Cardiomyopathy is estimated to be present in 10-20% of the general domestic feline population. In many affected cats there is some historical or clinical evidence that the disease is present, but in a number of cases the cardiomyopathy is completely occult until congestive heart failure (CHF) arises. Left-sided CHF is common in cats, and thoracic radiographs are often used to assess the cardiac silhouette and potential for other pathologic sequelae such as pulmonary edema and pleural effusion in cats with acute left-sided CHF. In the most common types of feline cardiomyopathy, the left ventricular walls may thicken concentrically so that the lumen size of the left ventricle is significantly diminished, leading to reduced cardiac output as the heart pumps, but the external silhouette of the left ventricle as visualized on radiographs will appear normal. The left atrial walls, however, are much thinner than those of the left ventricle and they may stretch significantly in a failing heart, causing a radiographic or echocardiographic image of the left atrium to be noticeably enlarged in a cat with left-sided CHF.
In this retrospective case series* of 100 cats with acute left-sided CHF, these investigators hypothesized that left atrial size as determined from thoracic radiography could be normal. In many general practice and primary emergency care settings, survey thoracic radiography may be the only imaging modality available to evaluate a cat in CHF, so the sensitivity and specificity of radiography in identifying left atrial enlargement in this patient population is important in reaching a diagnosis.
Left atrial size was assessed on both right lateral and ventrodorsal radiographic views. The left atrium was considered enlarged if there was an obvious bulge extending caudodorsally in the area of the left atrium on the right lateral projection, or if there was a bulge noted in the left border of the cardiac silhouette from 1:30 to 3 o'clock on the ventrodorsal view, or if the left atrial measurement on the vertebral heart scale was > 1.30 times the length of the 4th thoracic vertebra as visualized on the right lateral view. These measurements were compared with left atrial size as assessed by 2-dimensional echocardiography.
Left atrial enlargement was identified in 96% of the cats on echocardiography. Using thoracic radiography, left atrial enlargement was found in 48% on the lateral view, 53% on the ventrodorsal view, and in 64% on any radiographic view. There was no left atrial enlargement noted on any radiographic view in 36% of the patients. There was also poor agreement between echocardiographic and radiographic methods of left atrial size estimation. Cats with no left atrial enlargement identified on radiographs had similar disease presentations and similar triggers for their CHF when compared with cats who did have radiographic evidence of left atrial enlargement. Both groups also had similar histories of diuretic use, in that approximately half had been previously treated with diuretics and half had not.
Those cats with an extremely large left atrium (maximum left atrial dimension > 25 mm; n=10) on echocardiography also demonstrated left atrial enlargement on thoracic radiographs. Approximately 2/3 of cats with moderate to severe enlargement of the left atrium on echocardiography (maximum left atrial dimension >20 mm; n=49) demonstrated left atrial enlargement on thoracic radiographs. The use of thoracic radiographs to identify left atrial enlargement had the best balance between specificity and sensitivity in those animals with a maximum left atrial dimension of > 20 mm on echocardiography, which is fairly large (normal maximum left atrial dimension is < 15.7mm). Radiographic evidence of left atrial enlargement may therefore not be present in a clinically relevant number of cats with acute left-sided CHF.
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*Reference: Schober KE, Wetli E, Drost WT. Radiographic and echocardiographic assessment of left atrial size in 100 cats with acute left-sided congestive heart failure. Vet Radiol Ultrasound 2014;55(4):359-67.