**2.1 Idiopathic pericardial effusion**

The unknown causes (idiopathic) of pericardial effusion in dogs were accounting for 20–75%. This disorder is more common in large- and giant-breed dogs and is thought to be an inflammatory condition affecting the pericardial sac. One-time pericardiocentesis is curative in about half of cases. If multiple taps are required, pericardiectomy is recommended and is usually curative. The diagnosis is arrived after exclusion of possible causes of intrapericardial fluid accumulation thorough echocardiographic examination.

## **2.2 Congestive heart failure**

Although congestive heart failure is a common cause of pericardial effusion in cats, it is a less common cause in dogs. When congestive heart failure leads to intrapericardial fluid accumulation, cardiac tamponade is rare, and pericardiocentesis is almost never indicated. However, in few cases, modification of congestive heart failure medications may be warranted.

## **2.3 Infectious**

Pericardial effusion due to infectious cause is not so frequently observed. It may include multiple causes, viz. bacterial infection secondary to migrating foreign bodies. The most commonly isolated bacteria associated with the condition are *Actinomyces*, *Nocardia* [5] and *Coccidioides immitis* [6].

Further, constrictive and septic pericarditis also results in minimal pericardial effusion in dogs, mostly sequelae to infectious diseases.

### **2.4 Neoplasia**

In dogs cardiac neoplasia are rare; if present neoplastic pericardial effusion is commonly related with hemangiosarcoma, chemodectoma (common in brachycephalic breeds), and mesothelioma and less commonly related with ectopic thyroid carcinoma, lymphosarcoma and myxosarcoma. Prognosis depends on the type of tumour. Hemangiosarcomas are more aggressive and are likely to metastasize, and these tumours tend to effuse more rapidly. Palliative pericardiectomy is not usually recommended for hemangiosarcoma because the risk of severe and fatal intrathoracic hemorrhage is significant.

#### *Pericardial Effusion in Dogs DOI: http://dx.doi.org/10.5772/intechopen.89051*

However, in patients with aortic body tumours or mesotheliomas, tumours that effuse more slowly than hemangiosarcomas do, pericardiectomy is often palliative, allowing the fluid to be absorbed through the pleural surface and, thus, preventing the development of cardiac tamponade [4].

#### **2.5 Other causes**

#### *2.5.1 Atrial rupture*

In chronic mitral valve disease and severe left atrial enlargement, left atrial tearing is a possible cause of pericardial effusion. The pericardial effusion may contain a clot because hemorrhage is acute. The clot is often visible on an echocardiogram as a hypoechoic structure within the fluid-filled (anechoic) pericardial space. The hemorrhage rapidly leads to cardiac tamponade, cardiogenic shock, and possibly death. Pericardiocentesis in these patients may encourage continued bleeding into the pericardial space. Therefore, in general, pericardiocentesis is not recommended in these patients. If the patient destabilizes because of cardiac tamponade, pericardiocentesis is performed, but the prognosis is guarded at best.

Serositis and myocarditis caused by exposure to toxic metabolites that are eliminated by the kidneys result in **uremic pericarditis** [4].

There is a possibility of pericardial effusion caused by **hypothyroidism**, and hypothyroidism in human patients is a well-known cause of pericardial effusion, but cardiac tamponade is not a frequent clinical sign. However, the pathophysiology is not explained [7].

## **3. Pathophysiology**

The pericardium normally has minimal elasticity due to its fibrous nature. However, the pericardium can stretch when pressure is slowly placed on it. In pericardial effusion, as the fluid accumulates in the pericardial space, the ability of the pericardium to stretch is eventually exceeded, and further fluid accumulation subsequently results in increases in intrapericardial pressure. When the intrapericardial pressure increases to the pressure of the right atrium and ventricle (normally 4–8 mm Hg), cardiac tamponade develops [4] with variable degrees of haemodynamic collapse. The volume of fluid required to cause cardiac tamponade varies greatly, depending on the speed with which the fluid accumulates. In experimental canine models, as little as 25–100 mL of fluid rapidly injected into the pericardial space can raise intrapericardial pressure high enough to cause tamponade. In contrast, pericardial effusion that slowly increases in volume can result in a volume as high as 2 L in a large-breed dog before cardiac tamponade manifests.

Pericardial effusion is a resultant of several etiological factors, and the pathogenesis may vary depending on the aetiology. In neoplasias/nephritic syndrome/toxins, there is direct irritation of pericardium causing inflammation resulting in building of fluid. In case of systemic inflammatory diseases or infections, the inflammatory cells target and gets collected within the pericardium, whereas in atrial/ventricular rupture, direct addition of blood to the pericardial space results in effusion. In congestive heart failure, the fluid accumulation in pericardium is due to increase capillary hydrostatic pressure and altered starling forces.

Depending on the size of the enlarging pericardium (rapid accumulation of low volumes or slow accumulation of high volumes) and activation of pain fibers that are

#### **Figure 1.**

*Pericardial effusion-pathogenesis.*

responsible for expression of signs which may vary viz., asymptomatic or sharp pain with inspiration or dull pain. Increase in heart rate and peripheral vascular resistance can initially compensate these changes, thereby maintaining normal blood pressure. As the intrapericardial pressure rises further, left atrial and left ventricular filling are also compromised. Increased pericardium size (cardiomegaly) may compress the oesophagus, lungs and recurrent laryngeal nerve resulting in dysphagia, dyspnoea and hoarse voice. Further increase in the accumulation of pericardial fluid results in the compression of cardiac chambers, thereby increasing the filling pressures resulting in elevated intracardiac pressure. Impaired filling of the right heart results in venous congestion; thereby elevated jugular venous pressure, pedal oedema and hepatomegaly is evinced. Impaired filling of left heart/increased left ventricular end diastolic pressure results in respiratory distress. Impaired filling also impairs the cardiac output; thereby the blood pressure is decreased (hypotension) [8]. However, fixed ventricular volume increases the physiologic shift of septum towards left ventricle with inspiration, thereby lowering the left ventricular filling resulting in cardiac tamponade (**Figure 1**).

Cardiac tamponade results in decrease venous return, ventricular filling, stroke volume and cardiac output further resulting in shock/death of the patient.

#### **4. Clinical signs**

Clinical signs of pericardial effusion result from a combination of the volume of effusion, speed with which it accumulated, and underlying cause. Clinical signs may be vague until cardiac tamponade and associated cardiovascular

#### *Pericardial Effusion in Dogs DOI: http://dx.doi.org/10.5772/intechopen.89051*

decompensation develop. The interplay among these factors determines when, in the clinical course, intrapericardial pressure rise high enough to cause cardiac tamponade.

The most common presenting complaint of dogs with pericardial effusion is collapse, weakness, syncope, or lethargy. Dogs may present with abdominal distension and ascites (**Figure 2**) secondary to cardiac tamponade. Heart sounds are muffled, and lung sounds may also be muffled if there is associated pleural effusion. Femoral pulses are weak, and sometimes pulsus paradoxus may be palpated when the pulse is stronger during exhalation and weaker during inhalation. If there is cardiac tamponade, the animal may have signs of cardiogenic shock including pale mucous membranes, cold extremities, hypotension, tachycardia and collapse.

Dogs with chronic pericardial effusion typically have signs secondary to rightsided heart failure, including lethargy, exercise intolerance, respiratory difficulty (**Figure 3**), weight loss and abdominal distention. These signs may be progressive as the ability of the pericardium to stretch is exceeded, whereas dogs with acute pericardial effusion typically present with a history of acute collapse or weakness secondary to decreased cardiac output. Collapse sometimes occurs shortly after physical exertion, and syncope may also be noted.

**Figure 2.** *Distended abdomen in a dog with pericardial effusion.*

**Figure 3.** *Pericardial effusion affected dog with respiratory distress.*
