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with common differential diagnoses
TNCC, total nucleated cell count; FIP, feline infectious peritonitis (wet form), CHF; congestive heart failure
*Please note: cats with wet FIP typically have high protein effusions, but only moderate cell counts (500-5000 x10e9/L
Figure 3: Normal (top) and abnormal (bottom) left atrial size in the cat. The normal feline left atrium is relatively square in appearance and its size should appear to be approximately half the area of the left ventricle in long-axis and less than 1.5 times the aortic diameter in short-axis.
LA, left atrium; LV, left ventricle; PF, pleural fluid
NTproBNP can be measured quantitatively using reference laboratories (the best validated assay is Cardiopet proBNP Feline, run by IDEXX laboratories
Ltd, United Kingdom) or qualitatively using a benchtop, patient-side SNAP test (IDEXX). The patient-side test has a positive cut-off value that is useful for detecting subclinical cardiomyopathy (around 120-130 pmol/L, Machen et al. 2014). However, it has not been validated in a population of dyspnoeic cats and this relatively low cut-off may lead to a high rate of false positive results
in this population. Using the quantitative assay at a reference laboratory would be more useful in cats with respiratory distress because a significant increase (>265 pmol/L) has been shown to reliably differentiate cardiac from noncardiac causes of respiratory distress (Fox et al. 2009), but the transport time and laboratory turnaround mean that results take several days and are not useful in cats with acute dyspnoea. Despite these limitations, a negative NTproBNP SNAP test is highly likely to exclude CHF as the cause of respiratory distress. A positive test is less discriminatory.
Recent research has shown that the quantitative NTproBNP assay can be reliably run using pleural fluid (stored in an EDTA tube and handled as recommended by the laboratory for blood samples) instead of plasma (Humm et al. 2013). This is hugely advantageous
in cats with acute dyspnoea, because samples for laboratory submission can be achieved from therapeutic thoracocentesis instead of taking an additional blood sample from a patient where minimal handling is a priority. Although not validated, the SNAP test should have the same reliability on pleural fluid as the quantitative test (good negative predictive value, but likely to have a high rate of false positive results).
Considerations for cats with upper airway obstruction
Patients with suspected upper-respiratory tract obstruction usually require direct visual examination of the nasopharyngeal/laryngeal area, possibly with cross-sectional imaging (e.g. computed tomography) if available. Radiography may have limited value, owing to the complex bony anatomy of the area, and
ultrasonography of this area requires considerable expertise and high-frequency ultrasound transducers to obtain diagnostic images. A light plane of anaesthesia induction is required to facilitate examination of
the larynx, but the clinician should be prepared
for endotracheal intubation or even emergency tracheostomy if significant upper airway obstruction
is detected. If detected, nasopharyngeal polyps can
be removed by gentle traction (described in detail elsewhere). If suspected laryngeal neoplasia is present, fine needle aspiration might prove useful – laryngeal lymphoma should exfoliate sufficiently for a confident cytological diagnosis.
Empirical or initial treatment
Thoracocentesis is the definitive emergency treatment for stabilisation of cats with a pleural effusion or pneumothorax. It has a great therapeutic and diagnostic potential so should be performed wherever indicated.
Where a convincing obstructive respiratory pattern is present on observation, lower airway disease is most likely and empirical treatment with a bronchodilator (inhaled salbutamol or systemic terbutaline) and an anti- inflammatory dose of steroids should be considered. It is worth noting that beta-agonists can promote tachycardia and arrhythmias, which may lead to deterioration of
a cardiac patient, and corticosteroids can increase circulating volume and promote congestive signs. For these reasons, if the clinician is unsure of the likelihood of cardiac disease, left atrial assessment or NTproBNP measurement should be considered prior to empirical treatment for lower airway disease. Also, steroid use may lead to uncertainty in the interpretation of diagnostic bronchoalveolar lavage samples obtained at a later date, or a worsening of clinical signs if an infectious cause of respiratory signs is present (e.g. Mycoplasma spp).
If a cat with respiratory distress has an audible gallop sound, CHF is highly likely and furosemide should be administered empirically at a dose of 2mg/kg, with reassessment of respiratory rate and effort in 30 minutes. If a pleural effusion is present, thoracocentesis should
be performed. Furosemide not only reduces the severity of pulmonary oedema through its diuretic effect, but
has a rapid venodilatory effect if given intravenously
(if circumstances and stability permits), which helps to reduce left atrial pressure and pulmonary congestion. In addition, furosemide has mild bronchodilatory effects, so cats with signs of lower airway disease should not
be put at risk by a single dose and may in fact show a favourable response even when CHF is not the cause of respiratory distress. This positive response, referred to by some as a “furosemide response test”, may be interpreted as diagnostic for CHF. This test should be interpreted in light of furosemide’s non-cardiovascular effects and the patient’s other clinical findings (e.g. respiratory pattern at presentation).
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