Cardiac Cycle
By
Dr. Khaled Ibrahim Khalil
Objectives:
By the end of this lecture, you should :
Describe events in cardiac cycle.
Describe atrial, ventricular and aortic pressure changes during cardiac
cycle.
Describe the changes in ventricular volume & stroke volume during
cardiac cycle.
Relate ECG changes to the phases of cardiac cycle.
Describe the functions of cardiac valves and relate their state to the
production of heart sounds during cardiac cycle.
References: Textbook of Medical Physiology by Guyton 12th
ed. Pages: 104-107.
THE CARDIAC CYCLE
Definition:
 It is the cardiac events that occur from the beginning of one
heart beat to the beginning of the next beat.
 These events consists of periods of contraction called "systole"
and a period of relaxation called "diastole".
Duration:
 Assuming a heart rate of 75 beat/min. the average duration of
each cycle is 0.8 (60 / 75) second.
Phases of the cardiac cycle:
I- Atrial systole. (during which the ventricle is relaxed)
II- Ventricular systole, (during which the atrium is relaxed)
III- Ventricular diostole, (during which the atrium is relaxed) (i.e.,
relaxation of the whole heart).
Phases of the cardiac cycle:
I- Atrial systole. (during which the ventricle is relaxed)
II- Ventricular systole, (during which the atrium is relaxed)
It occurs in 3 phases :
a- Isometric (or isovolumetric) contraction phase.
b- Maximum ejection phase.
c- Reduced ejection phase.
III- Ventricular diostole, (during which the atrium is relaxed) (i.e.,
relaxation of the whole heart). It occurs in 3 phases:
a- Isometric (or isovolumetric) relaxation phase.
b- Rapid filling phase.
c- Reduced filling phase.
I- Atrial Systole
Duration
0.1 second
Atrial pressure
Increases temporarily from zero to 2 mmHg due to
atrial contraction. By the end of this phase, the
pressure returns back to zero due to relaxation of the
atrium and evacuation of blood into the ventricles.
The constriction of the circular muscle sleeve present
around the orifices (openings) of the superior and
inferior venae cavae and pulmonary veins prevents
blood regurgitation into these veins.
A-V valve
opened
↑slightly due to rush of blood from the atria, then
Intraventricular pressure. decreases again as the ventricles are still relaxed (it
dilates).
I- Atrial Systole
Ventricular volume.
Increases slightly due to entry of blood from
the atria into the ventricles.
Semilunar valves
closed
Aortic pressure
Decreases gradually due to continuous
flow of blood into the peripheral
circulation.
Heart sounds.
The 4th heart sound occurs in this phase.
This sound is normally inaudible, but can
be recorded by the phonocardiogram.
Electrocardiogram (ECG).
The P wave starts 0.02 second before
atrial systole.
II- Isometric contraction phase
Duration
0.05 second
Atrial pressure
Shows slight, but sharp increase due to sudden
closure of AV valve and ballooning of its cusps
towards the cavity of the atrium by the sudden
rise of intraventricular pressure.
A-V valve
Closes suddenly
Intraventricular pressure
As ventricular systole starts, the Ventricular
pressure very rapidly exceeds the atrial pressure,
leading to sudden closure of the AV valves.
Now, all the valves are closed and the ventricle
becomes as a closed chamber. So, the ventricle
contract isometrically. i.e without change in the
length of the muscle fibers. Intraventricular
pressure↑ from zero to 80 mm Hg in the left
ventricle.
II- Isometric contraction phase
Ventricular volume.
No change
Semilunar valves
Still closed
Aortic pressure
Is still decreasing and the aortic valve is
still closed.
Heart sounds.
Electrocardiogram (ECG).
Early part of the 1st heart sound is present
which is mainly due to sudden closure of
AV valves.
The Q wave starts 0.02 second before this
phase, and the remaining part of the Q R S
complex occurs during it.
III- Maximum Ejection Phase
IV- Reduced Ejection Phase
Duration
0.15 second
0.1 second
Atrial pressure
Shows a sharp decrease followed by
Shows gradual increase due to
gradual increase. The decrease is due continuous accumulation of
to shortening of ventricular muscle
venous return.
(by systole), pulling down the AV ring,
increasing atrial capacity and so
decreasing the atrial pressure. The
gradual increase of atrial pressure is
due to:
- Accumulation of venous blood in
the atria.
- Upward displacement of AV ring to
its normal position.
A-V valves
Remain closed
Ventricular
volume
↓rapidly due to ejection of Still ↓ing.
most of ventricular blood into
the aorta.
Remain closed
III- Maximum Ejection
Phase
IV- Reduced Ejection Phase
Markedly ↑ as a result of Slightly ↓ due to reduced
continuous contraction of force of pumping of blood
Intraventricular ventricular muscle. The into aorta.
pressure
ventricular pressure is
slightly higher than the
aortic pressure.
Semilunar
valves
Aortic pressure
opened
Still opened
Is increased due to ejection of
blood from the left ventricle.
The amount of blood entering
the aorta exceeds the amount
leaving it to the peripheral
circulation. So, the aortic
pressure increases, but remains
lower than the ventricular
pressure.
Drops slightly because the blood
leaving the aorta to the peripheral
circulation is greater than the
blood pumped into the aorta from
the ventricle.
III- Maximum Ejection IV- Reduced Ejection Phase
Phase
Heart sounds.
1st
Heart
sound No sounds are produced
continues, due to
continuous flow of
blood
from
the
ventricles to the aorta
causing vibration of its
walls
Electrocardiogram
(ECG)
The T wave starts in The top of T wave
the late part of this
phase
V- Isometric Relaxation Phase
Duration
0.06 second
Atrial pressure
Still increasing due to continuous venous return
A-V valve
Remain closed
Intraventricular
pressure.
Ventricular volume.
↓rapidly. The ventricle is now a completely
closed chamber. So, it relaxes isometrically i.e
without changing the length of its muscle
fibers. Therefore, there is no change in volume
but the pressure rapidly falls towards the zero
line.
No change
V- Isometric Relaxation Phase
Aortic pressure
Shows an initial sharp decrease due to sudden closure
of the aortic valve, called the “diacrotic notch”. This is
followed by secondary rise of pressure due to the elastic
recoil of the aorta. It is called the “diacrotic wave”.
semilunar valves
Closes suddenly
Heart sounds.
The 2nd heart sound is present, due to sudden
closure of the aortic valve.
Electrocardiogram
(ECG).
T wave ends during this phase.
VI- Rapid Filling Phase
VII- Reduced
Phase
Duration
0.1 second
0.2 second
Atrial pressure
At the beginning of this phase, Around zero or still increasing
atrial pressure is more than the due to continuous venous
ventricular pressure leading to return.
opening of the AV valve and
rushing of blood by its weight
into the relaxed ventricle. This
leads to rapid ventricular filling
and decrease in the atrial
pressure.
(A-V) valve
opened
Still opened
Intraventricular
pressure.
Around zero
Around zero line but
below atrial pressure.
Ventricular volume
Marked ↑ due to rapid gradual increase
ventricular filling with
blood from the atria.
Filling
VI- Rapid Filling Phase
Aortic pressure
State of
valves
VII- Reduced
Phase
Filling
Gradually decreases due to still decreasing.
continuous escape of blood
to
the
peripheral
circulation.
semilunar closed
Still closed
Heart sounds.
The 3rd heart sound is No sound is present
present
Electrocardiogram
(ECG).
U wave
present
may
be P wave, for the next
cardiac cycle begins.
N.B.:
Systolic B.P. in the left ventricle
= 130 mmHg
Diastolic B.P in the left ventricle
= zero
Systolic B. P. in the right ventricle = 35 mm Hg
Diastolic B. P. in the right ventricle = zero
Systolic B. P. in the aorta
= 120 mm Hg
Diastolic B. P. in the aorta
= 80 mm Hg
Systolic B. P. in pulmonary artery
= 30 mm Hg
Diastolic B. P. in pulmonary artery = 10 mm Hg