Introduction

Cardiac murmurs occur when the normal laminar flow of blood through the heart and great vessels is disturbed by any process that gives rise to turbulence. They are usually classified as:

 Pathological--due to congenital or acquired cardiovascular diseases

 Physiological--caused by changes in blood viscosity or velocity, e.g., in anaemic patients

 Innocent (or functional)--in patients where no diseases or defects can be detected (usually soft basal murmurs, localised, early systolic). These are encountered particularly in young animals (less than 16-24 weeks old), but also occasionally in adults.

Whereas systolic murmurs in older, small-breed dogs are common and in many cases of limited clinical significance, at least initially, the potential significance of a murmur in a cat should not be overlooked, and further investigation is often required.

Clinical Findings

The detection and characterisation of murmurs in cats may be particularly challenging, due to factors such as purring obscuring the heart sounds, a rapid heart rate, difficulty in localising the point of maximum loudness and radiation of the murmur to the sternum and sternal borders. Murmurs may also be extremely variable in intensity ('dynamic') at different heart rates in cats. Auscultation should be performed carefully and systematically, listening over the entire cardiac area, using a good quality stethoscope with a small diaphragm (e.g., a paediatric type) and making use also of the stethoscope's bell. There should be minimal background noise and the patient should be gently restrained in a standing position so as to be as relaxed as possible.

The finding of a murmur should prompt a thorough review of auscultatory findings and a thorough search for an underlying cause, based on history, general physical examination and appropriate further tests if indicated.

Murmur Characteristics

These are perhaps less useful in cats than in dogs in establishing a tentative diagnosis, but murmurs should be characterised as follows:

 Timing during the cardiac cycle. Systolic murmurs are by far the most common in cats, due to left or occasionally right atrioventricular (AV) valvular incompetence (regurgitation) or turbulence in the left or right ventricular outflow tracts caused by fixed or dynamic obstruction. Diastolic and continuous murmurs are rare in cats.

 Loudness may be subjectively graded from grade I (very faint) to grade VI (very loud). Grade III is defined as being equal in intensity to the normal heart sounds (S1 and S2). Loudness is not always correlated with clinical severity. In addition, the intensity of a murmur may vary from beat to beat with dysrhythmias and may be altered by heart rate (especially in cats as mentioned above) and vasoactive drugs, including sedatives.

 Point of maximum intensity (PMI). The point on the chest wall where a murmur is loudest (and where the thrill, if present, is palpable) can indicate the likely source of the sound within the heart:

 Caudal cardiac area (apex region)--murmurs from the left and right AV valves are heard best in this area on the left and right sides respectively; the right apex is slightly more cranial than the left. Murmurs from ventricular septal defects (VSDs) are best heard over the right apex, nearer the sternal border. As mentioned above, many murmurs in cats are best heard over the sternum and sternal borders.

 Cranial cardiac area (heart base)--murmurs from the aortic and pulmonary valves and patent ductus arteriosus (PDA) are best heard in this region, usually louder on the left, the pulmonic area being more cranial.

 Radiation. Murmurs may radiate away from their point of maximum intensity, particularly if loud or associated with cardiomegaly. Radiation to the ventral sternum and sternal borders is common in cats.

 Character (acoustic profile) and pitch. These are subjective assessments and difficult to make in cats. Murmurs may have a 'musical' quality if one frequency predominates, but they usually contain a mixed frequency spectrum.

The Other Heart Sounds

In addition to listening for and characterising cardiac murmurs, the clinician should pay attention to:

 The audibility of the heart sounds, e.g., muffling could indicate pleural or pericardial effusion

 Heart rate and rhythm and the pattern of any arrhythmia, comparing this to femoral pulse rate and rhythm. Sinus arrhythmia is a less common finding in cats in the examination room than in dogs. The finding of an arrhythmia will increase the potential significance of a murmur, indicating the possibility of significant myocardial or systemic disease.

 The presence of abnormal transient sounds, especially gallop sounds, which are audible ventricular filling (S3) or atrial contraction (S4) sounds. These are usually an indication of ventricular disease (abnormal filling pressures, abnormal diastolic function), although gallops are heard occasionally in healthy older cats. These lower-pitched sounds are often heard best with the bell of the stethoscope.

General Physical Examination

 Findings such as pallor or delayed capillary refill time (CRT), weak femoral pulses, cool extremities, jugular distension and ascites are all useful indicators of poor cardiac output and increased filling pressures.

 The general physical examination may also show signs of underlying systemic disease, such as weight loss and goitre with hyperthyroidism, abnormally shaped or sized kidneys with chronic kidney disease and ocular changes as an indication of systemic hypertension.

 Percussion of the thorax is useful to assess the area of cardiac dullness, the presence of pleural fluid or large masses.

 Lung and airway sounds should be assessed for abnormalities such as crackles and wheezing, which may be indicators of pulmonary oedema or small airway disease.

Causes of Pathological Murmurs

Congenital Heart Disease

A murmur in a young (<6 months old) cat has a high probability of being a congenital abnormality, but cardiomyopathy is occasionally seen in kittens of this age. The commonest congenital cardiac defects in cats are dysplasias of the AV valves and VSDs, with other defects seen including aortic stenosis, PDA, pericardioperitoneal diaphragmatic defects and complex cardiovascular anomalies such as endocardial cushion defects.

Acquired Heart Disease

Various forms of cardiomyopathy, especially primary (idiopathic) hypertrophic cardiomyopathy (HCM), are the most common forms of acquired cardiac disease in cats, although degenerative valvular disease is seen occasionally. Although cardiomyopathy is most often idiopathic, specific diseases such as those listed below may significantly affect cardiac anatomy and function in cats, giving rise to murmurs:

 Hyperthyroidism

 Hypertension, usually secondary to chronic kidney disease

 Taurine deficiency causing dilated cardiomyopathy (now rare)

 Other disease processes may affect the heart, e.g., chronic anaemia, acromegaly, feline infectious peritonitis (FIP), neoplasia

One survey from the US reported murmurs in 21% of apparently healthy cats and that 'many murmurs appear to be caused by structural heart disease that is in a clinically latent state'. A retrospective survey of symptomatic cats from Switzerland found that HCM was the most common diagnosis (67.6%). Congenital cardiovascular malformations were found in 11.8% of cases and VSD was the most frequent of these. Another survey by the same authors found, from a total of 144 cats undergoing screening prior to breeding, 8.3% with HCM and 6.9% with possible HCM. A congenital cardiac malformation was recognised in 4.2%, most often tricuspid valve dysplasia.

An important and very serious consequence of heart disease in cats is embolism of thrombi from the enlarged left atrium, which most frequently go to the terminal aorta.

Murmurs in Older Cats

Murmurs may arise, especially in older cats, due to dynamic right ventricular outflow tract obstruction. These may be functional (i.e., no evidence of cardiac or non-cardiac disease) or physiological, secondary to diseases associated with increased cardiac output, such as hyperthyroidism, anaemia and inflammation, and cases of chronic renal failure with and without systemic hypertension. Basal thickening of the interventricular septum, sometimes associated with radiographically visible tortuosity of the thoracic aorta, may be a cause or may be associated with dynamic left and right ventricular outflow tract obstruction in older cats, often without any clinical significance.

Investigation

 Blood sampling is useful where hypertension or other systemic disease is suspected. Older cats (<5 years) should have their T4 measured.

 Systemic arterial blood pressure should be measured in all cases if possible, especially if hypertension is suspected (renal disease or ocular lesions).

 Echocardiography to assess cardiac anatomy and function is the main tool for investigating the origin and significance of murmurs in cats.

 Other investigations may be indicated, e.g., thoracic radiographs to look for evidence of congestive heart failure or other thoracic disease, an electrocardiogram (ECG) for arrhythmias.

References

1.  Emerson D, Côté E, et al. Assessment of the prevalence of heart murmurs in overtly healthy cats. Journal of the American Veterinary Medical Association 2004; 225: 384-388.

2.  Riesen SC, Kovacevic A, et al. Prevalence of heart disease in symptomatic cats: an overview from 1998 to 2005. Schweizer Archiv für Tierheilkunde 2007; 149: 65-71.

3.  Riesen SC, Kovacevic A, et al. Echocardiographic screening of purebred cats: an overview from 2002 to 2005. Schweizer Archiv für Tierheilkunde 2007; 149: 73-76.

4.  Rishniw M, Thomas WP. Dynamic right ventricular outflow obstruction: a new cause of systolic murmurs in cats. Journal of Veterinary Internal Medicine 2002; 16: 547-552.