Electrocardiogram (ECG) findings can support a diagnosis if a marked volume of pericardial effusion is present, but ECG findings
are not pathognomonic. Electrocardiographic findings consistent with a diagnosis of pericardial effusion include
1. Decreased R-wave amplitude (due to increased fluid insulation between the heart and surface electrodes)
2. Sinus tachycardia (with concurrent cardiac tamponade)
3. Electrical alternans, particularly when large volumes of effusion are present. This is the phenomenon in which the R-wave
amplitude varies markedly in a beat-to-beat fashion (Figure 2).19 Electrical alternans, produced by swinging of the heart within the fluid-filled pericardial space, is usually only present
with large effusions. It has been shown in an experimental model that electrical alternans is rate-dependent and is most likely
to occur at relatively normal heart rates in dogs (90 to 144 beats/min).20
2. These lead II ECGs (25 mm/sec) from a dog show electrical alternans in the top image and a sinus rhythm without electrical
alternans after pericardiocentesis in the bottom image. Note the reduction in heart rate after pericardiocentesis.
4. Ventricular complexes may also be present.
With pericardial abnormalities such as peritoneopericardial diaphragmatic hernias, the ECG findings may be normal.
3 & 4. Lateral and dorsoventral thoracic radiographs from a dog with marked pericardial effusion. Note the large, round or
globoid cardiac silhouette typical with large amounts of pericardial effusion.
High-quality thoracic radiographs are invaluable for diagnosing pericardial effusion and usually demonstrate a globoid cardiac
silhouette. The size of the cardiac silhouette will depend on the volume of effusion (Figures 3 & 4). Pericardial effusion can often be distinguished from dilated cardiomyopathy because the cardiac silhouette is rounder and
more clearly demarcated. However, you must consider other possible differential diagnoses for a large, rounded cardiac silhouette,
such as tricuspid dysplasia. Also, occasionally small effusions can lead to cardiac tamponade, yet the radiographic findings
are ambiguous. Rarely, heart base tumors can be seen on radiographs; they may lead to tracheal deviation.