ECG remains key diagnosticfor cardiomyopathy in cats

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Nov 01, 2003

Q. Could you provide a brief review of car Could you provide a brief review of cardiomyopathies in cats?

Cardiomyopathies affect heart muscle. Changes in the heart muscle leads to loss of function and thereby clinical disease can occur. There are primary idiopathic forms of cardiomyopathy as well as secondary forms of cardiomyopathy that occur as a result of an underlying systemic-metabolic problem.

Cardiomyopathies may be classified according to the structural heart changes that occur. In Persian's and Maine Coon's, a familial predisposition to hypertrophic cardiomyopathy has been identified. The secondary forms do often occur in cats, especially in older cats.

Good examples of secondary cardiomyopathies are dilated cardiomyopathy in association with taurine deficiency and cardiac hypertrophy that occurs in association with hyperthyroidism or hypertension.

HypertrophicThe most frequent form of cardiomyopathy in cats is the hypertrophic form. In hypertrophic cardiomyopathy, the ventricular walls begin to hypertrophy and the heart muscle fibers become disorganized. With hypertrophy, the heart can no longer relax and the ventricular lumen is decreased.

Ischemia of the heart muscle also occurs quite common with infarction. There is also a specific manifestation of hypertrophic cardiomyopathy termed hypertrophic obstructive cardiomyopathy.

In these cases, there is dynamic obstruction of the aortic outflow tract during systole, leading to a situation similar to aortic stenosis. It is possible that this dynamic aortic stenosis leads to secondary generalized ventricular hypertrophy.

Restrictive and intermediate forms of cardiomyopathies are also recognized. Their classification does pose some challenges as there is inconsistency in nomenclature. Restrictive cardiomyopathy should probably be limited to those cases where severe fibrosis of the endocardium and subendocardium has occurred. The hearts are stiff and non-compliant, and as a result diastolic dysfunction exists with an inability to fill the ventricle properly.

Much of the normal cardiac muscle architecture is deformed. Restrictive cardiomyopathy may be secondary to a previous bout of myocardial inflammation, although it may also just represent an end stage form of cardiac disease where ischemia and infarction has led to widespread scar tissue formation in the myocardium.

Intermediate cardiomyopathyIntermediate cardiomyopathy is somewhat more difficult to define. It usually refers to cardiomyopathy that has features of both dilated and hypertrophic cardiomyopathy, that is, hypertrophy of the ventricular walls with chamber dilation and normal to slightly decreased contractility.

Whether intermediate cardiomyopathy represents a discrete disease process or is a stage in the progression of cardiomyopathy from hypertrophy to dilation is uncertain.

With the advent of nutritional supplementation of taurine, dilated cardiomyopathy has become a relatively uncommon diagnosis in cats. There are still occasional cases seen and taurine supplementation should be tried in these affected cats.

Treatment focuses on relieving signs of congestive heart failure with diuretics and possibly ACE inhibitor. Arrhythmias occur frequently. With supraventricular tachycardias, digoxin becomes a therapeutic option.

DifferentiatingHistory and clinical findings are not useful in differentiating between the various forms of cardiomyopathy since they tend to be similar. Clinical manifestations, when present, are a reflection of heart failure, arrhythmias, conduction abnormalities, or as a result of thromboembolism.

Physical examination may raise the possibility of secondary cardiomyopathy by finding a palpable thyroid nodule (thyrotoxic cardiomyopathy), small kidneys (hypertensive cardiomyopathy) or retinal degeneration (taurine-deficient dilated cardiomyopathy).

The natural progression of these various cardiomyopathies is variable so that it is possible for cats to remain asymptomatic for prolonged periods of time. Many times heart disease is detected on routine physical examination without overt clinical signs being noted. It is uncommon for cats to have a slowly progressive course as is seen with most dogs with congestive heart failure. It is also uncommon for cats to cough with cardiomyopathy, even when they have significant cardiomegaly and/or pulmonary edema. Sudden death can occur in some cases without prior signs of cardiac disease. In some instances, owners will detect decreased exercise tolerance and a tendency for a cat to open mouth breath after minor exercise before development of congestive heart failure.

Cardiovascular examination can detect the presence of heart disease, though it too can rarely differentiate between the various forms of cardiomyopathy. Examination of the cat should begin with careful inspection of the cat at rest. Abnormal breathing patterns both in regard to the respiratory rate and respiratory character may be noted. The area of the jugular vein should be examined for possible distension or a jugular pulse.

The neck should also be palpated as well for the presence of a thyroid nodule. The femoral pulses should be palpated, and in most cases of cardiomyopathy, they should be fairly normal.

With arrhythmias, pulse deficits may occur. With low output heart failure, and especially with dilated cardiomyopathy, pulses may be weak. If thrombus is present, pulses to one or both rear legs are poor to absent. In addition, the paw pads are pale or cyanotic. Cats rarely cough or have ascites in association with cardiomyopathy. Findings will depend on whether or not heart failure is present.

Without heart failure, at least a heart murmur should be present, usually with the point of maximal intensity near the left parasternal region. The murmur can be of variable intensity with it being loudest when the cat is excited. This is typical for hypertrophic obstructive cardiomyopathy. Gallop rhythms are not uncommon. They are a result of abnormal systolic filling such as occurs with dilated cardiomyopathy or atrial contraction in association with a stiff ventricle.

With heart failure, auscultation can reveal crackles with pulmonary edema, although this is less commonly the case in cats than in dogs. Even with severe dyspnea as a result of pulmonary edema, at times it can be difficult to auscult the sounds typically thought to be present with edema. Muffling of heart sounds in cats is almost always a result of pleural effusion. In dogs, one would need to consider pericardial effusion as well, however, in cats most cases of pericardial effusion are also secondary to cardiomyopathy.

Evaluating cardiac diseaseElectrocardiography can be helpful in the evaluation of a cat with suspected cardiac disease. The primary findings that can point toward a cardiomyopathy are enlargement patterns on the ECG, conduction disturbances and the identification of arrhythmias. It generally is not possible to differentiate the various forms of cardiomyopathy based on only the ECG findings.

Enlargement patterns in cats are mainly comprised of two ECG changes. An R-wave that exceeds 0.9 mV suggests heart enlargement. Less commonly a prolongation of the QRS interval will be seen (>0.045 seconds). A cranial axis deviation pattern is also typical in cardiomyopathy. This is reflected in a positive R-wave in Lead I and deep S-waves in Leads II, III and aVF. This may be an indicator of a left anterior fascicular block or may merely reflect a left ventricular enlargement pattern. If Lead I is negative (deep S wave) along with leads II, III, and aVF then this can be a sign of right ventricular enlargement and heartworm disease needs to be considered.

Conduction disturbancesSeveral conduction disturbances can be seen with feline cardiomyopathy. One of the more common conduction disturbances is third degree atrioventricular block.

Third degree atrioventricular block may be caused by the pronounced thickening of the intraventricular septum in which the atrioventricular node resides. This may be suspected whenever a cat has a heart rate that is very slow (<120 beats per minute) and needs to be confirmed with an ECG. Generally, cats do not show the typical signs that dogs with third degree atrioventricular block will show such as syncope or episodic weakness because the intrinsic heart rate of the ventricle in cats is sufficiently high enough to take over for the normal sinus rhythm and allow the cat to function without clinical signs even with this advanced conduction problems. Third-degree atrioventricular block will also be seen commonly in older cats as a result of fibrosis of the atrioventricular node.