Pulmonary thromboembolism (PTE) is the occlusion of the pulmonary vasculature (artery, vein, arteriole) that forms in the venous system (emboli) or the pulmonary artery system. The actual incidence of PTE is not known, and the cause depends on the underlying disease.
Risk factors associated with formation of PTE include:
disseminated intravascular coagulation
indwelling central venous catheters
These diseases lead to a hypercoagulable state causing thrombus formation and embolic obstruction.
Thrombi within the pulmonary venous system causes changes to pulmonary hemodynamics, causing hypoxemia. This includes ventilation-perfusion mismatch and shunting. Increased pulmonary vascular resistance may result, leading to pulmonary hypertension, decreased coronary circulation, and myocardial ischemia. This in turn leads to a decrease in cardiac output and cardiogenic shock.
In dogs, PTE is associated with:
protein-losing nephropathy (nephrotic syndrome) and enteropathy
canine cushing syndrome
Cardiomyopathy and neoplasia are the leading cause of PTE in cats.
Clinical Findings of Pulmonary Thromboembolism in Dogs and Cats
Clinical signs of pulmonary thromboembolism (PTE) are widespread and nonspecific depending on the extent of underlying disease and thromboembolic occlusion. Patients may become tachypneic, with increased lung sounds, cough, tachycardia, syncope, cyanosis, and sudden death. Clinical signs of PTE may be indistinguishable from the clinical signs associated with the underlying disease process.
Diagnosis of Pulmonary Thromboembolism in Dogs and Cats
Identification of risk factors
Supportive diagnostic testing (blood-gas analysis, radiography, echocardiography, angiography if available
Diagnosis of pulmonary thromboembolism (PTE) is often a challenge with traditional diagnostics available in veterinary medicine. Arterial blood gas analysis will identify hypoxemia present in 80% of dogs, although response to oxygen is variable. Thoracic radiographs can be normal in 9%–27% of dogs and in 9% of cats with PTE. Abnormal radiographic findings with PTE include alveolar or interstitial pulmonary infiltrates or regional hypovascular lung areas. However, thoracic radiographs that underestimate the degree of clinical respiratory compromise should raise suspicion of PTE. Blood gas analysis may reveal hypoxemia and hypocapnia, but the presence of normal blood gas values does not exclude PTE.
Echocardiography can aid in assessment for pulmonary hypertension that may be associated with PTE (dilation of the right ventricle, pulmonary artery, inferior vena cava; right ventricular hypokinesis; tricuspid regurgitation; abnormal septal wall motion). A normal echocardiogram does not exclude diagnosis of PTE. Spiral CT angiography or selective pulmonary angiography remain gold standards for diagnosis of PTE in people.
There is no gold standard for testing for PTE in small animals; therefore, a diagnosis is based on clinical suspicion, underlying disease with known risk factors, and excluding other causes of respiratory failure.
Treatment of Pulmonary Thromboembolism in Dogs and Cats
Supportive care should address hypoxemia, shock, and anxiety associated with PTE. Hypoxemic patients (PaO2 < 80 mmHg) should be placed on oxygen supplementation with either an oxygen cage or nasal cannula. If there is evidence of shock, bolus replacement crystalloid therapy should be considered until cardiovascular parameters have stabilized. If intravenous fluid therapy does not stabilize the animal, the addition of vasopressor support is an option. A patient may benefit from sedation (butorphanol 0.4 mg/kg, IV or IM) for anxiolysis. If hypoxemia is severe, positive-pressure ventilation may be considered.
Treatment with anticoagulants and antiplatelet drugs may be problematic because of the difficulty in confirming diagnosis of PTE. Therapeutic goals for patients with PTE are to discourage propagation of the thrombus and to prevent recurrence.
The dosage of unfractionated heparin in dogs is 250 U/kg, SC, every 6 hours and in cats is 150 U/kg, SC, every 4 hours.
Low-molecular-weight heparin: The dosage of enoxaparin in dogs is 0.8 mg/kg, SC, four times a day and in cats is 0.25 mg/kg, SC, four times a day. The dosage of dalteparin in dogs is 100–175 U/kg, SC, three times a day and in cats is 75 U/kg, SC, four times a day.
Warfarin is not recommended for use in dogs or cats.
The dosage of rivaroxaban in dogs is 1–2 mg/kg per day and in cats is 0.5–1 mg/kg per day.
Key Points on Pulmonary Thromboembolism in Small Animals
Diagnosis of PTE is challenging and should be considered in patients that have difficulty breathing but normal radiographs.
There are few evidence-based therapeutic recommendations for antithrombotic treatment in dogs or cats.
For More Information
Goggs, et al. American College of Veterinary Emergency and Critical Care (ACVECC) Consensus on the Rational Use of Antithrombotics in Veterinary Critical Care (CURATIVE) Guidelines: Sm An J Vet Emerg Crit Care. Dec 2018;29:12–36.
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