Heart Sounds & Heart Murmurs

Note: A bit wordy, but I've tried to make it as clear as possible, and also highlighted key points, if that's helpful.

Heart Sounds (aka Heartbeats)

First Heart Sound (S₁)

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.

Second Heart Sound (S₂)

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.

The 2nd heart sound tends to become softer as the aortic stenosis becomes more severe. This is a result of the increasing calcification of the valve preventing it from "snapping" shut and producing a sharp, loud sound.

Note to help in understanding the splitting of heart sounds:

Although S₁ and S₂ are considered to be discrete sounds, notice that each is created by the near-instantaneous closing of two separate valves.

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.

Remember that the pressure during systole in the left ventricle is much greater than in the right, and hence, 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.

Third Heart Sound (S₃)

Shortly after S₂, the closing of the semilunar valves, the AV valves open and diastole begins. Diastole is itself further divided into several stages, the first being that of rapid filling, where 80% of the blood stored in the atria during systole is transferred to the ventricles.

At the end of this stage, about 140-160 ms after S₂, an S₃ may be heard if the volume which has been transferred is abnormally large, as in mitral regurgitation.

It can be thought of as a sound which is generated when the ventricle is forced to dilate beyond its normal range because the atrium has overloaded volume.

An S₃ is usually heard best with the bell of the stethoscope placed at the apex while the patient is in the left lateral decubitus (i.e. lying down) position. The presence of an S₃ is usually normal in children and young adults, but pathologic in those over the age of 40.

Fourth Heart Sound (S₄)

The late stage of diastole is marked by atrial contraction, or kick, where the final 20% of the atrial output is delivered to the ventricles.

If the ventricle is stiff and non-compliant, as in ventricular hypertrophy due to long-standing hypertension (increases in left ventricular pressure), the pressure wave generated as the atria contract produces an S₄.

It is heard best with the bell of the stethoscope at the apex.

Heart Murmurs

Produced as a result of turbulent flow of blood, turbulence sufficient to produce audible noise. They are usually heard as a whooshing sound. The term murmur only refers to a sound believed to originating within blood flow though or near the heart.

Systolic Ejection Murmur in relation to Aortic Stenosis

Systolic Ejection Murmur is a sign of aortic stenosis, detected by auscultation.

Aortic valve stenosis causes significant turbulence to blood flowing during contraction of the left ventricle resulting in a loud murmur; Systolic ejection murmur occurs during ejection of blood through the ventricular outflow tracts into the great arteries.

The loudness of the murmur does not, however, correlate with the severity of stenosis. Patients with mild stenosis can have loud murmurs, while patients with severe stenosis and heart failure may not pump enough blood to cause much of a murmur.

Other causes of heart murmurs

Innocent heart murmurs: caused by blood flowing through the chambers and valves of the heart or the blood vessels near the heart. Situations that cause innocent heart murmurs are,

• Anxiety
• Stress
• Fever
• Pregnancy
• Anemia and thyrotoxicosis (the murmur is not pathologic itself, but indicates the underlying disease process)
• Children also frequently have innocent murmurs which are not due to underlying structural abnormalities.

Note: 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.

Pathologic heart murmurs: caused by structural abnormalities of the heart. Situations that cause pathologic heart murmurs are,

• Regurgitation through the mitral valve (however, about 20% of cases of mitral regurgitation, though obvious using echocardiography, do not produce an audible murmur)
• Regurgitation through the aortic valve
• Stenosis of the aortic valve
• Other audible murmurs are associated with abnormal openings between the left ventricle and right heart (septal defects) or from the aortic or pulmonary arteries back into a lower pressure heart chamber.

Note: Regurgitation through the tricuspid or pulmonic valves essentially never produces audible murmurs.

Sources:

http://www.medicinenet.com/aortic_stenosis/page4.htm#5whatmight
http://www.uni-duesseldorf.de/WWW/MedFak/Herz-Kreislauf-Physiologie/lehre/sounds/html/murmur_overview.html