Split S2
Red = aortic pressure
Blue = left ventricular pressure
Yellow = left atrial pressure.
Aortic valve closes at beginning of D.
A split S2 is a finding upon auscultation of the S2 heart sound.[1]
It is caused when the closure of the aortic valve (A2) and the closure of the pulmonary valve (P2) are not synchronized.[2]
Physiologic split
During inspiration, the chest wall expands and causes the intrathoracic pressure to become more negative (ie think of a vacuum). The increased negative pressure allows the lungs to expand and fill with air. While doing so, it also induces an increase in venous blood return from the body into the right atrium, via the superior and inferior vena cavae, and into the right ventricle by increasing the pressure gradient (blood is being pulled by the vacuum from the body and towards the right side of the heart). Simultaneously, there is a reduction in blood volume returning from the lungs into the left ventricle (the blood wants to stay in the lungs because of the vacuum surrounding the lungs). Since there is an increase in blood volume in the right ventricle, the pulmonary valve (P2 component of S2) stays open longer during ventricular systole due to an increase in ventricular emptying time, whereas the aortic valve (A2 component of S2) closes slightly earlier due to a reduction in left ventricular volume and ventricular emptying time. Thus the P2 component of S2 is delayed relative to that of the A2 component. This delay in P2 versus A2 is heard as a slight broadening or even "splitting" of the second heart sound; though it is usually only heard in the pulmonic area of the chest because the P2 is soft and not heard in other areas. During expiration, the chest wall collapses and decreases the negative intrathoracic pressure (compared to inspiration). Therefore, there is no longer an increase in blood return to the right ventricle versus the left ventricle and the right ventricle volume is no longer increased. This allows the pulmonary valve to close earlier such that it overlaps the closing of the aortic valve, and the split is no longer heard.
It is physiologically normal to hear a "splitting" of the second heart tone in younger people, during inspiration and in the "pulmonary area", i.e. the 2nd ICS (intercostal space) at the left edge of the sternum.
Recap:
1. Chest wall expands during inspiration
2. Intrathoracic pressure becomes more negative to form a vacuum
3. Venous return from the body to the right heart increases + Venous return from the lungs to the left heart decreases
4. Right ventricular volume and emptying time increases + Left ventricular volume and emptying time decreases
5. Pulmonic valve closure is delayed + Aortic valve closure is advanced
6. S2 splits into A2 and P2 respectively
7. Expiration equalizes filling and emptying times on both sides of the heart eliminating the splitting of S2
Pathologic split
- A bundle branch block will produce continuous splitting but the degree of splitting will still vary with respiration.
- If splitting does not vary with inspiration, it is termed a "fixed split S2" and is usually due to an atrial septal defect (ASD) or ventricular septal defect (VSD). The ASD or VSD creates a left to right shunt that increases the blood flow to the right side of the heart, thereby causing the pulmonary valve to close later than the aortic valve independent of inspiration/expiration.
- Reverse splitting indicates pathology. Aortic stenosis, hypertrophic cardiomyopathy, left bundle branch block, and a ventricular pacemaker could all cause a reverse splitting of the second heart sound.
References
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<references />, or <references group="..." />- ↑ "The Auscultation Assistant - Split S2". Retrieved 2009-01-09.
- ↑ "casemed.case.edu". Retrieved 2009-01-09.
