Pulmonary Thromboembolism

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Introduction

Pulmonary thromboembolism describes the occlusion of pulmonary vessels by a clot. It has usually arisen from the systemic venous circulation, embolises to the pulmonary circulation and causes obstruction of the arterial supply to an area of the lung. This results in ventilation perfusion mismatch occurring where there are areas of the lung which continue to be ventilated but do not receive adequate blood supply. This can result in severe hypoxia and marked signs of respiratory distress if the area of underperfused but ventilated lung is large.

It can lead to pulmonary hypertension and result in pleural effusion.

Causes of pulmonary thromboembolism include:

Cardiac disease: Dirofilaria immitis, dilated cardiomyopathy, chronic mitral valve insufficiency, endocarditis
Neoplasia: lymphosarcoma, bronchoalveolar carcinoma, pancreatic carcinoma
Disseminated Intravascular Coagulation
Sepsis
Hyperadrenocorticism
Protein-losing nephropathy: amyloidosis, glomerulonephritis
Protein-losing enteropathy
Pancreatitis
Eosinophilic lung disease
Air emboli
Autoimmune haemolytic anaemia
Iatrogenic: indwelling vascular catheters, transfusions
Idiopathic

Clinical Signs

Clinical signs are usually non-specific. There may be an acute onset of respiratory distress, with increased depth, rate and effort of breathing. Patients may also present with an acute onset of signs of right sided heart failure due to the sudden increase in pulmonary vascular resistance.

There may be signs of an underlying disease such as Cushing's or intestinal disease.

Diagnosis

Diagnosis is difficult ante-mortem.

History and clinical signs are usually vague and non-specific.

Radiography may demonstrate a diminution or loss of peripheral vessels and an increase in size of the central pulmonary artery, but are often normal.

Blood gas analysis will reveal the ventilation perfusion mismatch and there will be hypoxemia, hypocapnia and respiratory alkalosis.

Nuclear perfusion scintigraphy is a safe and sensitive test and will detect if there is a lack of perfusion of part of the lung.

Lung Ultrasound may reveal wedge shaped defects on the air-chest wall interface. Found to be 43% sensitive and 98% specific in a multi-center human trial compared to CT. [1]

Treatment

This primarily involves treating the underlying cause.

Supportive care includes oxygen supplementation, strict cage confinement and careful parenteral fluid therapy.

Anticoagulant therapy should be considered in severe cases to prevent extension of the clot within the pulmonary circulation. Heparin has a rapid onset and short-term effects.

Fibrinolytic therapy is very expensive and lacks selectivity, but includes drugs such as streptokinase, urokinase and tissue plasminogen activator.

Prognosis

Prognosis is poor to guarded in cats and dogs. Recurrence is possible, especially if the cause has not been resolved.


Pulmonary Thromboembolism Learning Resources
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Flashcards
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Small Animal Abdominal and Metabolic Disorders Q&A 01


References

Pasquini, C. (1999) Tschauner's Guide to Small Animal Clinics Sudz Publishing

Boswood, A. (2010) Pulmonary parenchymal disease RVC Student Notes




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