Cardiac cycle new

JOHNY WILBERT, M.SC[N]
LECTURER,
APOLLO INSTITUTE OF HOSPITAL
MANAGEMENT AND ALLIED SCIENCE

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:
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
Total = 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
Total0.53

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.

 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.
shows the correlation between atrial and
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 pressureincreases 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.
 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:
Ef =SV/ EDV=EDV- ESV/EDV
Ef = Ejection fraction
SV = Stroke volume
EDV = Enddiastolic volume
ESV = Endsystolic volume.
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

 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,

 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

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