Heartbeat - Shinon

The heart vibrates due to the opening and closing of valves and changes in blood flow as the heart beats, and these vibrations can be heard from the chest wall. These are called heart sounds. Heart sounds are noises in physical terms, and contain various frequency components, but are mainly low-frequency components below 500 Hz. Heart sounds last a short time, and are heard as a single sound by the human ear, and are distinguished from long-lasting heart murmurs heard in cases of valvular disease. The sensitivity of the human ear is good for sounds between 500 and 5,000 Hz, so listening to heart sounds is at a disadvantage for the human ear. Various types of stethoscopes are used as tools to compensate for this. When listening to heart sounds with a stethoscope, the following four areas are used, as they make it easy to distinguish the sounds of each valve and cause the least loss of acoustic energy. These are: (1) the aortic region (second intercostal space, right border of the sternum), (2) the pulmonary artery region (second intercostal space, left border of the sternum), (3) the tricuspid valve region (lower end of the sternum), and (4) the mitral valve region (apex of the heart). However, these regions do not necessarily correspond to their anatomical locations.

Normally, two heart sounds are heard per cardiac cycle: the first sound, which occurs at the beginning of ventricular systole, and the second sound, which occurs at the end of ventricular systole. The first sound is produced when the atrioventricular valves close, and consists of three components: a relatively high-pitched main sound, and two low-pitched anterior and posterior sounds before and after it. The second sound is produced by the closure of the semilunar valves, and consists of two components, an aortic valve component II _A and a pulmonary valve component II _P. Normally, the second sound is heard as a single sound, but even in healthy people, the second sound may be relatively widely split, and the two components, II _A and II _P , may be heard separately. This split becomes clear at the end of inspiration and becomes unclear during expiration. This is due to the amount of venous return associated with breathing, and is called respiratory splitting. Heart sounds other than the first and second sounds are called extra heart sounds, and include the third sound, fourth sound, atrioventricular valve opening sound, and ejection sound, and are often heard during illness. The third sound is said to be caused by the vibration of the ventricular wall during early diastole when the ventricles rapidly fill, and can be heard even in normal people. It is particularly common in young people, with more than half of those under 30 years old being heard, and it is said that it is not heard in those over 50 years old. The fourth sound is a sound related to atrial contraction and is also called the atrial sound. The third and fourth sounds are frequently heard in cases of heart failure. The atrioventricular valve opening sound is not heard in healthy people, but in cases of mitral valve stenosis, for example, a high-pitched, sharp mitral valve opening sound is heard. The ejection sound is also called the ejection phase click, and can be heard immediately after the first sound when there is expansion of the aorta or pulmonary artery.

[Hidenobu Mashima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

心臓は拍動に伴う弁の開閉や血流の状態の変化によって振動を生じるが、この振動は胸壁上から聴取することが可能である。これを心音という。心音は物理学的には雑音であり、さまざまな周波数成分を含むが、主として500ヘルツ以下の低周波数成分からなっている。心音は持続時間が短く、人間の耳で聴くと単一な音として聴取され、弁膜症などのときに聞こえる持続時間の長い心雑音と区別される。人間の耳の感度は500～5000ヘルツの音に対して良好であるため、心音聴取は人間の耳にとっては不利な条件下にある。これを補うための道具としてさまざまな型の聴診器が用いられている。聴診器で心音を聴取する際には、各弁膜の音の区別が容易、かつ音響エネルギー損失のもっとも少ない点として、次の四つの領域が用いられている。すなわち、(1)大動脈領域（第2肋間(ろっかん)胸骨右縁）、(2)肺動脈領域（第2肋間胸骨左縁）、(3)三尖弁(さんせんべん)領域（胸骨下端部）、(4)僧帽弁領域（心尖部）である。ただし、これらの領域は、かならずしも解剖学的な位置とは一致していない。

　正常である場合は、1心周期に2個の心音、すなわち心室収縮期の初めに生じるⅠ音と、心室収縮期の終わりに生じるⅡ音が聴取される。Ⅰ音は房室弁が閉鎖する際に生じる音で、比較的高調の主節と、その前後の低調な前節・後節の3成分からなる。Ⅱ音は半月弁の閉鎖によって生じ、大動脈弁成分Ⅱ_Aと肺動脈弁成分Ⅱ_Pの2成分からなる。通常、Ⅱ音は単一の音として聴取されるが、健常者でもⅡ音が比較的広く分裂し、Ⅱ_AとⅡ_Pの2成分が分かれて聴取されることがある。この分裂は吸気の終わりに明瞭(めいりょう)となり、呼気時には不明瞭になる。これは呼吸に伴う静脈還流量に起因するもので、呼吸性分裂とよばれる。Ⅰ音、Ⅱ音以外の心音は過剰心音とよばれ、Ⅲ音、Ⅳ音、房室弁開放音、駆出(くしゅつ)音などがあり、多くは病気の際に聴取される。Ⅲ音は、心室が急速に充満する拡張早期に心室壁が振動することによって生じるとされ、正常でも聴取されることがある。とくに若年者に多く、30歳未満では半数以上に認められ、50歳以上では聴取されないといわれている。Ⅳ音は心房収縮に関係した音で心房音ともよばれる。Ⅲ音、Ⅳ音は心不全の際などに高率に聴取される。房室弁開放音は、健常者では聴取されないが、たとえば僧帽弁狭窄(きょうさく)症では、高調で鋭い僧帽弁開放音が聴取される。駆出音は駆出期クリックともよばれ、大動脈または肺動脈の拡張のある場合に、Ⅰ音の直後に聴取される。

［真島英信］

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