Congenital diseases of the pericardium
Congenital disorders of the pericardium include peritoneopericardial diaphragmatic hernia in dogs and cats; intrapericardiac cysts, observed only in dogs; and complete absence or partial defects of the pericardium, which are rare in both dogs and cats. Absence of the pericardium is rare and does not precipitate clinical signs. Partial defects (perforations) of the pericardium are only slightly more common but they engender a risk of cardiac herniation. Peritoneal-pericardial diaphragmatic hernia (PPDH) is the most common congenital anomaly involving the pericardium of dogs and cats. Congenital PPDH occurs as an isolated defect and in association with umbilical hernias, malformed or absent sternebrae, pectus excavatum, and a variety of congenital heart defects. Clinical signs in dogs and cats with PPDH vary in relation to the organs that are displaced into the pericardium and the degree to which the blood supply and function of these organs are compromised. The liver is herniated most frequently, followed by the small intestine, spleen, and stomach. Thus, signs of gastrointestinal disease-vomiting, anorexia, and diarrhea-are common in dogs and cats with PPDH. Respiratory abnormalities, varying from coughing or wheezing to severe dyspnea, are the next most frequently observed clinical signs. Only rarely are signs of heart failure and cardiac tamponade observed.
Herniated segments of intestine and the stomach may be easily identified following oral administration of barium. When the spleen or portions of the liver have herniated into the pericardial sac, echocardiography is particularly useful to confirm the diagnosis. Liver function should be assessed by traditional methods prior to surgical intervention. Other techniques found useful for the diagnosis of PPDH and the identification of associated complications include selective or non-selective angiography, tomography, positive or negative contrast peritoneography, pneumography, magnetic resonance imaging, and computerized axial tomography. Surgical reduction of the hernia and repair of the diaphragmatic defect is advisable in most dogs and cats with a PPDH. Surgical imperatives include herniation of the stomach into the pericardial sac, bowel obstruction, and vascular embarrassment to the liver. In many cases only the omentum or small portion of the liver has herniated into the pericardial sac. In this circumstance, observation rather than surgery is often the prudent choice, particularly in older animals with other systemic disorders.
Acquired Pericardial Disease
Pericardial effusion is the most common cause of pericardial disease in the dog (much less common in cats) and accounts for more than 90% of pericardial disorders. Constrictive pericarditis is, by comparison, uncommon. Most (60 to 70%) clinically significant pericardial effusions are secondary to neoplasia, mainly hemangiosarcomas or heart base tumors (chemodectomas or thyroid carcinomas). Neoplastic effusions usually occur in dogs greater than 7 years of age (average 9+ years). German Shepherd dogs and Golden retrievers are predisposed to hemangiosarcoma. Brachycephalic breeds (Boxers, Boston terriers) are reportedly predisposed to heart base tumors. Mesothelioma is the most common primary tumor of the pericardium. Idiopathic pericardial effusions comprise about 30% of clinical cases. Most commonly affected are medium to large breed, middle-aged (average age 7 years) male dogs. The German Shepherd dog is over-represented in this disorder also. In cats, pericardial effusion is often associated with feline cardiomyopathy, feline infectious peritonitis, and neoplasia (primarily lymphosarcoma).
Pathophysiology. Increased pericardial pressure impairs diastolic filling of the heart. This causes a reduction of ventricular stroke volume, cardiac output, and in severe cases, a fall in aortic blood pressure. Systemic and/or pulmonary congestion develop as a consequence of elevated intrapericardial pressure and the operation of a number of compensatory responses. Pericardial effusate volumes in dogs with cardiac tamponade vary from 150ml to over 1500ml. The severity of signs depends mainly on the rate of fluid accumulation and the amount of pericardial fibrosis. The pericardial pressure volume relationship is such that there is a progressively greater rise in pericardial pressure as pericardial volume increases.
Figure 1. Hemodynamics of Cardiac Tamponade
Phase one: mildly compromised filling.
Phase two = right heart tamponade.
Phase three = left and right heart tamponade.
The right auricle and right atrium are the most common sites for hemangiosarcoma with rare reports of tumor in other locations such as the left atrium or left ventricle. Metastasis to lungs, spleen, and other organs is usually present at the time of diagnosis. Heart base tumors are typically located within the pericardium at the heart base often arising from and surrounding the aorta and brachiocephalic trunk. Tumors in this location are often slow to metastasize but may be locally invasive and can cause compression of the atria or vena cavae. Chemodectomas/aortic body tumors/nonchromaffin paraganglioma arise from chemoreceptor cells in the adventitia of the aorta or pulmonary artery. Thyroid/parathyroid tumors arise from ectopic thyroid rests located at the heart base or, more rarely, within the myocardium. Most cases of idiopathic pericardial effusion show variable amounts of vasculitis, inflammatory infiltrates, and fibrosis. No etiologic agent has been identified.
Most animals with pericardial effusion are presented with a vague clinical history of lethargy, weakness, exercise intolerance, and anorexia. More severely compromised animals evidence obvious abdominal distension, respiratory difficulty, or syncope. In some cases, signs of an underlying systemic illness dominate the clinical history. Rapidly developing effusions (atrial tear, traumatic laceration, bleeding neoplasm) cause acute hypotension, weakness, dyspnea, collapse, and, on occasion, sudden death.
The heart sounds are usually muffled, and lung sounds may also be diminished if a large pleural effusion is present. Dogs developing a left atrial tear secondary to chronic degenerative valvular disease have a loud systolic heart murmur, although it is decreased in intensity when compared to previous examinations. Careful examination usually reveals evidence of impaired venous return and elevated right heart filling pressures. Jugular venous distention or a positive hepatojugular reflux is almost invariably present. Measurement of central venous pressure (CVP) is valuable when evaluation of the jugular veins is equivocal. Measures of CVP usually exceed 10-12 mmHg (normal = < 6 mmHg) when cardiac tamponade is present. Hepatomegaly and free abdominal fluid are common findings unless the effusion is of very sudden onset.
Femoral arterial pulses are often weak as a consequence of peripheral vasoconstriction and reduced stroke volume. Pulsus paradoxus is a valuable clinical sign that may be seen in cardiac tamponade. It is defined as an inspiratory fall in systemic arterial pressure exceeding 10 mmHg. When the systemic veins are distended and ventricular filling is limited by cardiac tamponade, inspiration increases venous return to the right side of the heart and further raises intrapericardial pressure, reducing left ventricular distensibility. The interventricular septum is displaced toward the left ventricle, causing a significant reduction in left ventricular filling, stroke volume, and systemic blood pressure.
Low amplitude PQRS complexes are commonly observed in all leads, including the chest leads. Dampening of the PQRS complexes by the accumulated fluid and diminished cardiac filling are responsible for this observation. Electrical alternans, alternating variation in the amplitude of QRS complexes with every other beat, is sometimes observed and is a helpful clue in establishing the diagnosis. This phenomenon is caused by the rhythmic swinging of the heart within the fluid filled pericardial sac. Atrial arrhythmias, including atrial fibrillation, are sometimes observed. Nonspecific ST segment elevation or depression is also seen.
Characteristic findings include a large globular cardiac silhouette rounded in all views, enlarged vena cava, and occasionally distended pulmonary veins. With small effusions, these findings may not be evident. In animals with pleural effusion, the heart is often obscured and thoracentesis is required for evaluation of heart size and shape. Chest x-rays should always be carefully evaluated for evidence of a heart base mass and for metastatic disease.
Echocardiography is the most sensitive and specific non-invasive method of detecting pericardial effusion. Echocardiography can detect as little as 15 ml of intrapericardial fluid. An anechoic space between the epicardium and pericardium is the classic echocardiographic finding in pericardial effusion. The heart can often be observed swinging from side to side within the pericardium. This observation offers a logical explanation for electrical alternans as the heart can be seen to change position. Overall cardiac chamber size is diminished when pericardial effusion causes impaired cardiac filling. Echocardiography often permits visualization of intrapericardiac or cardiac mass lesions. Whenever possible, echocardiography should be performed before pericardiocentesis, as the pericardial fluid will greatly enhance the ability to visualize mass lesions on the heart or pericardium. It is important to realize that small tumors can be easily missed.
Pericardiocentesis and fluid analysis
Thoracic radiographs can be used as a guide for choosing a site to perform pericardiocentesis. A right-sided approach lessens the chance of coronary artery damage. While monitoring the ECG, an over-the-needle 6" catheter is advanced toward the heart. When fluid is obtained, the catheter is advanced into the pericardial sac, and the stylet is withdrawn. Aspirated fluid should be compared with peripheral blood (PCV, protein, xanthochromia, clotting) and the pericardial sac subsequently drained. Routine cytology and fluid analysis are prudently obtained to exclude bacterial, fungal, or obvious neoplastic etiologies. It is usually not possible to distinguish the most common neoplastic effusates hemangiosarcoma, heart base tumors) from idiopathic effusions on the basis of red cell or white cell counts, protein content, or by cytologic examination. Nor is the pH of the fluid a reliable indicator of the etiology of the effusion.
Surgery or repetitive pericardiocentesis are employed to palliate neoplastic effusions. Pericardiocentesis or pericardiectomy are often curative in cases of idiopathic pericardial effusion. If effusion continues to recur after several pericardiocenteses, pericardial biopsy should be performed together with a subtotal pericardiectomy. Subtotal pericardiectomy involves removing the pericardium below the phrenic nerves. Pericardial windows and biopsy of the pericardium and associated mass lesions can often be satisfactorily accomplished via thoracoscopic surgery, but there are many advantages to surgical exploration of the pericardium and heart. Small tumors and mesothelioma are often best diagnosed with this approach. Heart base tumors that are relatively small can sometimes be dissected free from the aorta and excised. Cysts and granulomas may also be removed at surgery, and left atrial tears or traumatic lesions may be repaired. Subtotal pericardiectomy is not always prudent for patients with hemangiosarcoma.
The prognosis is good for patients with idiopathic effusions or effusions due to pericardial cysts or peritoneopericardial hernias. The prognosis for dogs with hemangiosarcoma is poor even with aggressive surgical and medical therapy. The prospects for animals with heart base tumors are more variable. Many dogs with slow growing chemodectomas experience long survival times with good quality of life for a year or more following subtotal pericardiectomy.