Old Dogs with Murmurs
Degenerative mitral valve disease (DMVD) is very common in older, small breed dogs. The mitral valve is one of four valves in the heart that keep blood flowing in the right direction. The “lub-dub” we associated with a healthy heart is the sound of heart valves closing and should be all that a veterinarian hears when listening to a dog’s heart with a stethoscope.
The mitral valve is located between the heart’s left atrium and left ventricle. Small breed dogs have a genetic tendency to develop pathological changes to their mitral valves. The condition is sometimes called endocardiosis or myxomatous valvular degeneration. We don’t know exactly why or how it happens, but the valve leaflets that are normally thin and fit together precisely become irregularly thickened and “clunky,” for lack of a better word. These changes prevent the leaflets from working as they should, and the valve begins to leak, which causes turbulent blood flow around the valve. The sound of this turbulence is called a heart murmur. In the case of DMVD, the murmur occurs between the normal “lub” and “dub” heart sounds (i.e., a systolic murmur) and can be heard most clearly around a particular point on the left side of the dog’s chest.
The condition is so common (I’ve had two dogs with it myself) that when I’m presented with an older, small breed dog with a characteristic murmur, I assume that it’s caused by DMVD until proven otherwise. Practically, the diagnosis can be confirmed when an X-ray reveals an enlarged left atrium and no other potential causes for the murmur, but an echocardiogram (ultrasound of the heart) is sometimes necessary to reach a definitive diagnosis.
A cough is usually the first sign that owners notice in dogs with DMVD. The left atrium enlarges as a result of being overfilled with blood that is “backwashing” out of the left ventricle through the leaky valve. The abnormally big left atrium presses up on the dog’s trachea and bronchi leading to airway compression, irritation, and coughing. DMVD is a progressive disease. The mitral valve becomes increasingly distorted and unable to perform its job, which causes a worsening cough and sometimes a progression to congestive heart failure (CHF).
Providing information about treatment and prognosis after a dog has been diagnosed with DMVD can be a little dicey. If the patient already suffers from CHF, standard treatment (e.g., enalapril, furosemide, and pimobendan) for that condition should be started immediately. With a little luck, these dogs can often maintain a good quality of life for a year or so.
The best way to respond to a diagnosis of DMVD without CHF is to monitor the patient closely. Studies haven’t shown a clear benefit to starting any form of treatment before congestive heart failure is present. We want to catch CHF as soon as possible, of course, so I like to recheck these patients at least two or three times a year and at the first sign of an increase in coughing.
Some dogs with DVMV rapidly progress to CHF; others never do. It’s a crapshoot, but a recent study may help veterinarians predict which dogs are at the greatest risk for CHF and schedule more frequent rechecks for those individuals. Most veterinary clinics can measure two parameters that in the majority of cases accurately predicted progression of DVMV to CHF: a chest X-ray that reveals a vertebral heart size greater than 12 and a simple blood test measuring a NT-proBNP level over 1500 pmol/L.
Another study looked at the predictive value of changes in NT-proBNP levels after treatment. A decline to below 965 pmol/L on a follow-up exam was correlated with a greater chance that the dog would survive for more than one year after diagnosis.
These findings should help veterinarians determine which patients need the closest monitoring and improve the care of dogs with degenerative mitral valve disease.
Dr. Jennifer Coates
Reynolds CA, Brown DC, Rush JE, Fox PR, Nguyenba TP, Lehmkuhl LB, Gordon SG, Kellihan HB, Stepien RL, Lefbom BK, Meier CK, Oyama MA. Prediction of first onset of congestive heart failure in dogs with degenerative mitral valve disease: the PREDICT cohort study. J Vet Cardiol. 2012 Mar;14(1):193-202.
Wolf J, Gerlach N, Weber K, Klima A, Wess G. Lowered N-terminal pro-B-type natriuretic peptide levels in response to treatment predict survival in dogs with symptomatic mitral valve disease. J Vet Cardiol. 2012 Sep;14(3):399-408.