By Rebecca Malakoff, DVM, DACVIM (Cardiology)
Veterinarians commonly auscult heart murmurs during routine exams for pets presenting for non-cardiac reasons (for example annual vaccine appointments). Although it would be wonderful if every such pet could have a full cardiac workup, not every client may have the resources or inclination to follow up with an echocardiogram or visit to a cardiologist, requiring general practitioners to be able to provide information to enable their clients to make the best diagnostic decisions and plans for their pets. In 2015 a group of board certified cardiologists brought together as the “Working Group of the ACVIM Specialty of Cardiology on Incidentally Detected Heart Murmurs” published a reference article in JAVMA providing current information by species and age group to help veterinarians in this endeavor.1 This summary will highlight some of the key points made by the authors, although it is worth noting that the full article provides even more information that is well worth reading.
For both puppies and kittens with heart murmurs warranting further investigation, 2-D and Doppler echocardiography by a cardiologist are recommended as providing the best diagnostic and prognostic information.
The group described characteristics which aid in determining whether a murmur is pathological (resulting from a cardiovascular lesion) as opposed to nonpathological (associated with a structurally normal heart). A nonpathological murmur may be termed functional if there is a physiologic cause such as anemia, or innocent if no obvious physiologic cause is identified. The authors modified a “6-S” rubric used in human cardiology to indicate features more likely to be associated with a nonpathological murmur: murmurs which are 1. Soft (generally grade 1 or 2/6), 2. Systolic, 3. Small (localized to left heart base or to one location with no radiation), 4. Single (no other abnormal heart sounds such as clicks, gallops, or arrhythmias), 5. Short (predominantly heard in early or midsystole), and 6. Sensitive (absent or much softer at rest than with excitement or exercise). It is important to note that this rubric may be more useful in dogs, as with cats it is often not possible to classify systolic murmurs grades 1-3/6 as pathologic or nonpathologic.
Nonpathologic murmurs are commonly ausculted in puppies, but pathologic murmurs are heard as well, typically resulting from congenital heart disease. The most common congenital heart defects diagnosed in dogs in the United States are pulmonic stenosis, subaortic stenosis, patent ductus arteriosus, and ventricular septal defect, with some breed predispositions to particular disorders. Further investigation of an incidentally ausculted murmur in a young dog is warranted if the murmur is continuous, diastolic, prolonged such that it obscures the second or both heart sounds, or accompanied by transient abnormal heart sounds (such as a split second heart sound). Murmurs ausculted in locations other than the left heart base (such as radiating to the carotid region, over the right hemithorax, or best heard at the left apical region over the mitral valve), or are loud (grade 3/6 or louder) with PMI over the left heart base warrant further testing.
Young cats (< 6 months) may have nonpathological murmurs or pathological murmurs, and as previously emphasized, the specific characteristics that separate these two categories fail to reliably do so in most cats with grade 1 to 3/6 systolic murmurs. If such a murmur is ausculted, the authors suggest three different possible approaches: further cardiovascular testing (echocardiography), second opinion auscultation by a cardiologist, or reexamination and reauscultation after a period of 2 to 4 weeks. Any murmurs louder than a grade 3/6 or continuous in duration warrant further investigation. Murmur intensity and location is less helpful for predicting specific diagnosis and prognosis for cats than dogs. For example, cats with a small, clinically insignificant ventricular septal defect may have a loud (grade 5/6) systolic murmur.
For both puppies and kittens with heart murmurs warranting further investigation, 2-D and Doppler echocardiography by a cardiologist are recommended as providing the best diagnostic and prognostic information. ECG and thoracic radiographs may provide useful ancillary information, but cannot provide a definitive diagnosis of the cause of the murmur. Assessment of cardiac size on thoracic radiographs in cats can have limited accuracy, especially as concentric ventricular hypertrophy is not radiographically apparent.
For adult and geriatric dogs with an incidental heart murmur, considering signalment aids in determining most likely etiology of pathologic murmurs. The majority of small breed (< 20 kg) dogs with a systolic murmur over the left apex have degenerative mitral valve disease. Although echocardiography provides more precise and accurate information regarding the cause of the murmur and is ideal in these cases, thoracic radiographs are often performed first because of lower cost and greater availability. If the thoracic radiographic findings are normal, clinically important heart disease is uncommon (absence of cardiomegaly may suggest a nonpathologic murmur or mild/early degenerative mitral valve disease). If there is cardiomegaly or other cardiovascular abnormalities, echocardiography is more strongly recommended. For large breed dogs (>20 kg), fewer conclusions may be reached confidently based solely on physical exam findings. Large breed dogs with a left apical systolic murmur may have dilated cardiomyopathy or degenerative mitral valve disease, and degenerative valve disease can progress more rapidly in dogs of this size compared to small breed dogs. Therefore, for this size group of dogs, echocardiography should be recommended as initial diagnostic test of choice. Other findings to prompt recommendation for echocardiography in adult and geriatric dogs share some similarities as with juveniles: diastolic or continuous murmurs, murmur location other than left apical, murmur accompanied by arrhythmia, or murmur that is recent in onset and coexists with vague systemic signs (to rule out possible infective endocarditis).
Systolic heart murmurs are fairly common in healthy adult and geriatric cats. The most common form of heart disease causing a pathologic murmur in cats is hypertrophic cardiomyopathy, and the most common cause of a nonpathologic murmur is dynamic right ventricular outflow tract obstruction. Because there is so much overlap in how these murmurs sound, it is generally impossible to differentiate pathological and nonpathological murmurs in cats based on auscultation alone. Additional findings which would strengthen the recommendation for echocardiography in an adult cat include presence of a gallop sound, arrhythmia, a murmur which is diastolic or continuous, a murmur which is loud (grade 3/6 or louder), or increased strength of the apex beat. Thoracic radiography is less useful for determining whether significant cardiac disease is present, as concentric hypertrophy (such as with hypertrophic cardiomyopathy) may not result in an apparently enlarged cardiac silhouette. Certainly if cardiomegaly is apparent radiographically, this strengthens the recommendation for echocardiography, as would the finding of an abnormal NTproBNP level.
Finally, several considerations beyond physical exam findings or other test results can help in determining whether a juvenile or adult veterinary patient should have an echocardiogram. For example, client concern, anxiety, or desire to be as informed as possible about the cause of the murmur can influence whether an echocardiogram is pursued. The veterinarian should also consider whether the patient requires general anesthesia in the near future, or any treatments with relative cardiac contraindications. If the dog or cat is to be used for breeding, echocardiography to try to obtain a definitive diagnosis of the cause of murmur becomes of more paramount importance as well.
For more information, please contact Angell’s Cardiology service at 617-541-5038 or firstname.lastname@example.org. Dr. Malakoff works full time at our Waltham location.
- Cote E, Edwards NJ, Ettinger SJ et al. Management of incidentally detected heart murmurs in dogs and cats. J Am Vet Med Assoc 2015;246(10): 1076-1088.