Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body. Figure 1. The cardiac cycle begins with atrial systole and progresses to ventricular systole, atrial diastole, and ventricular diastole, when the cycle begins again. Correlations to the ECG are highlighted. Fluids, whether gases or liquids, are materials that flow according to pressure gradients—that is, they move from regions that are higher in pressure to regions that are lower in pressure.
Accordingly, when the heart chambers are relaxed diastole , blood will flow into the atria from the veins, which are higher in pressure. As blood flows into the atria, the pressure will rise, so the blood will initially move passively from the atria into the ventricles.
When the action potential triggers the muscles in the atria to contract atrial systole , the pressure within the atria rises further, pumping blood into the ventricles.
During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle. Again, as you consider this flow and relate it to the conduction pathway, the elegance of the system should become apparent.
At the beginning of the cardiac cycle, both the atria and ventricles are relaxed diastole. Blood is flowing into the right atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles.
Approximately 70—80 percent of ventricular filling occurs by this method. The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left. Contraction of the atria follows depolarization, represented by the P wave of the ECG. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular tricuspid, and mitral or bicuspid valves.
At the start of atrial systole, the ventricles are normally filled with approximately 70—80 percent of their capacity due to inflow during diastole. Atrial systole lasts approximately ms and ends prior to ventricular systole, as the atrial muscle returns to diastole. Ventricular systole see image below follows the depolarization of the ventricles and is represented by the QRS complex in the ECG. It may be conveniently divided into two phases, lasting a total of ms.
At the end of atrial systole and just prior to atrial contraction, the ventricles contain approximately mL blood in a resting adult in a standing position. This volume is known as the end diastolic volume EDV or preload. Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar pulmonary and aortic valves and be ejected from the heart.
However, blood pressure quickly rises above that of the atria that are now relaxed and in diastole. This increase in pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves.
Since blood is not being ejected from the ventricles at this early stage, the volume of blood within the chamber remains constant. Consequently, this initial phase of ventricular systole is known as isovolumic contraction , also called isovolumetric contraction see image below. In the second phase of ventricular systole, the ventricular ejection phase , the contraction of the ventricular muscle has raised the pressure within the ventricle to the point that it is greater than the pressures in the pulmonary trunk and the aorta.
Blood is pumped from the heart, pushing open the pulmonary and aortic semilunar valves. Pressure generated by the left ventricle will be appreciably greater than the pressure generated by the right ventricle, since the existing pressure in the aorta will be so much higher. Nevertheless, both ventricles pump the same amount of blood. This quantity is referred to as stroke volume. Stroke volume will normally be in the range of 70—80 mL. It can also detect enlargement of the heart, decreased blood flow, or the presence of current or past heart attacks.
ECGs are the primary clinical tool to measure electrical and mechanical performance of the heart. The ECG works by detecting and amplifying tiny electrical changes on the skin that occur during heart muscle depolarization.
The output for the ECG forms a graph that shows several different waves, each corresponding to a different electrical and mechanical event within the heart. Changes in these waves are used to identify problems with the different phases of heart activity. The first wave on an ECG is the P wave, indicating atrial depolarization in which the atria contract atrial systole. Increased or decreased P waves can indicate problems with the potassium ion concentration in the body that will alter nerve activity.
A missing P wave indicates atrial fibrillation, a cardiac arrhythmia in which the heart beats irregularly, preventing efficient ventricular diastole. This is generally not fatal on its own. The QRS complex refers to the combination of the Q, R, and S waves, and indicates ventricular depolarization and contraction ventricular systole.
In healthy young individuals, the volume of blood transported during the rapid filling phase E wave is greater than the volume transported during atrial contraction A wave. With age, however, the A wave becomes larger, which is explained by the fact that ventricular compliance diminishes and atrial contraction becomes increasingly important for atrial emptying. When the atrial contraction is completed, atrial myocardium begins to relax and atrial pressure drops.
The mitral valve closes when the atrial pressure is below the ventricular pressure. Thus, diastole includes two phases when there is no filling IVRT and diastasis and two phases with filling rapid filling and atrial contraction. Rapid filling is a passive process, propelled by the pressure gradient between the atrium and the ventricle. The final emptying of the atrium is achieved by active atrial contraction.
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The echocardiographic examination. Principles and Preparations for Echocardiographic Examinations. Left ventricular systolic function and contractility. Left Ventricular Function. Left ventricular diastolic function. Valvular heart disease. Tricuspid valve stenosis.
Miscellaneous conditions. Pericardial disease.
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