Friday, June 17, 2011

Heart Murmurs: What’s the Scoop?

It is not uncommon in my cat practice for me to detect a heart murmur in one of my feline patients during a routine physical examination. Everyone has heard of heart murmurs, but not everyone knows exactly what they are, and what their significance may be. I’m hoping this blog post can help.

Murmurs are vibrations caused by turbulent, high-velocity blood flow, usually through a (normal or abnormal) valve or vessel. Blood flow in the heart normally tends to be laminar, but it becomes less laminar (i.e. turbulent) as it picks up in speed. If the turbulence is severe enough, the sound becomes audible, and we hear this as a “murmur”.

It can be difficult for a veterinarian to know if a feline heart murmur is just a physiologic occurrence, or if it’s a pathologic finding. It is possible to have a condition that increases the rate of blood flow through the heart – for example, excitement, stress, pain, fever, anemia, hyperthyroidism – resulting in enough turbulence as to create a murmur, even though the heart is normal. Certainly, pathology within the heart, such as heart valve disorders or heart muscle disorders (cardiomyopathies) can lead to turbulent blood flow and an audible murmur.

Can a veterinarian tell the difference, just by listening with a stethoscope, between a benign physiologic murmur and a murmur that is associated with a true heart disorder? Simply put: no. Granted, if I hear what is called a “gallop” rhythm – a heart rate that sounds like horses galloping – in addition to a murmur, then the odds are that the murmur is pathologic rather than benign. But for the most part, the only way to really tell if a murmur is benign or pathologic is to perform echocardiography. Synonyms for this procedure are cardiac ultrasound or a sonogram.

The list of heart diseases in cats that can cause murmurs is long and includes long and complicated names like dynamic right ventricular outflow obstruction, hypertrophic cardiomyopathy, mitral valve endocardiosis, pulmonic stenosis, atrial or ventricular septal defects, and the like. Discussion of each of these disorders is beyond the scope of this blog. I just thought I’d impress you with the fancy terminology.

An interesting report came out about seven years ago, in the Journal of the AVMA, August 2004 (1)(2). Entitled “Assessment of the Prevalence of Heart Murmurs in Overtly Healthy Cats”, it was a study of 103 healthy domestic cats that were being screened as possible blood donors. Of the 103 cats, 22 had murmurs. Echocardiography was performed on 7 of the 22 cats, and 6 of them had true heart disorders. The most common disorder was hypertrophic cardiomyopathy, a common (and very serious) heart condition.

I think it’s pretty amazing that these researchers found murmurs in 22 of 103 cats. That’s 21%. Frankly, that seems pretty high to me. Heck, that’s one out of five cats! In my practice, I would say I hear murmurs in maybe 5 to 8 percent of cats. A more significant finding, to me, was that 6 of the 7 cats with murmurs had true heart disease.

The main conclusion drawn from the study is that heart murmurs occur commonly in apparently healthy cats and in many or most instances, the murmurs are caused by structural heart disease, the most common being hypertrophic cardiomyopathy. So, take your cat to your veterinarian twice yearly (“Twice a year for life”; I can’t emphasize that enough) and watch your vet as he/she listens to the heart with a stethoscope. Ask if there’s a murmur. Be proactive. But don’t panic. The prognosis is variable, and depends on the cause of the murmur. Remember, early detection is the key to successful outcomes.
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