Associate Professor, Department of Clinical Sciences, Faculty of Veterinary Medicine and Animal Science, Swedish University of Agricultural Sciences, Uppsala, Sweden
Heart murmurs in cats can be associated with structural heart disease (pathological heart murmurs), but a high percentage of cats with murmurs appear to have no evidence of structural heart disease (non-pathological heart murmurs). A systematic evaluation of murmur characteristics is indicated when a heart murmur is detected in a cat.
An exact cardiac diagnosis can seldom be established based solely on cardiac auscultation, but an optimal auscultation technique can substantially narrow down the list of differential diagnoses. Auscultation should be performed in a quiet environment, and the patient should preferably be standing on all four limbs during cardiac auscultation as abnormal sounds, which easily can be mistaken for heart murmurs, can arise in a recumbent animal due to rubbing of the heart against the chest wall. Furthermore, localization of area of origin of different cardiac sounds is more easily achieved in a standing animal. Breathing sounds can be mistaken for sounds originating from the heart, and differentiating heart sounds from breath sounds can be facilitated by watching the cat’s breathing pattern while simultaneously auscultating the heart.
Murmurs are auditory vibrations caused by high-velocity and turbulent blood flow through the heart and vessels. A systematic evaluation of murmur characteristics is indicated when a heart murmur is detected in a cat. The murmur’s auscultatory features can be characterized by timing within the cardiac cycle, anatomic location, radiation area, intensity (loudness), frequency (pitch), and shape of vibration pattern (which is most easily achieved by recording murmurs by phonocardiography). Point of maximum intensity (PMI) of a murmur is the location over the thorax where the murmur is loudest, and can serve as a guide to the origin of the murmur. The PMI is commonly associated with a particular valve region (i.e., pulmonic, aortic, mitral, tricuspid). However, the small size of the feline heart makes it difficult to distinguish the different valve areas on the thorax. Accordingly, localization of murmurs in cats is often described as sternal versus parasternal, base versus apex, and left versus right. The area of the ventral thorax/sternum should always be auscultated in cats, as murmurs often are heard best at this location. Lateralization can narrow the differential diagnosis list in some patients.
Most cardiovascular diseases in cats result in systolic murmurs, which most commonly start immediately after the first heart sound, S1, but they can also start later in systole. Systolic murmurs can last for only a portion of the systole (early, mid or late systolic murmurs), throughout systole from the end of S1 to the beginning or the second heart sound, S2, (holosystolic murmur), or extremely rarely; throughout systole from the beginning of S1 to the end of S2 (pansystolic murmur). Diastolic turbulent blood flow is rare in cats and can be relatively difficult to detect. Continuous murmurs last throughout systole and diastole.
For some cardiac diseases, grading of the murmur intensity serves as a rough estimate disease severity, whereas a lack of agreement exists between severity of cardiac disease and murmur grade for other diseases. The intensity of murmurs is most commonly graded on a 1 to 6 scale, where a grade 1 murmur is regarded as the softest and a grade 6 the loudest. A precordial thrill is palpated in animals with a grade 5 or grade 6 murmur.
Etiologies of Murmurs
Murmurs in cats frequently develop in conjunction with structural heart disease (pathological murmurs). However, presence of a heart murmur does not always signal presence of structural cardiac disease as murmurs can also arise due to various physiologic processes (nonpathological murmurs).
The most common underlying structural heart disease causing a heart murmur in adult cats is hypertrophic cardiomyopathy (HCM). Also young cats may have structural heart diseases commonly recognized in adult cats (such as HCM). The PMI of the murmur in HCM cats is usually heard over or just to the left of the sternum. Dynamic subvalvular aortic stenosis and mitral regurgitation associated with systolic anterior motion of the mitral valve are the most common sources of systolic murmurs heard in cats with HCM. However, dynamic subvalvular aortic stenosis can also be observed in volume - depleted cats and in other nonhypertrophic states.
Some congenital diseases commonly present with characteristic murmur features: A left-to-right patent ductus arteriosus (PDA) most commonly produce a continuous murmur best heard over the left heart base in both systole and diastole. A ventricular septal defect (VSD) commonly produce a pronounced systolic murmur with PMI over the right thorax.
Although the majority of canine patients with heart disease have an audible murmur, it is important to remember that some cardiac diseases (such as pericardial diseases, dilated cardiomyopathy, restrictive cardiomyopathy, atrial septal defects (ASD), and right-to-left shunting PDA) do not necessarily produce blood flow turbulence and an audible murmur.
Non-pathological murmurs can occur in cats of any age. Kittens may have “innocent” physiological murmurs assumed to arise due to large stroke volumes relative to the size of their great vessels. These murmurs usually disappear by approximately 5 months of age. High-output diseases, such as hyperthyroidism and anemia caused by various disease processes, can also give rise to a systolic murmur due to increased stroke volume and/or decreased blood viscosity.
Mild dynamic right ventricular outflow tract obstruction (DRVOTO) is a common non-pathological cause of physiologic murmur in healthy cats in stressful situations, such as examination at a veterinary hospital. DRVOTO can often be detected as a parasternal systolic murmurs in excited cats. Color Doppler echocardiography of these cats demonstrates a localized, turbulent systolic jet located within the right ventricular (RV) outflow region, originating just cranial to the tricuspid valve. Spectral Doppler tracings of the turbulent jets demonstrates abnormally high peak velocity, late systolic flow acceleration, and marked variability with heart rate, which is commonly seen with dynamic obstructions. Frame-by-frame examination of 2-dimensional (2D) echocardiographic images reveals systolic narrowing of the proximal outflow region of the right ventricle. DRVOTO in cats is generally benign and does not appear to be associated with progressive pathology.
Often, murmurs caused by cardiomyopathy and those resulting from non-pathological causes in cats cannot be differentiated from each other by auscultation alone as there is an overlap between auscultatory characteristics. Non-pathological murmurs tend to be fairly quiet (grade 1/6 to 3/6), but it is often not possible to classify systolic grade 1-3 murmurs as clearly pathological or nonpathological. Systolic grade 4/6 to 6/6 murmurs indicates the presence of structural heart disease. Murmurs in cats do commonly change in intensity with stress/excitement or variation in heart rate. However, this may occur with murmurs of either pathological or non-pathological causes. Identification of gallop sound or arrhythmia may add evidence to the possibility that the murmur is caused by a structural heart disease.
Additional Diagnostic Testing
Differentiation of causes of feline cardiac murmurs is difficult by auscultation alone without additional diagnostic testing. Thoracic radiography or measurement of circulating cardiac biomarkers can be helpful for detection of advanced cardiac disease, but a specific cardiac disease cannot be established by these techniques. Echocardiography is considered the diagnostic test of choice for pinpointing the cause of a murmur. The cause of the murmur must be evaluated to a degree that is appropriate for the patient, and acceptable to the veterinarian and client. Indications for further testing include: Detection of overt clinical signs that could be referable to cardiac disease, suspicion of a pathological murmur on the basis of auscultatory features, presence of gallop sounds and/ or arrhythmia, the cat’s intended use (such as a potential breeding purpose), whether an event (such as anesthesia) that could increase the risk of CHF, arrhythmia, or thromboembolism is imminent, client concern, or desire to be aware of the cause of the murmur, and associated prognosis and treatment options, in addition to availability, cost, and perceived value of diagnostic tests. The owner should be informed that the etiology of a murmur cannot be fully established in all cats, but lack of structural heart disease, when evaluated by echocardiography, indicates that the murmur is of a nonpathologic origin.
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