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 capable of providing an uninterrupted supply of tissues with energy substrates and oxygen. This is achieved due to the cardiac cycle, the sequence of phases of the organ, associated with the constant alternation of rest and stress.
This concept should be considered from several points of view. Firstly, from the morphological, that is, from the point of view of the basic description of the phases 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.
Heart Performance
Uninterrupted operation of the heart from the moment of its laying in embryogenesis to the death of the body is ensured by the alternation of systole with diastole. This means that the body does not work continuously. Most of the time the heart even rests, which allows it to meet the needs of the body throughout life. The work of some organ structures 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 heartbeat cycle from a morphological point of view.
Fundamentals of cardiac morphophysiology
The heart in mammals of animals and humans consists of two atria that enter the ventricular cavities (VP) through the atrioventricular (AB) openings with valves (AVC). Systole and diastole alternate, and the cycle ends with a general cardiac pause. As soon as blood escapes from the VP into the aorta and the pulmonary artery, the pressure in them drops. A retrograde current develops from these vessels back into the ventricles, which is quickly suppressed by valve opening. But at this time, the atrial hydrostatic pressure is higher than the ventricular pressure, and the AVK are forced to open. As a result, on the pressure difference, at the moment when the ventricular systole has passed, but it hasn’t come in the atria, ventricular filling takes place.
This period is also called a general cardiac pause, which lasts until the pressure in the cavities of the ventricles (RV) and atria (PP) of the corresponding side is equalized. As soon as this happened, atrial systole sets in to push the remaining portion of the blood into the pancreas. After it, when the remainder of the blood is squeezed into the ventricular cavity, the pressure in the PP decreases. This causes a passive blood flow: in the left atrium, a venous discharge from the pulmonary veins is carried out, and in the right - from the hollow.
Systemic Cardiac Cycle
The cycle of cardiac activity begins with a ventricular systole - the expulsion of blood from their cavities together with the simultaneous diastole of the atria and the beginning of their passive filling on the pressure difference in the delivery vessels, where at this moment it is higher than in the atria. After ventricular systole, a general cardiac pause occurs - the continuation of passive atrial filling with negative pressure in the ventricles.
Due to the higher hemodynamic pressure in the PP and low in the pancreas, 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 is equalized, passive current becomes impossible, and atrial replenishment ceases, which causes their reduction in order to pump an additional portion into the ventricular cavities.
From the atrial systole, the pressure in the ventricular cavities significantly increases, the ventricular systole is provoked - the muscle contraction of its myocardium. The result is increased pressure in the cavities and closure of the atrio-ventricular 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 bloodstream. This completes the cardiac cycle: the heart again begins to passively fill the atria into their diastole and then at the time of the general cardiac pause.
Heart pauses
In the work of the heart there are many episodes of rest: diastole in the atria and ventricles, as well as a general pause. Their duration can be calculated, although it very much depends on the heart rate. At 75 beats / min, the cardiac cycle time will be 0.8 seconds. Atrial systole (0.1 s) and ventricular contraction - 0.3 seconds are included in this period. This means that the atria rest for about 0.7 s, and the ventricles - 0.5. During rest, a general pause (0.5 s) is included.
About 0.5 seconds, the heart is passively filled, and 0.3 seconds is reduced. Atria have a relaxation time of 3 times longer than in the ventricles, although similar volumes of blood are pumped by them. However, they mostly enter the ventricles by passive current along a pressure gradient. Blood by gravity at a moment of low pressure in the cardiac cavities enters the cavity, where it accumulates for subsequent contraction and expulsion into the efferent vessels.
The meaning of periods of relaxation of the heart
Blood enters the cavity of the heart through the openings: in the atria through the mouths of the vena cava and pulmonary veins, and into the ventricles through the AVC. Their capacity is limited, and the actual filling takes longer than expelling it in the circulation. And the phases of the cardiac cycle are just those that are needed to sufficiently fill the heart. The smaller these pauses, the less the atria will be filled, the less blood will be sent to the ventricles and, accordingly, in 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 to a certain limit. Upon reaching a high frequency of contractions, the filling of the cavities due to the extremely short diastole will significantly fall, as well as 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 body's resources, then atrial fibrillation, supraventricular and ventricular tachycardia, ventricular fibrillation, as well as ventricular tachysystole with WPW syndrome lead to a drop in pressure.
The manifestation of the patient's complaints and the severity of his condition begins 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 muscles of the atria contract at the same time in accordance with the rhythm generated by the sinus node. Its importance lies in pumping approximately 15% of the blood in 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 into the atrial diastole. And only 12 ml is pumped by atrial systole, which allows you to increase the pressure level in order to close the valves during the period of 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 whole atrial systole. Because of this, arrhythmia makes a negative hemodynamic contribution - it depletes blood flow into the ventricular cavities by about 15-20%. Their filling is carried out by gravity with a general cardiac pause and during the period of ventricular systole. That is why a certain portion of the blood portion always lingers in the atria and constantly shakes, repeatedly increasing the risk of thrombosis in the circulatory system.
The retention of blood in the cavities of the heart, and in this case in the atria, leads to their gradual distension and makes it impossible to maintain a rhythm with successful cardioversion. Then arrhythmia will become constant, which accelerates the development of cardiac insufficiency with stagnation and hemodynamic disturbances in the circulatory system by 20-30%.
Ventricular systole phase
With a cardiac cycle duration of 0.8 s, the ventricular systole will be 0.3 - 0.33 seconds with two periods - stress (0.08 s) and expulsion (0.25 s). The myocardium begins to contract, but its efforts are not enough to squeeze blood from the ventricular cavity. But the pressure created already allows you to close the atrial valves. The phase of exile occurs at a time when systolic pressure in the ventricular cavities allows you to push out a portion of the blood.
The voltage phase in the cardiac cycle is divided into a period of asynchronous and isometric contraction. The first lasts about 0.05 s. and is the beginning of a whole reduction. An asynchronous (erratic) contraction of myocytes develops, which does not lead to an increase in pressure in the ventricular cavity. Then, after covering the entire mass of the myocardium with excitation, an isometric contraction phase is formed. Its importance lies in a significant increase in pressure in the ventricular cavity, which allows you to close the atrio-ventricular valves and prepare to push blood into the pulmonary trunk and aorta. Its duration in the cardiac cycle is 0.03 seconds.
The period of exile of the phase of ventricular systole
Ventricular systole proceeds to expel blood in 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 the maximum systolic, and muscle contraction pushes a portion of approximately 70% of the actual volume from their cavity. The second phase is slow expulsion (0.13 s.): The heart pumps the remaining 30% of the systolic volume into the efferent vessels, however, this happens even with a decrease in pressure, which precedes the ventricular diastole and a general cardiac pause.
Ventricular diastole phase
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 is made up of two phases - fast (0.08 sec) and slow (0.17 sec).
In the protodiastolic period (0.04 s.), The transitional stage between the systole and diastole of the ventricles, the pressure in the ventricular cavities drops, which causes the closure of the aortic and pulmonary valves. In the second phase, a period of zero pressure begins in the ventricular cavities with the valves closed at the same time.
During the period of rapid filling, the atrioventricular valves instantly open, and the blood flows along the pressure gradient into the ventricular cavities from the atria. In this case, the cavities of the latter are still constantly supplemented by the inflow along the bringing veins, which is why with a smaller volume of the atrial cavities, similar portions of blood, like the ventricles, are still pumped. After, due to the peak pressure in the ventricular cavities, the inflow slows down and a slow phase begins. It will result in atrial contraction, which occurs in the ventricular diastole.