is a consequence of either
suspected because this imaging technique eliminates increased hydrostatic or decreased oncotic pressure.
superimposition and silhouetting of intrathoracic struc- Liver disease, protein‐losing enteropathies and nephropa-
tures. Pleural abscesses on CT often appear as ring‐ thies, malnutrition, maldigestion, and malabsorption can
enhancing cystic structures with a fluid‐attenuating cause profound hypoalbuminemia with or without hypo-
core. Additional benefits of thoracic CT include identify- globulinemia, leading to decreased oncotic pressure. In
ing pleural thickness, small‐volume pneumothorax, addition, chronic blood loss, intestinal parasitism, and
parenchymal lung masses, and peripheral bronchop-
leural fistulae.

Pleural Fluid Evaluation
Table 34.4 Pyothorax bacterial isolates
Thoracocentesis is therapeutic and has the potential to
be diagnostic in any dog or cat with pleural effusion. In Organism Dogs Cats
the emergent respiratory patient, thoracocentesis should
be the first step after pleural effusion has been confirmed Aerobic Escherichia coli Pasteurella spp.
by TFAST with a goal to remove as much fluid as possi- Enterobacter cloacae Actinomyces spp.
ble, the exception being the coagulopathic patient. There Klebsiella pneumoniae Escherichia coli
are no absolute contraindications to perform thoraco- Pasteurella spp. Enterococcus faecalis
Actinomyces spp.
centesis but in coagulopathic patients, extreme caution Streptococcus canis
should be used if thoracocentesis is necessary and should Streptococcus
be reserved for patients in moderate to severe respira- intermedius
tory distress. In the absence of ultrasound availability, Mycoplasma spp.
thoracocentesis can be carefully performed in a dyspneic Nocardia spp.
patient prior to radiographs. The small risk associated Anaerobic Peptostreptococcus Bacteroides spp.
with thoracocentesis is outweighed by the major risk of anaerobius Peptostreptococcus
Bacteroides spp.
anaerobius
prolonged restraint for radiographs of a patient in severe Fusobacterium spp. Fusobacterium spp.
respiratory distress. Unilateral or bilateral thoracocente- Porphyromonas spp. Porphyromonas spp.
sis may be needed and is ideally performed with ultra- Prevotella spp. Prevotella spp.
sound guidance. Eubacterium spp. Filifactor villosus
Gross appearance of aseptically collected pleural fluid Propionibacterium spp. Fibrobacter succinogenes
should be recorded, as well as protein content and Source: Adapted from Walker, Jang, and Hirsch 2000.