This rare disorder occurs as a result of the thickening and decreased pliability of the pericardium. The thickened pericardium
causes decreased diastolic filling and clinical signs of right heart failure due to elevated right atrial pressure. This is
then reflected to the systemic veins, although the pericardial effusion may be minimal. Constrictive pericarditis is extremely
challenging to confirm and often requires cardiac catheterization for definitive diagnosis. Infectious etiologies (e.g. fungal infection) have been demonstrated, although the last major survey showed that most cases were idiopathic.13
Septic pericarditis is rarely diagnosed in small animals, but dogs and cats occasionally develop infective pericarditis secondary
to bacterial or fungal infections. Feline infectious peritonitis can also lead to clinically significant pericardial effusion
in cats. There have been reports of sterile pericardial effusion in cats with chronic uremia, but this is uncommon.9,14
The most common, clinically significant pericardial diseases in small animals result in pericardial effusion, and it is the
effusion that often leads to the clinical signs recognized by owners and practitioners. As pericardial effusion accumulates,
intrapericardial pressure may exceed the diastolic right atrial and ventricular pressure. When this occurs, there may be diastolic
right atrial and ventricular collapse, and right heart filling is inhibited. Because cardiac output is reduced, sympathetic
tone increases and sinus tachycardia results. If the process is chronic, the renin-angiotensin-aldosterone system will be
activated and will result in sodium and water retention by the kidneys. However, these mechanisms cannot compensate adequately
if the elevated intrapericardial pressure is not corrected. When acute cardiac tamponade occurs, clinical signs consistent
with low cardiac output often develop. Typical signs include lethargy, collapse, exercise intolerance, syncope, tachycardia,
poor pulse quality, and hypotension.15 Rapid accumulation of even small volumes of fluid, as seen with hemopericardium secondary to right atrial hemangiosarcoma,
can quickly lead to decompensation. This is because the pericardial membrane does not have time to stretch and intrapericardial
pressure rises rapidly.1 However, the pericardium will hypertrophy and stretch over time so that patients with a slower, more gradual accumulation
of fluid can often tolerate large volumes of pericardial effusion before cardiac tamponade develops. In these chronic cases,
signs of volume overload, such as ascites, will be present. Because cardiac tamponade inhibits right atrial filling, elevated
systemic venous pressure leads to engorged systemic veins. Cardiac tamponade and, therefore, most of these clinical signs,
rarely occur in cats.16
Pulses are usually weak, and pulsus paradoxus may be present if cardiac tamponade is present. Pulsus paradoxus is an increase
in pulse pressure on expiration and a decrease on inspiration.17 Although this variation in pulse strength is normally present during the respiratory cycle, pulsus paradoxus is an accentuation
of this phenomenon in patients with tamponade, so much so that the variation is palpable.
Muffled cardiac sounds and a decreased palpable precordial impulse are often noted with marked volumes of pericardial fluid.
Patients with cardiac tamponade and clinical signs of right heart failure usually have noticeable jugular vein distention.
Sinus tachycardia is often seen as a compensatory mechanism for decreased left ventricular stroke volume in patients with
cardiac tamponade due to elevated sympathetic tone.
Clinical signs specific to peritoneopericardial diaphragmatic hernias can be vague and are usually referable to the organ
that is herniated. Gastrointestinal signs predominate and include vomiting, anorexia, and diarrhea, but cough and dyspnea
may also be present. Patients with this type of hernia rarely present in congestive heart failure or cardiac tamponade, and
the problem is most often an incidental finding on thoracic radiographs.18
A thorough physical examination is usually adequate to make a tentative diagnosis when cardiac tamponade is present. However,
many animals with pericardial effusion have no clinical signs, and some forms of pericardial disease (e.g. peritoneopericardial diaphragmatic hernia) are clinically silent. You can use the following diagnostic modalities to confirm
a tentative diagnosis of pericardial effusion before you perform pericardiocentesis. However, if a patient is clinically unstable,
you may not have enough time to perform all of these tests before doing a pericardiocentesis.