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Heart Sounds: Advanced
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Introduction to Heart Sounds
| The first heart sound (S1) The first heart sound results from the closing of the mitral and tricuspid valves. The sound produced by closure of the mitral valve is termed M1 and the sound produced by closure of the tricuspid valve is termed T1. The M1 sound is much louder than T1 due to higher pressures in the left side of the heart, thus M1 radiates to all cardiac listening posts (loudest at the apex) and T1 is usually only heard at the left lower sternal border. The M1 sound is thus the main component of S1. CLINICAL PEARL: A split S1 is best heard at the tricuspid valve listening post since T1 is much softer than M1. M1 occurs slightly before T1. Since the mitral and tricuspd valves normally close almost simultaneously, only one single heart sound is usually heard. However, in about 40% of normal individuals, as well as in certain cardiac conditions, a "split S1" sound can be appreciated. This occurs when the mitral valve closes significantly before the tricuspid valve allowing both valves to make an audible, separate sound. Inspiration delays the closure of the tricuspid valve in a normal person (due to increased venous return) thus enhancing the splitting of the S1 sound. Also, a split S1 sound is common in the setting of a right bundle branch block (RBBB). A RBBB causes the electrical impulse to reach the left ventricle before the right ventricle. Dysynchrony then occurs resulting in the left ventricle contracting before the right ventricle thus the pressure in the left ventricle to rises before that of the right ventricle. This increased pressure forces the mitral valve closed before the tricuspid valve resulting in a split S1 sound. A left bundle branch block (LBBB) has the opposite affect on S1. In this setting, the electrical impulse reaches the right ventricle before the left ventricle, thus the pressure in the right ventricle rises before that of the left ventricle. This forces the tricuspid valve closed earlier resulting in complete overlap of M1 and T1 and thus no audible split S1 sound. CLINICAL PEARL: A RBBB causes a split S1 while a LBBB results in the absence of a split S1 |
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The second heart sound (S2)
The second heart sound is produced by the closure of the aortic and valves. The sound produced by closure of
the aortic valve is termed A2 and the sound produced by closure of the pulmonic valve is termed P2. The A2
sound is normally much louder than P2 due to higher pressures in the left side of the heart, thus A2 radiates
to all cardiac listening posts (loudest at the right upper sternal border) and P2 is usually only heard at the left
lower upper sternal border. The A2 sound is thus the main component of S2.
CLINICAL PEARL: A split S2 is best heard at the pulmonic valve listening post since P2 is
much softer than A2.
Like the S1 heart sound, the S2 sound is described regarding splitting and intensity. S2 is physiologically split
in about 90% of people. The S2 heart sound can exhibit persistent (widened) splitting, fixed splitting,
paradoxical (reversed) splitting, or absence of splitting.
CLINICAL PEARL: In severe hypertension, a loud S2 may be prolonged and slurred falsely
mimicking a split S2.
Physiologic split S2
Normally, A2 occurs just before P2 and the combination of these sounds make up S2. A physiological split S2
occurs when the A2 sound precedes P2 by a great enough distance to allow both sounds to be heard
separately. This happens during inspiration when increased venous return to the right side of the heart
delays the closure of the pulmonic valve and decreased return to the left side of the heart hastens the
closure of the aortic valve, thus further separating A2 and P2. During expiration, the distance narrows and
the split S2 is no longer audible.
Persistent (Widened) split S2
Persistent (widened) splitting occurs when both A2 and P2 are audible (split) during the entire respiratory
cycle, however the splitting becomes greater with inspiration and less prominent with expiration. This differs
from a fixed split S2 which exhibits the same amount of splitting throughout the entire respiratory cycle (see
below). A persistently split S2 occurs in the setting of a RBBB or severe mitral regurgitation. A RBBB causes
a delay in the closure of the pulmonic valve and thus a delay in P2 without any affect on A2. In severe mitral
reguritation, the A2 occurs early secondary to a large proportion of left ventricular stroke volume entering
the left atrium, thus causing left ventricular pressure to decrease faster. The P2 is not affected in severe
mitral regurgitation.
Fixed split S2
Paradoxically (reversed) split S2
Absence of split S2
The third heart sound (S3)
The fourth heart sound (S4)
Extra Systolic Heart Sounds
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