Responding to a Cardiac Emergency: Pericardial Effusion in Canine Patients

Pericardial effusion is considered a cardiac emergency situation that is most commonly seen in canine patients. It is very important, especially for veterinary technicians in emergency care facilities, to be able to recognize the signs of pericardial effusion and quickly respond.

Pathophysiology

The normal pericardium has an outer fibrous pericardium and inner serous pericardium, which consists of a parietal and visceral layer (Figure 1). The pericardial cavity—the area between these 2 layers of the serous pericardium—normally contains 1 to 15 mL of a plasma ultrafiltrate.¹

Even though animals can survive and function normally without an intact pericardium, the pericardium serves many important purposes. The pericardium:

• Structurally supports the heart in the thorax, preventing excessive motion of the heart
• Protects the heart from neoplasia and infections.

FIGURE 1. Diagram depicting relationship between the pericardium and heart.

Pericardial Effusion

Excessive fluid in the pericardial space is called pericardial effusion. When effusion accumulates slowly, the pericardium can enlarge to accommodate this increase in volume and, if intrapericardial pressure is low, clinical signs may not be present and cardiac function remains relatively normal.

When effusion accumulates quickly or intrapericardial pressure rises quickly, intrapericardial pressure surpasses the normal diastolic pressure in the right ventricle and cardiac tamponade occurs.² Pericardial effusions of large volumes can also compress the lungs and trachea, causing respiratory difficulties and coughing. When the esophagus is compressed by excess fluid buildup, dysphagia or regurgitation may occur.³

Cardiac Tamponade

Cardiac tamponade results in a decrease in venous return, stroke volume, cardiac output, and ventricular filling. Therefore:

• The body tries to compensate by increasing heart rate and peripheral vascular resistance to help maintain normal blood pressure.
• As intrapericardial pressure continues to rise, it causes collapse of the right side of the heart, preventing cardiac output into the pulmonary artery.
• Since blood is no longer leaving the right side of the heart, there is no venous return to the left atrium and ventricle,² eventually resulting in cardiogenic shock manifested by lethargy, hypotension, poor pulse quality, and tachycardia.

Causes

Congenital Causes

Congenital causes of pericardial effusion may include:¹

• Peritoneopericardial diaphragmatic hernia: A condition in which the septum transversum fuses with the pleuroperitoneal folds, creating an incomplete separation of the abdominal and thoracic cavities and usually resulting in a small amount of fluid accumulation. However, gastrointestinal signs are more commonly noted due to the presence of abdominal organs within the pericardial cavity.
• Pericardial cysts: Another congenital cause of pericardial effusion, but they are much rarer and often asymptomatic.

Acquired Causes

An acquired cause of pericardial effusion is pericarditis, which stems from neoplastic, immune, inflammatory, and, occasionally, infectious disease processes.

• Neoplastic processes are the most common cause of pericardial effusion and cardiac tamponade:
  • Hemangiosarcoma is commonly found in the right atrial region or right atrial appendage
  • Chemodectomas are often found at the heart base
  • Mesotheliomas and metastatic tumors may be identified.
• Idiopathic pericardial effusions often are linked to inflammatory or immune processes. In these idiopathic cases, the effusion is sterile, and often hemorrhagic. Thickening of the epicardium and pericardium may be seen during histologic examination, along with associated inflammation.
• Infectious processes occur least commonly, but can include fungal disease, such as coccidiomycosis, and, rarely, bacterial pericarditis.

Congestive heart failure, uremia, and decreased oncotic pressure can result in small-volume pericardial effusion, usually without cardiac tamponade.¹

Presentation

Prevalence

Pericardial effusion is commonly seen in golden retrievers and other medium to large breed dogs. Any age dog can be affected, although one study noted the median age of dogs with idiopathic pericardial effusion was 7 years, while in dogs with nonidiopathic effusions, the median age was 9 years.⁴ This same study found a higher prevalence of pericardial effusions in male dogs.

Clinical Signs

Patients experiencing chronic pericardial effusion may demonstrate weakness, abdominal enlargement, cough, tachypnea, and episodes of syncope. In those with acute pericardial effusion, acute episodes of collapse and weakness may be evident.² As pericardial fluid accumulates, shock and death can occur.

Physical Examination

Table 1 lists cardiac signs that may be noted upon physical examination of patients with pericardial effusion. The patient frequently has a decreased level of consciousness due to a decrease in oxygen delivery to the brain,¹ and patients with infectious pericarditis may be febrile.²

Types of Fluid

While it is important to understand which types of pericardial effusion are more likely to be seen with certain conditions, cytology of pericardial effusions is rarely able to differentiate between neoplastic and idiopathic causes. The etiology of pericardial effusions is most commonly determined by echocardiography (see Management).

Hemorrhagic effusions are common in dogs, while transudates are moderately frequent in dogs and cats. Exudative effusions are fairly rare in small animals.

Hemorrhagic Effusions

Hemorrhagic effusions appear dark red, with a:

• Packed cell volume > 7%
• Specific gravity > 1.015
• Protein concentration > 3 g/dL.

Hemorrhagic pericardial effusions in canine patients are commonly caused by hemangiosarcoma. Heart base tumors, pericardial mesotheliomas, malignant histiocytosis, lymphoma, and metastatic carcinomas may also cause hemorrhagic pericardial effusion.

Idiopathic pericardial effusion is another cause of hemorrhagic pericardial effusions, most commonly seen in medium to large breed dogs, such as golden retrievers, Labrador retrievers, and Saint Bernards. Male dogs of a median age of 7 years are commonly affected.⁴

Less common causes of hemorrhagic pericardial effusion include left atrial rupture, coagulopathy, penetrating trauma, and uremic pericarditis.⁵

Transudative Effusions

Pure transudative fluids are typically clear, with a specific gravity < 1.012 and protein concentration < 2.5 g/dL. In contrast, modified transudates often appear slightly cloudy or have a pink tinge, with a specific gravity of 1.015 to 1.030 and protein concentration of 2.5 to 5 g/dL.

Causes of transudative effusions include congestive heart failure, hypoalbuminemia, peritoneopericardial diaphragmatic hernia, pericardial cysts, and certain toxemias.²

Exudative Effusions

Exudative effusions appear cloudy to opaque, with a protein concentration > 3 g/dL and specific gravity > 1.015. Although not common in small animals, sterile exudative effusions have been seen in dogs with leptospirosis, distemper, bacterial pericarditis, and idiopathic pericardial effusion, and in cats with feline infectious peritonitis and toxoplasmosis.³

Diagnostics

Imaging

Although less sensitive than echocardiography, radiography can be a useful start for primary care facilities. The overall cardiac silhouette appears enlarged, and excessive amounts of fluid can cause a globoid-shaped heart on both the ventrodorsal and lateral views (Figure 2).

Advanced imaging, such as magnetic resonance imaging, is now being used more often to diagnose the presence of cardiac masses in patients with pericardial effusion.

FIGURE 2. Lateral thoracic radiograph of an enlarged and globoid-shaped heart due to an excessive amount of fluid accumulation in the pericardium.

Echocardiography

Echocardiography is the most frequently used test for detection of pericardial effusion and diagnosis of pericardial masses.

Pericardial Effusion. Smaller effusions are first seen over the posterobasal part of the left ventricle.⁵ The effusion appears as a sonolucent, echo-free void between the parietal pericardium and the epicardium (Figure 3). In patients with large quantities of fluid, the heart may appear to be swinging back and forth during echocardiography.³ When the effusion has increased and is circumferential, it may be graded as small, moderate, or large (Table 2).

Cardiac Tamponade. When cardiac tamponade occurs, diastolic collapse of the right atrium and, sometimes, the right ventricle may be seen during the echocardiogram. The heart may also “swing,” because it floats within the pericardial space in a phasic fashion. In these patients, right atrial collapse may last longer during the systolic phase. Lastly, hepatic vein distension and ascites can help support the presence of cardiac tamponade as well.

FIGURE 3. Echocardiogram image of a heart surrounded by pericardial fluid. RV = right ventricle; RA = right atrium; LV = left ventricle; LA = left atrium

Electrocardiography

Electrocardiography (ECG) can also be a useful diagnostic tool, especially when determining the presence of cardiac tamponade.

• Low-voltage (small) QRS complexes are common. Reduced voltage can also be caused by pneumothorax, emphysema, and infiltrative myocardial disease.⁵
• Electrical alternans occurs when the size or configuration of the QRS complex, and occasionally the T wave, changes beat-to-beat. This ECG finding may suggest a large volume of pericardial effusion, and results from the heart moving back and forth within the pericardium (Figure 4).
• Sinus tachycardia is common in cardiac tamponade; while less frequent, atrial or ventricular tachyarrhythmias may occur as well.²

FIGURE 4. Electrocardiogram demonstrating electrical alternans, in which the height of the QRS complex alternates with each beat due to the heart swinging within the pericardium. This finding is rate dependent and more commonly seen at a normal heart rate.

Management

Pericardiocentesis

When cardiac tamponade is present, immediate pericardiocentesis is indicated. Reduction of pericardial pressure by removal of fluid results in greater cardiac output and a decreased heart rate, which should improve the clinical condition of the patient. Diuresis—which takes place after cardiac tamponade is relieved—typically resolves any ascites. If ascites is severe, it can be removed to make the patient more comfortable and relieve ventilatory compromise.⁶

To assist with performing pericardiocentesis, the veterinary technician:

• Positions the patient in left lateral recumbency (procedure ideally performed on right side)
• Provides sedation with a benzodiazepine and opioid, if necessary
• Can provide oxygen supplementation via flow by or mask
• Places a peripheral IV catheter in the cephalic vein (Table 3)
• Shaves and scrubs a square over the fourth and fifth intercostal space, where the cardiac apex beat is palpated
• Watches the ECG to monitor for any arrhythmias while the procedure is being performed; a critical component to the process.

Once pericardiocentesis has been performed, signs associated with right-sided congestive heart failure tend to subside very quickly. When pericardial effusion is secondary to diseases causing congestive heart failure, congenital malformations, or hypoalbuminemia, the patient often improves once the underlying condition has been treated.³

Complications of pericardiocentesis include cardiac injury or puncture resulting in arrhythmias, pneumothorax or hemorrhage due to lung laceration, coronary artery laceration, and potential for dispersion of infection or neoplastic cells into the pleural space.⁵

Pericardiectomy

When pericardial effusion recurs after one or more pericardiocentesis procedures, pericardiectomy can be considered.

Traditionally, median sternotomy and right lateral thoracotomy are pursued. Open thoracotomy allows:

• Complete removal of the pericardium below the level of the phrenic nerve
• Accurate identification of masses arising from the right atrium
• Excision of the right auricular appendage when a mass is isolated to that area of the heart.

Thoracoscopic pericardiectomy is an alternative method, with a lower morbidity rate than traditional thoracotomies. However, the inability to visualize the right atrium as a whole is a disadvantage of this technique.

Percutaneous Balloon Pericardiotomy

A minimally invasive option is available: the application of percutaneous balloon pericardiotomy in which a balloon catheter is inserted percutaneously into the pericardial sac and inflated until it tears the pericardium, resulting in drainage of pericardial fluid into the pleural cavity.

Percutaneous balloon pericardiotomy requires only minimal sedation, usually a narcotic and benzodiazepine, and a local anesthetic, making it relatively safe for unstable or critical patients. However, it doesn’t allow exploration or biopsy of the heart or pericardium.⁷

This procedure was reported to be successful in 4 out of 6 canine patients with recurrent pericardial effusion,⁸ and may be considered in patients with a poor long-term prognosis as a result of neoplasia.

Summary

The ability to recognize potential history and clinical signs consistent with pericardial effusion can greatly improve the technician’s triage skills. Rapid detection of clinical signs of pericardial effusion, along with swift implementation of treatment, can lead to a more successful outcome for the patient.

ECG = electrocardiography