Physiology Of The Normally Moving Split

1. Does the normal split of the S2 widen on inspiration or expiration?

ANS: It widens on inspiration, so that the A2P2 becomes an A2-P2.

2. Does the split movement of the S2 occur because of the movement of the A2 or the movement of the P2?

ANS: Both. The P2 moves out, away from the A2, and the A2 moves inward, away from the P2.

3. Which component moves more, the A2 or the P2?

The P2 outward movement contributes more to the inspiratory widening of the S2 than does the inward movement of the A2.

4. Why does the P2 occur later with inspiration?

ANS: a. The right ventricle (RV) becomes larger with inspiration because inspiration lowers the intrathoracic pressure (makes it more negative), and this acts to draw more blood from the superior and inferior venae cavae into the right side of the heart. The lungs act as a bellows, i.e., when they expand, they function like a suction apparatus, which sucks blood from the inferior and superior venae cavae into the right atrium and ventricle. This increased RV volume on inspiration delays pulmonary closure because when a ventricle increases its volume and has only one outlet for systole, it takes longer to eject that extra volume.

b. On inspiration the pulmonary impedance falls because the capacitance of the pulmonary vasculature is increased. This also contributes to the delay in pulmonary valve closure.

5. Why does the A2 occur earlier with inspiration?

ANS: Because the LV becomes smaller with inspiration. This occurs because inspiration, by enlarging the chest volume, also enlarges the vascular capacity of the lungs so much that they cannot compensate by drawing enough blood from the RV. In other words, the lungs do not fill from the RV in proportion to their increase in blood space potential during inspiration. This excessive increase in lung capacity withholds some blood from the LV.

Note: Maximum widening of the split A2-P2 occurs at the peak of inspiration. Maximum narrowing occurs during mid-expiration.

6. Does the normally moving split phenomenon (i.e., widening on inspiration and narrowing on expiration) refer to (a) held expiration and inspiration or to moving respiration, and to (b) deep or normal respiration?

ANS: a. It refers to moving respiration.

b. It refers to normal depth of respiration.

Note: a. A split S2 at end-expiration is so rare after age 50 that it should be considered abnormal. b. It is easier to hear the normally split S2 in the upright position because upright posture exaggerates most respiratory effects.

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