## Explanation Of Normal Splitting Sequence Of S2

1. Draw the ventricular pressure curve and an aortic pressure curve.

2. Superimpose the aortic pressure curve on the ventricular pressure curve.

Note that when the LV pressure rise exceeds aortic pressure, the aortic valve will open and produce a single-chamber effect or an "aortoventricle." The point at which ejection is finished and the aortic and LV pressure curves separate is called the incisura and is simultaneous with the aortic second sound, or A2.

3. At what pressure in the left ventricle (LV) would you expect the aortic valve to open, assuming a normal blood pressure of 120/80 mmHg?

ANS: When pressure in the LV rises to just above aortic diastolic pressure (about 80 mmHg), the aortic valve will open.

4. After the aortic valve opens, what is the difference in pressure between the LV and the aorta?

ANS: Almost none. The LV and the aorta are almost a single pressure chamber as soon as the aortic valve opens. (It may be called an "aortoventricle" at this time.) Only in the presence of aortic stenosis (AS) is there a significant pressure difference (gradient) between the aorta and the ventricle.

Note: In fact, there is normally a slight positive gradient between the LV and the aorta during the first two-thirds to three-fourths of systole. It has been called an impulse gradient.

5. What do we call the notch on the carotid or aortic pressure tracing that occurs roughly at the time of aortic valve closure? How is it related to the heart sounds?

ANS: In the external carotid tracing taken by putting a pressure-sensitive pickup on the neck, it is called the dicrotic notch. In aortic pressure tracings, it is called the incisura. The incisura is simultaneous with the A2 if aortic root pressure tracings are used for timing.

6. At what pressure does the A2 occur, i.e., does it occur at aortic systolic, diastolic, or some intermediate pressure?

ANS: The aortic valve closes when the force of ventricular ejection decreases, and the peripheral resistance plus the elastic recoil of the expanded aorta overcomes the decreasing pressure in the LV. This occurs at just below aortic systolic pressure (e.g., if the systolic pressure in the aorta is 120 mmHg, the A2 probably occurs at a pressure of about 110 mmHg).

Note: The pulmonary artery pressure tracing also has a dicrotic notch or inci-sura where the P2 occurs. The normal pulmonary artery pressure is about 25/10 mmHg.

7. If the aortic valve closes at a pressure of about 100 mmHg, and the pulmonary valve closes at about 20 mmHg, why will aortic closure occur first?

ANS: The second heart sounds occur simultaneously with the incisuras of the pulmonary artery and aorta. The timing of the incisuras, in turn, has been shown to be related to the impedance to flow. For example, the less the arteriolar resistance and elastic recoil of the pulmonary arteries, the longer will forward flow continue and the later will the incisura occur on the pressure curve of the pulmonary artery. The pulmonary vascular resistance is about a tenth of that of the systemic resistance and the elastic recoil of the normal pulmonary artery is probably less than that of the aorta. Therefore, forward flow continues longer in the pulmonary circuit than in the aortic circuit after their respective pressure crossovers. This causes the pulmonary pressure and closure sound (P2) to occur later than the aortic incisura and A2.