Cardiac Cycle Introduction .
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.
Division & 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.
Events of Cardiac Cycle.
Events of cardiac cycle are classified into two:
1. Atrial events
2. Ventricular events.
Atrial Events of Cardiac Cycle.
Atrial events are divided into two divisions:
1. Atrial systole = 0.11 (0.1) sec.
2. Atrial diastole = 0.69 (0.7) sec.
Ventricular Events of Cardiac cycle.
Ventricular events are divided into two divisions:
1. Ventricular systole = 0.27 (0.3) sec.
2. 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.
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.
Atrial systole is also known as last rapid filling phase or pre-systole. 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.
Atrial Diastole.
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). 11This 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 0.5sec (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 of Cardiac Cycle.
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.
Ejection Period .
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.
End-systolic Volume.
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 end systolic volume. It is 60 to 80 mL per ventricle.
Measurement of end-diastolic volume.
End systolic volume is measured by radionuclide angiocardiography (multi-gated acquisition – MUGA scan) and echocardiography. It is also measured by cardiac catheterization, computed tomography (CT) scan and magnetic resonance imaging (MRI) .
Ejection Fraction.
Ejection fraction refers to the fraction (or portion) of end diastolic volume (see below) 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 (Ef) 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).
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 .
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 filling 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 place 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 filling becomes slow. Now, it is called the slow filling. It is also called diastasis. About 20% of filling occurs in this 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.
End-diastolic volume is the amount of blood remaining in each ventricle at the end of diastole. It is about 130 to 150 mL per ventricle.
Measurement of end-diastolic volume.
End-diastolic volume is measured by the same methods, which are used to measure end-systolic volume .
Intra-atrial Pressure changes during cardiac Cycle.
Pressure in the atria is called the intra-atrial pressure. Intra-atrial pressure is responsible for opening of the atrioventricular valves and ventricular filling. It is also the main factor for the development of venous pulse.
Method of Study for Intra-atrial pressure.
Right atrial pressure is recorded directly by cardiac catheterization . Left atrial pressure is determined indirectly by measuring pulmonary capillary wedge pressure, which reflects the left atrial pressure accurately.
Pulmonary Capillary Wedge Pressure.
Pulmonary capillary wedge pressure is the pressure exerted in the pulmonary capillary bed after obstructing the proximal part of pulmonary artery. Pulmonary capillary wedge pressure is measured by using a balloon tipped multilumen cardiac catheter. (Swan-Ganz catheter). Tip of the catheter is not open but a pressure transducer is attached to it. By means of venous puncture, the catheter is guided through right atrium into right ventricle.
From the right ventricle, it is advanced towards the proximal portion of pulmonary artery and the balloon is inflated with air by using a syringe. This occludes the pulmonary artery. Then, the catheter alone is advanced further into distal portion of pulmonary artery, leaving the inflated balloon at the proximal portion. It allows the catheter to float in a wedge position. Now the pressure existing in the pulmonary capillary bed ahead of catheter is called pulmonary capillary wedge pressure (the word wedge refers to being obstructed).
When the proximal part of pulmonary artery is obstructed, pressure in the distal part falls rapidly and after about 10 seconds, it becomes equal to left atrial pressure. It is because of the absence of any valve between pulmonary capillary bed and left atrium. So, the left atrial pressure can be determined by measuring pulmonary capillary wedge pressure.
Maximum and minimum pressures in the left and right atria .
Area Maximum pressure Minimum pressure
Left atrium 7 to 8 mm Hg 0 to 2 mm Hg.
Right atrium 5 to 6 mm Hg 0 to 2 mm Hg.
Left ventricle 120 mm Hg 5 mm Hg.
Right ventricle 25 mm Hg 2 to 3 mm Hg.
Systemic aorta 120 mm Hg 80 mm Hg.
Pulmonary artery 25 mm Hg 7 to 8 mm Hg.
Intra-atrial Pressure Curve.
Intra-atrial pressure curve is similar to the tracing of jugular venous pulse, which is known as phlebogram. It has three positive waves, a, c and v and three negative waves, x, x1 and y .
‘a’ Wave.
‘a’ wave is the first positive wave and occurs during atrial systole. The pressure rises sharply up to 5 mm Hg in right atrium and 7 mm Hg in left atrium. After reaching the peak, the pressure starts decreasing.
‘x’ Wave.
‘x’ wave is the first negative wave and appears during the onset of atrial diastole. Because of relaxation of atria, the pressure falls. Atrioventricular valves close at the end of this wave.
‘c’ Wave.
‘c’ wave is the second positive wave and this appears during isometric contraction. Rise in pressure is due to the closure of atrioventricular valves and the increased intraventricular pressure. When atrioventricular valves close, there is a little back flow of blood towards atria. When the intraventricular pressure increases, there is bulging of AV valves into the atria. Because of these two factors, the atrial pressure rises.
‘X1′ Wave.
‘X1 ’ wave is the second negative wave and appears during ejection period. During ejection period, the contraction of ventricular musculature pulls the atrioventricular ring towards the ventricles. This causes fall in atrial pressure.
‘v’ Wave.
‘v’ wave is the third positive wave, which is obtained during atrial diastole. It shows a gradual increase in atrial pressure due to filling of blood in atria (venous return).
‘y’ Wave.
‘y’ wave is the third negative wave and appears after the opening of AV valves when the blood rushes from atria into ventricles. So, the pressure in the atria falls.
Intraventricular Pressure Changes During Cardiac Cycle.
Intraventricular pressure is the pressure developed inside the ventricles of the heart. It is essential for the circulation of blood, because the flow of blood through systemic and pulmonary circulation depends upon the pressure at which the blood is pumped out of ventricles.
Thus, intraventricular pressure is essential for the circulation of blood.
Method of Study of Intraventricular pressure.
Intraventricular pressure is measured by cardiac catheterization.
Maximum & Minimum Pressure in Ventricles .
There is some difference in the pressure in right ventricle and left ventricle. The pressure is always more in left ventricle than in the right ventricle.
Intraventricular Pressure Curve.
Intraventricular pressure curve has seven segments.
‘A-B’ Segment.
‘AB’ segment is a positive wave and appears during atrial systole. Rise in pressure during this period is due to the entry of a small amount of blood into the ventricles because of atrial systole. The pressure rises to about 6 to 7 mm Hg in the right ventricle and to about 7 to 8 mm
Hg in the left ventricle. ‘B’ indicates the closure of atrioventricular valves.
‘B-C’ Segment.
‘BC’ segment appears during isometric contraction. During isometric contraction period, there is a sharp rise in the intraventricular pressure.
‘C’ denotes the opening of semilunar valves.
‘C-D’ Segment.
‘CD’ segment appears during ejection period. During ejection period, the pressure in the ventricles rises to the peak and then falls down. First part of the curve indicates the maximum ejection and the pressure increases to the maximum. Second part of the curve represents the slow
ejection phase when the pressure decreases.
Maximum pressure rise in right ventricle is about 25 mm Hg and the maximum pressure rise in left ventricle is about 120 mm Hg, during the peak of this wave. Maximum pressure in the left ventricle is 4 to 5 times more than that in the right ventricle, because of the thick wall of the left ventricle.
‘D-E’ Segment.
‘DE’ segment appears during protodiastole. Pressure decreases slightly due to the starting of ventricular relaxation. ‘E’ indicates the closure of semilunar valves.
‘E-F’ Segment.
‘EF’ segment is obtained during isometric relaxation. There is a sharp fall in the intraventricular pressure during this phase. Pressure in the ventricle falls below the pressure in the atria and this causes the opening of atrioventricular valves. ‘F’ represents the opening of atrioventricular valves.
‘F-G’ Segment.
‘FG’ segment appears during rapid filling phase. In spite of filling of blood, pressure decreases in the ventricles. It is because of the relaxation of the ventricles.
‘G-A’ Segment.
‘GA’ segment is the last part of intraventricular pressure curve. It is obtained during slow filling phase. Because of continuous relaxation of ventricles during slow filling period, the ventricular pressure decreases further.
Aortic Pressure Changes During Cardiac Cycle.
Aortic pressure is the pressure developed in the aorta. It is necessary to maintain the blood flow through the circulatory system.
Method of Study for Aortic Pressure.
Changes in aortic pressure during the cardiac cycle are recorded by using catheter.
Maximum & Minimum Pressure in Aorta.
Pressure in systemic aorta is always higher than that of pulmonary artery. It is because of the higher pressure in left ventricle than in the right ventricle. Minimum pressure in systemic aorta is much greater than the minimum pressure in the left ventricle. It is due to the presence of elastic tissues in the aorta, which enable the aorta to recoil and maintain the minimum pressure at a higher level.
Aortic Pressure Curve .
During the ejection period of the cardiac cycle, the pressure in the aorta increases and reaches the peak . During diastole, it reduces gradually and reaches the minimum level. At the time of closure of semilunar valves, an incisura occurs due to back flow of some blood towards the ventricles .
Ventricular Volume Changes During cardiac Cycle.
Volume of blood in the ventricles is an important factor to maintain cardiac output and blood circulation.
Method of Study for Ventricular Volume Changes.
1. By using Henderson Cardiometer.
This study is done only in animals. Cardiometer is a cupshaped device with an outlet. At the top, it is closed by means of a rubber diaphragm. A small hole is made in the diaphragm, through which the ventricles of the animal are pushed.
Cardiometer is connected to a recording device like Marey tambour (a small stainless steel capsule covered by rubber membrane) or polygraph, to record the volume changes.
2. By Angiography.
Angiography is the radiographic study of heart and blood vessels using a radiopaque contrast medium. During angiography, it is possible to measure the ventricular dimensional area and thickness of ventricular wall. From the values obtained, the ventricular volume is calculated.
Volume of Blood in right & Left Ventricles.
End-diastolic Volume and End-systolic Volume.
Amount of blood is the same in both right and left ventricles. Maximum volume of blood in each ventricle after filling (end-diastolic volume) is 130 to 150 mL. Minimum volume of blood left in the ventricles at the end of ejection period (end of systolic volume) is 60 to 80 mL.
Ejection Fraction.
Ejection fraction (Ef) is the stroke volume divided by end-diastolic volume, expressed in percentage.
Ventricular Volume curve .
Ventricular volume curve recorded by using Henderson Cardiometer has seven segments .
‘A-B’ Segment.
‘AB’ segment wave is because of atrial systole or last filling phase of ventricles, during which a small amount of blood enters the ventricles from the atria. It increases the ventricular volume slightly. ‘B’ indicates the closure of atrioventricular valves.
‘B-C’ Segment.
‘BC’ segment is a positive wave, which is obtained during isometric contraction. Actually, the ventricular volume is not altered during isometric contraction. However, the slight upward deflection of this wave is an artifact. It is because the heart thrusts itself into the Cardiometer during isometric contraction. ‘C’ represents the opening of semilunar valves.
‘C-D’ Segment.
‘CD’ segment occurs during ejection period. Initially, there is a sharp fall in the ventricular volume. This occurs during rapid ejection. Later, during slow ejection period, the blood leaves the ventricles slowly. So the ventricular volume decreases slowly.
‘D-E’ Segment.
‘DE’ segment part of the ventricular volume curve is recorded during protodiastole. There is no change in the ventricular volume during protodiastole. ‘E’ denotes the closure of semilunar valves.
‘E-F’ Segment.
‘EF’ segment appears during isometric relaxation period of the cardiac cycle. Actually, the ventricular volume is not altered during isometric relaxation. However, there is a slight upward deflection in the curve due to artifact. It is because of the entrance of blood into coronary artery from aorta during this period. It increases the pressure within the Cardiometer. ‘F’ indicates the opening of atrioventricular valves.
‘F-G’ Segment.
‘FG’ segment appears during rapid filling phase. Rapid rise in ventricular volume is due to sudden rush of blood after the opening of atrioventricular valves.
‘G-A’ Segment.
‘GA’ segment is recorded during slow filling phase. Ventricular volume increases slowly because of slow filling.
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