Download THE CARDIAC CYCLE

THE CARDIAC CYCLE
At the end of lecture, the student should be able to:
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Define what is cardiac cycle?
Describe the phases of cardiac cycle.
Describe the relationship of the electrocardiogram to the Cardiac
Cycle.
Tell us the Pressure Changes in the Atria.
Explain the Period of Isovolumic (Isometric) Relaxation.
Relationship of the Heart sounds with cardiac cycle.
Lecture Outline
Definition: CARDIAC CYCLE.
The cardiac events that occur from the beginning of one heartbeat to
the beginning of the next are called the cardiac cycle.
Each cycle is initiated by spontaneous
generation of an action potential in the sinus
node.
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Atria act as primer pumps for the ventricles.
Ventricles in turn provide the major source of
power for moving blood through the body’s
vascular system.
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Diastole:
A period of relaxation called diastole, during which
the heart fills with blood.
Systole:
A period of contraction called systole.
Phases of Cardiac Cycle:
It can be divided into four distinct phases:
(I) Contraction Phase and
(Ii) Ejection Phase, Both Occurring In
Systole;
(Iii) Relaxation Phase and
(Iv) Filling Phase, Both Occurring In Diastole.
At the end of phase IV, the atria contract.
ATRIAL SYSTOLE
Definition:
Atrial systole occurs in late part of
ventricular diastole,
 Main force driving blood from the atria
to the ventricles is the decrease in
ventricular pressure that occurs during
ventricular diastole.
 The drop in ventricular pressure that
occurs during ventricular diastole allows
the atrioventricular valve to open,
emptying the contents of the
atrium into the ventricle.
 Contraction of the atrium confers a
relatively minor, additive effect toward ventricular filling.
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Function of the Atria as Primer Pumps:
About 80 per cent of the blood flows directly
through the atria into the ventricles even before
the atria contract.
Then, atrial contraction usually causes an
additional 20 per cent filling of the ventricles.
Therefore, the atria simply function as primer
pumps that increase the ventricular pumping
effectiveness as much as 20 per cent.
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Relationship of the Electrocardiogram:
The P wave is caused by
spread of depolarization
through the atria, and this
is followed by atrial
contraction, which causes
a slight rise in the atrial
pressure curve
immediately after the
electrocardiographic P
wave.
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• The PR segment is
electrically quiet as the depolarization proceeds to the AV node.
This brief pause before contraction allows the ventricles to fill
completely with blood.
Pressure Changes in the Atria—The a Wave.
The a wave is caused by atrial contraction.
right atrial pressure increases 4 to 6 mm Hg during atrial contraction,
and
 Left atrial pressure increases about 7 to 8 mm Hg.
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Heart Sounds:
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Fourth heart sound (S4) is abnormal and is associated with the
end of atrial emptying after atrial contraction.
It occurs with hypertrophic congestive heart failure, massive
pulmonary embolism, tricuspid incompetence, or cor pulmonale.
Ventricular systole:
Atrio-ventricular delay in impulse conduction.
Phases:
1. Isovolumic contraction.
2. Rapid ejection - 70% emptying.
3. reduced/slow ejection - 30% emptying.
Period of Isovolumic (Isometric) Contraction:
The ventricular pressure rises
abruptly after the beginning of
ventricular contraction causing
the A-V valves to close.
 0.02 to 0.03 second is required
for the ventricle to build up
sufficient pressure to push the
semilunar valves open against the
pressures in the aorta and pulmonary artery.
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Therefore, contraction is occurring in the ventricles, but there is no
emptying so called the period of Isovolumic or isometric contraction,
meaning that tension is increasing in the muscle but little or no
shortening of the muscle fibers is occurring.
Pressure Changes:
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 but 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.
Relationship of the Electrocardiogram:
About 0.16 second after the onset of the P wave, the QRS waves appear
as a result of electrical depolarization of the ventricles, which initiates
contraction of the Ventricles and causes the ventricular pressure to
begin rising, as also shown in the figure. Therefore, the QRS complex
begins slightly before the onset of ventricular systole.
Heart Sounds
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The first heart sound (S1, "lub") is due to the closing AV valves and
associated blood turbulence.
Period of rapid ejection:
The first third of the period of
ejection is called the period of
rapid ejection because 70 per
cent of the blood emptying
occurs during this period.
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The semilunar (aortic and pulmonary) valves open at the beginning
of this phase.
While the ventricles continue
contracting, the pressure in the
ventricles (red) exceeds the
pressure in the aorta and
pulmonary arteries (green); the
semilunar valves open, blood
exits the ventricles, and the
volume in the ventricles
decreases rapidly (white).
As more blood enters the
arteries, pressure there builds
until the flow of blood reaches
a peak.
The Period Of Slow Ejection:
The last two thirds is the period of slow ejection
The remaining 30 per cent emptying occurs during the next two thirds
of the period of ejection.
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At the end of this phase the semilunar (aortic and pulmonary)
valves close.
After the peak in ventricular and arterial pressures (red and
green), blood flow out of the ventricles decreases and ventricular
volume decreases more slowly (white).
When the pressure in the ventricles falls below the pressure in the
arteries, blood in the arteries begins to flow back toward the
ventricles and causes the semilunar valves to close. This marks the
end of ventricular systole mechanically.
Relationship of the Electrocardiogram:
The ventricular T wave in the electrocardiogram 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.
Ventricular diastole:
Phases:
-isovolumic relaxation
- rapid filling
- slow filling
- atrial systole.
Period of Isovolumic (Isometric) Relaxation:
Ventricular relaxation begins suddenly.
At the end of systole, 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.
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.
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The second heart
sound (S2, "dup")
occurs when the
semilunar (aortic and
pulmonary) valves
close. S2 is normally
split because the
aortic valve closes
slightly earlier than
the pulmonary valve.
RAPID VENTRICULAR
FILLING
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Once the AV valves
open, blood that has
accumulated in the
atria flows rapidly
into the ventricles.
Ventricular volume
(white) increases
rapidly as blood flows
from the atria into
the ventricles.
SLOW REDUCED
VENTRICULAR FILLING:
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Rest of blood that has
accumulated in the
atria flows slowly into
the ventricles.
Ventricular volume
(white) increases
more slowly now. The ventricles continue to fill with blood until
they are nearly full.
References:
Text book of medical Physiology. GUYTON & HALL. Eleventh Edition.
Color Atlas of Physiology. Stefan Silbernagel, Agamemnon Despopoulos.
6th Edition.
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