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.

Obtaining high-quality images for radiographic evaluation of the cardiac silhouette in cats presenting with possible CHF can be challenging for a number of reasons. Many of these cats present in severe respiratory distress and supportive care such as oxygen, sedatives, diuretics and cautious handling must be provided before and during radiography. Therefore, these investigators did not exclude from the study cats who had received recent prior administration of low-dose oral or parenteral diuretics. In addition, the presence of concurrent respiratory or intrathoracic disease such as allergic airway disease, heartworm-associated respiratory disease, noncardiogenic or cardiogenic pleural effusion that may obscure all or part of the cardiac silhouette, and any pulmonary edema may confound the clinician’s ability to discern cardiac from respiratory disease in these patients.

Acute onset left-sided CHF was diagnosed in the study cats based on history and clinical findings, principally tachypnea, labored breathing and respiratory distress; evidence of pulmonary edema and/or pleural effusion on survey radiographs; and response to diuretic therapy. The cats were evaluated within 12 hours of presentation using both thoracic radiographs and echocardiography. Most of the cats included in the study, which evaluated thoracic radiographs and echocardiograms obtained from 2003 to 2012 at a veterinary teaching hospital in the USA, were mixed breed cats; Himalayan, Maine Coon, and Siamese breeds were represented by three patients each. Mean age of the cats was 8.1 years (range 0.3-19 years). Twenty-eight of the patients were female and 72 were male.

The primary diagnoses established in these patients included hypertrophic cardiomyopathy (n=62), unclassified cardiomyopathy (n=7), restrictive cardiomyopathy (n=6), congenital heart disease (n=7), dilated cardiomyopathy (n=6), hyperthyroidism (n=3), and bacterial endocarditis (n=1). Several other cats had diagnoses that were more unusual in terms of potential association with CHF: chronic renal failure with systemic hypertension (n=4), chronic anemia (n=2), diabetes mellitus with complete atrioventricular block (n=1), and no underlying or concurrent disease (n=1). Thirty-six of the cats had been previously stable prior to developing acute left-sided CHF, and potential triggers for their decompensation included: parenteral fluid administration (n=15), acute arterial thromboembolism (n=12), parenteral depot corticosteroid administration (n=7), general anesthesia (n=1), and possible myocardial infarct (n=1). Almost half (n=43) of the patients had received furosemide prior to any imaging studies.

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 4^th 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. [PJS]