Cardiac cycle is defined as the succession of (sequence of) coordinated events taking place in the heart during each beat. Each heartbeat consists of two major periods called systole and diastole. During systole, heart contracts and pumps the blood through arteries. During diastole, heart relaxes and blood is filled in the heart. All these changes are repeated during every heartbeat, in a cyclic manner.
Events Of Cardiac Cycle
Events of cardiac cycle are classified into two:
- Atrial Events
- Ventricular Events
Divisions And Duration Of Cardiac Cycle
When the heart beats at a normal rate of 72/minute, duration of each cardiac cycle is about 0.8 second.
1- Atrial Events
Atrial events are divided into two divisions:
- Atrial Systole = 0.11 (0.1) sec
- Atrial Diastole = 0.69 (0.7) sec
2- Ventricular Events
Ventricular events are divided into two divisions:
- Ventricular systole = 0.27 (0.3) sec
- Ventricular Diastole = 0.53 (0.5) sec
In clinical practice, the term ‘systole’ refers to ventricular systole and ‘diastole’ refers to ventricular diastole. Ventricular systole is divided into two subdivisions and ventricular diastole is divided into five subdivisions.
1- Ventricular Systole
- Isometric Contraction = 0.05 sec
- Ejection Period = 0.22 sec
- Total Time = 0.27 sec
2- Ventricular Diastole
- Protodiastole = 0.04 sec
- Isometric Relaxation = 0.08 sec
- Rapid Filling = 0.11 sec
- Slow Filling = 0.19 sec
- Last Rapid Filling = 0.11 sec
- Total Time = 0.53 sec
Among the atrial events, atrial systole occurs during the last phase of ventricular diastole. Atrial diastole is not considered as a separate phase, since it coincides with the whole of ventricular systole and earlier part of ventricular diastole.
Description Of Atrial Events
Atrial systole is also known as last rapid filling phase or presystole. It is usually considered as the last phase of ventricular diastole. Its duration is 0.11 second.
During this period, only a small amount, i.e. 10% of blood is forced from atria into ventricles. Atrial systole is not essential for the maintenance of circulation. Many persons with atrial fibrillation survive for years, without suffering from circulatory insufficiency. However, such persons feel difficult to cope up with physical stress like exercise.
Pressure And Volume Changes
During atrial systole, the intra-atrial pressure increases. Intraventricular pressure and ventricular volume also increase but slightly.
Fourth Heart Sound
Contraction of atrial musculature causes the production of fourth heart sound.
After atrial systole, the atrial diastole starts. Simultaneously, ventricular systole also starts. Atrial diastole lasts for about 0.7 sec (accurate duration is 0.69 sec). This long atrial diastole is necessary because, this is the period during which atrial filling takes place. Right atrium receives deoxygenated blood from all over the body through superior and inferior venae cavae. Left atrium receives oxygenated blood from lungs through pulmonary veins.
Atrial Events Vs Ventricular Events
Out of 0.7 sec of atrial diastole, first 0.3 sec (0.27 sec accurately) coincides with ventricular systole. Then, ventricular diastole starts and it lasts for about 0.5 sec (0.53 sec accurately). Later part of atrial diastole coincides with ventricular diastole for about 0.4 sec. So, the heart relaxes as a whole for 0.4 sec.
Description Of Ventricular Events
Isometric Contraction Period
Isometric contraction period in cardiac cycle is the first phase of ventricular systole, It lasts for 0.05 second. Isometric contraction is the type of muscular contraction characterized by increase in tension, without any change in the length of muscle fibers. Isometric contraction of ventricular muscle is also called isovolumetric contraction.
Immediately after atrial systole, the atrioventricular valves are closed due to increase in ventricular pressure. Semilunar valves are already closed. Now, ventricles contract as closed cavities, in such a way that there is no change in the volume of ventricular chambers or in the length of muscle fibers. Only the tension increases in ventricular musculature.
Because of increased tension in ventricular musculature during isometric contraction, the pressure increases sharply inside the ventricles.
First Heart Sound
Closure of atrioventricular valves at the beginning of this phase produces first heart sound.
Significance Of Isometric Contraction
During isometric contraction period, the ventricular Pressure increases greatly. When this pressure increases above the pressure in the aorta and pulmonary artery, the semilunar valves open. Thus, the pressure rise in ventricle, caused by isometric contraction is responsible for the opening of semilunar valves, leading to ejection of blood from the ventricles into aorta and pulmonary artery.
Due to the opening of semilunar valves and isotonic contraction of ventricles, blood is ejected out of both the ventricles. Hence, this period is called ejection period. Duration of this period is 0.22 second. Ejection period is of two stages:
1- First Stage Or Rapid Ejection Period
First stage starts immediately after the opening of semilunar valves. During this stage, a large amount of blood is rapidly ejected from both the ventricles. It lasts for 0.13 second.
2- Second Stage Or Slow Ejection Period
During this stage, the blood is ejected slowly with much less force. Duration of this period is 0.09 second.
Ventricles are not emptied at the end of ejection period and some amount of blood remains in each ventricle. Amount of blood remaining in ventricles at the end of ejection period (i.e. at the end of systole) is called endsystolic volume. It is 60 to 80 mL per ventricle.
Measurement of End-Diastolic Volume
End-systolic volume is measured by radionuclide angiocardiography (multigated acquisition — MUGA scan) and echocardiography. It is also measured by cardiac catheterization, computed tomography (CT) scan and magnetic resonance imaging (MRI).
Ejection fraction refers to the fraction (or portion) of end-diastolic volume that is ejected out by each ventricle per beat. From 130 to 150 mL of end-diastolic volume, 70 ml is ejected out by each ventricle (stroke volume). Normal ejection fraction is 60% to 65%.
Determination Of Ejection Fraction
Ejection fraction (E) is the stroke volume divided by end-diastolic volume expressed in percentage. Stroke volume (SV) is end-diastolic volume (EDV) minus end-systolic volume (ESV). Ejection fraction is calculated as:
E(f) = SV/EDV = EDV – ESV / EDV
E(f) = Ejection Fraction
SV = Stroke Volume
EDV = End-diastolic Volume
ESV = End-systolic Volume
Significance Of Determining Ejection Fraction
Ejection fraction is the measure of left ventricular function. Clinically, it is considered as an important index for assessing the ventricular contractility. Ejection fraction decreases in myocardial infarction and cardiomyopathy.
Protodiastole is the first stage of ventricular diastole, hence the name protodiastole. Duration of this period is 0.04 second. Due to the ejection of blood, the pressure in aorta and pulmonary artery increases and pressure in ventricles drops.
When intraventricular pressure becomes less than the pressure in aorta and pulmonary artery, the semilunar valves close. Atrioventricular valves are already closed. No other change occurs in the heart during this period. Thus, protodiastole indicates only the end of systole and beginning of diastole.
Second Heart Sound
Closure of semilunar valves during this phase produces second heart sound.
Isometric Relaxation Period
Isometric relaxation is the type of muscular relaxation, characterized by decrease in tension without any change in the length of muscle fibers. Isometric relaxation of ventricular muscle is also called isovolumetric relaxation.
During isometric relaxation period, once again all the valves of the heart are closed. Now, both the ventricles relax as closed cavities without any change in volume or length of the muscle fiber. Intraventricular pressure decreases during this period. Duration of isometric relaxation period is 0.08 second.
Significance Of Isometric Relaxation
During isometric relaxation period, the ventricular pressure decreases greatly When the ventricular pressure becomes less than the pressure in the atria, the atrioventricular valves open. Thus, the fall in pressure in the ventricles, caused by isometric relaxation is responsible for the opening of atrioventricular valves resulting in fitting of ventricles.
Rapid Filling Phase
When atrioventricular valves are opened, there is a sudden rush of blood (which is accumulated in atria during atrial diastole) from atria into ventricles. So, this period is called the first rapid filling period. Ventricles also relax isotonically. About 70% of filling takes during this phase, which lasts for 0.11 second.
Third Heart Sound
Rushing of blood into ventricles during this phase causes production of third heart sound.
Slow Filling Phase
After the sudden rush of blood, the ventricular becomes slow. Now, it is called the slow filling. It also called diastasis. About 20% of filling occurs in phase. Duration of slow filling phase is 0.19 second.
Last Rapid Filling Phase
Last rapid filling phase occurs because of atrial systole. After slow filling period, the atria contract and push a small amount of blood into ventricles. About 10% of ventricular filling takes place during this period. Flow of additional amount of blood into ventricle due to atrial systole is called atrial kick.
End-diastolic volume is the amount of blood remain in each ventricle at the end of diastole. It is about 130tc 150 mL per ventricle.
Measurement Of End-Diastolic Volume
End-diastolic volume is measured by the same me which are used to measure end-systolic volume.