Figure 7-2 Lead II ECG tracing from a dog indicating atrial fibrillation and left ventricular enlargement. Note the irregular rhythm, lack of P waves, and widened QRS complex. 50 mm/sec; 10 mm/mV.

Chest Radiography

• Chest radiographs are relatively insensitive to mild increases in the heart size. A single set performed in the asymptomatic occult phase contributes relatively little to the immediate diagnosis; however, serial radiographs are well suited to monitor progressive heart enlargement and progression of disease. In the overt clinical phase, radiographs are invaluable in helping to diagnose congestive failure and to monitor response to treatment.

• The chest radiographs in Doberman Pinschers are often misleading in that heart enlargement is less striking than in other breeds with similar clinical signs. In these instances, cardiac ultrasound helps to determine the magnitude of left-sided heart enlargement and systolic dysfunction.

• The chest radiographs in Boxers with ARVC are usually normal.

Echocardiography

• Echocardiography is widely used to quantify heart enlargement and systolic function. Routine echocardiography is not particularly sensitive in detecting early changes in occult disease, nor is it particularly helpful after a diagnosis of end-stage disease is made. As such, the utility of echocardiography increases as disease moves out of the occult phase to the overt clinical phase then declines again as the patient advances into end-stage disease. Early in the course of disease, many dogs possess normal echocardiographic examinations, despite having a significant numbers of ventricular arrhythmias.

• Echocardiographic criteria are used to help diagnose occult DCM.

• In Doberman Pinschers, a left ventricular end-diastolic diameter (LVIDd) > 46 mm or a left ventricular end-systolic diameter (LVIDs) > 38 mm is highly suggestive of early disease. These values may not be applicable to Dobermans with very large body weights, and LVIDd > 49 mm or LVIDs > 42 mm may be better guides in this population.

• In Doberman Pinschers, fractional shortening is an unreliable index for occult DCM, as this breed typically has fractional shortening values in the mid or low 20% range. Similarly, large-breed dogs with athletic lifestyles commonly display fractional shortening values in the mid 20% range, and longitudinal echocardiographic studies or Holter monitoring is required to determine whether disease is truly present.

• In Irish Wolfhounds, occult disease is defined as LVIDd > 61.2 mm, LVIDs > 41 mm, fractional shortening < 25%, end-systolic volume index > 41 ml/m², or E-point to septal separation > 10 mm.

• Newer ultrasound modalities such as Doppler tissue imaging may be more sensitive in detecting early abnormalities of myocardial contractility.

• As disease progresses from occult to overt, echocardiography helps monitor heart enlargement, assess contractility, and evaluate secondary mitral regurgitation. Echocardiography is used in conjunction with chest radiographs to help decide when to initiate therapy.

• Common echocardiographic findings in dogs with advanced occult or overt clinical disease include the following:

• Moderate to severe left ventricular and atrial enlargement (Figure 7-3).

• Reduced systolic motion of the left ventricular wall and interventricular septum (Figure 7-4).

• Mild to moderate mitral regurgitation secondary to mitral annulus dilation.

• Incomplete systolic opening of the aortic valves.

• Decreased aortic blood flow velocity.

• Increased mitral valve E-point to septal separation (normal < 6 mm) (Figure 7-5).

• Decreased systolic thickening of the left ventricular wall and interventricular septum.

• The echocardiogram in Boxers with ARVC is usually normal. Subtle right ventricular dilation or wall motion abnormalities may be noted.

Figure 7-3 2-dimensional echocardiogram of the left ventricle (LV) and atrium (LA) of a Great Dane with dilated cardiomyopathy. Note the dilated ventricular and atrial chambers. RV, Right ventricle; RA, right atrium.

Figure 7-4 M-mode echocardiogram of the left ventricle of a Cocker Spaniel with dilated cardiomyopathy. Note the decreased systolic motion of the interventricular septum (arrow) and left ventricular free wall (arrowhead).

Figure 7-5 M-mode echocardiogram of the left ventricle and mitral valve of a Brittany Spaniel with dilated cardiomyopathy. Electronic calipers are being used to measure the E-point to septal separation (EPSS), which is markedly increased over normal.