Valerie asked these questions....
Murmurs are interesting things. While they indicate that there is an abnormality with the heart they say nothing about the severity of the problem. All murmurs should be investigated further, but not all murmurs are dangerous or even require treatment.
What is a heart murmur? As most people will know, there are four chambers in the heart and therefore four sets of valves. When the heart contracts, valves close to prevent blood from flowing backwards and to keep everything moving in one direction. If a valve doesn't close properly a doctor will hear a "swish" kind of sound meaning that some of the blood flowed backwards. The "lub-dub" sound of the heart is actually the sound of the parts of the valve striking each other. If we don't hear a crisp, clean sound then part of the valve didn't function properly.
In veterinary medicine we grade murmurs on a scale of loudness. Most vets use a 1 to 6 (or I to VI) scale, with 1 being the mildest and barely audible. How well we can hear the murmur depends on the ambient noise around us when we're listening as well as the direction of the blood flow. If the "jet" of the backflowing blood is pointed towards the chest wall it will sound louder than if it's pointed away. So a Grade 1 or Grade 4 murmur could both be due to an equal amount of valve failure, but one is louder because it's moving towards the stethoscope. This principle also means that the "grade" of the murmur doesn't mean anything about the severity of disease, as a Grade 2 may be due to a more serious problem than a Grade 4. And that's why you should never treat a patient based only on the grade of the murmur.
The severity of the murmur can also vary from exam to exam. When the heart beats faster a murmur will become more obvious. I have seen patients that have no audible murmur when they are calm, but when they get excited the murmur can be heard (I've actually had chances to hear this happen as I was listening, the murmur coming and going). A mild murmur might be worsened if a pet is dehydrated. And I'm sure there are other factors involved that aren't immediately obvious.
Many things can cause murmurs: birth defects, tumors, and bacterial plaques are probably the most common reasons that would have true consequences. Dehydration and anemia can cause a temporary murmur because of a reduction in the amount of blood flowing. Very young pets may have a transient murmur because the heart walls are growing at a different rate than the valves themselves; if this is the cause it should always resolve within a few weeks to a few months of being first heard as the heart catches up with itself.
There can also be "false" murmurs. If a pet is breathing hard or loudly I will sometimes start to think that they have a murmur. Experience and careful listening usually will bring me to the conclusion that it's only respiration and not the heart. I have initially heard arrhythmias and murmurs because the owner or my assistant was tapping on the chest or rubbing the pet's body, causing sounds to transmit through the pet and then my stethoscope. On the flip side you can miss a murmur because you are hard of hearing or don't have a good quality stethoscope. Barking dogs and talking staff can also interfere with the ability to hear a Grade 1 murmur. When I have a question about whether or not I hear a murmur, I will take the patient into a quiet room and try to listen again, eliminating ambient noise. I will also listen for a minute or two rather than 30 seconds or less, trying to follow the heart sounds and mentally eliminate causes other than valvular disease.
So what does that mean with Valerie's cat? Well, we can't completely ignore the possibility that either the first vet misdiagnosed it or the more recent vet didn't hear it even though it's there. However, I generally chose to believe my colleagues when they say the did or did not hear a murmur. If the cat was very young when it was first diagnosed, she could have indeed grown out of it. The recent vet may have had a noisy room when listening, making it difficult to hear a very soft murmur. The cat may have been calmer or otherwise in better health on the latest visit, making the murmur less evident.
So let's say that a patient has a murmur. What then? I have attended lectures by two different cardiologists over the last 18 months, and both were in agreement on further testing. If possible both a chest x-ray and an ultrasound of the heart (echocardiogram) should be performed. The x-ray does a better job of showing changes in the shape and size of the heart, the surrounding blood vessels, and any changes to the lungs. An ultrasound can show each individual valve and valve flap, as well as measure the contraction of the heart, the thickness of the walls, and the flow of blood. They compliment each other and a full picture of the heart needs both. But if that is not possible due to finances or logistics and you can only do one of the two, specialists agree that the x-ray is the test of choice. And that's something pretty much every vet will have.
If the patient isn't showing any outward clinical signs and there are no changes to the heart or lungs on an x-ray, the current consensus is that treatment isn't necessary, but follow-up is. Those patients should have a repeat of the x-ray in six months and then annually as long as there aren't any progressive changes.
Valerie, I hope that helps try to explain the mystery!