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ECG
H.R Tohidypour
OVERVIEW
Cardiac Physiology
Electrocardiography
Cardiac Physiology
Electrocardiography Diagnosis
ARTERIES
VEINS
distributes
blood from
heart
brings blood
back to heart
Cardiac Physiology
Electrocardiography Diagnosis
Atria
Ventricles
Cardiac Physiology
Electrocardiography Diagnosis
Sinoatrial
Node
Atrioventricular
Node
Conduction System
of the Heart:
A Conceptual Model
for Illustration
SA
Node
Inter-nodal
Tract
Left
Bundle
Branch
AV
Node
Bundle
of HIS
James Fibers
Right
Bundle
Branch
Bundle
of Kent
Septal
Depolarization
Fibers
Anterior
Superior
Fascicle
Posterior
Inferior
Fascicle
Basic conduction mechanisms
• Sinoatrial node (SA
node)- primary
pacemaker of the
heart
• Atrioventricular node
(AV node)
• His Bundle
• Bundle branches
• Purkinje fibers
Extrinsic Innervation of the Heart
• Heart is stimulated
by the sympathetic
cardioacceleratory
center
• Heart is inhibited
by the
parasympathetic
cardioinhibitory
center
Cardiac Cycle
• Cardiac cycle refers to all events
associated with blood flow through the
heart
– Systole – contraction of heart muscle
– Diastole – relaxation of heart muscle
Cardiac Physiology
Electrocardiography Diagnosis
Introduction to Electrocardiography (ECG, EKG)
•
•
•
•
•
Electrocardiography - graphic recording of the electrical activity
(potentials) produced by the conduction system and the myocardium
of the heart during its depolarization / re-polarization cycle.
During the late 1800's and early 1900's, Dutch physiologist Willem
Einthoven developed the early electrocardiogram. He won the Nobel
prize for its invention in 1924.
Hubert Mann first uses the electrocardiogram to describe
electrocardiographic changes associated with a heart attack in 1920.
The science of electrocardiography is not exact. The sensitivity and
specificity of the tool in relation to various diagnoses are relatively low
Electrocardiograms must be viewed in the context of demographics,
health histories, and other clinical test correlates. They are especially
useful when compared across time to see how the electrical activity of
the heart has changed (perhaps as the result of some pathology).
Design considerations: differential recordings
• ECG recording is differential = recorded as potential
difference between two leads.
• This is due to presence of significant electric noise.
Typically 60 Hz noise is present and equally
distributed
over the entire body patients body. Noise amplitude
~100mV, ECG signal amplitude ~1-5 mV <<100mV!!!
Subtractions of two signals, recorded from two
• different locations will eliminate noise.
Differential recording of ECG
Accessories used
Cardiac Physiology
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T
P
Q
S
Electrocardiography Diagnosis
Cardiac Physiology
R
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Electrocardiography Diagnosis
Electrical Conduction in Heart
1
1
SA node
AV node
2
THE CONDUCTING SYSTEM
OF THE HEART
1 SA node depolarizes.
SA node
3
Internodal
pathways
2 Electrical activity goes
rapidly to AV node via
internodal pathways.
3 Depolarization spreads
more slowly across
atria. Conduction slows
through AV node.
AV node
A-V bundle
Bundle branches
4
Purkinje
fibers
5
4 Depolarization moves
rapidly through ventricular
conducting system to the
apex of the heart.
5 Depolarization wave
spreads upward from
the apex.
Purple shading in steps 2–5 represents depolarization.
Electrical Activity
P wave: atrial
depolarization
START
P
The end
R
PQ or PR segment:
conduction through
AV node and A-V
bundle
T
P
P
QS
Atria contract.
T wave:
ventricular
Repolarization
Repolarization
R
T
P
ELECTRICAL
EVENTS
OF THE
CARDIAC CYCLE
QS
P Q wave
Q
ST segment
R
R wave
R
P
QS
P
R
Ventricles contract.
Q
P
S wave
QS
Cardiac Physiology
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1 sec
T
P
Q
S
0.5 Sec
Electrocardiography Diagnosis
• The normal electrocardiogram is composed of a P
wave, a QRS complex, and a T wave.
• The QRS complex is often, but not always, three
separate waves: the Q wave, the R wave, and the S
wave.
• The P wave is caused by electrical potentials
generated when the atria depolarize before atrial
contraction begins.
• The QRS complex is caused by potentials
generated when the ventricles depolarize before
contraction, that is, as the
depolarization wave spreads through the
ventricles.Therefore, both the P wave
and the components of the QRS complex are
depolarization waves.
Normal Voltages in the Electrocardiogram.
• The recorded voltages of the waves in the normal
electrocardiogram depend on the manner in which
the electrodes are applied to the surface of the body
and how close the electrodes are to the heart.
• When one electrode is placed directly over the
ventricles and a second electrode is placed
elsewhere on the body remote from the heart, the
voltage of the QRS complex may be as great as 3 to
4 millivolts.
• When electrocardiograms are recorded from
electrodes on the two arms or on one arm
and one leg, the voltage of the QRS complex
usually is 1.0 to 1.5 millivolt from the top of
the R wave to the bottom of the S wave; the
voltage of the P wave is between 0.1
and 0.3 millivolt; and that of the T wave is
between 0.2 and 0.3 millivolt.
Comparison of an ECG and a myocardial
action potential
• P-Q or P-R Interval. The time between
the beginning of the P wave and the
beginning of the QRS complex is the
interval between the beginning of
electrical excitation of the atria and the
beginning of excitation of the
ventricles. This period is called the P-Q
interval. The normal P-Q interval is
about 0.16 second. (Often this interval
is called the P-R interval because the Q
wave is likely to be absent.)
• Q-T Interval. Contraction of the ventricle
lasts almost from the beginning of the Q
wave (or R wave, if the Q wave is absent) to
the end of the T wave. This
interval is called the Q-T interval and
ordinarily is about 0.35 second.
Rate of Heartbeat as Determined
from the Electrocardiogram
• The rate of heartbeat can be determined easily from
an electrocardiogram because the heart rate is the
reciprocal of the time interval between two
successive heartbeats. If the interval between two
beats as determined from the time calibration lines is
1 second, the heart rate is 60 beats per minute. The
normal interval between two successive QRS
complexes in the adult person is about 0.83 second.
This is a heart rate of 60/0.83 times per minute, or 72
beats per minute.
Limb Leads
• Standard ECG Leads (The Einthoven limb
leads) are defined as follows
According to the Einthoven triangle and Kirchhoff’s voltage
law, the standard lead voltages have the following
relationship:
Hence only two of these three leads are independent.
Wilson Central Terminal
• Unipolar potential definition by Frank Norman Wilson (1890-1952):
unipolar potentials should be measured with respect to the central
terminal (CT).
•
To satisfy the conservation law of current, the total current into the CT from
the limb leads must add to zero. Thus, we have:
Goldberger Augmented Lead
• Three additional limb leads, VR, VL, and VF are obtained by
measuring the voltage between each limb electrode and the
Wilson CT. For instance, the left leg lead is given by:
E. Goldberger observed in 1942 that these signals can be
augmented by omitting that resistance from the Wilson CT, which is
connected to the measurement electrode. In this way, the
aforementioned three limb leads, VR, VL, and VF may be replaced
with a new set of leads that are called augmented leads. The equation
for augmented left leg lead is:
Three additional leads can be obtained by comparing each
limb lead potential with the central terminal voltage. For
example, from (*) we have, for the right arm,
If, in creating the CT voltage, the connection to RA is dropped,
then in place of (VR) we have:
• A comparison of Eq. VR with Eq. aVR
shows the augmented signal to be 50%
larger than the signal with the Wilson CT
chosen as reference.
Electrocardiographic Leads
• Three Bipolar Limb Leads
• Chest Leads (Precordial Leads)
• Augmented Unipolar Limb Leads
Three Bipolar Limb Leads
• Lead I. In recording limb lead I, the negative terminal
of the electrocardiograph is connected to the right
arm and the positive terminal to the left arm.
• Lead II. To record limb lead II, the negative terminal
of the electrocardiograph is connected to the right
arm and the positive terminal to the left leg.
• Lead III. To record limb lead III, the negative terminal
of the electrocardiograph is connected to the left
arm and the positive terminal to the left leg.
Chest Leads (Precordial Leads)
• Often electrocardiograms are recorded with one
electrode placed on the anterior surface of the
chest directly over the heart at one of the
points.This electrode is connected to the
positive terminal of the electrocardiograph, and
the negative electrode, called the indifferent
electrode, is connected through equal electrical
resistances to the right arm,
left arm, and left leg all at the same time, as also
shown in the figure. Usually six standard chest
leads are recorded, one at a time, from the
anterior chest wall, the chest electrode being
placed sequentially at the six
points shown in the diagram. The different
recordings are known as leads V1, V2, V3, V4,
V5, and V6.
• In leads V1 and V2, the QRS recordings of the
normal heart are mainly negative because
, the chest electrode in these leads is nearer
to the base of the heart than to the apex, and the
base of the heart is the direction of electronegativity
during most of the ventricular depolarization
process.
• the QRS complexes in leads V4,V5, and V6
are mainly positive because the chest
electrode in these leads is nearer the heart
apex, which is the direction of
electropositivity during most of
depolarization.
Augmented Unipolar Limb Leads
• Two of the limbs are connected through
electrical resistances to the negative
terminal of the electrocardiograph, and the
third limb is connected to the positive
terminal.
• When the positive terminal is on the right
arm, the lead is known as the aVR lead; when
on the left arm, the aVL lead; and when on
the left leg, the aVF
lead.
•Normal recordings of the augmented unipolar limb
leads, are all similar to the standard limb lead
recordings,
•except that the recording from the aVR lead is
inverted.
Principles of Vectorial Analysis
of Electrocardiograms
• heart current flows in a particular direction in
the heart at a given instant during the cardiac
cycle.
• A vector is an arrow that points in the
direction of the electrical potential generated
by the current flow, with the arrowhead in the
positive direction.
• Also, by convention, the length of the arrow
is drawn proportional to the voltage of the
potential.
The Concept of a “Lead”
RA & LA
-
Augmented Voltage Leads
AVR, AVL, and AVF
LEAD AVR
RA
By combining certain
limb leads into a central
terminal, which serves
as the negative
electrode, other leads
could be formed to "fill in
the gaps" in terms of the
angles of directional
recording. These leads
required augmentation
of voltage to be read
and are thus labeled.
LEAD AVL
+
+
-
RA & RL
LA
LL & LA
LL +
LEAD AVF
The Concept of a “Lead”
Summary of the
“Limb Leads”
Each of the limb
leads (I, II, III, AVR,
AVL, AVF) can be
assigned an angle
of clockwise or
counterclockwise
rotation to describe
its position in the
frontal plane.
Downward rotation
from 0 is positive
and upward rotation
from 0 is negative.
LEAD AVR
LEAD AVL
-150o
-30o
0o
LEAD I
60o
120o
LEAD III
90o
LEAD AVF
LEAD II
Hexaxial Array for Axis Determination – Example 1
Lead I
If lead I is mostly
positive, the
axis must lie in the
right half of
of the coordinate
system (the main
vector is moving
mostly toward the
lead’s positive
electrode)
Hexaxial Array for Axis Determination – Example 1
Lead AVF
If lead AVF is
mostly positive, the
axis must lie in the
bottom half of
of the coordinate
system (again, the
main vector is
moving mostly
toward the lead’s
positive electrode
Hexaxial Array for Axis Determination – Example 1
I
AVF
Combining the two
plots, we see
that the axis must
lie in the bottom
right hand quadrant
Cardiac Physiology
Electrocardiography Diagnosis
Atrioventricular Block
• Ischemia
• Nodal Compression
• Nodal Inflamation
• Extreme Stimulation
Cardiac Physiology
Electrocardiography Diagnosis
Normal ECG
ECG with Atrioventricular Block
Cardiac Physiology
Electrocardiography Diagnosis
Preventricular Contractions
• Coffee
• Cigarettes
• Sleep deprivation
• Pathology
Cardiac Physiology
Electrocardiography Diagnosis
Normal ECG
ECG with Preventricular Contractions
Cardiac Physiology
Electrocardiography Diagnosis
Ventricular Fibrilation
• Ischemia
• Electric Shock
Cardiac Physiology
Electrocardiography Diagnosis
Normal ECG
ECG during Ventricular Fibrillation
Abnormal Sinus Rhythms
• Tachycardia : means fast heart rate, usually
defined in an adult person as faster than 100
beats per minute
• Bradycardia : means a slow heart rate, usually
defined as fewer than 60 beats per minute.
ECG Changes : Ischemia
•
•
•
•
T-wave inversion ( flipped T)
ST segment depression
T wave flattening
Biphasic T-waves
Baseline
ECG Changes: Injury
• ST segment elevation of greater than 1mm in at least 2
contiguous leads
• Heightened or peaked T waves
• Directly related to portions of myocardium rendered
electrically inactive
Baseline
Evolving MI and Hallmarks of
AMI
Q wave
ST Elevation
1 year
T wave
inversion
Now…The Circuit
• PreAmp
• Low Pass Filter
• High Pass Filter
Noise
• Several sources
•60Hz power lines – shielding, filtering
•Other biopotentials – filtering
•Motion artifacts – relaxed subject
•Electrode noise – high quality electrodes, good contacts
•Circuit noise – good design, good components
When measuring biopotentials (say ECG),
EVERYTHING else creates noise
– power
line interference
– even other biopotentials (like EEG, EMG, EOG)
are noise sources. These have characteristic
frequencies. So use Band Pass Filters.
Pass only
fL to fH
attenuate
the others.
fL
fH
Frequencies of Biopotentials
Signal
ECG
Frequency
range (Hz)
0.01 – 300
Amplitude
range(mV)
0.05 – 3
EEG
0.1 – 100
0.001 – 1
EOG
0.1 – 10
0.001 – 0.3
EMG
50 – 3000
0.001 – 100
QRS Detector Components
• Why detect QRS Complex?
1. Most Rhythm analysis algorithms are based
on QRS complex
2. Used in the diagnosis of Tachycardia
3. Largest amplitude and sharpest waveform
the can be extracted from the ECG
Preprocessing Stage filters the signal down to the range of 10Hz to 25Hz.
Activation Currents in Cardiac
Tissue
• However, the electrocardiogram (ECG) is
a recording of the electric potential,
generated by the electric activity of the
heart, on the surface of the thorax. The
ECG thus represents the extracellular
electric behavior of the cardiac muscle
tissue.
• There are two important properties of cardiac tissue
that we shall make use of to analyze the potential
and current distribution associated with these
propagating waves. First, cells are interconnected by
low-resistance pathways (gap junctions), as a result
of which currents flowing in the intracellular space
of one cell pass freely into the following cell.
Second, the space between cells is very restrictive
(accounting for less than 25% of the total volume).
As a result, both intracellular and extracellular
currents are confined to the direction parallel to the
propagation of the plane wavefront.
Io+Ii=0
Integrating from x = – inf
Vm = phi(i) – phi(o)
, to +inf x = x gives
Thank you