Page 205 - Small Animal Internal Medicine, 6th Edition

CHAPTER 9 Pericardial Disease and Cardiac Tumors 177

to support an infectious cause in dogs. Idiopathic pericardial normal cardiac filling pressure. This accumulation impedes
effusion is reported most frequently in medium- to large- venous return and cardiac filling. As long as intrapericar-
VetBooks.ir breed dogs. Golden Retrievers, Labrador Retrievers, and dial pressure remains low, cardiac filling and output remain
relatively normal. If fluid accumulates slowly, the pericar-
Saint Bernards may be predisposed. Although dogs of any
age can be affected, the median age is 6 to 7 years. More cases
pericardial fluid volume at relatively low pressure. However,
have been reported in males than females. Mild pericardial dium may distend enough to accommodate the increased
inflammation, with diffuse or perivascular fibrosis and focal pericardial tissue is relatively noncompliant. Rapid fluid
hemorrhage, is common on histopathologic examination. accumulation or a large effusion causes a steep rise in intra-
Layers of fibrosis suggest a recurrent process in some cases. pericardial pressure, leading to cardiac tamponade. Pericar-
Constrictive pericardial disease is a potential complication. dial fibrosis and thickening further limit the compliance of
Other, less common causes of intrapericardial hemor- this tissue.
rhage include left atrial (LA) rupture secondary to severe A pericardial effusion of extremely large volume can
mitral insufficiency (see Chapter 6, p. 130), coagulopathy cause clinical signs by virtue of its size, even without overt
(mainly rodenticide toxicity or disseminated intravascular cardiac tamponade. Lung and/or tracheal compression can
coagulation), penetrating trauma (including iatrogenic lac- compromise ventilation and stimulate cough; esophageal
eration of a coronary artery during pericardiocentesis), and, compression can cause dysphagia or regurgitation.
possibly, uremic pericarditis.
CARDIAC TAMPONADE
TRANSUDATES Cardiac tamponade develops when pericardial fluid accu-
Pure transudates are clear, with a low cell count (usually mulation raises intrapericardial pressure to or above the
<1000 cells/µL), specific gravity (<1.012), and protein normal cardiac diastolic pressure. This external compres-
content (<2.5 g/dL). Modified transudates might appear sion of the heart progressively limits filling, initially of the
slightly cloudy or pink tinged. Their cellularity (≈1000-8000 more compliant right heart, then the left. Cardiac output
cells/µL) is still low, but total protein concentration subsequently falls while systemic venous pressure rises. Pres-
(≈2.5-5.0 g/dL) and specific gravity (1.015-1.030) are higher sure in all cardiac chambers and the great veins eventu-
than those of a pure transudate. Transudative effusions occur ally becomes equilibrated during diastole. Neurohormonal
in some dogs and cats with CHF, hypoalbuminemia, PPDH, compensatory mechanisms are activated as tamponade
pericardial cysts, or toxemias that increase vascular perme- develops. Gradual pericardial fluid accumulation results
ability (including uremia). These conditions usually are asso- in signs of CHF because of compensatory volume reten-
ciated with relatively small-volume pericardial effusion; tion and the direct effects of impaired cardiac filling. Mani-
cardiac tamponade is rare. festations of systemic venous congestion and right-sided
CHF (ascites and pleural effusion) usually predominate
EXUDATES because of the right heart’s thinner wall and lower pres-
Exudative effusions are cloudy to opaque or serofibrinous to sures. Pericardial effusion does not typically affect cardiac
serosanguineous. They typically have a high nucleated cell contractility directly, but reduced coronary perfusion during
count (usually much higher than 3000 cells/µL), protein tamponade can impair both systolic and diastolic func-
content (often much above 3 g/dL), and specific gravity tion. Low cardiac output, arterial hypotension, and poor
(>1.015). Cytologic findings are related to the etiology. Exu- organ perfusion can ultimately lead to cardiogenic shock
dative pericardial effusions are rare in small animals, except and death.
in cats with FIP. The rate of pericardial fluid accumulation and the disten-
Infectious pericarditis usually is related to plant awn or sibility of the pericardial sac determine whether and how
porcupine quill migration, extension of a pleural or medias- quickly cardiac tamponade develops. Rapid accumulation of
tinal infection, bite or other penetrating wounds, or possibly even a relatively small volume can raise intrapericardial pres-
bacteremia. Various bacteria (aerobic and anaerobic), acti- sure sharply. A gradual process is implied when the pericar-
nomycosis, coccidioidomycosis, aspergillosis, disseminated dial fluid volume is large. Cardiac tamponade is relatively
tuberculosis, and, rarely, systemic protozoal infections have common in dogs but rare in cats.
been identified. Sterile exudative effusions have been Pulsus paradoxus is the term used to describe the exag-
reported in association with leptospirosis, canine distemper, gerated variation in arterial blood pressure that occurs
and idiopathic pericardial effusion in dogs, and with FIP and during the respiratory cycle because of cardiac tamponade.
toxoplasmosis in cats. FIP is the most important cause of During inspiration intrapericardial and right atrial (RA)
symptomatic pericardial effusion in cats. Chronic uremia pressures fall, which facilitates right heart filling and pulmo-
occasionally causes a sterile, serofibrinous, or hemorrhagic nary blood flow. At the same time, left heart filling is reduced
effusion. as more blood is held in the pulmonary vasculature and the
interventricular septum bulges leftward from the inspiratory
Pathophysiology increase in right ventricular (RV) filling; consequently, left
Fluid accumulation within the pericardial space causes clini- heart output and systemic arterial pressure decrease during
cal signs when it raises intrapericardial pressure to or above inspiration. The variation in systolic arterial pressure between