Physiology Of The First Sound Components

1. Draw a left ventricular (LV) pressure curve.

An LV pressure curve begins at a pressure of about 0 mmHg and rises to the same systolic pressure as in the aorta, i.e., normally about 120 mmHg.

Draw a left atrial pressure curve on the ventricular pressure curve and show where the mitral valve closes. Why does it close here?

ANS: The mitral valve closes when LV pressure rises above left atrial pressure, which is about 10 mmHg. If the left atrial pressure at the beginning of ventricular contraction is 10 mmHg, then as soon as the LV reaches a pressure of slightly more than 10 mmHg, the mitral valve will close.

The most important component of the S1, which is the mitral component of M1, occurs as the result of this valve closure, but the sound should not be thought of as due to the slapping together of leaflets. It is more probably due to sudden cessation of mitral valve flow at the time of maximum leaflet tension that occurs immediately after leaflet apposition, setting the entire cardiohemic system into vibration.

3. Draw an aortic pressure curve on the ventricular pressure curve and show where the aortic valve opens. (See question 4.)

4. When does the aortic valve open if the aortic diastolic pressure is 80 mmHg?

ANS: The aortic valve opens when the LV pressure exceeds the aortic diastolic pressure of 80 mmHg.

Note: The time between closure of the mitral valve and the opening of the aortic valve is the isovolumic contraction period.

5. How do these pressure curves relate to the first heart sound, or Sj?

ANS: The events associated with closure of the mitral valve and ejection of blood into the aorta are probably responsible for the two major components of the Sj. However, many cardiologists believe that the second major component of the split Sl is due to closure of the tricuspid valve, ignoring the fact that mitral and tricuspid closure occur 0 020.03 s apart, which can be distinguished only with difficulty even by expert auscultators.

6. What are the components to the usual split of Sl?

ANS: If the components are close together—fast enough to say "ta-da" as quickly as possible as if you were rolling the syllables on your tongue— they are probably due to mitral followed by tricuspid closure, i.e., M j-Tj. If the components are fairly well separated, as in saying "pa-ta" as quickly as possible, then the components are probably mitral closure followed by aortic valve opening, or Ml-Aj [1]. The second component of a wide split, or Al, is more commonly known as an ejection sound, and it is occasionally so short and sharp that it is then known as an ejection click.

Aortic

Aortic curve

When the LV pressure reaches aortic diastolic pressure (about 80 mmHg), the aortic valve opens, and an aortic valve opening sound may be produced and become the second component of a split S1. When aortic recoil power becomes stronger than LV ejection, the aortic valve closes to prevent regurgitation. The S2 occurs at this time.

7. When is it likely that tricuspid closure is the second component of a split Sj?

ANS: a. Whenever the right ventricle (RV) has a volume or pressure overload, as in ASD or in the presence of pulmonary hypertension. b. Ebstein's anomaly.

8. In what percentage of normal subjects is splitting of the Sj audible?

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