There are common misconceptions about heart murmurs that persist. Here are some links, and I could have listed many more. I’ve included some quotes from each site, and some comments of my own at the end.

“A heart murmur is an abnormal heart sound, usually heard by listening to the heart with a stethoscope.

“The loudness of a murmur reflects the amount of turbulence that is present in the heart. However, the loudness of a heart murmur does not always correlate directly with the severity of disease.”

“Although listening to the heart for abnormal heart sounds such as murmurs or gallops is an important way to look for heart disease, this only gives you an indication that the animal may have underlying heart disease. It doesn’t necessarily tell you which type of heart disease or how severe it is.

“Based on the type of abnormal heart sound, the location and the breed of animal, your veterinarian can often make an educated guess as to what type of heart disease your pet has based on what’s most likely, but this is still not definitive.”

Very technical site written by board certified cardiologist:
Cardiac Auscultation 101
Terri DeFrancesco, DVM, DACVIM (Cardiology), DACVECC
Associate Professor in Cardiology and Critical Care
NC State University College of Veterinary Medicine

This is a subjective determination of the loudness of the murmur. In most cases, murmur intensity doesn’t correlate with the severity of the heart disease.

“Although there is not an exact correlation, the louder the heart murmur in a dog with suspect MVD, the more persuasive to be with diagnostics.”

“Veterinarians rank the intensity or loudness of a heart murmur in grades from one to six, with one being barely audible and six being the loudest. There is also a one-to-five ranking system that works the same way. These grades do not necessarily correlate with the severity of the heart condition; they are merely one of several ways that veterinarians attempt to characterize the murmur.”

Comments from Pat – There are cases where a murmur can actually become softer as the heart disease worsens. This is because the heart becomes increasingly enlarged so that there is less turbulence from regurgitant blood flow and thus less noise. Remember that a heart murmur is just a noise – it can give clues about the underlying disease and severity, but it is not diagnostic for heart failure. If it were, we wouldn’t need to do radiographs, electrocardiograms and echocardiograms in order to stage heart disease (along with considering the presence or absence of symptoms). My 16 year old shih tzu was diagnosed with heart failure a few weeks ago, and she has NO heart murmur at all. Radiographs showed an enlarged heart and pulmonary edema. She also had symptoms of labored breathing. The absence of a murmur was confirmed by two GP vets, an ER critical care specialist, and a cardiologist. This is unusual, but it is possible. (This is also a reason to get baseline chest radiographs on elderly dogs with no murmurs.)

The point is that you CANNOT diagnose the severity of underlying heart disease or whether a dog is in heart failure based on the grade of a murmur. Similarly, no vet should decide whether to prescribe meds or not based only on the grade of a murmur. A murmur is simply a noise that gives you a notice to monitor and do appropriate follow up and testing.