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Heart Murmurs

What’s that Sound? Approaching Heart Murmurs in Dogs and Cats
Danielle Laughlin, DVM, DACVIM-Cardiology

Unexpectedly finding a heart murmur during a routine wellness examination is common. Determining what, if any, additional diagnostics are warranted can be difficult, and many owners will want to know possible causes before they decide to visit a cardiologist. Recommendations vary on a case-by-case basis, but generally considering the patient’s signalment, clinical signs and the client’s goals can help recognize which cases are most likely to benefit from advanced diagnostics including echocardiography.

Dogs

 Systolic murmur in a puppy

Benign or “innocent” murmurs are not uncommon in puppies. These murmurs are generally soft (I-II/VI), systolic, loudest at the left base and typically resolve by 4-6 months of age. Importantly, these murmurs are not associated with clinical signs. It is reasonable in an otherwise healthy puppy with no familial history of heart disease and a quiet left basilar murmur to recommend waiting and rechecking when they are older. Offering echocardiography at the time of initially hearing the murmur is a good idea as some owners will want to be more proactive and pursue imaging sooner. For purebred puppies about to be sold or those with heart disease documented in their lineage, echocardiography is reasonable when the murmur is first heard.

Puppies with murmurs and clinical signs of heart disease including lethargy, weakness, collapse or breathing difficulties should be referred for an echocardiogram. Additionally, puppies with arrhythmias, abnormal heart sounds, right-sided or apical murmurs, moderate to loud murmurs especially associated with a palpable precordial thrill (grade III-VI), and those with continuous murmurs should all receive recommendation for echocardiography as soon as possible.

Systolic murmur in an adult small breed dog

The most common murmur in a small breed middle-aged to older dog is chronic degenerative valvular disease causing mitral and/or tricuspid regurgitation. This classically leads to a systolic left (mitral) and/or right (tricuspid) apical murmur. With moderate to severe degenerative valvular disease the murmurs are typically at least moderate (grade III/VI or above). Since the risk of congestive heart failure and the associated clinical signs of tachypnea at rest, coughing and dyspnea correlates with cardiac (and specifically left atrial) size, thoracic radiographs are a reasonable initial diagnostic in an adult small breed dog with a systolic apical murmur. Generally, if the heart size is normal (VHS < 10.5 in most breeds) and the patient is asymptomatic, monitoring with thoracic radiographs every 12 months is recommended. With radiographic cardiomegaly, rechecking every 6 months is indicated or sooner if clinical signs develop. An initial echocardiogram to confirm the underlying disease type is reasonable even when the heart size is normal but should certainly be considered when there is radiographic evidence of cardiomegaly. Benefits of echocardiography not apparent on radiographs include assessing for pulmonary hypertension, systolic function and additional flow abnormalities. Small breed adult dogs with murmurs that are atypical in location or accompanied by arrhythmias or clinical signs should always be offered echocardiography and consultation with a specialist.

Systolic murmur in an adult large breed dog

Large breed dogs can also be affected by degenerative mitral and tricuspid valve disease, but dilated cardiomyopathy is considerably more common in these breeds than in small breeds and can be difficult to diagnose without echocardiography. Large breed dogs with dilated cardiomyopathy can have no audible murmur or a very low-grade murmur despite having severe cardiac disease. For this reason, any large dog with clinical signs suggestive of cardiac disease, radiographic evidence of cardiomegaly, or any cardiac murmur should be offered echocardiography.

 Diastolic murmurs in dogs

Diastolic murmurs are uncommon in small animals and auscultation of these murmurs always warrants referral to a cardiologist for additional imaging. These are typically associated with severe aortic regurgitation such as that seen with bacterial endocarditis. Other aortic valve abnormalities or, in theory, severe pulmonic insufficiency with pulmonary hypertension and mitral/tricuspid stenosis could also cause diastolic murmurs.

Cats

 Benign murmurs are very common in cats and are often related to relatively rapid blood flow through the right ventricle (called dynamic right ventricular outflow tract obstruction). These murmurs are systolic in timing and can be heard on either the right or left parasternal areas. They typically are mild to moderate in intensity (grade I-III/VI) and may vary with heart rate or be intermittently audible. Unfortunately, the only way to differentiate a benign feline murmur from a pathologic murmur associated with cardiomyopathy or valvular dysplasia is echocardiography. Radiographs can offer support for underlying heart disease if there is clear cardiomegaly (VHS >8, “valentine” shape) though normal thoracic radiographs do not rule out the possibility of heart disease. It is reasonable, therefore, to offer owners of feline patients referral for echocardiography at the time a murmur is ausculted. This recommendation is especially important if the patient requires general anesthesia for surgery or dental cleaning. Cats with loud murmurs (> grade IV/VI), arrhythmias, or clinical signs should always be offered echocardiography. Cats can also have cardiomyopathy without an audible murmur so some patients, especially those with a family history of cardiac disease or those intended for breeding, may require an echocardiogram despite not having a murmur.

A recent summary released in the Journal of Small Animal Practice (E.Coté et al. Incidentally detected heart murmurs in dogs and cats: executive summary 2015) is available online and provides more information and frequently asked questions regarding incidental murmurs in small animal patients. Please contact our cardiologists, Dr. Laughlin and Dr. Pogue, at BluePearl-GVS with any questions about your patients.