Radiography of Feline Cardiac Disease

CLINICAL RADIOLOGY

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RADIOGRAPHY OF FELINE CARDIAC DISEASE Mark Rishmw, BVSc, MS

Despite the advent of dynamic imaging of the heart (echocardiography), thoracic radiography remains an integral part of the diagnosis and management of cardiac disease. Radiography offers one of the best noninvasive methods of evaluating extracardiac thoracic structures, including the bronchi, lungs, pleurae, and mediastinum. Additionally, most clinicians have ready access to radiographic equipment. This allows rapid preliminary evaluation of the cardiovascular and respiratory systems as well as timely institution of therapy in diseased patients. As such, thoracic radiographs should be part of the diagnostic approach in any cat with evidence of cardiopulmonary disease. As is the case with any diagnostic procedure, however, there are certain advantages and limitations of radiography in evaluating a patient with suspected cardiopulmonary disease, including the ability to accurately interpret results. This article reviews the use of radiography in the feline cardiopulmonary patient, limitations of the techmque, and interpretation of general and specific radiographic findings. INDICATIONS AND CONTRAINDICATIONS

Thoracic radiographs are indicated in any cat with clinical signs of cardiac or pulmonary disease. The degree to which a clinician recognizes these clinical signs reflects the severity of the abnormalities and the experience of the clinician. Common pulmonary clinical signs include dyspnea, tachypnea, crackles, wheezes, absence of anticipated breath From the Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York

VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 30 • NUMBER 2 • MARCH 2000

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sounds in various regions of the thorax, and loss of compliance of the ventral thoracic cage. Common cardiac clinical signs include gallop sounds, muffled heart sounds, and signs consistent with aortoiliac thromboembolism. Many clinicians also routinely radiograph the chest of patients with thoracic trauma (e.g., motor vehicle accidents, "highrise" syndrome) because of the high incidence of thoracic injury in these patients. Less clear indications for thoracic radiography in cats include coughing and heart murmurs or arrhythmias without more overt clinical signs. The main strength of thoracic radiography in evaluating the feline cardiac patient is in its ability to identify congestive heart failure. Radiographic diagnosis of changes in cardiac size or shape is less accurate but may be the best diagnostic option available in some cases. Radiography is a somewhat stressful procedure for most animals because it requires restraint. In many cases, chemical restraint is not recommended when dealing with cardiopulmonary disease; thus, animals must be restrained physically. If sedation is necessary, acepromazine (0.03 mg/kg) offers relatively stable sedation with little hemodynamic compromise. Clinicians need to assess the benefits of a thoracic radiograph compared with the risks involved to the patient if such a procedure is performed. This risk-benefit assessment can be expressed in terms of the probability of a disease as shown in Figure 1. This type of assessment process can be demonstrated with feline cardiac patients. For example, a middle-aged cat with open-mouthed

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Figure 1. The risk-benefit analysis of a diagnostic procedure. As the probability of a disease in a patient increases, the risk of not treating that patient also increases. However, additional diagnostics are indicated to increase the probability of the disease, so that appropriate treatment can be initiated. When the probability approaches 100%, additional diagnostic procedures may be contraindicated. As the probability of a disease approaches 0%, a diagnostic procedure may be harmful, or an unnecessary waste of resources.

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breathing, respiratory distress, a gallop sound, a systolic murmur, and tachycardia has a high probability of cardiac disease with congestive heart failure. There may be limited benefit in obtaining radiographs or an echocardiogram in this patient to confirm the disease process at the time of presentation. Indeed, the stress of restraining such a cat and depriving it of acute medical support may compromise the patient and increase the risk of death. Here, the risk of not treating the patient outweighs the benefit of additional diagnostic testing. At the other end of the spectrum, a cat with a diagnosis of hemobartonellosis and severe anemia that presents with a murmur is unlikely to have a primary cardiac cause for the clinical signs. In this case, radiography offers little to the diagnosis or management of the condition and unduly stresses the patient or, at the very least, incurs an unnecessary expense to the client. This does not mean that clinicians should avoid radiography in cats with severe congestive heart failure. Once such a patient is stabilized medically, radiographs may provide valuable information about the thoracic contents. The procedure should simply be timed appropriately to minimally compromise the patient. As Ronald Burk eloquently stated, "The patient must not be diagnosed to death." 4 TECHNIQUE

The specifics of obtaining a radiograph of diagnostic quality are outside the scope of this article. Suffice it to say that clinicians should develop a technique chart to properly and consistently expose radiographs. Radiographs are part of the medical record and should be of diagnostic quality (for clinical and medicolegal purposes). Consulting a radiologist to assist with establishing a radiographic facility and optimizing radiographic technique is extremely cost-effective and beneficial. Radiographic processing is becoming an automated procedure in many clinics, decreasing "laboratory" error of developing or fixing. Because cats are generally of similar body shape and thoracic cage size, many practices establish "cat thorax" settings that reliably expose good-quality radiographs. Positioning feline patients is of little importance when evaluating the thorax. One study showed that there is no difference in diagnostic quality between dorsoventral and ventrodorsal views. 5 Other clinicians believe that dorsoventral positioning offers a more anatomically correct orientation of the heart. The study by Carlisle and Thrall5 supported this idea, suggesting that there may be more variability in the cardiac silhouette in a ventrodorsal position, especially of the right cardiac border. Further, caudal lobar pulmonary vasculature, especially as it extends over the diaphragm and abdominal viscera, is more visible in a dorsoventral view. Because a cat' s thoracic cage is relatively compressible, however, dorsoventral positioning tends to flatten the sternum and "squash" the chest so that the heart lies more horizontally along the

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sternum than normal. 5 On the other hand, Toal et aP0 suggested that there was more variability in cardiac silhouette in the dorsoventral view than in the ventrodorsal view, a finding associated in part with the cardiac cycle. Nevertheless, the authors stressed that the changes in shape and size were small and did not falsely suggest cardiac disease. Lateral views differ even less than dorsoventral and ventrodorsal views. The cardiac silhouette and pulmonary fields are similar in both views. By convention, right lateral views are standard at many institutions for routine thoracic examination. Either view can satisfactorily display the cardiothoracic structures unless there are specific localized pulmonary or pleural lesions. Most clinicians prefer radiographs taken at end inspiration when examining the cardiac system. Maximally aerated lungs effectively provide a negative contrast to pulmonary parenchymal lesions. At end inspiration, the thoracic cage is maximally distended; thus, the cardiac silhouette seems relatively smaller within it. Finally, radiographs taken at end inspiration decrease the risk of misinterpreting normal pulmonary interstitium as an interstitial infiltrate. In reality, few radiographs are exposed at true end inspiration, but clinicians should attempt to obtain inspiratory radiographs. In cats, tracheal and bronchial collapse is rare; thus, expiratory phase views are of little diagnostic benefit. As is the case in other species, obese cats create technical difficulties. First, radiographs tend to be underexposed, because the perithoracic fat absorbs more X-rays than does air. Second, obese cats tend to hypoventilate; thus, their end inspiratory tidal volume is small, and their chests fail to expand (Fig. 2). One method of avoiding these problems is to radiograph the patient anesthetized with positive-pressure ventilation and to increase the kilovolt peak slightly. Assessing film quality and recognizing potential technical errors or artifacts is an art in itself. I assess adequate exposure of lateral views by examining the vertebrae of the mid-thoracic spine. The vertebral bodies in this region should be semiopaque but not too translucent or too opaque if the radiograph is exposed appropriately. This assessment is subjective, and other clinicians may prefer lighter or darker films. In the dorsoventral projection, exposure quality is much more difficult to standardize. There is a range of exposures that produce diagnostic radiographs. Clinicians should determine if there are technical or artifactual reasons why lesions may be "invented" or overlooked. If there are, these factors should be accounted for, or the radiographs should be reacquired. The most common error is to overexpose the lung and parenchymal markings. The black image often gives the impression of a well-aerated lung with no disease. Obliquity of the patient can be easily evaluated by looking for the superimposition of the sternum and spine on the ventrodorsal or dorsoventral view and examining for parallelism of the costal arches in the lateral view. Some clinicians assess phase of respiration by examining the cardiosternodiaphragmatic angle on the lateral view. With an inspiratory phase exposure, the feline cardiac silhouette should be separated from the

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diaphragm. 20 This method may be unreliable, because structures in this region often change shape with disease (e.g., cardiomegaly might displace the cardiac silhouette caudally; ascites might displace the ventral diaphragm cranially; sternal deformities, which are common in cats, may displace the sternum dorsally). Thus, I generally evaluate thoracic structures that are less likely to alter their relationships with cardiopulmonary disease. Specifically, I assess the association of the dorsocaudal lung tips and thoracic vertebrae (lumbodiaphragmatic recess) on a lateral view to assess phase of respiration. In cats that have been radiographed with adequate pulmonary inflation, the lung tips should extend past the cranial border of the thirteenth thoracic vertebra (i.e., between T13 and Ll) (Fig. 3). Usually, the dorsocaudallung tips reach the curve of the last rib. Lung tips that extend beyond cranial L1 may suggest hyperinflation, although this can be commonly observed in normal cats. Lung tips that fail to extend to T13 generally indicate underinflation. Assessing inflation in the dorsoventral or. ventrodorsal projection is much more difficult. I examine the position of the most cranial edge of the diaphragm in relation to the ribs. In cats, if any part of the diaphragm lies cranial to the eleventh rib, the lungs are likely underinflated. Again, various disorders such as ascites or diaphragmatic paralysis may affect this. The reader should realize these are subjective criteria that have not been critically evaluated. Unfortunately, standard criteria for judging respiratory phase are also largely unsubstantiated. 20 Some clinicians mistakenly identify pleural effusion on a lateral view along the dorsocaudal lung border just ventral to the caudal thoracic vertebrae. In cats, a prominent psoas muscle group originates within the thorax and forms a small triangular wedge that extends caudally along the bottom of the vertebrae (see Fig. 3). Thus, clinicians should be wary of diagnosing pleural effusion solely on the basis of soft tissue opacity in this region and should look for other signs of effusion. NORMAL RADIOGRAPHIC CARDIOPULMONARY ANATOMY

Radiographic anatomy of the feline heart has been detailed previously.9, 16, 20, 28, 31 Cats have less variability in their thoracic cage shape and size than dogs; thus, it is easier to standardize measurements of feline cardiovascular structures. Despite this, many feline cardiac diseases that cause substantial clinical signs often have small changes in cardiac dimensions. For example, hypertrophic cardiomyopathy (HCM) often results in an increase in left ventricular wall thickness of 2 to 4 mm, with much of this thickening directed into the chamber lumen. This small a change in ventricular dimensions may be difficult to appreciate radiographically. The normal feline heart is biconvex on both lateral and dorsoventral views, and it is longer and more slender than the canine cardiac silhouette. It tapers towards the apex more than towards the base. The apex is

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Figure 2. Lateral and ventrodorsal radiographs obtained from the same obese cat during (A) spontaneous conscious respiration and (B) positive pressure ventilation with general anesthesia. During spontaneous respiration, the patient hypoventilated and failed to expand the thoracic cage and lungs. This artifactually increases the opacity of the pulmonary parenchyma and gives the false impression of cardiomegaly. These changes disappear with proper inflation of the lungs (in this case, under anesthesia). Illustration continued on opposite page

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Figure 3. Lateral (A) and ventrodorsal (B) views of a normal feline thorax obtained during adequate inspiration. Lateral (C) and ventrodorsal (D) views of an obese feline thorax obtained during adequate inspiration. A, The caudodorsal lung tips separate slightly from the ventral aspects of the caudal thoracic vertebrae at the origin of the psoas muscles {1). The caudodorsal lung tips extend to the last rib {2). The cardiac silhouette is separated from the diaphragm (3). The cardiac silhouette tapers toward the apex and less toward the base (4). The silhouette is supine, with substantial sternal contact. The caudal vena cava courses down from the diaphragm to the heart (5). B, The cranial border of the diaphragm is caudal to T11 (1). The cardiac apex points to the left {2). The cranial pulmonary vasculature is indistinct (3). C, The caudodorsal lung tips extend to the last rib (1 ). The cardiac silhouette is not separated from the diaphragm (2). The cardiac silhouette is supine with substantial sternal contact (3). The cardiac silhouette tapers toward the apex and less toward the base (4). The pulmonary vasculature is relatively indistinct (5). Illustration continued on opposite page

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generally positioned slightly towards the left of midline in the dorsoventral view, but this is variable in cats. 16 Caudal and cranial cardiac waists are invisible in cats as are auricular silhouettes and the pulmonary trunk. Most investigators agree that the dorsoventral dimension of the cardiac silhouette in normal cats on a lateral view occupies approximately 70% of the depth of the chest at the same level (see Fig. 3). 16 Lord and Zontine16 also demonstrated that many diseased feline hearts occupied a similar proportion of the thoracic cage, negating the usefulness of this measurement. Recently, Litster and Buchanan15 developed a vertebral heart score (VHS) for cats similar to that developed by Buchanan for dogs. These authors showed a normal VHS for cats of 7.5 ± 0.3. The relationships of the cardiac chambers in cats are similar to those in dogs. In the lateral view, the right ventricle wraps around the front of the heart from the right side such that the cranial cardiac border defines the cranial edge of the right ventricle. The caudal cardiac border defines the caudal edge of the left ventricle, which lies slightly caudal to the right ventricle. The two atria are relatively parallel in the cat and are located just below the carina at the base of the heart (Fig. 4). The caudal vena cava descends from the diaphragm to the heart on a lateral view, roughly in a line parallel to the spine directly above it (see Fig. 3). 16 Normal variations of feline cardiac silhouettes exist. Some authors believe that the feline heart is more upright in the thorax. 4 Others mention an "almost supine cardiac orientation" in some cats (Fig. 5). 20 In my experience, the feline cardiac silhouette is less upright than in dogs, which is consistent with the observations of Moon et aP 9 and Hamlin et al9 that the feline heart on average, lies at an angle of 50° from the longitudinal axis of the body. Hamlin et al9 found that this angle becomes even more acute if the· forelimbs are extended cranially out of the pulmonary fields, suggesting an even more supine position for the heart in many feline thoracic radiographs. Moon et aP 9 found that the supine orientation becomes more pronounced with age. In their study, 40% of cats over 10 years old had more horizontally aligned (supine) hearts than younger cats using Hamlin et al's method/ and the angle between the heart and the sternum in these cats was also more acute (40° vs 31°). The normal feline aorta also changes with age. In cats with supine hearts, the aorta tends to bulge or become "redundant." 19 This is thought to be a normal aging change, that is, possibly secondary to the altered cardiac geometry or elongation of the aorta. In human beings, this change often accompanies systemic hypertension, a condition that was not excluded in this study (see Fig. 5). Several authors have attempted to objectify the assessment of cardiac size in cats. 9' 12, 15, 16 Common comparisons are made in several texts to rib spaces traversed by the cardiac silhouette or to the number of sternebrae contacted by the cardiac border. These criteria are insensitive in cats because of the dynamic nature of the ribs during respiration and the more variable position of the heart within the thorax, Other methods such as comparing cardiac width to length or cardiac width to thoracic

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cage dimensions are tedious, and reference ranges are often difficult to remember. The authors of one study showed a correlation between the maximum width of the cardiac silhouette and the distance from the cranial border of the fifth rib to the caudal border of the seventh rib on a lateral view but did not provide any details regarding the degree of correlation.31 They suggested a ratio of 1:1 as being normal and a ratio of 1.3:1 as being evidence of cardiomegaly. The VHS proposed by Litster and Buchanan15 may offer a more user-friendly objective means of assessing cardiac size, especially for the clinician who is inexperienced in evaluating the feline heart, but a subjective evaluation of cardiac size and shape remains critical in establishing the presence of cardiac disease and narrowing the list of differential diagnoses. Pulmonary vasculature is inconsistently identified on feline thoracic radiographs. The relationship between pulmonary veins, arteries, and bronchi is the same as in dogs (veins are ventral and central to the arteries in the lateral and dorsoventral views, respectively}, but vessels are more difficult to identify and trace to the periphery of the lung fields. The caudal lobar vessels are best evaluated in the dorsoventral view, and the cranial lobar vessels are best evaluated in the lateral view (see Fig. 3). RADIOGRAPHIC PATTERNS OF CARDIAC DISEASE Noncardiac Changes

The likelihood of correctly identifying cardiac disease from thoracic radiographs in cats (and other species) relies on systematic and thorough examination of all visible structures and body systems (i.e., if it is on the film, evaluate it). Obviously, some regions are not going to be exposed ideally or centered within the field of interest, but they should be evaluated to the best of one's ability. Noncardiac structures offer valuable information that may support or refute the diagnosis of cardiac disease, primarily by demonstrating the presence of congestive heart failure. It is advisable to evaluate the noncardiac structures first and in order of increasing importance. This reduces the likelihood of overlooking a significant finding in a system unassociated with the heart. Musculoskeletal System

A cursory examination of the musculoskeletal system may allow the detection of rib fractures or tumors that might account for clinical signs of dyspnea or alter the prognosis and therapeutic options for cardiac disease. Additionally, body condition (obese, normal, or emaciated) can be determined. This may help to qualify the interpretation of the lung fields, abdominal contents, and cardiac silhouette.

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Figure 4. Lateral and ventrodorsal views of a normal feline thorax. A, Approximate location of the left atrium. 8, Approximate location of the right atrium. C, Approximate location of the left ventricle. Illustration continued on opposite page

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Figure 4 (Continued). D, Approximate location of the right ventricle. E, Approximate location of the left and right atria on the ventrodorsal view.

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Figure 5. Lateral (A) and ventrodorsal (B) views of a cat with a prominent aorta. The cardiac silhouette is relatively supine on the lateral view and consequently appears somewhat elongated on the ventrodorsal view. The edges of the aorta are identified (arrows). Note the separation of the caudodorsal lung fields from the spine at the origin of the psoas muscles and the extension of the lung tips to the last rib.

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Abdominal Cavity

Evaluation of the abdominal cavity may identify ascites, which is associated with right-sided failure. Characteristic changes associated with ascites include loss of abdominal detail and abdominal distention without obesity. Hepatomegaly is much less commonly observed in cats with right heart failure than in dogs. It should not be used as a sole indicator of right heart disease, but in conjunction with other changes, it can support the diagnosis. Absence of viscera from its usual location may suggest a peritoneopericardial diaphragmatic hernia (Fig. 6), and visceral displacement may suggest tumors. Pleural Space

This is an important extracardiac structure in cats, because left-sided congestive heart failur~ often manifests as pleural effusion in the cat. 13 Indeed, 10% of cats with pleural effusion in one study had HCM, a condition that primarily affects the left heart. 6 Pleural effusion also occurs with biventricular congestive failure. On the other hand, isolated right-sided failure rarely causes pleural effusion without concurrent ascites. 14 An older study ascribed all pleural effusion to right heart disease but did not critically evaluate the involvement of the right or left heart in the pathogenesis of the congestive failure. 16, 1 7 The cats in this study likely had a relatively high incidence of dilated cardiomyopathy with biventricular failure. More recently, echocardiographic and

Figure 6. Lateral view of a cat with a peritoneopericardial diaphragmatic hernia. There are multiple soft-tissue opacities within the pericardia! silhouette, suggesting the presence of multiple viscera. The dorsal peritoneopericardial mesothelial remnant is identified (arrow). Note the apparent absence of the liver within the abdomen. The pulmonary fields appear normal.

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intracardiac pressure studies in cats with congestive heart failure have demonstrated primary left heart disease with pleural effusionP The cause of pleural effusion in left heart disease is thought to be associated with the vascular anatomy of the visceral pleura. In cats, the visceral pleura is thin and is supplied by the pulmonary arteries and drained by pulmonary veins. 18 Thus, an increase in pulmonary venous hydrostatic pressure (left-sided heart failure) could lead to extravasation of fluid from the visceral pleural surface into the pleural space. The parietal pleura is drained by systemic veins, explaining why pleural effusion can occur with right heart failure and why biventricular failure produces pleural effusion most readily (both venous circulations are compromised). All cardiac diseases capable of causing left-sided congestive heart failure can result in pleural effusion. Radiographic findings associated with pleural effusion depend on the quantity of effusate. Small effusions generally cause some rounding of the costophrenic angles, slight retraction of lung lobes from the thoracic wall, and slight "leafing" of the lung lobes (filling of the interlobar fissures), resulting in loss of sharp detail of the cardiac silhouette. Large effusions cause retraction and rounding of lung lobes, separation of the lobes, and loss of the cardiac silhouette and the continuity of the diaphragmatic border. Substantial pleural effusions obscure the heart and make assessment of the cardiac silhouette virtually impossible. 16• 24 Further, noncardiac causes of effusion such as mediastinal lymphoma may remain unidentified. In these cases, clinicians should perform thoracocentesis to empty the pleural space and aid respiration, and then they should obtain further radiographs. Respiratory System

Evaluation of the respiratory system is the prime motivation for obtaining thoracic radiographs in a cat with suspected cardiac disease. No other single readily available diagnostic test offers as much information as a radiograph when evaluating the respiratory system. The respiratory system can be divided into three components: the large airways, the small airways and parenchyma, and the pulmonary vasculature. These should all be examined individually. In cats, large airway disease is usually limited to intimal inflammation (so-called "reactive airway disease," "feline asthma," or "bronchitis"); thus, dynamic studies of the trachea are generally unnecessary. The large airways are usually normal in feline cardiac patients. Cardiogenic pulmonary edema in cats primarily involves the interstitium and, if severe enough, the alveoli and small airways. Unlike dogs, the pattern of distribution of pulmonary edema and the radiographic changes are extremely variable in cats. Pulmonary edema does not localize to the hilar and perihilar regions but distributes more unevenly and inconsistently. 16 It can be focal, multifocal, peripheral, or asymmetric. When it comes to pulmonary edema in cats, "anything goes." I have seen edema radiographically localized to cranial lung

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lobes, have a generalized interstitial pattern, have a miliary pattern similar to that of pulmonary carcinomatosis or lungworm infestation, have an alveolar pattern, or present like canine pulmonary edema (Fig. 7). This variability and inconsistency in the radiographic appearance of the pulmonary parenchyma can add to the diagnostic and therapeutic dilemma that the clinician must address. The ability of the clinician to recognize the many faces of pulmonary edema in cats and to assimilate these data with other clinical evidence of cardiac disease (i.e., history, signalment, physical examination findings, other diagnostic test results) ultimately determines the effectiveness and accuracy with which heart failure is recognized and treated or excluded from the differential diagnosis in the feline patient. It is important to note that cardiomegaly, especially left atrial enlargement, accompanies most cases of cardiogenic pulmonary edema in cats. Pulmonary vasculature should be evaluated separately from the parenchyma. As stated previously, pulmonary vasculature is less consistently identified or traced to the terminal branches in cats than in dogs. Both views (lateral and dorsoventral) should be examined. Cranial lobar vessels can be differentiated in the lateral view, and caudal lobar vessels can be differentiated in the dorsoventral view. A generalized increase in vascularity of the caudal lung fields can often be appreciated in the lateral view (see Fig. 7). Theoretically, nonshunting congestive left-sided heart failure should produce pulmonary venous enlargement because of pulmonary venous hypertension. Radiographically, this presents as pulmonary veins that are larger than their corresponding arteries. This finding is inconsistent in cats, and pulmonary veins often appear unremarkable in cats with congestive heart failure (see Fig. 7). Left-to-right shunting lesions (e.g., patent ductus arteriosus, ventricular and atrial septal defects) result in increased pulmonary blood volume throughout the pulmonary vascular system. This produces enlargement of both the pulmonary arteries and pulmonary veins. The vessels often taper further out in the periphery than is normal (Fig. 8). Pulmonary (arterial) hypertension is an arterial disorder that is not reflected in the venous circulation. Radiographically, pulmonary arterial hypertension presents with enlarged pulmonary arteries and normal or small pulmonary veins. Dirofilariasis also causes similar changes because of arteriolar obstruction and intimal proliferation?, 23 Cardiac Changes

There are two cardiac changes that can occur with cardiac disease: generalized cardiomegaly and changes in the size or shape of specific chambers. Clinicians therefore attempt to identify the existence of cardiac disease by documenting one or both of these changes. They should be aware of the basis for many of the "rules" defining the identification of heart disease and the potential pitfalls or limitations of radiography in

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8 Figure 7. A, Lateral view of a cat with hypertrophic cardiomyopathy (HCM) and severe left atrial enlargement. There is a left atrial bulge along the caudal border of the silhouette, and the caudal vena cava is elevated. The trachea is also elevated. The ventricular apex still tapers, producing the ice-cream cone appearance. 8 , Lateral view of a cat with HCM and pulmonary edema. Note the prominent pulmonary vessels and generalized interstitial pattern. Illustration continued on opposite page

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Figure 7 (Continued). C, Lateral and ventrodorsal views of a cat with HCM and pulmonary edema. In this case, the pulmonary vessels are unremarkable, but there is a generalized interstitial pattern. There is no pleural effusion. Note the prominence in the region of the right atrium on the ventrodorsal view. This is likely displacement of the right atrium by the left atrial enlargement.

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Figure 8. Lateral (A) and ventrodorsal (B) views of a cat with a patent ductus arteriosus (PDA). The apex is deviated to the right (note the descending aorta on the left in the ventrodorsal view); the heart is very enlarged and elongated (note the increased sternal contact with this case of left heart disease), and the pulmonary arteries and veins are enlarged. (Arrows denote the edges of the pulmonary arteries and veins on the ventrodorsal view; there is a generalized vascular pattern on the lateral view, especially in the cranial lung fields.)

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correctly identifying diseased patients. By doing so, they stand to increase the accuracy of their interpretations. Cardiomegaly, the generalized enlargement of the heart for unspecified reasons and via unspecified means, is the radiographic hallmark of cardiac disease. In radiologic parlance, cardiomegaly generally refers to an increase in the size of the cardiac silhouette. Importantly, clinicians should remember that not all heart disease produces cardiomegaly and that not all cardiomegaly is associated with primary cardiac disease. Nonetheless, cardiomegaly is still used a guide to the presence of cardiac disease. To this end, cats with radiographic evidence of cardiomegaly usually have cardiac disease (few false-positive results), but many cats without obvious cardiomegaly also have cardiac disease (many falsenegative results). Historically, radiographs provided the primary means of assessing cardiac size in animals, and clinicians became adept at assessing radiographic changes. They could not determine the accuracy and validity of their interpretations, however. The advent of echocardiography provided a means of verifying the radiographic interpretation of cardiomegaly and soon became the "gold standard" of assessing cardiac size, demonstrating the limits of radiography in this field. Interestingly, no objective studies have been performed to determine the accuracy of radiography in identifying generalized cardiomegaly or specific chamber enlargement since the introduction of echocardiography. A single study by Wortman et aP2 demonstrated that trained observers could accurately identify cardiac disease in cats using a combination of cardiac and extracardiac radiographic criteria and, similarly, could exclude cardiac disease with even greater accuracy. The diseased cats selected in this study were not described, however; thus, cats with heart disease but without cardiomegaly might not have been included, increasing the specificity of the test. This study also demonstrated that unskilled observers failed to identify diseased cats more frequently than skilled observers. Generalized cardiomegaly can be suspected if the VHS exceeds 8.1 (mean ± 2 SD)15 or if the ratio of maximum cardiac width to the distance from the fifth through seventh ribs on a lateral view exceeds 1.3.31 Many hearts that are subjectively considered to be enlarged by experienced observers may fall within these limits. Clinicians should recognize that these limits probably have a high specificity (few false-positive results) but a low sensitivity (many false-negative results), although neither the sensitivity nor specificity of either method has been determined. Conversely, Suter and Lord27 found that observers using subjective methods tended to overdiagnose cardiomegaly in dogs. An adjunct to the determination of overall cardiac (silhouette) size is the assessment of specific chamber enlargement. Correct identification of specific chamber enlargement is even more fraught with error. There are several reasons for this. First, the radiographic silhouette of the heart is effectively a shadow cast by a complex three-dimensional object. The information that the observer can obtain from examining this shadow is substantially less than that obtained from examining the object itself.

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Second, although cardiomegaly can be assessed objectively using various measurement techniques, changes in the size or shape of specific chambers are assessed purely subjectively. Therefore, the experience of the observer determines the accuracy of interpretation. Clinicians who have repeatedly confirmed their radiographic impressions with echocardiography are probably more likely to correctly identify enlargement patterns. Additionally, observers who integrate extracardiac radiographic findings with likely disease pathophysiology are probably more likely to correctly identify changes in chamber size or shape. For example, the "valentine-shaped" heart associated with HCM might be interpreted as biatrial enlargement; however, the presence of left-sided congestive failure on radiographs, lack of caudal vena cava distention or ascites, and repeated echocardiographic confirmation of isolated left heart disease in most of these cases should lead the clinician to re-evaluate the criteria for specific chamber enlargement and to decide that this radiographic pattern is associated with marked left atrial enlargement in most cases. There are several generalizations that can be made with heart disease to assist the observer in correctly interpreting radiographs. First, in cats as in dogs, left heart disease is far more common than right heart disease or biventricular /biatrial heart disease. This is especially true because of the decrease in the incidence of feline dilated cardiomyopathy. Thus, by simply "playing the odds" and observing clinical and extracardiac radiographic signs, an observer is correct more often than not if he or she diagnoses left heart changes. . Second, diseases that cause left atrial enlargement usually also affect the left ventricle; diseases that cause right atrial enlargement usually also affect the right ventricle. No single cardiac disease causes right atrial enlargement and left ventricular enlargement simultaneously and exclusively; thus, an interpretation consisting of only these two findings should be questioned. Likewise, isolated left atrial and right ventricular enlargement is a rare combination of findings. Thus, clinicians should consider the pathophysiology of the suspected disorder when interpreting radiographs. Nevertheless, HCM and unclassified (restrictive) cardiomyopathy can both cause isolated atrial (usual left or bilateral) enlargement without corresponding ventricular enlargement. Third, the left atrium is correctly identified as enlarged or normal most consistently from radiographs, and the right heart is most often identified incorrectly as enlarged from radiographs. Thus, when right heart enlargement is suspected, the observer should question the interpretation and try to substantiate the impression with clinical and extracardiac findings. Fourth, mild heart enlargement, detectable echocardiographically, cannot be detected radiographically. A 1- to 2-mm change in chamber dimensions falls within the differences observed with radiographs taken at different points of the cardiac cycle.30 Left Atrial Enlargement On the lateral view, the feline left atrium sits just below the carina (see Fig. 4A). This is different from the dog, where the body of the left

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atrium is located further caudally below and between the mainstem bronchi. The left auricle points ventrally at the sternum. Consequently, left atrial enlargement has a different radiographic appearance than that seen in dogs, and mild enlargement is more readily identified on the dorsoventral view than on the lateral view. 2• 26 The dorsal displacement of the left mainstem bronchus seen in dogs with an enlarged left atrium is not seen in cats, but the trachea may be elevated (see Fig. 7A). There is less of a straightening or bulging of the caudal border of the heart than is seen in dogs, but a notch may be seen at the atrioventricular junction. Instead, the atrium, when large, occupies the base of the heart. In cases of HCM or unclassified cardiomyopathy, where the ventricle is not markedly enlarged, the heart takes on the appearance of a tilted icecream cone (see Fig. 7). The caudal vena cava may be elevated to a more horizontal plane (see Fig. 7A). On the dorsoventral view, the left atrium sits in the left cranial quadrant and extends slightly over the midline (see Fig. 4E). As it enlarges, it forms a bulge in the left cranial quadrant that is accentuated by the enlarging left auricle. A markedly enlarged left atrium can also displace the right atrium to the right, resulting in the valentine shape that has been described with HCM (Fig. 9). 10• 16• 28 This should not be identified as right atrial enlargement. Finally, with marked left atrial enlargement, the heart can be rotated clockwise, displacing the apex to the right of the midline (see Fig. 9). Left Ventricular Enlargement

The left ventricle occupies most of the apical half of the cardiac silhouette on the lateral view (see Fig. 4C). The thin right ventricle wraps around the left; thus, it accounts for a substantially smaller portion of the cardiac silhouette (see Fig. 4D). The left ventricle enlarges in two ways: concentrically and eccentrically (and rarely by dilatation). Concentric hypertrophy, an increase in the wall thickness, occurs secondary to aortic stenosis (which is uncommon in the cat) or HCM. Eccentric hypertrophy, an increase in the chamber volume, occurs secondary to volume overloads or dilated cardiomyopathy. Dilatation occasionally occurs secondary to dilated cardiomyopathy. Concentric left ventricular hypertrophy often produces few radiographic changes, as the increase in cardiac size is relatively modest in many cases. When it does increase substantially, the caudal border of the cardiac silhouette becomes more convex, tapering to an apex, and the right ventricle is displaced craniaventrally, increasing sternal contact (see Fig. 7). Suter26 described the left atrial and left ventricular enlargement of HCM as producing a "twohumped [caudal] border." Eccentric hypertrophy causes a rounding of the apex, an increase in the length and width of the cardiac silhouette, and an elevation of the trachea. This cannot be differentiated from dilatation of the left ventricle. In almost all cases, except for isolated aortic stenosis, left atrial enlargement accompanies the left ventricular enlargement.

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Figure 9. Lateral (A) and ventrodorsal (B) views of a cat with unclassified (restrictive) cardiomyopathy and biatrial enlargement (valentine heart). The apex is shifted to the right of the midline in the ventrodorsal view, and there is little ventricular enlargement visible. This radiographic pattern is indistinguishable from isolated left atrial enlargement secondary to hypertrophic cardiomyopathy. The pulmonary vasculature appears unremarkable.

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On the dorsoventral view, the left ventricle elongates, and the apex often becomes rounded. The convexity of the left lateral wall increases occasionally with severe concentric hypertrophy. Right Ventricular Enlargement

On the lateral view, the right ventricle is thin and wraps around the front and right side of the left ventricle (see Fig. 4D). Thus, the cranial border of the cardiac silhouette represents the cranial aspect of the right ventricle. Right ventricular enlargement can also be caused by eccentric or concentric hypertrophy. Generally, only marked right ventricular enlargement can be consistently identified correctly. In my experience, most cases of "mild right ventricular enlargement" demonstrate echocardiographically normal right ventricles. Marked concentric hypertrophy causes an increased convexity to the cranial border and an elevation of the apex from the sternum. Marked eccentric right ventricular hypertrophy has an appearance similar to that of pericardia! effusion (Fig. 10). The cranial border of the cardiac silhouette protrudes forward and along the sternum, and the caudal border remains unaffected. The trachea may be elevated. The reader should note that increased sternal contact does not signify right ventricular enlargement. It may occur as a normal finding in older cats or may be associated with marked left atrial or left ventricular enlargement26; therefore, it should be ignored as an indicator of specific chamber enlargement. Indeed, most critical reviews detailing radiographic changes in cats do not include this criterion. 16, 20, 26 On the dorsoventral view, marked concentric right ventricular hypertrophy causes a rounding of the cardiac silhouette along the diaphragmatic border and a shift of the apex cranially to the left. Marked eccentric right ventricular hypertrophy causes a generalized rounding and enlargement of the right side, but the apex remains relatively fixed. Right Atrial Enlargement

Isolated right atrial enlargement does not occur, except in rare cases of tricuspid stenosis or atresia. Thus, a radiographic diagnosis of isolated right atrial enlargement should be viewed with suspicion. Biatrial enlargement is seen primarily with unclassified (restrictive) cardiomyopathy. Otherwise, right ventricular enlargement should accompany right atrial enlargement. On the lateral view, the right atrium sits in approximately the same plane as the left atrium (see Fig. 4B). The right auricle of the cat does not wrap around the front of the heart as it is reported to do in the dog. Therefore, right atrial enlargement in cats is not usually distinguishable from left atrial enlargement on the lateral view. On the dorsoventral view, marked right atrial enlargement causes a rounding of the right cranial quadrant. Mild right atrial enlargement cannot be identified radiographically. Generally, the caudal vena cava should enlarge in animals with a markedly enlarged right atrium.

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Figure 10. Lateral (A) and ventrodorsal (B) views of a cat with right heart enlargement secondary to severe tricuspid dysplasia. The caudal vena cava is elevated on the lateral view, with a caudal right atrial bulge at the junction of the caudal vena cava (note that this is not the left atrium). Also note the similarity in the cardiac silhouette to Figure 78 (cat with HCM and left heart enlargement). The pulmonary fields are unremarkable. On the ventrodorsal view, the apex is shifted away from the diaphragm to the left (arrow).

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RADIOGRAPHIC FINDINGS IN SPECIFIC FELINE CARDIAC DISEASES Acquired Cardiac Diseases Hypertrophic Cardiomyopathy

This is currently the most common cardiac disease recognized in feline patients (see Fig. 7). Radiographic findings in cats with HCM range from completely normal radiographs to radiographs demonstrating severe congestive heart failure and marked cardiomegaly. Early or mild HCM generally has no detectable radiographic abnormalities. As the disease progresses, left atrial enlargement becomes more noticeable, and the cardiac silhouette may elongate, especially in the dorsoventral view. Gradually, the left ventricle becomes more convex (see Left Ventricular Enlargement section), pulmonary veins may become more prominent, and the left atrium becomes progressively larger, with the heart assuming an ice-cream cone appearance in the lateral view. Ultimately, left-sided congestive heart failure in the form of pulmonary edema or pleural effusion develops. Occasionally, atypical changes such as globoid enlargement or rounding of the ventricular apex may be observed. 3 Dilated Cardiomyopathy

This disease is diagnosed rarely since the identification and correction of taurine deficiency as a cause in the late 1980s. Radiographically, when it presents, it is usually in the advanced stages and demonstrates marked left and right atrial enlargement and a rounded enlarged left ventricle. Congestive heart failure is usually present, with findings similar to those in cats with HCM. Unclassified (Restrictive) Cardiomyopathy

This disease is also less common than HCM. It is characterized by noncompliant ventricles and most commonly affects both left and right sides of the heart (see Fig. 9). Consequently, radiographs reveal marked biatrial enlargement without ventricular enlargement, usually congestive heart failure, and often pleural effusion. In general, this condition cannot be differentiated radiographically from HCM. Hyperthyroid Cardiomegaly

Hyperthyroidism causes overcirculation of the systemic and pulmonary vasculature as well as cardiac hypertrophy. Radiographic findings include predominantly left-sided cardiomegaly with or without congestive heart failure. These findings are similar to those of HCM but usually have less pronounced left atrial enlargement.

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Hypertension

In people, chronic systemic hypertension can lead to cardiac concentric hypertrophy and ultimately to myocardial failure. Recent echocardiographic studies of hypertensive cats showed minor changes in cardiac dimensions that did not exceed normal reference ranges. 21 • 25 Thus, these cats should appear radiographically normal. There is a possibility that the redundant or bulging aortae that are seen in older cats (see Fig. 5) may be associated with hypertension, but this has not been proven. 21 Mitral Insufficiency

Acquired degenerative mitral valve disease is rare in cats and occasionally results in congestive heart failure. Radiographic findings include variable degrees of left ventricular eccentric hypertrophy and left atrial enlargement as well as signs of left-sided congestive heart failure in severely affected animals. Right Ventricular Dysplasia

Recently, right ventricular dysplasia has been reported in cats. 8 These cats have isolated· right ventricular cardiomyopathy and rightsided heart failure. Echocardiographically, they resemble cats with severe tricuspid dysplasia (i.e., marked eccentric right ventricular hypertrophy and right atrial enlargement). Radiographically, these cats would be expected to have similar findings, although radiographic findings were not detailed in the report. Additionally, right-sided congestive heart failure would be anticipated (see Fig. 10). Heartworm Disease

Several studies have examined the radiographic changes in cats experimentally exposed to Dirofilaria immitus. The most notable changes included more prominent caudal lobar pulmonary arteries and increased density in the pulmonary parenchyma. The pulmonary parenchymal changes subsided in 53% of the cats within 14 months of inoculation in one study7 but persisted in all inoculated cats for 9 months in another study. 23 Cardiomegaly was reported in 10 of 15 cats in one study7 and in 5 of 11 cats in another study, 23 but specific chamber enlargement was not identified. The changes in the caudal lobar arteries were best identified on the dorsoventral view. Pericardia/ Disease

As is the case with dogs, severe pericardia! disease produces a globoid cardiomegaly in cats on thoracic radiographs. The caudal border of the cardiac silhouette tends to curve into the carina, suggesting a lack of left or right atrial enlargement. The pulmonary fields become

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compressed, increasing the parenchymal opacity, which should not be misinterpreted as pulmonary edema. The caudal vena cava becomes enlarged, and ascites may be evident. Radiographically, severe pericardia! effusion can be mistaken for tricuspid dysplasia or right ventricular dysplasia. Pericardia! effusion in cats is often secondary to severe left heart disease such as the cardiomyopathies. In these cases, the effusion is usually small and remains radiographically undetected. 22 Congenital Cardiac Diseases Patent Ductus Arteriosus

This is an infrequent but not rare diagnosis in cats. In most cases, clinical findings of a loud continuous murmur are sufficient in making the diagnosis. Radiographic findings include marked left ventricular eccentric hypertrophy, often with an elongated heart, and proportionally little left atrial enlargement (as compared with dilated cardiomyopathy, HCM, or mitral insufficiency) (see Fig. 8). The pulmonary vasculature is pronounced in many cases because of the volume overload to the pulmonary circulation. This is often best appreciated in the dorsoventral view. Congestive heart failure may be present. I have not appreciated the "ductus bump" of patent ductus arteriosus in cats. Ventricular Septal Defect

Most septal defects in cats are relatively small and well tolerated by the patients. Hemodynamically important defects cause left ventricular enlargement similar to that seen with patent ductus arteriosus. Rarely, the right ventricle enlarges. The pulmonary vasculature is pronounced because of pulmonary overcirculation. Peritoneopericardial Diaphragmatic Hernia

This is a congenital anomaly that results in the displacement of various abdominal viscera within the pericardia! space. It is a common incidental finding on routine radiography of older cats. Therefore, it is often unassociated with clinical signs but may occasionally cause heart failure.U Radiographic characteristics of peritoneopericardial diaphragmatic hernia are similar to those of pericardia! effusion; however, various soft tissue opacities can often be discerned within the pericardium, and viscera may be absent in the abdomen (e.g., the liver might be "missing"). If the intestines have herniated into the pericardium, a contrast study of the gastrointestinal tract should reveal their presence anterior to the diaphragm. There is a loss of continuity of the diaphragmatic border in the area of the hernia. Berry et aP described the identification of a dorsal peritoneopericardial mesothelial remnant as a radiographic diagnostic aid (see Fig. 6).

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Miscellaneous Cardiac Diseases

Various other cardiac congenital anomalies have been described in cats. Most authors documented cardiomegaly but were generally unable to differentiate the cause of the cardiomegaly from survey radiographs. Similarly, Yaphe et aP3 reported generalized cardiomegaly associated with severe anemia in a kitten. Both right and left sides of the heart were enlarged secondary to the massive volume overload, which is similar to findings of systemic arteriovenous fistulae in cats. A single report exists of a chemodectoma in a cat in which the radiographic findings were similar to those of dogs with chemodectomas.29 CONCLUSIONS

Thoracic radiography continues to play an important role in the diagnosis and management of feline cardiac disease. It allows rapid differentiation between congestive heart failure and primary pulmonary disease as a cause of respiratory clinical signs in many cases. Further, it allows detection of cardiomegaly and specific chamber enlargement in many cases, although echocardiography has largely supplanted this function because of its substantially greater accuracy. Nonetheless, it still remains one of the best methods available to the clinician for monitoring the management of congestive heart failure in cats and for assessing anesthetic risk in feline patients with cardiac disease. References 1. Berry CR, Koblick PD, Ticer JW: Dorsal peritoneopericardial mesothelial remnant as an aid to the diagnosis of feline congenital peritoneopericardial diaphragmatic hernia. Vet Radio! 31:239, 1990 2. Bolton GR: Feline cardiology. Feline Pract 5:42, 1975 3. Bright JM, Golden AL, Daniel GB: Feline hypertrophic cardiomyopathy: Variations on a theme. J Small Anim Pract 33:266, 1992 4. Burk RL: Radiographic examination of the cardiopulmonary system. Vet Clin North Am Small Anim Pract 13:241, 1983 5. Carlisle CH, Thrall DE: A comparison of normal feline thoracic radiographs made in dorsal recumbency versus ventral recumbency. Vet Radiol23:3, 1982 6. Davies C, Forrester SD: Pleural effusion in cats: 82 cases (1987-1995). J Small Anim Pract 37:217, 1996 7. Donahoe JMR, Kneller SK, Lewis RE: Hematologic and radiographic changes in cats after inoculation with infective larvae of Dirofilaria immitus. JAVMA 168:413, 1976 8. Fox PR, Liu S-K: Arrhythmogenic right ventricular cardiomyopathy/ dysplasia. In Proceedings of the 16th American College of Veterinary Internal Medicine Forum, San Diego, 1998, p 89 9. Hamlin RL, Smetzer DL, Smith CR: Radiographic anatomy of the normal cat heart. JAVMA 143:957, 1963 10. Harpster NK: Feline cardiomyopathy. Vet Clin North Am Small Anim Pract 7:355, 1977 11. Hay WH, Woodfield JA, Moon MA: Clinical, echocardiographic and radiographic findings of peritoneopericardial diaphragmatic hernia in two dogs and a cat. JAVMA 195:1245, 1989

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12. Kimura H, Suganuma T, Ogata M, et al: Radiographic anatomy of Japanese Domestic cats. J Jpn Vet Med Assoc 47:123, 1994 13. Kittleson MD: Hypertrophic cardiomyopathy. In Kittleson MD, Kienle RD (eds): Small Animal Cardiovascular Medicine. St. Louis, Mosby, 1998, p 347 14. Kittleson MD: Radiography of the cardiovascular system. In Kittleson MD, Kienle RD (eds): Small Animal Cardiovascular Medicine. StLouis, Mosby, 1998, p 47 15. Litster AL, Buchanan JW: Vertebral scale system to measure feline heart size in radiographs [abstract]. In Proceedings of the 17th American College of Veterinary InteTI)~tl Medicine Forum, Chicago, 1999, p 731 16. Lord PF, Zontine WJ: Radiologic examination of the feline cardiovascular system. Vet Clin North Am Small Anim Pract 7:291, 1977 17. Lord PF, Wood A, Tilley LP, et al: Radiographic and hemodynamic evaluation of cardiomyopathy and thromboembolism in the cat. JAVMA 164:154, 1974 18. McLaughlin RF, Tyler WS, Canada RO: A study of the subgross pulmonary anatomy in various mammals. Am J Anat 108:149, 1961 19. Moon ML, Keene BW, Lessard P, et al: Age related changes in the feline cardiac silhouette. Vet Radio! Ultrasound 34:315, 1993 20. Myer WC, Bonagura JD: Survey radiography of the heart. Vet Clin North Am Small Anim Pract 12:213, 1982 21. Nelson OL, Riedesel EA, Ware WA: Echocardiographic and radiographic changes associated with systemic hypertension in cats [abstract]. In Proceedings of the 17th American College of Veterinary Internal Medicine Forum, Chicago, 1999, p 712 22. Rush JE, Keene BW, Fox PR: Pericardia! disease in the cat: A retrospective evaluation of 66 cases. JAm Anim Hosp Assoc 26:39, 1990 23. Selcer BA, Newell SM, Mansour AE, et al: Radiographic and 2-D echocardiographic findings in eighteen cats experimentally exposed to D. immitus via mosquito bites. Vet Radio! Ultrasound 37:37, 1996 24. Snyder PS, Takashi S, Atkins CE: The utility of thoracic radiographic measurement for the detection of cardiomegaly in cats with pleural effusion. Vet Radio! 31:89, 1990 25. Stepien RL, Henik RA, Weichert JL: Echocardiographic findings in 41 cats with systemic hypertension [abstract]. In Proceedings of the 42nd British Small Animal Veterinary Association Congress, Birmingham, UK, 1999, p 315 26. Suter PF: Cardiac diseases. In Thoracic Radiography: A Text Atlas of Thoracic Diseases of the Dog and Cat. Wettswil, Switzerland, PF Suter, 1984, p 351 27. Suter PF, Lord PF: A critical evaluation of the radiographic findings in canine cardiovascular diseases. JAVMA 158:358, 1970 28. Thrall DE, Losonsky JM: Dyspnea in the cat: Part III-Radiographic aspects of intrathoracic causes involving the heart. Feline Pract 9:36, 1979 29. Tillson DM, Fingland RB, Andrews GA: Chemodectoma in a cat. JAm Anim Hosp Assoc 30:586, 1994 30. Toal RL, Losonsky JM, Coulter DB, et al: Influence of cardiac cycle on the radiographic appearance of the feline heart. Vet Radio! 26:63, 1985 31. Van den Broek AHM, Darke PGG: Cardiac measurements on thoracic radiographs of cats. J Small Anim Pract 28:125, 1987 32. Wortman JA, Chang-Winterkorn P, Knight DH, et al: Receiver operating characteristic curve analysis of the utility of thoracic radiographs for the diagnosis of feline cardiac disease. Vet Radio! 28:121, 1987 33. Yaphe W, Giovengo S, Moise NS: Severe cardiomegaly secondary to anemia in a kitten. JAVMA 202:961, 1993

Address reprint requests to Mark Rishniw, BVSc, MS Department of Clinical Sciences College of Veterinary Medicine Cornell University Ithaca, NY 14853 e-mail: [email protected]