The Normal Pericardium
The pericardium is the fibrous envelope of tissue surrounding the heart. In health, the pericardium contains a small amount of fluid and the heart. Normally, a small volume of pericardial fluid allows the heart to move easily within the sac as it beats. Several functions are attributed to the pericardium, but it is clear that the pericardium serves no vital function because it can be removed surgically when diseased without untoward effect.
Pericardial effusion is an abnormal accumulation of fluid or blood within the pericardial sac. Because the sac is fibrous and does not stretch, accumulation of fluid within the sac will cause increased intrapericardial pressure and decreased cardiac filling. If the heart does not fill with blood, it cannot effectively pump blood throughout the body. When there is enough fluid and pressure within the pericardial space to adversely affect heart function, this is called cardiac tamponade. Pericardial effusion that is severe enough to cause tamponade may result in sudden death.
The most common causes of pericardial effusion are neoplasia (cancer) and idiopathic. Pericardial effusion can also result from infectious, metabolic, toxic, cardiovascular, and traumatic causes.
The most common cause of neoplastic pericardial effusion is due to cardiac hemangiosarcoma. Hemangiosarcoma is a highly malignant neoplasm of blood vessel cells and often affects the liver, spleen, and right side of the canine heart. Because of its highly malignant nature, hemangiosarcoma should always be considered a systemic disease (throughout the whole body). However, gross masses are not always identified when the diagnosis of peridardial effusion is first made. Pulmonary (lung) metastases and concurrent hepatic (liver) and splenic involvement are confirmed by radiographs and abdominal ultrasound in only about 25% of cases. Other neoplastic causes of pericardial effusion include heart based tumors, chemodectomas, aortic body tumors, ectopic thyroid tumors, mesothelioma, cardiac lymphosarcoma, rhabdomyosarcoma, and fibrosarcoma. These are much less common than cardiac hemangiosarcoma
The second most common cause of pericardial effusion in dogs is “idiopathic” meaning that no cause for the pericardial effusion is established. These dogs may ultimately require surgery to prevent cardiac tamponade and decreased cardiac function.
Rarely, bacterial or fungal infections can cause pericardial effusion. Kidney failure with resulting uremia can cause pericardial effusion as can severe hypothyroidism with cholesterol-based pericardial effusion. Coagulation disorders leading to pericardial effusions occasionally occur with anticoagulant rodenticide toxicity, warfarin toxitiy, secondary to disseminated intravascular coagulation, and other coagulopathies. Dogs in congestive heart failure frequently have some pericardial effusion, but it is seldom in sufficient quantity to cause significant hemodynamic compromise. Occasionally, dogs will develop pericardial effusion secondary to an intrapericardial foreign body.
Clinical Signs and Presentation
Dogs with pericardial effusion commonly present with acute (sudden) onset of vague and nonspecific signs. Common presenting signs include lethargy, respiratory difficulty, collapse, reduced appetite, vomiting, and abdominal distention. Less commonly dogs have increased thirst, weakness, or coughing.
Physical examination findings in dogs with pericardial effusion range from vague and mild to complete hemodynamic collapse. Common physical examination findings include muffled heart sounds, depression, weak pulses, tachycardia (fast heart rate), and abdominal distention with a fluid wave. Mucous membranes are frequently pale, and the jugular veins may be distended. Careful palpation of the pulses usually discloses variations in pulse quality associated with phase of respiration.
Thoracic radiographic (x-rays) may support a diagnosis of pericardial effusion, but can also appear normal if the effusion is very acute and of a small volume. Pulmonary metastasis may be present, but are not commonly found at the time of diagnosis.
Echocardiography (ultrasound examination of the heart) is the most sensitive and specific noninvasive method to confirm the presence of pericardial effusion. A mass on the heart may, or may not be visible on the initial examination. In patients who are not hemodynamically stable, it is important to alleviate the cardiac tamponade using a pericardiocentesis technique to improve cardiac outflow before a more complete echocardiographic examination is performed. This means that your emergency veterinarian may need to remove fluid from the pericardium to stabilize your pet before they are able to look more closely for a mass on the heart.
The initial treatment for any pericardial effusion that is causing hemodynamic compromise is pericardiocentesis. Pericardiocentesis is performed by placing a long needle through the chest wall and into the pericardial sac to remove blood or fluid. Some dogs may need only a local anesthetic in the chest wall in order to perform this procedure. Other dogs will need systemic sedation or pain medication. Following stabilization by pericardiocentesis and any other appropriate supportive care (usually IV fluids), treatment is directed toward the specific cause.
Treatment for cardiac hemangiosarcoma is challenging and the diagnosis confers an extremely poor prognosis. Pericardiocentesis is usually associated with marked clinical improvement, but cardiac tamponade typically recurs within a few days, often resulting in death or euthanasia. More aggressive treatment for cardiac hemangiosarcoma includes various combinations of tumor resection, pericardectomy, chemotherapy, and splenectomy (when concurrent splenic involvement is identified). These therapies appear to provide a survival benefit, but average survival times are disappointingly short (56 to 229 days). Chemotherapy, especially with doxorubicin, following tumor resection may prolong survival, but the benefit is typically short. Despite the poor prognosis associated with pericardial effusion caused by cardiac hemangiosarcoma, management should always involve consultation with a veterinary cardiologist and an oncologist to take advantage of continually emerging modalities for the treatment of this highly malignant tumor.
Pericardial effusion caused by heart base tumors, chemodectomas, and aortic body tumors may have a slightly better prognosis. These tumors may be locally infiltrative and typically cannot be completely surgically removed. However they may be slow growing so treatment with a surgical pericardectomy (removal of the pericardial sac) may allow several months of good quality life.
Treatment of Idiopathic Pericardial Effusion
Treatment of idiopathic pericardial effusion is initially through repeated pericardiocentesis followed by pericardectomy in recurrent cases. This is because following pericardiocentesis alone, cardiac tamponade often recurs within 1 to 2 month and may be fatal in some cases. Recurrent tamponade with progressive fibrosis of the pericardium may also develop and necessitates pericardectomy for treatment.