Heart murmurs are common in cats. In some cats, a murmur may be a sign of life-threatening heart disease, whereas in other cats it may be a completely innocent finding. Ideally, we need to be able to identify which cats with murmurs are most at risk.

Causes of Heart Murmurs

An important distinction is to differentiate those murmurs associated with structural heart disease from those that are physiological.

Diagnostic Tests

Auscultation can narrow the differential list in some cases, but is less useful in cats than in dogs.

Murmurs that vary with stress are characteristic of hypertrophic cardiomyopathy (HCM) and functional murmurs. These murmurs may vary from moment to moment, so that a murmur present at the start of a physical exam may have disappeared by the end if the cat has calmed down.

Very loud murmurs (≥ 5/6) are most likely to be associated with congenital heart disease.

Some causes of murmurs can be ruled out with simple tests such as haematology (to rule out anaemia), palpation of a goitre and T4 concentrations (to rule out hyperthyroidism), and blood pressure measurement (to rule out hypertension).

Identifying the 'High-Risk' Cat

Obtaining a definite diagnosis in cats can be difficult, so instead it is better to try to identify cats at high risk of developing congestive heart failure (CHF) or arterial thromboembolism (ATE) from low-risk cats or normal cats with a physiological murmur. Echocardiographic examination by a cardiologist is the gold standard, but this is not an option chosen by every owner. How can 'high-risk' cats be identified conveniently and inexpensively?

Cats with hypertrophic cardiomyopathy (HCM) but without clinical signs and with normal LA size generally have a good prognosis, with a median survival time of > 9.9 years reported in one study.¹ Cats with HCM and LA enlargement have consistently been shown to have an increased risk of cardiac mortality, so this is an important prognostic indicator.^1,2 Identifying LA enlargement is also a useful way of identifying a cardiac cause of respiratory distress in cats. Cats with heart disease in Stage B2, C or D are at increased risk of a cardiac death and may benefit from treatment such as antiplatelet therapy and stress-avoidance strategies.

Physical Examination

Auscultation of a murmur is not a reliable way to spot a subclinical 'high-risk' cat. The louder the murmur, the more likely the cat is to have heart disease,³ but not all cats with HCM should be considered 'high risk'. Up to 50% of apparently healthy cats with a heart murmur may have normal hearts on echocardiography, and only about 50% of cats presented with CHF will have a heart murmur. So auscultation of a heart murmur may help identify cats with heart disease, but it won't necessarily identify 'high-risk' cats. Gallop sounds and audible arrhythmias indicate 'high-risk' cats more reliably.⁴

Thoracic Radiography

The advantages of thoracic radiography are its wide availability and ability to demonstrate cardiomegaly in 'high-risk' cats. The disadvantages are the challenges of positioning cats with heart disease in order to obtain diagnostic films, the challenges of finding a sedative combination that will enable such films to be taken, and the relatively poor sensitivity at detecting mild–moderate LA dilation.⁵

ECG

For cats with a regular heart rhythm, there is little value from an ECG for identifying high-risk cats.

Plasma Biomarkers

High-sensitivity troponin-I and NT-proBNP assays may be changing the way we identify 'high-risk' cats. Biomarkers do not offer the same reliability in staging heart disease as echocardiography carried out by an experienced cardiologist, but they may be more reliable than some longer established diagnostic techniques.⁶ Biomarkers may help with pre-screening which cats with a murmur should be referred and which cats can be considered 'low-risk' and therefore do not need further diagnostic tests.

'In-House' Echocardiography

Many practices have ultrasound machines, but relatively few use them on a regular basis for feline echocardiography. Yet echocardiography is the ideal technique to identify left atrial (LA) enlargement, which is one of the most powerful prognostic indicators we have for cats with HCM.⁴ Most people prefer a short-axis right parasternal view that includes the LA and the aortic valve. The LA diameter should be no more than 1.6 x the aortic diameter (Ao). Most 'high-risk' cats have an unequivocally dilated LA. Identifying mild LV hypertrophy or identifying the source of a murmur in a cat is much more challenging.

Referral Echocardiography

Referring to a cardiologist has the disadvantages of taking time and can be expensive. In experienced hands, however, an echocardiogram can provide more reliable information on risk than any other technique. For owners that want peace of mind after being told their cat may have heart disease, echo performed by a specialist is the most reliable route. If the echocardiography results show the cat is at high risk of CHF or ATE, then antiplatelet therapy should be started. Appropriate therapy may be selected for cats with a history of CHF, and some high-risk asymptomatic cats may also be started on other therapy. In some cases, long-term followup can be based on serial biomarker testing without need for frequent repeat echo exams. In all cases considered to be 'high risk', the owner should be instructed on how to measure respiratory rate at home in order to identify the early signs of CHF.

Management of Asymptomatic Cats with a Murmur

Stage B. Asymptomatic Cats (HCM)

There are few studies reporting the effects of drug therapy in cats with preclinical heart disease, and treatment in this population is controversial. As HCM is believed to be very common³, and prognosis is good in many cats,⁴ for many cats no treatment is necessary.

Asymptomatic cats that may require treatment:

 Cats with congenital heart disease
As with dogs, PDAs should be closed. In contrast with dogs, ventricular septal defects (VSDs) are the most common defect, and the prognosis for VSDs is good, and intervention of any kind is rarely needed.

 Cats with outflow tract obstruction
It is not clear whether cats with dynamic left ventricular outflow tract obstruction (DLVOTO) are at any increased risk of adverse outcome, although DLVOTO causes chest pain and exertional breathlessness in people. Beta-blockers are the most effective choice for relieving obstruction, and atenolol (2–12.5 mg q12h per cat) is most commonly used but does not appear to improve survival.⁷ It is not known whether cats experience exertional chest pain with HCM. Atenolol can relieve or prevent this chest pain in some human HCM patients, and this may be a better reason for giving atenolol to cats than improving survival. For cats with DLVOTO that are syncopal, the use of atenolol may be valuable if the syncope is associated with ischaemia-related ventricular arrhythmias. Note that although the murmur may disappear in many cats, systolic function may deteriorate in cats with HCM over time, and for some cats the atenolol may no longer be useful once this happens and DLVOTO resolves spontaneously. In some cats, atenolol may adversely affect LA function and may increase the risk of CHF.

Cats with LA Enlargement (Stage B2)

The prognosis is worse in cats with left atrial (LA) enlargement, as they are predisposed to both CHF and ATE. There are no studies reporting any benefit with any treatment in preventing CHF in cats with HCM. Cats at risk of ATE should receive antithrombotic treatment; currently the first-choice treatment is clopidogrel (18.75 mg per cat, q24h PO).

References

1.  Payne J, Luis Fuentes V, Boswood A, et al. Population characteristics and survival in 127 referred cats with hypertrophic cardiomyopathy (1997 to 2005). Journal of Small Animal Practice. 2010;51:540–547.

2.  Rush JE, Freeman LM, Fenollosa NK, et al. Population and survival characteristics of cats with hypertrophic cardiomyopathy: 260 cases (1990–1999). Journal of the American Veterinary Medical Association. 2002;220:202–207.

3.  Wagner T, Fuentes VL, Payne JR, et al. Comparison of auscultatory and echocardiographic findings in healthy adult cats. Journal of Veterinary Cardiology. 2010;12:171–182.

4.  Payne JR, Borgeat K, Connolly DJ, et al. Prognostic indicators in cats with hypertrophic cardiomyopathy. Journal of Veterinary Internal Medicine. 2013;27:1427–1436.

5.  Schober KE, Wetli E, Drost WT. Radiographic and echocardiographic assessment of left atrial size in 100 cats with acute left-sided congestive heart failure. Veterinary Radiology & Ultrasound. 2014;55:359–367.

6.  Borgeat K, Sherwood K, Payne JR, et al. Plasma cardiac troponin I concentration and cardiac death in cats with hypertrophic cardiomyopathy. Journal of Veterinary Internal Medicine. 2014;28:1731–1737.

7.  Schober KE, Zientek J, Li X, et al. Effect of treatment with atenolol on 5-year survival in cats with preclinical (asymptomatic) hypertrophic cardiomyopathy. Journal of Veterinary Cardiology. 2013;15:93–104.