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Electrical Activity
of the Heart
Different expression levels and
Different types of ion channels
University of Jordan
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Electrocardiogram
– ECG or EKG
– Composite record of
action potentials
produced by all the
heart muscle fibers
– Compare tracings
from different leads
with one another and
with normal records
– 3 recognizable
waves
• P, QRS, and T
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The Electrocardiogram
• The major deflections and intervals
in a normal ECG include:
– P wave - atrial depolarization
– P-Q interval - time it takes for
the atrial kick to fill the
ventricles
– QRS wave - ventricular depolarization
and atrial repolarization
– S-T segment - time it takes to empty the
ventricles before they repolarize (the T wave)
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Correlation of ECG Waves and
Contraction
 Systole means ventricular contraction
 Diastole means ventricular relaxation
Cardiac cycle events:
1. Cardiac action potential arises in the SA node → P
wave
2. Atrial systole (Atrial contraction)
3. Action potential enters AV bundle and leaves to the
ventricles → QRS complex which masks atrial
repolarization (un-recordable)
4. Contraction of ventricles (systole)
• Begins shortly after QRS complex appears and
continues during S-T segment
5. Repolarization of ventricular fibers → T wave
6. Ventricular relaxation (diastole)
Cardiac Cycle
Heart switching between
relaxed and contracted states
Time-dependent changes in:
Atrial Pressure
Aortic Pressure
Ventricular Pressure
Aortic Flow
Ventricular Volume
Heart Sounds
Venous Pulse
Cardiac contraction
Electrical events (ECG)
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Definition of a Cardiac Cycle
Is a sequence of coordinated events that
occur during a single heartbeat.
Each beat consists of an electrical
stimulus followed by a contraction
(systolic) and relaxation (diastolic) period.
Cardiac cycle consists of
3 important events
• Generation of electrical activity as the heart
auto-rhythmically depolarizes & repolarizes
• Mechanical activity consisting of alternate
periods of contraction (systole) and
relaxation (diastole)
• Uni-directional blood flow through the heart
chambers
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Cardiac Cycle
• All events associated with one heartbeat
• Systole and diastole of atria and ventricles
• In each cycle, atria and ventricles alternately
contract and relax
– During atrial systole, ventricles are relaxed
– During ventricle systole, atria are relaxed
• Forces blood from higher pressure to lower
pressure
• During relaxation period, both atria and ventricles
are relaxed
– The faster the heart beats, the shorter the
relaxation period
– Systole and diastole lengths shorten slightly
Heart Valves
Valves prevent blood backflow during a
cardiac cycle
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DIASTOLE
Semilunar
valves closed
AV valves
opened
Passive ventricular filling. The AV valves open and blood flows
into the relaxed ventricles, accounting for most of the ventricular
filling.
SYSTOLE
Semilunar
valves opened
AV valves
closed
Period of ejection. Continued ventricular contraction pushes blood
out of the ventricles, causing the semilunar valves to open.
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Cardiac Cycle
• Atrial systole 0.1 second
• Atrial diastole 0.7 second
• Ventricular systole 0.3 second
– Isovolumic contraction phase
 Rapid ejection period
 Slow ejection period
• Ventricular diastole 0.5 seconds
– Isovolumic relaxation phase
 Rapid filling phase
 Slow filling (Diastasis)
 Atrial contraction phase
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Phases of the Cardiac Cycle
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Aortic Pressure Curve
0
0 .1
0 .2
Time ( sec)
0 .3 0 .4 0 .5
0 .6
0 .7
0 .8
mmHg
Aorta
120
atrium
100
Pressure
Aort ic
80
R
60
P
40
T
Q
20
S
0
Ventricular and Aortic Pressures
0
mmHg
120
0 .1
Aortic
valve
opens
0 .2
Time ( sec)
0 .3 0 .4 0 .5
0 .6
0 .7
0 .8
Aortic valve
closes
Dichrotic
notch
Aorta
atrium
100
Aort ic
80
Pressure
60
40
Mitral valve opens
20
0
Vent ricular
Mitral valve closes
8
Left atrial pressure
mmHg
120
100
Aort ic
80
60
40
20
At rial
0
Atrial
systole
Ventricular
contraction
Atrium
filling from pulmonary veins
Left Atrial Pressure Waves
Atrial Systole
(a wave)
Venous c wave: Mitral valves close
and bulge toward the atrium.
Mitral valves open
Aorta
A t rial
atrium
Pressure
v wave: Passive filling of the
atrium from pulmonary veins
during systole
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Ventricular Filling (Diastole)
Rapid filling phase
Slow Filling Phase:
Diastasis
mmHg
120
100
Aort ic
80
120 ml
60
Stroke
volume
Ventricular volume
40
50 ml
20
At rial
0
Valve Events
0
mmHg
120
100
0 .1
0 .2
Time ( sec)
0 .3 0 .4 0 .5
0 .6
0 .7
0 .8
Semilunar
valves close: 2nd heart sound
80
60
Semilunar
valves open (outflow)
40
Pressure
20
Mitral/Tricuspid open
0
Mitral/Tricuspid close: 1st heart sound
10
R
T
P
(a) ECG
1
4
8
Q
0.3 sec
Ventricular
systole
120
0.4 sec
Relaxation
period
Atrial depolarization
2
Begin atrial systole
3
End (ventricular) diastolic volume
4
Ventricular depolarization
5
Isovolumetric contraction
6
Begin ventricular ejection
7
End (ventricular) systolic volume
8
Begin ventricular repolarization
9
Isovolumetric relaxation
9
Dicrotic wave
100
Aortic
pressure
5
80
6
(b) Pressure
(mmHg)
1
S
0.1
sec
Atrial
systole
Left
ventricular
pressure
60
40
Left atrial
pressure
10
20
2
0
(c) Heart sounds
S1
S2
S3
S4
3 End (ventricular) diastolic volume
130
10 Ventricular filling
Stroke
volume
(d) Volume in
ventricle (mL)
60
7
0
(e) Phases of the
cardiac cycle
Atrial
contraction
Isovolumetric
contraction
UniversityIsovolumetric
of Jordan
Ventricular
ejection
relaxation
Ventricular
filling
Atrial
contraction
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