As fluid accumulates within the pericardial sac, the pressure within the sac increases and progressively compresses the chambers of the heart. As the right-sided chamber are thinner walled than the left-sided chambers, they are compressed to a greater degree.
Compression of the right-sided chambers has two major consequences: 1) venous return is significantly decreased causing jugular venous distension and ascites; and 2) blood flow to the lungs is significantly decreased causing hypoxia and tachypnea. Once the pericardial pressure equals or exceeds the cardiac chamber pressures, the condition is referred to as cardiac tamponade. This is a life-threatening condition and will result in death by cardiovascular collapse if not treated.

Pericardial effusion is uncommon compared to other acquired cardiovascular diseases, but is not rare. This disorder occurs in both small and large animals. There are no feline breed predilections. Labrador retrievers and golden retrievers are the most commonly affected canine breeds. Overall, most cases involve large breed and giant breed dogs (90%) and there is a predilection for males (62%). Cattle most often develop pericardial effusion secondary to traumatic reticuloperitonitis or lymphoma. In horses, septic and idiopathic are the most common types reported.

The severity of clinical signs depends upon the rate of pericardial fluid accumulation. Historical features include exercise intolerance, inappetance, listlessness, and abdominal swelling. In horses, there is often a history of respiratory tract infection, fever, anorexia, and depression. Physical examination findings include lethargy, jugular venous distension, muffled heart sounds, and occasionally pericardial friction rubs. Ascites is consistently present in affected dogs. The two most common causes are neoplastic (hemangiosarcoma, heart-based tumor) and idiopathic or benign. Other, less common causes, are infectious (FIP in cats), trauma, chamber rupture, and secondary to congestive heart failure. In horses, septic and idiopathic are the most common types reported.

Results of a complete blood count, serum chemistry profile, and urinalysis are usually within normal limits. A mild anemia, neutrophilic leukocytosis, hyperfibrinogenemia, and hyperproteinemia may occur in horses with septic pericarditis and effusion.
Cytological evaluation of the pericardial fluid can be misleading if the effusion is serosanguinous (95% of all canine effusions). In benign effusions, activated mesothelial cells resemble neoplastic cells and a false positive may be reported. Conversely, neoplastic cells rarely exfoliate, therefore a false negative report is likely in cases of neoplastic effusion. In horses with suspected septic pericarditis, a culture and sensitivity of the fluid should be performed. There will be a large number of neutrophils, some degenerated, with septic pericarditis. Protein content of the fluid will be high and bacteria may be observed. Cytologic features of idiopathic pericardial effusion in horses is variable, with neutrophils, eosinophils, and macrophages present in variable numbers.

Radiographic findings include an increase in the size of the cardiac silhouette which takes on a roundish shape (there is a loss of contour caused by the cardiac chambers). The caudal vena cava is dilated. The interstitial density is increased, not from pulmonary edema, but secondary to loss of lung volume caused by the enlarged pericardial sac.

Echocardiography is the ideal test to definitively diagnose pericardial effusion. A tumor can be visualized in many cases of neoplastic effusion, but not all. When cardiac tamponade is present, the walls of the right atrium and right ventricle appear to collapse and flutter. The left-sided chambers are often decreased in size secondary to poor return from the lungs. Echocardiography can be used to guide the catheter during pericardiocentesis and to confirm that the effusion is absent following pericardiocentesis.
Electrocardiographically, the rhythm in most cases is normal sinus rhythm to sinus tachycardia. Occasional atrial premature and ventricular complexes may occur. The height of the R-waves is often decreased (less then 1 mV) and there may be a pattern of alternating heights of the R-waves, referred to electrical alternans. This electrocardiographic feature is virtually pathognomonic for pericardial effusion and results from the swinging motion of the heart within the fluid-filled pericardial sac.

Animals with cardiac tamponade should receive urgent treatment. The only effective treatment is to remove the effusion with a catheter, a procedure termed pericardiocentesis. Cardiac medications play no role in the treatment of this condition. Diuretics are contraindicated as they decrease blood volume and cause further collapse of the cardiac chambers. When performing pericardiocentesis, it is advisable to sedate the dog to allow full recovery of the effusion. The catheter should enter the chest on the right side, just above the costochondral junction at the fourth to fifth intercostal space. Attach a syringe or extension set with
stopcock and syringe (latter preferred) to the catheter. The system must be closed to avoid a pneumothorax. Pass the catheter directly towards the heart while intermittently aspirating. When the pericardial sac is entered, fluid (usually hemorrhagic) will freely flow into the syringe. Carefully advance the catheter over the needle into the pericardial sac. If arrhythmias develop, withdrawing the needle slightly usually suffices. Antiarrhythmic therapy is rarely needed. Place a sample into an EDTA tube for analysis. Remove as much fluid as possible. When performing pericardiocentesis in the horse, it is recommended that the left fifth intercostal space be used. This side is preferred to avoid the atria, coronary arteries, and right ventricle. Post-pericardiocentesis lavage, with or without antibiotics, is often performed in horses. Pericardiocentesis is easily performed and relatively safe, even for novices. A reluctance to perform this procedure because of inexperience should be tempered with the knowledge that medications are ineffective and contraindicated. It is advisable to confirm the presence of pericardial effusion echocardiographically, if possible, prior to performing pericardiocentesis.

Broad-spectrum antibiotics and parenteral fluids may be given in the immediate pre-pericardiocentesis interval and following pericardiocentesis. There are no reports that corticosteroids play a beneficial role in canine benign pericardial effusion, although they have been used with success in horses. Most tumors that cause neoplastic effusion are non-responsive to chemotherapy.

Most animals are ready for discharge the day following pericardiocentesis. In cases where benign effusion is suspected (no mass visible by echocardiography), the owner should be instructed to carefully monitor the animal for any signs of recurrence. Should this occur, a repeat pericardiocentesis is indicated. It is generally recommended that surgical exploration be performed if the animal is presented a third time.