Cardiac cycle is defined as the succession of (sequenceof) 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:
1. Atrial
events
2.
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.
ATRIAL EVENTS
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
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.
Ventricular Systole
Time (second)
1. Isometric
contraction = 0.05
2. Ejection period =
0.22
0.27
Ventricular Diastole
1. Protodiastole =
0.04
2. Isometric
relaxation = 0.08
3. Rapid filling =
0.11
4. Slow filling = 0.19
5. Last rapid filling
= 0.11
0.53
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 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 intraatrial 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). 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.
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 endsystolic volume. It is 60 to
80 mL per ventricle. Measurement of end-diastolic volume Endsystolic 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
Ejection
fraction refers to the fraction (or portion) of enddiastolic volume (see below)
that is ejected out by each ventricle per beat. From 130 to 150 mL of
enddiastolic 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
enddiastolic volume expressed in percentage. Stroke
volume
(SV) is, enddiastolic volume (EDV) minus endsystolic volume (ESV).
Ejection
fraction is calculated as:
SV EDV – ESV
Ef = =
EDV EDV
Where,
Ef =
Ejection fraction
SV = Stroke volume
EDV = Enddiastolic volume
ESV = Endsystolic 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
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
atrionventricular 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
Enddiastolic
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
Enddiastolic
volume is measured by the same methods, which are used to measure endsystolic
volume.
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