The stronger the ventricular contraction, the stronger the S1. Strong contractions that have a high dP/dT (i.e., a sharp increase in pressure over time) strengthen S1 as the valves close with more force and produce more vibration in the cardiohemic system.6,7 As ventricular preload increases, the ventricle becomes swollen and, more broadly, myocardiocytes are also stretched. This stretching brings the actin and myosin components of the muscle fiber to a more optimal level. Therefore, muscle fibers contract with greater force to pump the extra blood. Note, however, that this principle is only valid up to an optimal point. Any additional stretching beyond this point dissociates the actin-myosin complex, making it difficult to contract. The cardiac cycle is defined as a sequence of alternating contraction and relaxation of the atria and ventricles to pump blood through the body. It begins at the beginning of one heartbeat and ends at the beginning of another. The process begins as early as the 4th week of pregnancy, when the heart begins to contract for the first time. Figure 1.

The cardiac cycle begins with the atrial sysstole and progresses to the ventricular systole, atrial diastole and ventricular diastole when the cycle begins again. Correlations with ecg are highlighted. There is a secondary area of concentrated conductive tissue known as the atrioventricular node (AV node), which is located medial and posterior to the tricuspid valve. Like the SA node, the AV node has autonomous properties and is capable of generating action potential. However, these cells are slower than those of the SA node and therefore respond to the activity of the SA node. There are preferred internodal pathways that exist for more efficient transmission of the pulse to the AV node. Consider the beginning of the ventricular volume curve at the beginning of the diastole. Here there is a residual volume of about 50 ml of blood in the ventricle. At this time, the pressure curve decreases sharply during isovolumetric relaxation. As soon as the ventricular pressure is lower than the atrial pressure, the atrioventricular valve opens. There is a rapid increase in ventricular volume, followed by a slow and gradual increase (in line with the passive filling phase).

During this time, the ventricular pressure remains unchanged, as the chamber is able to absorb the increasing volume. In people at rest, the heart beats about once a second. With each beat, the heart undergoes a series of four hemodynamic events caused by changes in pressure and volume (E-Fig. 47-4) as well as the electrical activity represented by the ECG. When the heart muscle at the terminal diastole is relaxed, the ventricular pressure is at its resting level (terminal diastolic pressure) and the ventricular volumes are at their maximum value (final diastolic volume). The aortic pressure decreases as the blood expelled into the aorta during the previous ventricular contraction flows into the peripheral circulation. Atrial contraction provides a final boost to the ventricular volume immediately before the ventricular systole. Ventricular contraction increases the pressure in the ventricle; If this pressure exceeds the pressure in the atrium, the mitral valve closes. However, as the ventricular pressure remains lower than the aortic pressure, the aortic valve remains closed and, during this first phase of the cardiac cycle, the isovoluminous contraction phase, no blood enters or leaves the ventricle. During systole, the ventricular pressure eventually exceeds the aortic pressure, at which point the aortic valve opens, blood is expelled into the aorta, and the ventricular volume decreases during the sputum phase of the cycle. At the end of the systole, when the contraction is maximum, the ejection ends and the ventricular volumes are the lowest (endsystolic volume).

The volume of blood expelled, called stroke volume (SV), is defined as the difference between terminal and final systolic diastolic volumes. The ejection fraction (EF), defined as the percentage of final diastolic volume (PDE) expelled during contraction (EF = 100 × SV/EDP), is an index of cardiac function. The next phase of the cycle occurs when the heart muscle relaxes, the ventricular pressure is lower than the aortic pressure, and the aortic valve is closed. During this isovolume relaxation phase, the ventricular volumes remain constant, as the mitral and aortic valves are closed. When the ventricular pressure falls below the atrial pressure, the mitral and tricuspid valves open, and during the filling phase, blood flows from the atria into the ventricles. Atrial relaxation (atrial diastole) occurs when the ventricle contracts (ventricular systole). However, before ventricular contraction begins, the spread of the action potential of the atrium into the AV canal is briefly delayed. This AV delay ensures that the blood has enough time to come out of the atrium and complete the ventricular filling. Ventricular contraction ejects blood into the flow tract beyond an open bulboventricular valve (BV).

The cardiac cycle is completed when the ventricle relaxes (ventricular diastole). The ear pressure wave shows the change in ear pressure during systole and diastole. There are three important pressure changes represented by the letters a, v and c. The change in pressure produced when the atria fill with blood is represented by the „v“ wave towards the end of the ear pressure wave. There is a slight decrease in ear pressure, which corresponds to the opening of the atrioventricular valve. This is followed by the „a“ wave, which represents the contraction of the atria. The A wave is followed by a downward tilt when the atrioventricular valves close. This is followed by another rise called wave „c“.

This represents a bulge of the atrioventricular valves in the atria during ventricular contraction. Although the curves shown in Figure 6.3 do represent changes in left ventricular pressure in response to increased blood volume, they do not really show an acute picture of the normal changes that occur during a single cardiac cycle. This is illustrated in Figure 6.4 as a standard print volume loop divided into four phases. Phase I is called the filling period and begins at point A when diastolic filling begins. .