Cats with pleural effusion often have severe respiratory compromise at the time of presentation. Cats with respiratory compromise should be identified early, handled as little as possible and stabilized. Fortunately, most or all of the information required to localise the dyspnoea can be obtained from observation of the patient and a minimally invasive physical examination. The clinician has to juggle stabilization, localisation and owner communication while remaining vigilant for clues which allow ranking of the differentials to formulate a diagnostic plan.

Stabilisation

Triage at reception is important to identify dyspnoeic cats early. All sick cats should be observed in their carriers on arrival at the clinic regardless of the owner’s description of the presenting complaint. This is because dyspnoea may be subtle or absent prior to travelling. Such is the propensity of the cat to compensate for gradual-onset respiratory compromise by reduced activity that signs may not be seen by even the most observant owner. It is important that all members of the caregiving team, including reception staff, are trained to be vigilant for dyspnoeic emergencies and to take appropriate action.

Where respiratory distress is noted or suspected, immediate stabilisation is indicated. The techniques used and the order in which they are carried out depend on assessment of the individual patient.

• Reduce oxygen requirements by placing the patient in a cool, quiet environment and minimising handling to reduce oxygen demand.
• Supplemental oxygen. Options for short-term oxygen delivery to the dyspnoeic cat include oxygen chamber, mask and flow-by. The method that is best tolerated by the patient should be used. Struggling must be avoided. An oxygen chamber or cage is a useful way to deliver oxygen without the need for restraint.
• Intravenous access should be achieved at the earliest opportunity.
• Light sedation may be beneficial for dyspnoeic cats to reduce anxiety (e.g., butorphanol).
• Therapeutic thoracocentesis is carried out after imaging except where suspicion for pleural effusion is high and the dyspnoea is severe. In this case, unguided needle thoracocentesis can be life-saving, and the risk of causing significant harm is low.
• Monitoring of respiratory rate and depth should be initiated early so that changes can be readily appreciated.
• Where hypoventilation cannot be controlled by other means, the patient can be anaesthetised, intubated and ventilated.

Presenting Signs of Pleural Effusion

Observation. Cats with significant pleural space disease adopt a sternal position with abducted elbows. A restrictive respiratory pattern with increased inspiratory effort is typical. A restrictive respiratory pattern is rapid and shallow. Conditions which prevent the lungs from fully expanding cause a restrictive respiratory pattern. An increased respiratory rate and decreased inspiratory volume minimise respiratory effort in non-compliant lungs. Another major cause of restrictive respiratory patterns in cats is pulmonary parenchymal disease (e.g., pulmonary oedema, pneumonia).

Auscultation helps to distinguish pleural space disease from pulmonary parenchymal disease. Breath sounds are decreased or absent with pleural space disease. Differential diagnoses then include pleural effusion, pneumothorax, intrathoracic mass or diaphragmatic hernia. Where there is effusion, the reduction in breath sounds is more pronounced ventrally, and a fluid line may be appreciated on auscultation or percussion. Concurrent pulmonary oedema may contribute to the dyspnoea in left-sided heart failure and produce pulmonary crackles dorsally. Pleural effusion or pericardial effusion can cause muffled heart sounds.

Imaging. Pleural effusion can be confirmed with radiography (a single DV view, if patient permits) or thoracic ultrasonography.

Major Differential Diagnoses for Pleural Effusion in the Cat

• Congestive heart failure
• Neoplasia
• Pyothorax
• Feline infectious peritonitis
• Idiopathic chylothorax

These are not the only differentials, but they are the most common causes of pleural effusion of sufficient volume to cause dyspnoea. Data obtained from signalment, history and physical examination can be used to rank this list of common differentials.

On clinical examination of cats with suspected pleural space disease, the following should be noted:

• Jugular veins—are they distended and pulsating? This occurs with increased right heart filling pressures (congestive heart failure, high output failure [e.g., hyperthyroidism]) or with pulmonary hypertension.
• Are the jugular veins distended without pulsation? This occurs with occlusion of right heart inflow (cranial mediastinal mass, large non-cardiogenic pleural effusion)
• Lung sounds—pleural effusion will often result in reduced lung sounds, particularly ventrally. It may be possible to determine a fluid line on auscultation.
• Heart sounds—are they muffled? Consider a pleural exudate (e.g., pyothorax). Transudates and modified transudates contribute less to muffling of heart sounds.
• Is the cardiac apex beat displaced? An intrathoracic mass or focal accumulation of fluid may displace the cardiac apex beat (e.g., a left-sided intrathoracic mass would displace the cardiac apex beat, resulting in an abnormally loud apex beat on the right and an absent or muffled apex beat on the left).

Ranking of differential diagnoses is further informed by the gross characteristics (smell, colour, turbidity) of fluid obtained at thoracocentesis. Thoracocentesis is both a diagnostic and therapeutic procedure. A butterfly needle attached to extension tubing, a 3-way tap and a 50-ml syringe. Local anaesthetic can be used. The ventral third of the 7th–9th intercostal space avoiding the caudal rib margins can be used for blind thoracentesis.

Diagnostic samples collected in EDTA and plain tubes and direct smears can be submitted for total protein, total and differential cell count and cytology. Additional testing will be directed by clinical data and gross evaluation of the fluid. Once the patient is stable, additional testing such as repeat imaging (radiographs or CT), NT-proBNP and echocardiography may be indicated.

Indwelling chest drains suitable for cats include www.milainternational.com/index.php/products/guidewire-inserted-chest-tube.html. A video of placement is available at this site.

References

1.  Beatty J, Barrs V. Pleural effusion in the cat: a practical approach to determining aetiology. Journal of Feline Medicine and Surgery. 2010;12(9):693–707.

2.  Wurtinger G, Henrich E, Hildebrandt N, Wiedemann N, Schneider M, Hassdenteufel E. Assessment of a bedside test for N-terminal pro B-type natriuretic peptide (NT-proBNP) to differentiate cardiac from non-cardiac causes of pleural effusion in cats. BMC Veterinary Research. 2017;13(1):394.