Dr Nuala Summerfield

BSc BVM&S DipACVIM (Cardiology) DipECVIM CA(Cardiology) MRCVS
RCVS, American and European Specialist in Veterinary Cardiology

You are presented with a 9 year old male neutered domestic longhair at a routine vaccination. He’s a “big boy” but not particularly fat, and there’s nothing abnormal on your physical exam until you take a quick listen to his heart. Then you hear a soft heart murmur which you classify as a grade 2 out of 6, left sided systolic murmur. His heart rate is 170 beats per minute and the rhythm is regular. The rest of his physical examination is unremarkable. His owner reports that he is in excellent health and seems normal at home.

How significant is this murmur likely to be?
What are the differential diagnoses?
How would you proceed?

Have a think about what you would recommend and then read on for the answers!

How significant is this murmur likely to be?

The vast majority of heart disease in adult cats is caused by cardiomyopathy (heart muscle disease), primarily hypertrophic cardiomyopathy (HCM). The prevalence of HCM increases with age, affecting approximately 25-30% of apparently healthy cats older than 9 years of age. The prevalence of heart murmurs in cats also increases with age, with around 60% of apparently healthy cats older than 9 years of age having a heart murmur.

There are two important points that need highlighting – not all cats with HCM have a heart murmur AND a significant number of cats without underlying heart disease have heart murmurs!

Frustratingly in cats, it’s often not possible to distinguish between a benign flow murmur and a murmur associated with underlying heart disease from auscultation alone.

A heart murmur is caused by turbulent blood flow in the cardiac chambers or great vessels. Various factors can cause blood to become turbulent including structural heart disease and high cardiac output states (e.g. hyperthyroidism, anaemia). Thickening of the myocardium per se does not cause turbulent blood flow, hence many cats with HCM do not have a heart murmur. However, approximately one third of cats with HCM have left ventricular outflow tract (LVOT) obstruction due to systolic anterior motion (SAM) of the mitral valve. This is often also associated with some degree of mitral regurgitation, and these causes of turbulent blood flow account for the systolic murmur audible in some cats with HCM. These cats are said to have “obstructive HCM”, or HOCM.

It is important to remember that the loudness of the murmur does not correlate with the severity of HCM. Many ‘high-risk’ asymptomatic HCM cats (i.e. those cats with advanced underlying heart disease that have not yet developed symptoms) have no audible murmur. Gallop sounds are a much more specific finding for HCM than heart murmurs in cats.

Another important consideration is that cats are relatively prone to physiologic murmurs (i.e. benign flow murmurs). Also iatrogenic murmurs can be produced in cats by applying too much pressure with the stethoscope against the highly compliant feline thoracic cage, resulting in compression of the right ventricle and leading to partial right ventricular outflow tract (RVOT) obstruction. So this heart murmur in your patient could simply be because you pushed the stethoscope against his ribs a little too hard. As a result, it’s vital to auscultate cats in a standing or sitting posture and to minimise the pressure applied with the stethoscope on the chest wall.

What are the differential diagnoses?

Assuming that you rule out an iatrogenic murmur, it’s definitely reasonable to consider the possibility of HCM, especially since older male domestic cats are over-represented.

There are of course other types of cardiomyopathy that affect cats, although much less frequently than HCM.

Restrictive cardiomyopathy (RCM) is a possibility, with left ventricular fibrosis leading to severe diastolic dysfunction. Murmurs are reportedly common in this condition and although most cats present with signs of CHF, it is still possible that this cat is an early asymptomatic case.

Dilated Cardiomyopathy (DCM) is rare in cats. Unlike in dogs, DCM in cats is typically due to a taurine deficiency secondary to an imbalanced diet.

Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) is also rare, but is recognised in cats and not just limited to Boxer dogs! It is caused by extensive fibrofatty infiltration into the myocardium, ARVC usually leads to ventricular arrhythmias, but these are not necessarily apparent from auscultation alone. If this condition is suspected, ECG / Holter monitoring is indicated.

How would you proceed?

HCM has a variable progression and many affected cats can remain symptom-free for long periods. However clinical signs can develop at any time and typically include respiratory distress from congestive heart failure (CHF) and / or acute hindlimb paralysis from aortic thomboembolism (ATE). Therefore in view of these potentially serious sequelae, further investigation appears to be indicated in your patient.

At this stage, your patient is asymptomatic and there is no clinical evidence of CHF. However, thoracic radiography can be a very useful test to quickly and easily check for cardiac enlargement. Also in patients with suspected CHF, it allows assessment of the lung fields for pulmonary oedema and/or pleural effusion.

Systemic hypertension and hyperthyroidism should always be excluded in middle aged and older cats suspected of having underlying HCM. Biochemistry and haematology are rarely diagnostic in feline cardiac cases. However you may detect mild pre-renal azotaemia due to cardiac dysfunction and secondary decreased renal perfusion, and can exclude anaemia. These blood tests are also useful for future monitoring.

Another blood test that has demonstrated some clinical utility for identifying those asymptomatic ‘high risk’ HCM cats is the cardiac biomarker NT-proBNP. However, this biomarker seems to be less useful for identifying asymptomatic cats with early HCM.

Electrocardiography is in many ways an under-utilised diagnostic tool; however, in cats with cardiomyopathies it is rarely useful in isolation. In these conditions, ECG changes are highly variable and unless an obvious arrhythmia is present, not usually diagnostic.

The gold standard test for the diagnosis for cardiomyopathy in the cat is heart ultrasound (echocardiography). This is the only technique that will enable you to assess the severity of left ventricular hypertrophy in HCM, as well as confirming the presence of left ventricular outflow tract obstruction in HOCM, together with other important risk factors such as left atrial dilation and evidence of spontaneous echo contrast (‘smoke’) or thrombi within the left atrium.

Many cats with asymptomatic HCM have a relatively benign clinical course and will remain symptom-free for years, whereas in other cats HCM progression is more rapid. Clinical signs can develop at any age but are more common in older cats with HCM. It is vital to identify these ‘high risk’ asymptomatic HCM cats (i.e. those at increased risk for CHF or ATE), since interventions such as intravenous fluid therapy and general anaesthesia can precipitate CHF. Also prophylactic antithrombotic therapy in asymptomatic cats with obvious LA dilation may reduce the risk of ATE.

How did you do? If you’d like to know more or have a case that you’d like to discuss,
contact VVS and talk to our Cardiologists.