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A split S2 is a finding upon auscultation of the S2 heart sound.〔(【引用サイトリンク】title=The Auscultation Assistant - Split S2 )〕 It is caused when the closure of the aortic valve (A2) and the closure of the pulmonary valve (P2) are not synchronized normally. In a healthy person A2 is followed by P2.〔(【引用サイトリンク】title=casemed.case.edu )〕 ==Physiological split== During inspiration, the chest wall expands and causes the intrathoracic pressure to become more negative (think of a vacuum). The increased negative pressure allows the lungs to fill with air and expand. While doing so, it also induces an increase in venous blood return from the body into the right atrium via the superior and inferior venae 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 atrium (the blood wants to stay in the lungs because of the vacuum surrounding the lungs, and PVR is higher because of lung expansion). Since there is an increase in blood volume in the right ventricle during inspiration, 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 (Source: heart valve sound location mnemonic APT M2245). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Split S2」の詳細全文を読む スポンサード リンク
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