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 25-28 September, 2018 | Singapore
effective regardless of the cause of the effusion. It may also be performed blind where a clinical suspicion
of pleural space disease exists but ultrasound is unavailable, as a diagnostic procedure.
Thoracocentesis may be carried out in the calm conscious patient, or one lightly sedated with butorphanol (0.2-0.4mg/kg IM or SC). We recommend pre-oxygenation during equipment set-up and flow-
by oxygen administration during drainage. A butterfly needle, three way tap and 10ml syringe are the equipment of choice, with a strict aseptic technique if time and patient stability permits. At the very least, hair should be clipped and surgical spirit applied with a short contact time to reduce the likelihood of contamination by skin flora.
To drain pleural effusion, ultrasound guidance may
be used to select an accessible pocket of fluid and position the needle. Alternatively, needle placement blind may be achieved by using intercostal spaces 7-9, in the ventral one-third of the thorax. Slowly inserting the needle perpendicular to the skin (tolerated better than a fast movement) whilst applying gentle negative pressure will result in fluid entering the hub of the needle and extension tubing once the pleural space is reached. At this point, the butterfly wings may be used to flatten the needle against the inner aspect of the pleural, to minimise risk of trauma to the lung surface. The procedure for draining a pneumothorax is identical, but the needle should be positioned dorsally where air accumulates. Fluid should be drained as completely as possible, sampling some in EDTA and plain tubes for fluid analysis, cytology and bacteriology. Fluid analysis can help to narrow the possible list of differential diagnoses (Table 2). A fresh, air-dried smear may assist cytologists by minimising fluid preservation artefacts. Also, if possible, a second EDTA sample should be obtained for measurement of pleural fluid NTproBNP
(a cardiac biomarker, see below). It is anecdotally reported that 20ml/kg pleural fluid causes clinical
signs of tachypnoea and 50ml/kg is associated with severe dyspnoea. Although these figures are likely to be a crude representation of the individual patient’s pathophysiology, they may be a guide as to how much fluid the clinician should expect to drain in a particular patient. For example, in a 4kg cat with orthopnoea and a large pleural effusion on ultrasound, drainage of 80ml is unlikely to resolve clinical signs. In our experience, 200- 250ml could be expected in a cat with severe dyspnoea caused by a pleural effusion.
Identifying cardiac disease
Once haemothorax, pyothorax, diaphragmatic rupture and trauma are excluded, the most likely differential diagnoses for the dyspnoeic cat are cardiac disease, lower airway disease and neoplasia. Detection of a chylothorax may be associated with CHF, neoplasia or
idiopathic disease. In cats with an obstructive pattern, lower airway disease is highly likely and empirical treatment should be considered to help stabilisation and facilitate further imaging of the thorax, such as radiography or computed tomography. In patients with either a pleural effusion or a restrictive respiratory pattern, lower airway disease can be all-but excluded.
Where analysis of a pleural effusion detects a modified transudate, or thoracic ultrasound/thoracocentesis
have not yielded a diagnosis in cats with a restrictive
or paradoxical dyspnoea, CHF and neoplasia should
be considered the most likely causes. Significant heart disease is the easiest rule-out in this scenario, and two tests should be considered: focused assessment of left atrial size and measurement of NTproBNP (N-terminal pro B-type natriuretic peptide; a hormone released by the atrial and ventricular myocardium by the stimulus of wall stretch or stress).
Subjective assessments of atrial size in cats with respiratory distress are often preferable to absolute measurements, because standard echocardiographic views in lateral recumbency are rarely safe to obtain in dyspnoeic patients. Standard images of the left atrium are best obtained from the right hemithorax, over the palpable apical impulse. It is reassuring that the dilation of the left atrium associated with CHF is rarely subtle or equivocal (Figure 3). Whilst normal left atrial to aortic root ratio (LA:Ao - usually measured in short axis where the three cusps of the aortic valve are symmetrical
and clear, like a Mercedes-Benz logo) is less than 1.5, many cats with CHF have an LA:Ao ratio over 2. Where the findings of echocardiography are unclear, cardiac biomarkers offer a good alternative to identify significant heart disease, but there is an increase observed in the circulating levels associated with respiratory distress in cats, which leads to a “grey-area” in measurements (Fox et al. 2009). For this reason, it should be considered only after thorough assessment of respiratory pattern and cardiac auscultation, and attempts to assess left atrial size using ultrasound.
 Total protein (g/L)
  TNCC (x10e9/L)
  Fluid type
  Differential diagnoses
   Pure transudate
  Clear Transparent
    Decreased oncotic pressure
  1000- 5000
  Modified transudate
 Pink or
yellow tinged Transparent or slightly turbid
   Increased hydrostatic pressure
  CHF Lymphatic or vascular obstruction (neoplasia)
 Turbid Colour associated with pathogenesis e.g. chylous, purulent, haemorrhagic
   Inflammation Increased vascular permeability
  Neoplasia Chylothorax Pyothorax Haemothorax
Lung lobe torsion Diaphragmatic rupture (longstanding) FIP*
 Table 2: Characteristics of pleural fluid and their association

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