QUARTERLY BEAT / JULY 2025
to the negative effects of repetition artifact that results from gas within the pulmonary parenchyma. Additionally, thoracic radiographs provide a global view of the thorax, evaluating structures that may influence interpretation of cardiopulmonary or pleural space pathology. As a result, thoracic radiographs are essential and should be acquired when safe to do so. A complete thoracic radiographic study involves acquiring three projections: right lateral, left lateral, and either ventrodorsal (VD) or dorsoventral (DV) projections. A DV projection is likely safer in most patients as it maximizes ventilation. The only downside is that if pleural effusion is present, it is more likely to decrease visualization of the cardiac silhouette on a DV projection as compared to VD. The author prefers to start with the DV (or VD) projection for all thoracic studies as it will decrease the odds of atelectasis (resulting from lateral recumbency) causing increased opacity throughout the pulmonary parenchyma that could be confused with true disease. Finally, it is prudent to keep in mind that each lateral projection only evaluates the pulmonary parenchyma and pleural space of the opposite side because of recumbent atelectasis. For example, when looking at a right lateral radiograph, only the left cranial and left caudal lung lobes are evaluated. Beyond imaging, other routine diagnostics helpful during the initial work up of dyspneic cats include a complete blood count, a chemistry profile, urinalysis, retrovirus testing, and heartworm antigen/ antibody testing. More advanced testing includes echocardiography, diagnostic thoracic ultrasound, and computed tomography with contrast. THE NORMAL FELINE THORAX The feline cardiac silhouette is smaller appearing within the thoracic cavity as compared to the dog, with less interpatient variation in size and shape. Objectively, the cardiac silhouette spans 2-3 ribs spaces on the lateral projection and < 50% of the thoracic cavity width on the VD/DV projection. In most cats, the vertebral heart score (VHS) measured in lateral recumbency should be < 8.1. However, the sensitivity and specificity of the VHS in predicting cardiac enlargement is highly dependent on the cut-off value used with a trade-off favoring either one or the other. 2,3 Unfortunately, different studies have yielded very different cut-offs when determining 100% sensitivity (best for screening) versus 100% specificity (best for discriminating). Additionally, the VHS is no better than subjective evaluation of cardiac size, making the routine use of objective cardiac measures of questionable value at this time. The VHS and other measurements may be better for monitoring changes over time on serial radiographic studies for the individual patient. In geriatric cats, the cardiac silhouette often rotates cranioventrally, causing it to occupy more intercostal spaces without actually increasing in size. The feline pulmonary parenchyma is more lucent and “less busy” relative to the dog due to decreased visibility of pulmonary markings. Similar to the dog, a thin body condition will make the lungs appear more lucent, whereas obesity will cause an artificial increase in soft tissue opacity throughout the lungs, giving the false appearance of an unstructured interstitial pulmonary pattern. Cats do not commonly develop age-related pulmonary fibrosis, so the presence of a bronchial pattern should raise suspicion for airway disease. Finally, the pleural space is not normally visible unless it pathologically contains gas or fluid. Excessive fat within the mediastinum can also widen pleural fissures as well as make the cardiac silhouette appear enlarged.
The radiographic findings of specific etiologies will be discussed below. However, it is important to keep in mind that most imaging findings in isolation (e.g., unstructured interstitial pulmonary pattern on radiographs or ring-down artifact/B-lines on ultrasound) are incredibly non-specific with multiple differentials possible. As is this case for any diagnostic test, the diagnosis comes from the combination of historical clues, physical exam findings, other diagnostic test results, and most importantly, the veterinarian’s clinical acumen. It is well established that the accuracy of any imaging modality increases with ancillary information and experience level of the interpreter. HEART FAILURE Left sided heart failure is one of the most common causes of dyspnea in the cat. Acquired cardiomyopathies (e.g., hypertrophic, thyrotoxic, restrictive, non-specific, and dilated) are the most common cause of left sided congestive heart failure in cats. Heart failure is a clinical diagnosis that comes from physical exam and radiographic findings. Although echocardiography is useful to determine the underlying etiology of heart failure, it is often not necessary for diagnosing failure or initially treating the dyspneic patient. The presence of hypothermia on triage exam in a dyspneic cat should greatly raise the index of suspicion for heart failure and/or aortic thromboembolism. A recent study 4 demonstrated that the combined findings of hypothermia, tachycardia, tachypnea, and a gallop rhythm are strongly suggestive of heart failure and warrant diuretic therapy until further testing can be performed. The hallmark radiographic findings of feline left sided heart failure include a patchy (or less commonly diffuse) unstructured interstitial or alveolar pulmonary pattern and enlarged pulmonary lobar veins. Unfortunately, cats love to be very different than dogs in many ways. First, cardiomegaly may be only mild in the face of fulminant heart failure. When present, cardiomegaly is often generalized without appreciable chamber enlargement. A valentine-shaped cardiac silhouette is commonly seen on DV or VD radiographs and is most often caused by left atrial enlargement. 5 Second, the presence of enlarged pulmonary lobar veins is inconsistent in cats and further masked with a single dose of a diuretic. Third, pleural effusion commonly occurs with left sided heart failure in cats as compared to right sided failure in dogs. Finally, cardiogenic pulmonary edema will sometimes increase pulmonary opacity only around the bronchi (i.e., peribronchial cuffing) in cats, mimicking the bronchial pulmonary pattern that occurs with inflammatory airway disease. Without cardiac or pulmonary lobar venous enlargement, the findings of a pulmonary pattern with or without pleural effusion are very nonspecific and caused by other etiologies such as neoplasia and infectious disease. In these cases, a diuretic trial can be performed by administering an appropriate dose of a diuretic and repeating radiographs 6-24 hours later. Radiographic improvement of the pulmonary pattern in a short period of time following diuretic administration is strongly supportive of left sided heart failure. A clinical response to furosemide is not pathognomonic for heart failure as patients with airway disease will respond to the drug’s bronchodilatory effects.
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