Normally, the atria and ventricles of the heart alternately contract and relax in a rhythmic fashion. This cycle of contraction and relaxation is controlled by the electrical conduction system of the heart (figure 1). Therefore the cardiac cycle can be monitored by examining the electrical activity of the heart via an electrocardiogram (ECG). The cardiac cycle results in the rhythmic opening and closing of the valves of the heart. This activity produces heart sounds which also tell much about the functioning of the heart.

The electrical events of the cardiac cycle begin with depolarization of the sinoatrial node (SA node) in the right atrium. From the sinoatrial node, the electrical impulse spreads across the right and left atria. This electrical activity can be recognized as the P wave on an electrocardiogram (figure 2). This electrical impulse results in contraction of the atria, pumping the blood into the ventricles. Normally, the sinoatrial node depolarizes spontaneously at a rate of about 70 times per minute. This sets the normal rhythm of the heart, and so the sinoatrial node is called the natural pacemaker of the heart. Sometimes the sinoatrial node fails because of disease and a new pacemaker develops elsewhere in the atria or ventricles. This new pacemaker is called an ectopic pacemaker.

The ventricles are electrically insulated from the atria by the connective tissue which supports the valves of the heart (figure 3). Therefore, the electrical impulse from the atria cannot immediately pass into the ventricles. There is a delay, which allows time for the blood to flow from the atria into the ventricles. At the bottom of the right atrium is the atrioventricular node (AV node). The atrioventricular node picks up the electrical impulse from the right atrium and passes it into the atrioventricular bundle (Bundle of His) which runs into the ventricles. As the electrical impulse or depolarization spreads through the ventricles, it can be recognized as the QRS complex on an electrocardiogram. This electrical activity causes contraction of the ventricles, which is also known as ventricular systole. The ventricles are also capable of depolarizing spontaneously, without stimulation from the right atrium. However, because the SA node is depolarizing fairly rapidly, the signal from the right atrium usually causes the ventricles to contract before they can depolarize spontaneously. Sometimes, however, the ventricles contract before receiving "instructions" from the right atrium. This is known as a premature ventricular contraction (PVC).

As the ventricles contract, blood is pumped into the pulmonary trunk and the aorta. At the same time, the right and left atrioventricular valves close to prevent blood from flowing back into the atria (figure 4). The closing of the atrioventricular valves makes a noise which we recognize with a stethoscope as the first heart sound (S1). This phase of the cardiac cycle during which the ventricles contract is known as ventricular systole. After the wave of depolarization has spread throughout the ventricles, the ventricles return to their original electrical state. This event is called repolarization and appears on the ECG as the T wave. At the same time, the ventricles relax - a process called ventricular diastole. During diastole, the aortic semilunar valve and pulmonary semilunar valve close to prevent blood from flowing back into the ventricles. The closure of these valves produces the second heart sound (S2).

Heart sounds and the ECG tell us immediately about the rate and rhythm of the heart. Abnormal heart sounds, called murmurs, suggest that some part of the heart is not functioning normally. The ECG can give very specific information about each part of the electrical conduction system of the heart, including which parts of the system may have been damaged by heart disease.

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English - Japanese Glossary 

aorta: 大動脈 (daidoumyaku); aortic semilunar valve: 大動脈弁 (daidoumyakuben); atrioventricular bundle: 房室束 (boushitsusoku); atrioventricular node: 房室結節 (boushitsukessetsu); atrioventricular valve: 房室弁 (boushitsuben); atrium: 心房 (shinbou); bundle of His: ヒス束 (hisusoku); contraction: 収縮 (shuushuku); depolarization: 脱分極化 (datsubunkyokuka); diastole: 心臓拡張期 (shinzoukakuchouki); electrical conduction system: 刺激電導系 (shigekidendoukei); electrocardiogram: 心電図 (shindenzu); first heart sound: 第I音 (daiichion); heart sounds: 心音 (shinon); murmur: 心臓雑音 (shinzouzatsuon); P wave: P 波 (P ha); pulmonary semilunar valve: 肺動脈弁 (haidoumyakuben); pulmonary trunk: 肺動脈 (haidoumyaku); QRS complex: QRS群 (QRS gun); rate: 心拍数(shinpakusuu); repolarization: 再分極化 (saibunkyokuka); rhythm: リズム(rizumu), 調律 (chouritsu); second heart sound: 第II音 (dainion); sinoatrial node: 洞房結節 (douboukessetsu); stethoscope: 聴診器 (choushinki); systole: 心収縮期 (shinshuushukuki); T wave: T 波 (T ha); ventricle: 心室 (shinshitsu)