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Dyspneic cats: triage and differentiation of cardiogenic versus noncardiogenic causes (Proceedings)

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Aug 01, 2011

Respiratory abnormalities are relatively common in cats, who may suffer from a wide range of disease processes. An initial triage step in the management of a dyspnic cat is to determine whether the dyspnea is cardiogenic or due to extracardiac (primary pulmonary) disease. This task is in no way a simple one, as cats often have non-specific history and physical examination abnormalities. Dyspnic cats are notoriously very fragile patients who can become critically decompensated with too much stress and diagnostic testing. A family history of hypertrophic cardiomyopathy, sudden death, or arterial thromboembolism, or signalment including breeds predisposed to develop familial heritable hypertrophic cardiomyopathy (i.e. Ragdoll, Maine Coon cat, Scottish fold, Turkish Van, Sphynx, Rex, British Shorthair, American Shorthair, and Siberian cat) may raise the suspicion of heart disease. A history of coughing, retching, or wheezing is more consistent with airway disease or primary pulmonary disease than heart failure, since cats with heart failure infrequently cough. Clients should be questioned if they administer monthly heartworm preventative to the cat since heartworm disease is an important cause of respiratory abnormalities in cats.

Cardiovascular examination in cats with heart failure may be non-specific since often heart sounds are masked by adventitious lung sounds or muffled by pleural effusion. Depending on the heart disease present, the incidence of heart murmurs range from high (~60%) in cats with hypertrophic cardiomyopathy, to only 20% of cats with dilated cardiomyopathy. A gallop heart sound (S3 or S4 heart sound) is commonly heart in cats with dilated cardiomyopathy (80%) and invariably present with other heart diseases (33% in HCM). An arrhythmia is invariably present in cats with severe heart disease, and may include premature beats, tachycardia, or irregularly irregular rhythm. Cats with a mediastinal mass have firm, non-compliant chests identified by palpation of the cranial mediastinum.

The respiratory disease can be anatomically localized based on physical examination and thoracic radiographs. Upper airway disease (nasal, pharyngeal, extrathoracic trachea) produces a stridorous breathing pattern with marked increase in inspiratory effort. Lower airway disease involving the bronchioles is extremely common in cats, and produces an expiratory breathing pattern, wheezy pulmonary auscultation, and cough. Pulmonary parenchymal disease causes a mixed inspiratory and expiratory breathing pattern, and includes a wide range of diseases including cardiogenic pulmonary edema, infectious diseases, inflammatory diseases, neoplasia, parasitic lung disease, or pulmonary fibrosis, and can be very challenging to distinguish. Pleural disease (i.e. pleural effusion, pleural fibrosis) produces a restrictive breathing pattern characterized by shallow rapid respirations. Common pathophysiologic processes causing pleural effusion include: increased systemic and pleural capillary hydrostatic pressures (congestive heart failure), decreased plasma oncotic pressure (hypoalbuminemia <1.5 g/dL), increased capillary permeability (inflammation), lymphatic obstruction or dysfunction, infectious causes, trauma, coagulopathy, parasitic (heartworm or aleurostrongylus), and intrathoracic neoplasia.

Initial triage of a dyspnic cat includes a triage thoracic ultrasound to evaluate for significant pleural effusion, which should be immediately removed to stabilize the cat. This can be done with the cat in sternal recumbency with minimal manipulation or restraint. If the cat is fractious or extremely anxious, an opioid +/- low dose acepromazine (0.01 mg/kg) can be given as a sedative. Pleural effusion should be submitted for fluid analysis, which if often non-specific in cats with heart failure, and may include: transudate, modified transudate, pseudochylous, or chylous effusion. An exudate (septic vs. aseptic) is another important cause of pleural effusion in cats and can be identified on fluid analysis. During the triage ultrasound, assessment of atrial size is essential to determine whether the dyspnea is due to severe heart disease or non-cardiac disease. Left atrial size can be quantified by measurement of the left atrial diameter and aortic diameter, and a ratio of LA:AO < 1.5 is normal. Typically cats with heart failure have significantly increased LA:Ao of >1.8. Cats with normal left atrial size and dyspnea do not have congestive heart failure as a cause of the dyspnea. Assessment of left ventricular myocardial function is also important. Athough dilated cardiomyopathy or significant myocardial failure is relatively uncommon in cats (10% of cardiomyopathy cases), it has important treatment implications including avoidance of beta blockers and using pimobendan and possibly digoxin. The anterior mediastinum should also be imaged to assess for a mediastinal mass as a cause of dyspnea and pleural effusion.

Thoracic radiographs are invaluable for assessment of respiratory disease, but should never be a terminal event. Sometimes there are pathognomic pulmonary patterns of bronchial disease, or cats may have obvious cardiomegaly, pulmonary venous congestion, and pulmonary edema. Probably the majority of cats have more ambiguous abnormalities including pulmonary infiltrates, possibly a bronchial pattern, and possibly pleural effusion. Cardiogenic pulmonary edema does not follow a typical pattern of distribution of perihilar to caudodorsal lung lobes, and is most often diffuse. It always includes more than one lung lobe, and is typically asymmetrical. Detection of a lobar consolidation raises suspicion of pneumonia or neoplasia and places heart failure to the bottom of the list. Identification of cardiomegaly and atrial dilation can also be challenging in cats since the left atrium is located more centrally in the lateral radiography. Using subjective assessment, left atrial dilation was only identified in 48- 72% of cats compared to the gold standard of echocardiographic measurement of left atrial size. Often the dorsoventral or ventrodorsal views are most useful to identify atrial dilation, which appear as a valentine shaped heart. Sometimes the heart is obscured by pulmonary infiltrates, and an echocardiogram is essential to assess if there is significant cardiac disease. Pulmonary venous distension is often identified (71%) on radiographs in cats with heart failure. Presence of right heart enlargement, bicavitary effusion, hepatomegaly, and dilation of the caudal vena cava are consistent with right heart failure, with top differentials including dilated cardiomyopathy, tricuspid valve disease, arrhythmogenic right ventricular cardiomyopathy. Often there is a combination of pleural effusion and pulmonary edema in cats with heart disease from many etiologies.

Secondary diagnostic modalities used to refine the cause of dyspnea includes an advanced echocardiogram, computed tomography, bronchoscopy and bronchoalveolar lavage, and additional diagnostic tests such as heartworm antigen and antibody test or fine needle aspirate and cytologic evaluation of a pulmonary or mediastinal mass. An advanced echocardiogram is used to determine the etiology of heart disease present, assess for intracardiac thrombus or spontaneous echocardiographic contrast, and assess for heartworms in the main pulmonary artery and proximal left and right caudal lobar branches. Echocardiography is a specific and relatively sensitive tool in cats for diagnosis of heartworm disease, and is essential in cats that have lower airway disease.