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Managing incidental heart murmurs

Coté E, Edwards NJ, et al.  Management of incidentally detected heart murmurs in dogs and cats.  J Am Vet Med Assoc 2015;246(10):1076-88.

Heart murmurs in small animals are often detected unexpectedly on routine physical examinations, preanesthetic evaluations, and examination of patients presenting for noncardiac health problems.  The finding of an incidentally detected cardiac murmur often poses a dilemma as to its clinical importance, as it may arise from a cardiovascular lesion and be considered a pathological murmur, or may be associated with a structurally normal heart (a nonpathological murmur).  Nonpathological murmurs may be further characterized as functional murmurs, wherein there is a plausible physiological explanation for the murmur such as anemia, or innocent murmurs, which have no obvious physiological explanation.

Careful auscultation will often allow the clinician to decide whether a heart murmur is pathological or nonpathological in dogs, but in cats, making this distinction with systolic murmurs of intensity grades 1/6 through 3/6 may be difficult to impossible.  In small animal patients with murmurs that could be physiological or nonphysiological, assessment of all of the murmur’s auscultatory features (such as point of maximal intensity [PMI], areas of radiation, frequency, sound quality, and duration) as well as patient signalment, other physical examination findings, and laboratory findings, will help facilitate the creation of a ranked differential diagnosis list ranging from most likely to least likely conditions. In this reference article, management of incidentally detected cardiac murmurs in puppies, adult small and large breed dogs, geriatric dogs, kittens, and adult and geriatric cats is reviewed.  Factors which may influence the importance and probable benefit of cardiovascular diagnostic testing of an animal with an incidentally detected heart murmur include the clinician’s level of certainty regarding the cause of the murmur and its probable clinical importance, the potential for impending cardiovascular stress such as general anesthesia or airline travel, the use of the animal (breeding stock and animal athletes may warrant more aggressive cardiovascular diagnostic testing than household pets), the degree of client concern regarding the murmur, and the presence of other clinical signs that could be associated with cardiovascular disease.

Pediatric cats (< 6 months of age) may have the same heart diseases as adult cats (in particular hypertrophic cardiomyopathy).  Nonpathologic murmurs are possible in cats of any age.  Systemic disturbances such as anemia or excitement can cause nonpathologic murmurs in pediatric cats, and nonpathologic murmurs also can be ausculted in kittens with no identifiable systemic disturbances or structural cardiovascular disease. In most cats with systolic heart murmurs of 1/6 to 3/6 intensity, the characteristics that often serve to distinguish pathological and nonpathological murmurs are not reliable.  In these situations, a recommendation for further cardiovascular diagnostic testing such as echocardiography, referral to a cardiologist for second opinion auscultation, or re-examination and re-auscultation after a waiting period of 2-4 weeks, can be made.  The authors recommend the use of a pediatric stethoscope for the most accurate auscultation of kittens and small cats.  When a kitten’s heart murmur is identified as clearly pathological, cardiovascular diagnostic testing is definitely warranted for both diagnosis and prognosis.  Almost all congenital cardiac malformations in cats, except for patent ductus arteriosus with normal pulmonary artery pressures, will produce a systolic murmur. Thoracic radiographs are useful but still potentially inaccurate in pediatric cats. A normally sized cardiac silhouette in the absence of pulmonary abnormalities on thoracic radiographs may be somewhat reassuring that severe congenital heart disease is not present, but they do not completely rule it out. Moreover, pathological murmurs caused by lesions that generate minimal turbulence can be mistaken for nonpathological murmurs.

In overtly healthy adult cats, systolic cardiac murmurs are found in a significant percentage of animals; the reported incidence varies from 16% to 44% of animals ausculted in hospitals and shelters.  Anywhere from 16% to 77% of adult cats with incidentally detected heart murmurs but no clinical signs of heart disease have been reported to have structural heart disease, depending on various factors employed in the studies evaluating the cats. Hypertrophic cardiomyopathy (HCM) is the most common underlying heart disease in adult cats with incidentally detected murmurs; reported percentages of cats with incidentally detected murmurs having HCM range from 15% to 62%.  Dynamic right ventricular outflow tract obstruction, the most common cause of nonpathological cardiac murmurs in adult cats, has been reported to occur in 8% to 16% of adult cats with incidentally detected murmurs.  As the auscultatory characteristics of heart murmurs associated with cardiomyopathy and those with nonpathological causes overlap significantly, pathological and nonpathological murmurs in adult cats usually cannot be distinguished by auscultation alone.  The presence of a gallop sound or arrhythmia is more likely than a murmur to be a useful auscultatory finding for identifying adult cats with possible structural heart disease.

Precordial palpation may reveal an increase of the force of the heartbeat at the apex, which in turn can occur with high cardiac output conditions such as hyperthyroidism, or a precordial thrill, which is associated with high intensity heart murmurs > 5/6.  Pale mucous membranes may be associated with anemia.  The cat should be evaluated for the presence of jugular distension, ascites, and pulse quality; the latter is highly variable in cats, however.  While it can be difficult to discern PMI due to the small size of the feline heart, it is very important to auscult a cat’s heart parasternally and over the sternum specifically.  Both pathological and nonpathological feline heart murmurs can vary in intensity over time and with excitement or heart rate.

The authors provide several useful algorithms in the form of  flow diagrams outlining diagnostic pathways for incidentally detected heart murmurs, including one flow diagram specific to adult and geriatric cats.  Particularly in geriatric cats, extracardiac clinical signs that suggest hyperthyroidism, hypertension of any etiology, or anemia may explain the presence of a murmur that is nonpathological and resolves with treatment of the underlying disorder.  In cats, thoracic radiography can be useful for detection of advanced cardiac disorders.  Cats who present in respiratory distress and that have a vertebral heart score > 9.3 are likely to have heart disease, but applying this criterion to clinically healthy cats with incidentally detected heart murmurs is likely to result in a higher percentage of false-negative results.  Bloodborne cardiac biomarkers alone cannot identify the cause of a heart murmur, but circulating NT-proBNP assays in conjunction with thoracic radiography may be useful in identifying the potential for structural heart disease when echocardiography is not feasible.  Although all small animals with incidentally detected heart murmurs should ideally be further evaluated with echocardiography, the realities of primary care practice do not always allow for this. Echocardiography is the most accurate modality when compared to thoracic radiography or electrocardiography in identifying left atrial enlargement, the most important risk factor for adverse cardiac events and shortened lifespan in cats. [PJS]

See also:
Wagner T, Fuents VL. et al. Comparison of auscultatory and echocardiographic findings in health adult cats. J Vet Cardiol 2010;12:171-82.

Nakamura RK, Rishniw M, et al.  Prevalence of echocardiographic evidence of cardiac disease in apparently healthy cats with murmurs.  J Feline Med Surg 2011;13:266-71.