Straightening of the caudal border of the laterally projected cardiac silhouette is a reliable marker of left atrial dilation.
In the ventrodorsal view, left atrial enlargement may be apparent as "splitting" of the mainstem bronchi or by a prominence
of the cardiac silhouette at the 3 o'clock position.
Distention of the pulmonary veins provides indirect evidence of venous hypertension and though this finding is relatively
inconsistent, it heralds the development of edema. When present, pulmonary edema often has a central or perihilar distribution.
Interstitial edema causes blurring of vascular detail and often precedes the development of alveolar edema that is indicated
by the presence of air bronchograms.
Radiography is the single most informative diagnostic test in most cases of MVD. However, thoracic radiographs must be obtained
and interpreted with careful attention to the dog's body type, phase of respiration during the exposure and radiographic technique.
The individual chambers of the heart are poorly resolved by plain radiography. Despite this, the thoracic radiograph usually
provides an estimate of left atrial size that is adequate for clinical purposes. This is fortunate because assessment of left
atrial size is crucial in making the determination of the relative importance of cardiac disease to the clinical presentation
of coughing dogs with murmurs of mitral regurgitation.
With very few exceptions, a diagnosis of heart disease or failure as a cause of cough is simply untenable in the absence of
radiographic left atrial enlargement. MVD is a chronic disease and, therefore, enlargement of the left atrium and ventricle
precedes the development of pulmonary edema.
Obviously, the left atrium must be enlarged before it can be incriminated as a cause of bronchial compression. In the absence
of radiographic left atrial enlargement, cough is related to primary respiratory tract disease.
Echocardiographic evaluationEchocardiography can be used to assess the degree of left atrial and ventricular dilation and can provide estimates of systolic
myocardial function (contractility). Doppler studies can be used to document the presence of disturbed systolic flow within
the left atrium and, therefore, provide noninvasive confirmation that the murmur is indeed caused by mitral valve regurgitation.
However, mitral regurgitation is by far the most likely cause of an acquired murmur in older small breed dogs and, most often,
Doppler studies are simply confirmatory.