The cardiac cycle describes the sequence of events that occur with each heartbeat. It involves mechanical, electrical, and pressure changes in the heart. The cycle begins with ventricular relaxation and atrial filling in diastole. Upon electrical stimulation, the ventricles contract in systole to pump blood out of the heart. This is accompanied by characteristic pressure and volume changes in the chambers as well as events on an electrocardiogram like the P, QRS, and T waves. The cycle repeats with each heartbeat to sustain blood circulation.

1. Cardiac Cycle
Dr. Md. Mostafizur Rahman Bhuiyan
Resident (Phase: A)
Dept. of Paediatric Cardiology

2. Definition:
The cardiac events that
occur from the beginning
of one heartbeat to the
beginning of the next are
called the cardiac cycle.
Cardiac Cycle

3. Events in cardiac cycle :
 Electrocardiogram
 Mechanical changes
 Atrial systole & diastole
 Ventricular systole & diastole
 Pressure Changes
 Aortic pressure
 Ventricular pressure
 Atrial pressure
 Pulmonary artery pressure
 Volume changes
 Heart sounds
A Wiggers diagram, named after its developer, Dr. Carl J. Wiggers, is a standard diagram
used in teaching cardiac physiology. In the Wiggers diagram, the X axis is used to plot time,
while the Y axis contains all of the following on a single grid.

5. Relationship of the Electrocardiogram
to the Cardiac Cycle
• Each cardiac cycle is initiated by
spontaneous generation of an action
potential in the SA node.
The P wave
 Is caused by spread of depolarization
through the atria
 It is followed by atrial contraction
 It causes a slight rise in the atrial
pressure curve immediately after the P
wave.
About 0.16 s after the P wave, the QRS
appear

6. Relationship of the Electrocardiogram
to the Cardiac Cycle
QRS complex:
• Caused by electrical depolarization of
the ventricles
• It initiates contraction of the ventricles
i.e ventricular systole
T wave:
• It represents the stage of repolarization
of the ventricles when the ventricular
muscle fibers begin to relax.
• Therefore, the T wave occurs slightly
before the end of ventricular
contraction.

7. • A-V Valves Open; Semilunar Valves Closed
• Atrial systole (1): About 80% of blood normally
flows continually from the great veins into the
atria. Then, atrial contraction causes an
additional 20% filling of the ventricles.
• Atrial diastole : At the end of systole diastole
occurs and during this period blood enters into
atria from great veins.
• 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.
Mechanical events in Cardiac cycle

8. • Ventricular systole: starts at the
end of atrial sytole.
• Isovolumic contraction (2):
– Immediately after ventricular
contraction begins, the ventricular
pressure rises abruptly, causing the
A-V valves to close, produces the
1st heart sound
– during this period, ventricle
contracts but muscle does not
shorten and no blood passes out of
ventricle, thus pressure increases.
– It strarts from the closure of AV
valve and terminated by opening
of semilunar valve

9. • Period of rapid ejection (3) : the first third
of ejection is called the period of rapid
ejection as 70 per cent of the blood
emptying occurring during this time.
• Period of slow ejection (4): remaining 30
per cent emptying occurs during the last
two third of cardiac cycle known as the
period of slow ejection.
Period of Ejection:
When the left ventricular pressure rises slightly
above 80 mm Hg (and the right ventricular
pressure slightly above 8 mm Hg), the
ventricular pressures push the semilunar
valves open. Immediately, blood begins to
pour out of the ventricles.

10. • Ventricular diastole:
• Protodiastolic phase :
– pressure in ventricles
falls
– semilunar valve closes
due to high aortic
pressure
– produces 2nd heart
sound

11. Isovolumic relaxation (5) :
• interval between closure of semilunar valve and
opening of AV valve
• At the end of systole, ventricular relaxation begins
suddenly, allowing both the right and left
intraventricular pressures to decrease rapidly.
• The elevated pressures in the distended large
arteries that have just been filled with blood from
the contracted ventricles immediately push blood
back toward the ventricles, which snaps the aortic
and pulmonary valves closed.
• For another 0.03 to 0.06 second, the ventricular
muscle continues to relax, even though the
ventricular volume does not change, giving rise to
the period of isovolumic or isometric relaxation.
• During this period, the intraventricular pressures
decrease rapidly back to their low diastolic levels.
• Then the A-V valves open to begin a new cycle of
ventricular pumping.

12. • Rapid filling phase(6):
– ventricular pressure fall markedly and the
blood rushes from atria to ventricle.
• Blood was accumulated in the rt & lt atria during
ventricular systole for the closed A-V valves &
the moderately increased atrial pressure push
the A-V valves open and allow blood to flow
rapidly into the ventricles
– 3rd heart sound produces
• Slow filling phase /diastasis (7):
– amount of filling is minimum (blood from
the veins and passes through the atria
directly into the ventricles)
– ventricular pressure slowly rises
• Last rapid filling (Atrial systole):
– due to atrial contraction blood rushes from
atria to ventricle. It accounts for about 20
per cent of the filling of the ventricles during
each cardiac cycle.

13. Mechanical events in Cardiac cycle
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 Time VENTRICULAR EVENTS Time
Atrial systole 0.11
(0.1)
Ventricular systole 0.27
(0.3)
1.Isovolumetric
contraction
0.05
2. Ejection period 0.22
Atrial diastole
=
0.69
(0.7)
Ventricular diastole 0.53
(0.5)
1. Protodiastole 0.04
2. Isometric relaxation 0.08
3. Rapid filling 0.11
4. Slow filling /
diastasis
0.19
5. atrial systole / Last
rapid filling
0.11

14. 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.

15. Pressure Changes in the Atria
In the atrial pressure curve three minor elevations, called the
a, c, and v atrial pressure waves, are noted.
The a wave
• caused by atrial contraction.
• right atrial pressure increases 4 to 6 mm Hg during
atrial contraction, and the left atrial pressure
increases about 7 to 8 mm Hg.
The c wave
• occurs when the ventricles begin to contract;
• it is caused partly by slight backflow of blood into
the atria at the onset of ventricular contraction
• and mainly by bulging of the A-V valves backward
toward the atria because of increasing pressure in
the ventricles.
The v wave
• occurs toward the end of ventricular contraction;
it results from slow flow of blood into the atria
from the veins while the A-V valves are closed
during ventricular contraction

16. Pressure change in Ventricle:
Intra ventricular Pressure Change During
Ventricular Systole:
• When the heart is in diastole, pressure in
the systemic arteries averages about 80
mmHg.
• In Isometric contraction period,
intraventricular pressure sharply rises as
ventricle contracts in a closed cavity due to
closure of both AV & Semilunar valve.
• In maximum ejection phase, pressure in the lt
ventricle and aorta rises about 120 mmHg,
when blood passage out & volume decreases
• But the force of contraction is stronger than
out flow, so intraventricular pressure rises .

17. Pressure change in ventriular diastole:
• In protodiastolic phase, pressure continue
to fall.
• In isomertic relaxation phase, LV pressure
sharply falls blow the pressure in Aorta,
semilunar valves snap shut. The pressure
in the aorta falls to 80 mmHg, while
pressure in the left ventricle falls to 0
mmHg.
• Rapid filling phase: When the pressure in
the ventricles falls below the pressure in
the atria, the AV valves open and a phase
of rapid filling of the ventricles occurs.
• last rapid filling phase, corresponds with
atrial systole, blood pumps into ventricle
& pressure suddenly rises.
Pressure change in Ventricle:

18. Pressure changes during cardiac cycle
Pressures observed within cardiac chambers during systole and
diastole
Heart region Pressure (mmHg)
Right atrium 0-4
Right ventricle 25 systolic; 4 diastolic
Pulmonary artery 25 systolic; 10 diastolic
Left atrium 8-10
Left ventricle 120 systolic; 10 diastolic
Aorta 120 systolic; 80 diastolic

19. • During diastole, normal filling of the ventricles increases the volume of
each ventricle to about 110 to 120 milliliters. This volume is called the
end-diastolic volume.
• Then, as the ventricles empty during systole, the volume decreases
about 70 milliliters, which is called the stroke volume.
SV=EDV−ESV
• The remaining volume in each ventricle, about 40 to 50 milliliters, is
called the end-systolic volume.
• The fraction of the end-diastolic volume that is ejected is called the
ejection fraction - usually equal to about 60 per cent.
ejection fraction (EF) is used to evaluate stroke volume and
contractility. It is described as:
EF=(SV / EDV)×100%
A higher EF suggests more efficient heart activity.
Volume changes in ventricles:

20. Phonocardiogram
A graphic recording of cardiac
sound
A specially designed
microphone on the chest wall.
Sound waves amplified,
filtered and recorded.
Doppler Echocardiography has
replaced the
phonocardiography

21. 1st heart sound(lubb)
•The 1st heart sound, marks the
beginning of systole (end of diastole).
•Related to the closure of the mitral
and tricuspid valves.
2nd heart sound (Dub)
• The 2nd heart sound, marks the
end of systole (beginning of
diastole).
• Related to the closure of the aortic
and pulmonic valves
Heart sound in cardiac cycle

22. Heart sound in cardiac cycle
Third Heart Sound
• The third heart sound (S3) is a low-
pitched, early diastolic sound
audible during the rapid entry of
blood from the atrium to the
ventricleFourth Heart Sound
The fourth heart sound (S4)
• is a late diastolic sound that
corresponds to late ventricular
filling through active atrial
contraction. It is a low-intensity
sound heard best with the bell of
the stethoscope.