CARDIAC CYCLE :-
INTRODUCTION :-
The series of synchronized processes that occur in the heart with each beat is known as the cardiac cycle. The two main phases of each heartbeat are known as the diastole and systole. The heart beats during systole, pumping blood via the arteries. The heart contracts and fills with blood during diastole. Every heartbeat repeats all of these alterations in a cycle.
Events of cardiac cycle :-
(1). changes in arterial chambers- Arterial events of cardiac cycle and (2). changes in ventricle chambers – Ventricles events of cardiac cycle
An average rhythm cycle takes roughly 0.8 seconds when the cardiac muscle beats at a regular pace of 72 beats per minute.
Arterial Event of Cardiac cycle :
1. Arterial systole- 0.1 second and 2. Arterial diastole- 0.7 second
Ventricle Event of cardiac cycle :
1. Ventricle systole- 0.3 second and 2. Ventricle diastole- 0.5 second
In clinical practice, ventricular diastole is commonly referred to as “diastole” and ventricular systole as “systole.” Atrial systole is one of the atrial events that transpires in the final stages of ventricular diastole. Since atrial diastole occurs during both the early portion of ventricular diastole and the entirety of the ventricular systole, it does not qualify as a distinct phase.
Cardiac cycle :-
Steps-
1. Atrial Systole
2. Isometric Contraction
3. Ejection Period
4. Protodiastole
5. Isometric Relaxation
6. Rapid and Slow filling
1. Arterial Systole of Cardiac cycle :-
“A tiny amount of blood entering the ventricles while the atria close.”
Pre-systole, or last rapid contraction phase, is another name for atrial systole. It is typically regarded as the final stage of diastole in the ventricles. It lasts for 0.11 seconds. At this point, only 10% percent the blood is forced into the ventricles from the atria. Atrial systole is necessary for the maintenance of circulation. Many persons with atrial fibrillation do not have circulatory insufficiency for extended periods of time. However, these individuals have trouble managing physical stress, such as working out.
Pressure, volume and Heart sound- The intraatrial pressure rises during atrial systole. The ventricular volume and pressure within the ventricle both modestly rise. The fourth beating of the heart is produced by the atrial musculature contracting.
2. Isometric Contraction period of cardiac cycle :-
“Every valve is closed and The isometric stretching of the ventricles results in a rise in intraventricular pressure.” The initial stage of the ventricular contractions is known as the isometric contraction. The duration is 0.05 seconds. Muscle contractions that are isometric are defined by an increase in tension but not by a change in the length of the muscle fibers. Another name for the isovolumetric contraction of ventricular muscle is isometric contraction.
The rise in ventricular blood pressure that follows atrial systole causes the atrioventricular valves to shut. Semilunar valves are closed at this point. Currently, the ventricular chambers’ volume and the muscle fibers’ length remain unchanged when the ventricles contract as closed cavities. Only the ventricular musculature’s tension rises. The pressure inside the ventricles raises dramatically during isometric contraction due to increased strain in the ventricular muscles.
First Heart Sound- A crucial part of cardiac auscultation, the first heart sound, or S1, is mainly linked to the closing of the atrioventricular, also called the AV, the valves, including involve both the mitral and tricuspid valves. Ventricular systole begins with a sound of this.
DISCRPTION- Throughout the isometric narrowing phase, the pressure inside the increases noticeably. When this pressure increases above the pressure in the aorta and pulmonary arteries, the semilunar valves open. As a result, the increase in ventricle pressure brought on by isometric contraction causes semilunar valves to open, causing blood to flow from the ventricles into the aorta and pulmonary artery.
3. Ejection Period of cardiac cycle :-
“Semilunar valve are opened. ventricles contract and blood is shot out out.”
Blood exits through both ventricles as a result of the semilunar valves opening and the ventricles contracting isotonically. Thus, the duration of this frame is known as the “ejection period.” This session lasts for 0.22 seconds.
This is the period when the ventricle pressure is at its highest. A C wave appears on the venous pulse curve as the right ventricle contracts and the tricuspid valve enlarges into the right atrium.
Whenever the pressure that exists inside the ventricle is higher than the pressure outside, the aortic valve opens. Rapid blood ejection into the channel is caused by pressure differences between the ventricle and the aorta. This phase causes a dramatic decrease in ventricular capacity since most of the stroke volume is being evacuated.
The atrial filling procedure begins.
When the T wave appears, it indicates that the ventricles have repolarized and that there is no longer any fast ventricular ejection or contraction.
End systolic volume- At the conclusion of the expelling period, ventricles are not completely empty, and some blood is still present in each ventricle. Endsystolic volume is the volume of blood in the ventricles at the conclusion of the ejection phase, or at the end of systole. A ventricle holds 60 to 80 milliliters.
Ejection fraction- The amount (or part) of enddiastolic volume (as described below) that is expelled out from each ventricle per beat is referred to as the ejection fraction. Each ventricle expels 70 mL (the stroke volume) from 130 to 150 mL of enddiastolic volume. An ejection fraction of 60% to 65% is typical. The left ventricle’s ejection fraction is used to assess its function. It is regarded as a crucial indicator for evaluating ventricular contractility in clinical settings. In cardiomyopathy and myocardial infarction, the ejection fraction falls.
4. Protodiastole period of cardiac cycle :-
“The diastole begins at this point. At the conclusion of this time, the semilunar valves close”
The term “protodiastole” refers to the initial phase of ventricular diastole. This period lasts for 0.04 seconds. Blood is ejected, which causes a drop in ventricular pressure and an increase in the aorta and pulmonary artery pressure.
The semilunar valves close when the intraventricular pressure drops below the pressure in the aorta and pulmonary artery. As mentioned previously, the valves of the atrioventricular system are already closed. The heart doesn’t undergo any more alterations throughout this time. Therefore, protodiastole merely represents the start of diastole and the completion of systole.
Second Heart Sound- The second heart sound, or S2, is a crucial component of cardiac auscultation that indicates the end of systole and the beginning of diastole. Second heart sound is produced during this phase when the semilunar valves close.
5. Isometric Relaxation Period of cardiac cycle :-
“All of the valves are closed. When the ventricles experience isometric relaxation, their internal pressure drops”
Muscle unwinding that is characterized by a decrease in pressure but not an modification in the length of the muscle strands is known as isometric unwinding. Another title for isovolumetric loosening up of ventricular muscle is isometric relaxation.
All of the heart’s valves are closed amid the isometric unwinding stage. At this point, not one or the other the length nor the volume of the muscle fiber changes as the ventricles unwind into closed chambers. A drop in intraventricular weight happens all through this time. The isometric unwinding stage endures approximately 0.08 seconds.
Description- Ventricular pressure significantly drops during the isometric relaxation phase. The atrioventricular valves are open when the blood pressure in the atria is greater than the pressure in the ventricles. Hence, the opening of the atrioventricular valves and subsequent filling of the ventricles are brought on by the decrease in pressure within the ventricles brought on by isometric relaxation.
6. Rapid and Slow filling Period of cardiac cycle :-
“The ventricles of the atria are opened. Ventricles open up and fill up”
A quick surge of blood flows from the atria into the ventricles (where it has accumulated during atrial diastole) when the atrionventricular valves open. Thus, the term “first rapid filling period” refers to this time frame. Moreover, ventricles relax isotonically. Approximately 70% of the filling occurs in this 0.11-second phase.
The rapid blood surge is followed by a slowdown in ventricular filling. It’s known as the sluggish filling these days. Another name for it is diastasis. In this stage, about 20% of filling happens. The sluggish filling phase lasts about 0.19 seconds.
The cause of the last rapidly expanding phase is atrial systole.
Following a phase of gradual filling, the atria constrict, forcing a tiny volume of blood into the ventricles. During this time, 10% of ventricular filling occurs. The term “atrial kick” refers to the extra blood flow into the ventricle caused by atrial systole.
Various times of Cardiac Cycle :-
It has been demonstrated that the cardiac cycle time and heart rate are inversely related. The phases of the cardiac cycle, however, do not all vary proportionately. Compared to the systole duration, the diastole duration varies considerably more. In this case, a heartbeat of 120 rhythms per minute has a cardiac cycle duration of 0.5 seconds. The systolic and diastolic times will be shortened to 0.23 and 0.27 seconds, respectively, for a cycle length of one second at a speed of sixty beats per minute. This will have a diastole of 0.67 seconds and a systole of 0.33 seconds. Therefore, the diastole decreases by 0.4 seconds but the systole only decreases by 0.1 seconds when the rate increases from 60 to 120 beats per minute. Stated differently, heart rate fluctuates more during diastole than during systole.