Physiology of MurmursBefore trying to decipher what may be the underlying cause of a murmur, it is important to first understand what the normal heart sounds are, and what normal variations of these sounds may occur. It is assumed that you already understand the anatomy of the heart, and have read a basic physical examination textbook which describes the standard methods for auscultation.
The most obvious of the heart sounds are the first and second sounds, or S1 and S2, which demarcate systole from diastole. The heart sound playing in the background on the introduction page of this site is a normal sinus rhythm, with a sharp S1 and S2 and no other significant sounds. S1 is the sound which marks the approximate beginning of systole, and is created when the increase in intraventricular pressure during contraction exceeds the pressure within the atria, causing a sudden closing of the tricuspid and mitral, or AV valves. The ventricles continue to contract throughout systole, forcing blood through the aortic and pulmonary, or semilunar valves. At the end of systole, the ventricles begin to relax, the pressures within the heart become less than that in the aorta and pulmonary artery, and a brief back flow of blood causes the semilunar valves to snap shut, producing S2.
Although S1 and S2 are considered to be discrete sounds, you will notice that each is created by the near-instantaneous closing of two separate valves. For the most part, it is enough to consider that these sounds are single and instantaneous. However, it is worth remembering the actual order of the closures, because certain conditions can split these sounds into the separate valve components. During S1, the closing of the mitral valve slightly precedes the closing of the tricuspid valve, while in S2, the aortic valve closes just before the pulmonary valve. Rather than memorize this order, if you remember that the pressure during systole in the left ventricle is much greater than in the right, you can predict that the mitral valve closes before the tricuspid in S1. Similarly, because the pressure at the start of diastole in the aorta is much higher than in the pulmonary artery, the aortic valve closes first in S2. Knowing the order of valve closure makes understanding the different reasons for splitting of heart sounds easier.
When listening to a patient’s heart, the cadence of the beat will usually distinguish S1 from S2. Because diastole takes about twice as long as systole, there is a longer pause between S2 and S1 than there is between S1 and S2. However, rapid heart rates can shorten diastole to the point where it is difficult to discern which is S1 and which is S2. For this reason, it is important to always palpate the PMI or the carotid or radial pulse when auscultating. The heart sound you hear when you first feel the pulse is S1, and when the pulse disappears is S2.
When a valve is stenotic or damaged, the abnormal turbulent flow of blood produces a murmur which can be heard during the normally quiet times of systole or diastole. This murmur may not be audible over all areas of the chest, and it is important to first note where it is heard best and where it radiates to. Next, you should try to discern if the murmur occurs in systole or diastole by timing it against S1 and S2. Then, listen carefully to tell if the murmur completely fills that phase of the cycle (i.e., holosystolic), or if it has discrete start and end points. Regurgitant murmurs, like mitral valve insufficiency, tend to fill the entire phase, while ejection murmurs, like aortic stenosis, usually have notable start and end points within that phase. The quality and shape of the murmur is then noted. Common descriptive terms include rumbling, blowing, machinery, scratchy, harsh, or musical. The intensity of the murmur is next, graded according to the Levine scale:
* I - Lowest intensity, difficult to hear even by expert listeners
* II- Low intensity, but usually audible by all listeners
* III - Medium intensity, easy to hear even by inexperienced listeners, but without a palpable thrill
* IV - Medium intensity with a palpable thrill
* V - Loud intensity with a palpable thrill. Audible even with the stethoscope placed on the chest with the edge of the diaphragm
* VI - Loudest intensity with a palpable thrill. Audible even with the stethoscope raised above the chest.
Finally, it is important to decide if this murmur is clinically significant or not. Just as a murmur can be caused by normal flow through a stenotic valve, it may also be created by high flow through a normal valve. Pregnancy is a common high-volume state where these physiologic flow murmurs are often heard. Anemia and thyrotoxicosis can cause high-flow situations where the murmur is not pathologic itself, but indicates an underlying disease process. Children also frequently have innocent murmurs which are not due to underlying structural abnormalities. How can a physician determine if a murmur is significant?
The most important thing to consider is the clinical scenario. In a population of unreferred young adults, the prevalence of systolic murmurs ranges from 5% to 52%, with 86% to 100% of these patients having normal echocardiograms. Important questions to ask would include the presence of symptoms such as effort syncope, chest pain, palpitations, shortness of breath, or paroxysmal nocturnal dyspnea. In terms of the examination, there is no one way to rule in or out a murmur as being physiologic, but in general, physiologic murmurs tend to be located between the apex and left lower sternal border, have minimal radiation, occur during early to mid-systole, have a crescendo-decrescendo shape, and a vibratory quality. They will usually change intensity with positional maneuvers, becoming quieter on standing and louder with squatting. A Valsalva maneuver will decrease the intensity of the murmur because the increase in intrathoracic pressure will decrease venous return, which will decrease flow through the heart and lessen the turbulence. Additionally, they will not be correlated with additional audiologic findings, such as an S3 or S4.
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