The human body functions due to the presence of the circulatory system and cellular nutrition. The heart as the main organ of the circulatory system is able to provide uninterrupted supply of tissues with energy substrates and oxygen. This is achieved through the cardiac cycle, the sequence of phases of the work of the body, associated with the constant alternation of rest and load.
This concept should be considered from several points of view. Firstly, from the morphological point of view, that is, from the point of view of the basic description of the phases of the work of the heart as an alternation of systole with diastole. Secondly, with hemodynamic, associated with the decoding of capacitive and barometric characteristics in the cavities of the heart at each stage of systole and diastole. Within the framework of these points of view, the concept of the cardiac cycle and its constituent processes will be considered below.
Characteristics of the work of the heart
The uninterrupted work of the heart from the moment of its laying in embryogenesis until the death of the organism is ensured by the alternation of systole with diastole. This means that the body does not work constantly. Most of the time, the heart even rests, which allows it to provide for the needs of the body throughout life. The work of some structures of the body occurs at the time of rest of others, which is necessary to ensure the constancy of blood circulation. In this context, it is appropriate to consider the cycle of heartbeats from a morphological point of view.
Fundamentals of morphophysiology of the heart
The heart in mammals and humans consists of two atria flowing into the ventricular cavities (VP) through the atrioventricular (AV) openings with valves (AVK). Systole and diastole alternate, and the cycle ends with a general cardiac pause. As soon as blood is ejected from the VP into the aorta and pulmonary artery, the pressure in them drops. A retrograde current develops from these vessels back into the ventricles, which is quickly stopped by the opening of the valves. But at this time, the atrial hydrostatic pressure is higher than the ventricular pressure, and the AVKs are forced to open. As a result, on the pressure difference, at the moment when the ventricular systole has passed, but the atria have not come, ventricular filling occurs.
This period is also called a general cardiac pause, which lasts until the pressure in the ventricular (RV) and atrial (AA) cavities of the corresponding side equalizes. As soon as this has happened, atrial systole begins to push the remaining portion of blood into the pancreas. After it, when the rest of the blood is squeezed out into the ventricular cavities, the pressure in the right ventricle drops. This causes passive blood flow: into the left atriumvenous discharge is carried out from the pulmonary veins, and into the right - from the hollow ones.
Systemic view of the cardiac cycle
The cycle of cardiac activity begins with ventricular systole - expulsion of blood from their cavities along with simultaneous atrial diastole and the beginning of their passive filling on the pressure difference in the afferent vessels, where at this moment it is higher than in the atria. After ventricular systole, there is a general cardiac pause - the continuation of passive atrial filling with negative pressure in the ventricles.
Due to higher hemodynamic pressure in the RA and low in the RV, along with the continuation of passive atrial filling, the AV valves open. The result is passive ventricular filling. As soon as the pressure in the atrial and ventricular cavities equalizes, passive current becomes impossible, and atrial replenishment stops, which causes them to contract in order to continue pumping an additional portion into the ventricular cavities.
From the atrial systole, the pressure in the ventricular cavities increases significantly, ventricular systole is provoked - the muscle contraction of its myocardium. The result is an increase in pressure in the cavities and the closure of atrioventricular connective tissue valves. Due to the discharge at the mouth of the aorta and the pulmonary trunk, pressure is formed on the corresponding valves, which are forced to open towards the blood flow. This completes the cardiac cycle: the heart again begins passive filling of the atria in theirdiastole and further at the time of the general cardiac pause.
Heart pauses
There are many episodes of rest in the work of the heart: diastole in the atria and ventricles, as well as a general pause. Their duration can be calculated, although it depends very much on the heart rate. At 75 beats / min, the cardiac cycle time will be 0.8 seconds. This period included atrial systole (0.1s) and ventricular contraction - 0.3 seconds. This means that the atria rest for approximately 0.7 s, and the ventricles for 0.5 s. During the rest, a general pause (0.5 s) also enters.
About 0.5 seconds the heart passively fills, and 0.3 seconds it contracts. Atria, the relaxation time is 3 times longer than in the ventricles, although they pump similar volumes of blood. However, they mostly enter the ventricles by passive current along the pressure gradient. Blood by gravity at the moment of low pressure in the heart cavities enters the cavities, where it accumulates for subsequent contraction and expulsion into the efferent vessels.
Meaning of periods of relaxation of the heart
In the cavity of the heart, blood enters through the holes: into the atria - through the mouths of the hollow and pulmonary veins, and into the ventricles - through the AVC. Their capacity is limited, and the actual filling takes longer than its expulsion through the circulation. And the phases of the cardiac cycle are exactly what are needed for sufficient filling of the heart. The smaller these pauses, the less the atria will fill, the less bloodwill be sent to the ventricles and, accordingly, to the circles of blood circulation.
With an increase in the actual frequency of contractions, which is achieved by shortening the relaxation period, the filling of the cavities decreases. This mechanism still remains effective for the rapid mobilization of the functional reserves of the body, but an increase in the frequency of contractions gives an increase in the minute volume of blood circulation only up to a certain limit. Upon reaching a high frequency of contractions, the filling of the cavities due to the extremely short diastole will drop significantly, as will the level of blood pressure.
Tachyarrhythmias
The mechanism described above is the basis for reducing physical endurance in a patient with tachyarrhythmias. And if sinus tachycardia, if necessary, allows you to increase pressure and mobilize the resources of the body, then atrial fibrillation, supraventricular and ventricular tachycardia, ventricular fibrillation, as well as ventricular tachysystole in WPW syndrome lead to a drop in pressure.
The manifestation of the patient's complaints and the severity of his condition starts from discomfort and shortness of breath to loss of consciousness and clinical death. The phases of the cardiac cycle, discussed above in terms of the importance of pauses and their shortening in tachyarrhythmias, are the only simple explanation why arrhythmias should be treated if they have a negative hemodynamic contribution.
Features of atrial systole
Atrial (atrial) systole lasts about 0.1 s: the atrial muscles contract simultaneously in accordance with the rhythm generated by the sinusnode. Its importance lies in pumping approximately 15% of the blood into the cavity of the ventricles. That is, if the systolic volume of the left ventricle is about 80 ml, then about 68 ml of this portion passively filled the ventricle in the atrial diastole. And only 12 ml is pumped out by atrial systole, which allows the pressure level to increase in order to close the valves during ventricular systole.
Atrial fibrillation
In conditions of atrial fibrillation, their myocardium is constantly in a state of chaotic contraction, which does not allow the formation of a solid atrial systole. Because of this, arrhythmia makes a negative hemodynamic contribution - it impoverishes blood flow to the ventricular cavities by about 15-20%. Their filling is carried out by gravity during a general cardiac pause and during the period of ventricular systole. That is why some part of the portion of the blood always stays in the atria and is constantly shaken up, multiplying the risk of thrombosis in the circulatory system.
Blood retention in the cavities of the heart, and in this case in the atria, leads to their gradual stretching and makes it impossible to maintain the rhythm with a successful cardioversion. Then the arrhythmia will become constant, which accelerates the development of cardiac insufficiency with stagnation and hemodynamic disturbances in the circulation by 20-30%.
Ventricular systole phases
With the duration of the cardiac cycle of 0.8 s, the ventricular systole will be 0.3 - 0.33 seconds with two periods - tension (0.08 s) and expulsion (0.25 s). Myocardium begins to contract, but its efforts are not enoughfor squeezing blood out of the ventricular cavity. But the pressure created already allows the atrial valves to close. The ejection phase occurs at the moment when the systolic pressure in the ventricular cavities allows a portion of blood to be expelled.
The phase of tension in the cardiac cycle is divided into the period of asynchronous and isometric contraction. The first lasts about 0.05 s. and is the beginning of an integral contraction. An asynchronous (random) contraction of myocytes develops, which does not lead to an increase in pressure in the ventricular cavity. Then, after excitation covers the entire mass of the myocardium, the phase of isometric contraction is formed. Its importance lies in a significant increase in pressure in the cavity of the ventricles, which allows you to close the atrioventricular valves and prepare to push blood into the pulmonary trunk and aorta. Its duration in the cardiac cycle is 0.03 seconds.
Ejection period of the ventricular systole phase
Ventricular systole proceeds to the expulsion of blood into the cavity of the efferent vessels. Its duration is a quarter of a second, and it consists of a fast and a slow phase. First, the pressure in the ventricular cavities rises to maximum systolic, and muscle contraction pushes out of their cavity a portion of about 70% of the actual volume. The second phase is slow ejection (0.13 s): the heart pumps the remaining 30% of systolic volume into the efferent vessels, however, this occurs already with a decrease in pressure, which precedes ventricular diastole and a general cardiac pause.
Ventricular diastole phases
Ventricular diastole (0.47 s) includes a period of relaxation (0.12 seconds) and filling (0.25 seconds). The first is divided into protodiastolic and myocardial isometric relaxation phase. The filling period in the cardiac cycle consists of two phases - fast (0.08 sec) and slow (0.17 sec).
During the proto-diastolic period (0.04 sec.), the transitional phase between ventricular systole and diastole, pressure in the ventricular cavities drops, causing the aortic and pulmonary valves to close. In the second phase, there is a period of zero pressure in the ventricular cavities with simultaneously closed valves.
During the period of rapid filling, the atrioventricular valves instantly open, and blood flows along the pressure gradient into the ventricular cavities from the atria. At the same time, the cavities of the latter are constantly supplemented by inflow through the bringing veins, which is why, with a smaller volume of the cavities of the atria, they still pump similar portions of blood, like the ventricles. After that, due to the peak value of pressure in the ventricular cavities, the inflow slows down, a slow phase begins. It will end with an atrial contraction that occurs in ventricular diastole.
