Pericardial Effusion

Introduction

The pericardium is a sac that surrounds the heart and serves to protect the heart as well as maintain its position and shape. The pericardium is comprised of an outer fibrous layer and an inner serous layer. The fibrous outer layer is composed of collagen and elastin. The serous layer is composed of a single layer of mesothelial cells. The serous pericardium lines the inside of the fibrous layer, where it is known as the parietal layer, and also overlies the heart, where it is referred to as the visceral layer of the serous pericardium. The pericardial cavity is the space between the visceral and parietal layers, which usually contains a very small amount of fluid.

Pericardial effusion is a pathological accumulation of fluid within the pericardial sac. The rate of this fluid accumulation within the pericardium determines the haemodynamic effects, and therefore clinical signs. A slow accumulation is initially well-tolerated, but can eventually lead to increased pressure causing compression of the right atrium and right ventricle (cardiac tamponade). Rapid fluid accumulation results in acute tamponade.

Pericardial effusions may be idiopathic, neoplastic, infectious, traumatic, toxic or may result from atrial rupture in dogs with myxomatous mitral valve disease (MMVD).

Neoplasia affecting the heart, heart base or pericardium is the most common cause of pericardial effusion in dogs.

Idiopathic pericardial effusions are the second most common cause of pericardial effusion in the dog. Effusions are usually haemorrhagic and the cause is unknown. One study has demonstrated progression of idiopathic pericardial effusion to mesothelioma over time in a series of Golden Retrievers.

Bacterial pericardial effusions have been reported, usually associated with penetrating wounds and migrating foreign bodies.

Coagulopathies, such as those resulting from rodenticide toxicity, can manifest as haemorrhagic pericardial effusion.

Hypoproteinaemia can cause pericardial effusion in both the dog and cat.

In cats, the majority of pericardial effusions are secondary to congestive heart failure caused by cardiomyopathy. However, cardiac lymphoma, heart based tumours and a variety of metastatic tumours have been reported as causes of pericardial effusion. Additionally, feline infectious peritonitis virus (FIP) may cause pericardial effusions.

Signalment

Haemangiosarcoma has a predilection for the right atrium, particularly in German Shepherd dogs.

Heart based tumours, such as chemodectomas, are most common in Boxers.

Mesotheliomas have a higher prevalence in small-medium breeds.

Idiopathic pericardial effusion is most common in middle-aged, male, large and giant breed dogs. Golden Retrievers, Labrador Retrievers, Great Danes, Newfoundlands and St. Bernards are overrepresented.

History & Clinical Signs

Signs are vague and the diagnosis is easily missed. It is important to rule out pericardial disease in any animal presenting with exercise intolerance, collapse or ascites. Animals with low volume effusions, or those that are accumulating slowly may have no clinical signs.

Clinical signs include:

- Inappetence - Lethargy - Exercise intolerance - Weakness - Collapse - Dyspnoea - Weight loss - Abdominal enlargement

Physical Exam

- Muffled heart sounds - Right-sided congestive heart failure: jugular venous distension, jugular pulses, ascites, pleural effusion - Weak pulses due to poor cardiac output - Pulsus paradoxicus (pulse quality varies excessively with respiration) - Sinus tachycardia, compensatory mechanism for reduced cardiac output in order to maintain systemic blood pressure

Radiography

-Enlarged, rounded cardiac silhouette

-Signs of right-sided heart failure:

• ascites

• pleural effusion

• distension of the caudal vena cava

• hepatomegaly

Electrocardiography (ECG)

- Electrical alternans: variation in height of the QRS complexes, due to the heart swinging within the pericardial fluid

Echocardiography

Pericardial effusion is visible as a hypoechoic (black) area surrounding the heart. Cardiac tamponade is characterized by collapse of the right atrium +/- the right ventricle during diastole or systole. In severe cases, there is also underfilling of the left ventricle which gives it the appearance of being thickened (pseudohypertrophy).

Echocardiography is also useful to establish the cause of the pericardial effusion. Mass lesions may be small and difficult to distinguish in some cases, but may be very easily identified in others. Haemangiosarcomas can be seen infiltrating the right atrium, whereas heart base tumours typically surround the aorta and pulmonary artery.

Pericardial Fluid Analysis

Fluid should always be sent for further analysis, including total and differential cell counts, packed cell volume, specific gravity, cytology, bacterial culture and sensitivity. Pericardial effusions can be classified as haemorrhagic, transudate, modified transudate and exudate and the classification informs the likely underlying cause. Differentiating reactive mesothelial cells from neoplastic mesothelial cells can be incredibly difficult, sometimes a diagnosis can only be made following histopathological and immunohistochemical examination of the excised pericardium.

Differentials are as follows:

- Haemorrhagic: idiopathic, neoplastic (Haemangiosarcoma, heart base tumour, lymphoma, mesothelioma), trauma, coagulopathy, ruptured left atrium - Transudate: hypoproteinaemia - Modified transudate: right-sided congestive heart failure, neoplasia - Exudate: feline infectious peritonitis (FIP), infection (bacterial, fungal), foreign body

Treatment

Pericardiocentesis

Pericardiocentesis is a useful diagnostic test as well as treatment. This is the only effective treatment for cardiac tamponade. Diuretics should not be given prior to pericardiocentesis, as they will reduce circulating fluid volume and therefore preload. This will further decrease cardiac pressures, cardiac output and exacerbate tamponade. It is advisable to have continuous ECG monitoring throughout the procedure, as any contact with the myocardium may result in arrhythmias.

The effusion frequently has a haemorrhagic appearance, regardless of underlying aetiology. It is possible to inadvertently sample blood from the right ventricle during attempted pericardiocentesis. The effusion sample can be differentiated from right ventricular blood by the fact it does not clot.

Removal of large volumes of pericardial effusion can result in a dramatic improvement in preload and a sudden increase in the size of the right atrium. In some cases, this results in a transient atrial fibrillation which lasts 24-72 hours and does require treatment.

Risks of the procedure include: - Haemorrhage - Ventricular arrhythmias - Accidental puncture of the right ventricle and removal of right ventricular blood, resulting in hypovolaemic shock - Coronary laceration (reduce risk by using the right side for pericardiocentesis) - Tumour laceration - Infection

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Prognosis

-Good (idiopathic effusion, foreign bodies)

-Poor (neoplasia)

References

Kittleson, Mark D., Kienle, Richard D. (1998) Small Animal Cardiovascular Medicine Missouri:Mosby