When presented with a murmur in a dog, in the vast majority of cases the author would undertake a colour Doppler echocardiographic examination, primarily because they have been referred for diagnosis. Doppler colour flow mapping provides a visualisation of blood flow through all four valves. If one valve has either regurgitation or stenosis this will almost always provide a clear visualisation of the site of the murmur. Additionally assessment of each of the chambers on two-dimensional (2-D) echocardiography demonstrates the presence of any chamber dilation or hypertrophy which assists in deciding if the murmur is clinically significant. However that is maybe not the question that is meant by this lecture title nor is it necessarily useful to practitioners that do not have ready access to colour Doppler echocardiography or the requisite experience and knowledge to use it.
When a dog is initially found to have a murmur it can seem quite daunting. It is probably best to discuss murmurs in puppies and adults separately as these are two different scenarios.
Puppy Murmurs
The majority of congenital murmurs are systolic, except for a patent ductus arteriosus (PDA) which is continuous, running through systole and diastole.
Murmurs may be characterised by their intensity, point of maximum intensity and timing. Breed predisposition is of great value in predicting the most likely cause of a murmur. For example, in the UK, if a Boxer dog presents with a left-sided systolic murmur, the most likely cause of this is subaortic stenosis. Lists of breed predispositions are found in many textbooks.
A PDA is the single most important murmur to identify because the 50% mortality rate is approximately 1 year and it can be corrected. Although the PDA has a very characteristic sound it can be easily missed if careful auscultation of the region around the left heart base, and above, is not performed. PDAs can be corrected by interventional catheterisation techniques ('key-hole' surgery) and embolisation of the duct by either coils or Amplatzers (avoiding thoracotomy); thus early referral is mandatory. Breeds predisposed to a PDA are: German Shepherd Dog, collie, Cavalier King Charles Spaniel, Labrador Retriever and spaniels.
A quiet systolic murmur (grade 1 or 2) that does not become quieter with age (as a flow murmur might) could be associated with a congenital defect. For example, mild aortic stenosis may present with a quiet murmur, although it may become a little louder as time passes. What action should be taken very much depends upon the owner. In many cases it is reasonable to re-examine in a few weeks time to determine if the murmur persists or is becoming louder, or quieter. However if the owner is concerned, then further investigations of the murmur should be recommended.
A murmur that is loud (grades 3 to 6) would most commonly be associated with a congenital defect. In these cases, further investigations should be considered. However echocardiography of very tiny and young puppies is difficult and the diagnostic yield can be low in comparison to waiting until the puppy is a little older and bigger, even if only a few weeks. Also do be aware that a small ventricular septal defect (VSD) that could yet close (i.e., a good prognosis) may present with a very loud murmur, hence do not say to owners that a loud murmur definitely carries a poor prognosis! And conversely, some congenital defects may not be associated with a murmur, such as right-to-left shunting defects. In these cases, cyanosis may be the most useful clue. Fortunately, however, these are rare.
Puppy Flow Murmurs
It is not uncommon for puppies to present with flow murmurs (physiological or innocent murmurs) which result from an increase in blood flow through the aorta or pulmonary artery. These are usually fairly quiet, although they tend to vary in intensity, often with a variation in heart rate, whereas pathological murmurs are usually of consistent loudness regardless of heart rate. Thus, examining the heart until the animal relaxes, and the heart rate slows, is important, since flow murmurs often disappear or become very quiet. Flow murmurs usually disappear by 6 months of age. Re-examination at that time to check if the murmur persists or not might be useful.
Further Tests
Doppler echocardiographic examination performed by an experienced echocardiographer is necessary for definitive diagnosis in the vast majority of cases. Radiography and electrocardiograms (ECGs) usually provide too little information, although they can occasionally provide a diagnosis, particularly if a post-stenotic bulge is evident on the dorsoventral (DV) view. They cannot provide the more reliable and definitive diagnosis that Doppler echocardiography does and do not provide an assessment of severity. However, in the absence of this facility, ECG and radiography may help to narrow the differential diagnosis or screen for evidence of congestive heart failure.
Murmurs in Adult Dogs
The approach and advice will depend upon whether the murmur is associated with clinical signs or not. The most common cause of a systolic murmur in this group will be mitral regurgitation due to mitral valve disease (MVD) and as a general rule, the loudness of the murmur is proportional to the severity of the MVD. Approximately half the dogs with dilated cardiomyopathy (DCM) do not have an audible murmur and of the other half that do, the vast majority of have a quiet murmur (grade 1-3). It should also be remembered that dogs may also present in adulthood with a congenital murmur.
A Murmur and No Clinical Signs
In this situation a murmur is discovered at a routine health check or vaccination. The significance will be greatly influenced by the breed. For example, if it is a Dobermann, then DCM is possible and thus an urgent approach is indicated, but if it is a Cavalier King Charles Spaniel with a quiet murmur then it may be the onset of MVD and simple monitoring maybe all that is required, e.g., the owner watching for early clinical signs and chest radiographs every 6-12 months. Whilst Doppler echocardiography remains the best diagnostic test, chest radiographs and/or a serum N-terminal pro-B-type natriuretic peptide (NTproBNP) test may help to screen for cardiomegaly. In dogs with MVD that do not have clinical signs or evidence of cardiomegaly, there is currently no treatment that delays the onset of signs. In dogs with preclinical DCM there is some debate if treatment (angiotensin converting enzyme (ACE) inhibitors and/or pimobendan) is indicated. However, frequent and close monitoring of dogs with preclinical DCM is indicated, e.g., echocardiography every 3-6 months, as well as the owner watching for the onset of early clinical signs.
A Murmur with Clinical Signs
The clinical signs of exercise intolerance, breathlessness and coughing are common to both cardiac and respiratory disease; hence this group of dogs can be a diagnostic, and thus a therapeutic, challenge. Without doubt, good-quality inspiratory chest radiography is a key diagnostic test in such cases. If the radiographs are clear cut and unambiguous, then the diagnosis is easy and treatment can follow. Additionally the severity of pulmonary oedema will dictate the initial dose of furosemide required for that individual. But radiographs can be ambiguous. For example, a Yorkshire Terrier may present with coughing and have a mitral murmur; such small-breed dogs often have a heart that appears to be relatively large on chest radiographs, leaving it difficult to decide whether the cough is due to the MVD or its tracheal collapse. In such ambiguous cases a serum proBNP might provide assistance, providing its result is clearly high (>750 pmol/l) or low (<120 pmol/l). Otherwise the next step is Doppler echocardiography to rule in/out the presence of heart disease or respiratory diagnostics (e.g., bronchoscopy).
References
1. Martin M, Corcoran B. Notes on: Cardiorespiratory Diseases of the Dog and Cat (second edition). Oxford: Blackwell Publishing, 2006.