Subtle Indications of Pre-clinical Heart Disease in Cats

Veterinary Cardiology Tip

Cats give little indication of their heart disease – be suspicious of any potential cardiac abnormality, no matter how small, to catch pre-clinical heart disease.

Cats do not adhere to basics that are useful when evaluating the canine heart.  From the physical exam to screening tests, such as chest radiographs and the electrocardiogram, cats can thwart our best attempts to diagnose heart disease prior to signs of congestive heart failure (CHF) or arterial thromboembolism. Practitioners should be aware of nuances of feline heart disease and heed the warning of any potential indication of cardiac dysfunction.
Reliance upon the presence of a heart murmur alone is insufficient for determining the presence or absence of heart disease in cats.  Though specificity of murmurs for detecting heart disease is relatively high (87%), the sensitivity of murmurs for heart disease is relatively low (31%) in asymptomatic cats.¹  Even with heart disease severe enough to cause CHF, only half of these cats will exhibit heart murmurs.²  Finding other auscultable abnormalities, such as gallop sounds and arrhythmias, may be of great benefit, as the addition of these findings may improve sensitivity and specificity for detecting heart disease in cats.  Focused auscultation at the parasternal border through variable heart rates and an adequate time frame will enhance heart disease detection in cats.

Chest radiographs enable assessment of the cardiac silhouette, pulmonary vessels, and lung fields. Unfortunately, chamber enlargement patterns are not as helpful in detecting heart disease in cats, as left atrial enlargement, a marker of severe disease and prerequisite for left heart failure is only radiographically detected in half of cats with left sided CHF.  Furthermore, irregularities in silhouette contour may represent an artifact secondary to fat accumulation.  Vertebral heart size can be helpful, with the normal range being 6.9-8.1 vertebrae.³  Inspection of pulmonary veins and arteries may provide additional benefit in detecting a cat with heart disease.  Enlargement of either may be indicative of severe heart disease, with pulmonary artery distention more common than venous distention in cats with CHF.

Though many cats with heart disease have normal ECGs, the presence of an arrhythmia can be specific for heart disease.  In one study, nearly all cats with ventricular ectopy were found to have echocardiographic abnormalities.⁴  Cats with hypertrophic cardiomyopathy (HCM) are known to have more frequent and complex arrhythmias than normal cats,⁵ and a link between HCM and atrioventricular block has been proposed.⁶  Furthermore, patterns of chamber enlargement (e.g., increased R-wave amplitude) or axis shifts make underlying heart disease more likely (Fig 1).  ECG assessment is reasonable for any cat suspected of heart disease.

Fig 1: Lead II ECG in a cat showing overall negative QRS complex (decreased R-wave amplitude, prominent S-wave) consistent with a ventricular conduction disturbance. Prolonged PR interval (inset) indicates 1st degree atrioventricular block. Both findings suggest the possibility of cardiomyopathy.

Biomarker testing is a new frontier for assessing the heart.  In particular, NT-proBNP levels reflect cardiac stretch that occurs with heart disease. This exhibited 91% sensitivity and 86% specificity for detection of occult heart disease with 91% positive predictive value and 87% negative predictive value.⁷  This is not the gold standard for diagnosis of heart disease but can be helpful in guiding further diagnostics when used in cats at risk for cardiac disease (e.g., with abnormal heart sounds, arrhythmias, family history of heart disease).  The SNAP® Feline ProBNP test provides a more immediate surrogate to the NT-proBNP value.  A positive result suggests the presence of moderate to severe heart disease,⁸ but a negative result may fail to reflect mild heart disease that may still need monitoring.  BNP testing is very reasonable for any cat suspected of heart disease, but the potential limitations should be considered.

References:

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