A heart murmur is the presence of sound in a portion of the cardiac cycle that is meant to be silent. In the course of a normal heartbeat, the sounds that can be heard are those of the valves slamming shut. This creates the "lub-dub" sound that most people understand as a typical heartbeat. If something disturbs the silence that comes in between these beats, it is a murmur, and may or may not be a problem. To fully understand a murmur, and whether or not it is a cause for concern, one must consider the normal workings of the heart, where the murmur is coming from, and what implications that has for the heart in which the sound is heard.
The heart is divided into four chambers -- the right and left atria and ventricles. In between each atrium and ventricle is an atrioventricular valve. On the left, this valve is known as the mitral valve. On the right, it is called the tricuspid valve. The other important valves are those that close the aorta and the pulmonary artery. These are known as the semilunar valves because of their crescent-like shape, and are named by the vessel that they functionally occlude.
Heartbeats are defined by the rhythmic movement of blood into and out of the ventricles. Thus, the heart sounds are dependent on the movement of blood as well. During systole, or the ejection of blood from the ventricles, one should hear the S1 sound of the AV valves closing due to increased pressure. This also forces the blood out into either the aorta or the pulmonary artery. During diastole, where the ventricles relax and are refilled with blood from the atria, the S2 sounds are heard as the semilunar valves snap shut and the AV valves open to allow blood through. In very large animals, there are two additional possible heart sounds. S3 is the sound of the turbulent entrance of blood into the ventricle during early diastole, and S4 is the sound of the atria contracting and forcing blood into the ventricles immediately prior to systole. If these sounds are heard in an animal that does not have a heart the size of a basketball, it is generally assumed that they are pathologic and associated with an unfortunate dilation of the heart.
When Things Go South:
A heart murmur is made when there is some sort of difference in this process that causes excess turbulence or vibration in the heart. This is the noise being heard through the stethoscope. Murmurs can be classed as either physiologic or pathologic.
A physiologic murmur is a noise that is not caused by an actual physical problem in the heart, but is rather associated with some general weirdness of the blood. This may include a change in blood viscosity associated with anemia or dehydration, an altered cardiac output associated with certain drugs or exercise, or it may be due to the relative stenosis of trying to press a very large amount of blood from the very large ventricle to the much smaller vessels. This last type, known as an ejection murmur, is quite common in young animals and is associated with their high resting heart rate.
This is a murmur that is in fact a problem. These are associated with an abnormality in the heart resulting in turbulent flow. This is due to a stenosis of a valve or vessel or an insufficiency of a valve, with the latter being far more common. Other possible causes are an acquired or congenital shunt, or a congenital defect in the septum of the heart. In most cases, a louder sound is indicative of a worse problem. This is not, however, always the case. For instance, in general, a septal defect that is loud is considered to have a better prognosis than one that is very quiet. The louder a sound, the more turbulent it is, meaning that the hole through which the blood is shunting is smaller. In terms of abnormal holes in your heart, bigger is not in fact better.
Valvular insufficiency is a common cause of pathological murmur and is just as stated. The valve in question is insufficient in preventing backflow from the ventricle to the atrium, and thus a lot of noise is heard during either systole or diastole, depending on the valve that is failing. Insufficiency may be a degenerative change associated with aging, or it may be associated with more problematic issues such as endocarditis or myocarditis, as well as with myocardial failure. There may also be issues associated with rupture of the chordae tendinae, which usually function to hold the valves and which can allow them to flop wildly around if not tethered. Stenosis of the valves is much less common -- so much so that we did not cover it in class. In human medicine, it is associated with valvular problems that make them unable to open as widely as normal function would dictate.
Now that we know that some murmurs are bad and some are not, it is important to learn how to recognize and grade them.
The first step is to recognize the points of maximal intensity -- this is the area where the murmur is loudest. Because murmurs are associated with valves, it is only logical to assume that we should listen in at the valves to see if that is where the issue is coming from. Valves occur on both sides of the heart, with 3 of the four being easier to auscult on the left side of the heart. In horses, the mitral valve can be heard most reliably in the fifth intercostal space, approximately one-third of the distance between the point of the elbow and the point of the shoulder. This requires the stethoscope to be placed under the triceps muscle, or up in the axilla of the animal. The pulmonic valve is heard in the third intercostal space, directly in front of and slightly higher than the mitral. The aortic valve splits the difference between these two valves and is heard in the fourth intercostal space, only slightly further up toward the shoulder than the other two valves. On the right side, the tricuspid valve can be ausculted at the same level as the aortic valve. In cows, these landmarks are all basically the same but one intercostal space up-- to accurately auscult a cow, one must really shove up under the elbow. In smaller animals, who have realistically smaller hearts, these sounds will be heard at the base of the left heart, with minor fidgeting around to distinguish the sounds.
The next step comes in determining the timing of the murmur. This is done with a combination of auscultation and pulse palpation. Pulse is felt as the blood is ejected from the ventricles and courses through the arteries of the body during systole. This is the biggest hint on timing-- if your murmur is occurring at the same time you are feeling a pulse, it is a systolic murmur. If it does not, and occurs during the long(er) period between beats, this is a diastolic murmur. As heart rate increases, it becomes harder to determine if a sound is systolic or diastolic, so pulse can be a helpful aid for determination between the two.
The sound should also be named according to how much of the normal sound it obscures. A holo- murmur occurs between normal beats. That is, a holosystolic murmur would be heard between S1 and S2 without obscuring either sound. A pan- murmur blends into the normal sounds and makes it difficult to differentiate between them. A pansystolic murmur will begin with S1 and blend into S2. Murmurs may be present at the beginning, middle or end of either systole or diastole, or may be continuous throughout both phases. In general, a murmur that is longer, as in is continuous or runs throughout diastole, is a more serious problem.
The final criteria for significance is intensity. Murmurs are graded on a scale of 1-5 or 1-6, though for simplicity I will use the 1-5 scale.
- I - the murmur is focal (over a specific area) and is only appreciable after careful auscultation. This may take several attempts or someone who actually knows what they are doing to appreciate.
- II- murmur is quiet and is still a focal sound, but can be heard by people who do not have board certification or the hearing of bats.
- III- murmur is moderately loud and the sound may no longer be focal -- it may radiate to other areas of the heart, with the point of maximal intensity being located at one valve.
- IV- murmur is loud, radiating, and creates a palpable vibration that can be felt on the chest.
- V- murmur is so loud that it can be heard before the scope is even placed on the chest.
As one might imagine, the lower the grading of intensity, the better off the patient is. It is possible to have a low intensity and a large problem, such as a large septal defect, but in general a grade of II or less is considered to be less worrisome than a higher grade.
Now that the murmur has been elucidated, demarcated, located, and vociferated, we need to talk about what to do about the issue. Not all murmurs are a concern, as we learned with physiologic murmurs. There are several markers that are used to hint that a bigger issue might be at hand and require further evaluation. This is usually done with an echocardiogram. Indicators for a diagnostic workup include
- A murmur graded at 3/5 or higher
- A murmur accompanied by signs of cardiac disease including
- Sudden onset of murmur
- Murmur with associated change in pulse quality
- Murmur with a pathological arrythmia
- Murmur associated with a fever of unknown origin
Echocardiograms and ultrasound
investigation may help to determine the severity of the issue and what can be done to treat it.
Results of the echocardiogram, as well as other cardiac disease specific tests, will point to the underlying etiology behind the murmur. Treatment will be based according to this etiology. Not all problems need management -- a nonpathologic murmur does not need to be treated. A simple age-related pathologic murmur may not need to be treated until years down the road, while an infection in the myocardium requires immediate attention. Each treatment is different based on what the underlying pathology is, which is an entirely different set of nodes. By following the general rules of identifying the source of the murmur and its intensity and following up on likely pathologic murmurs, a diagnosis can be made and appropriate treatment initiated.
If this seems like a lot of information, or sorely lacking in human-related facts, it is. But now, several thousand characters later, you are just like me. You too know all there is to vaguely understand about heart murmurs. I hope that in the future, this serves you just as well as it serves me.