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Electrocardiogram
Interpretation
ISMAIL HAMAM, MD
DEPARTMENT OF CARDIOLOGY
JORDAN UNIVERSITY HOSPITAL
Objectives
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To review Basic Concepts for the 12-Lead
ECG
To know Normal Criteria for the ECG
To perform a Systematic Approach to ECG
Interpretation.
To discuss a deferent Rhythms on the 12Lead ECG
Basic Concepts
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The heart is a pump with an electrical
conduction system
2 basic types of cardiac cells in the heart
1. Myocardial cells or “muscle” cells
2. Specialized cells of the conduction system or
“pacemaker” cells
The Electrical Cycle
1.The S-A Node generates
an action potential.
2. The action potential
propagates in the atria
and causes a contraction.
It is also transmitted to
the AV Node.
3.The action potential is
delayed at the A-V Node.
4.The action potential is
transmitted to the ventricles
and causes contraction
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The electrocardiogram (ECG) is a standardize
way to measure and display the electrical activity
of the heart.
Physicians can diagnose problems with the heart
by analyzing its ECG and comparing it to the
ECG
12-lead ECG
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Simultaneous measurements from:
– Three limb leads
– Three augmented limb leads
– Six chest leads
Process data from all leads to obtain ECG
waveform
Structure of a 12-Lead Recording
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The three standard or limb leads 1,2 and 3 have
been in use since the late 1800’s, but the
augmented limb leads aVR, aVL and aVF, and
the precordial or chest leads V1-6 only came
into use in the 1930’s.
The six precordial electrodes create a six-lead
image of the heart on the transverse plane
.Each of the pericardial leads is unipolar (1
electrode constitutes a lead).
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Limb leads are three electrodes that are placed on the
two arms and left leg create a 360o six-lead electrical
image of the heart on the frontal plane on the ECG
recording (1, 2, 3, aVR, aVL, aVF).
The lead on the right leg is system ground.
Accurate lead placement is important. Malposition can
lead to significant interpretation errors, especially while
comparing serial recordings from the same patient.
Limb Leads
Lead
Relative
angle
l: RA-LA
0°
ll: RA-LL
60°
lll: LA-LL
120°
Augmented Limb Leads:
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aVR = (LA-LL) vs. RA(+)
aVL = (RA-LL) vs. LA(+)
aVF = (RA-LA) vs. LL(+)
Locating the leads
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The four limb leads are placed on the four limbs.
V1 in the 4th intercostal space adjacent to the right
sternum.
V2 in the 4th intercostal space adjacent to the left
sternum.
V4 in the 5th intercostal space in the mid- clavicular
line.
V6 90o to the sternal axis at the same level as V4 and
on the mid- axillary line.
V3 halfway between V2 and V4.
V5 halfway between V4 and V6, in the anterior axillary
line.
Paper Speed and Voltages
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Time is measured for cardiac heartbeats in milliseconds
on the x-axis.
The standard 12-lead recording is made with the paper
running through the machine at rate of 25 mm/sec.
This produces tracings along the X-axis with a duration
of 40 msec/mm (“one small square”), or 200 msec for
each 5 mm thick line (“one large square”) .
Strength of the electrical signal from the heart is
measured in millivolts on the y-axis.
The ECG normally records a gain of 1 mV/cm on the
Y-axis .
paper
.1 mv
Voltage
.5 mv
.04 seconds
Time
Paper speed = 25mm / second
.20 seconds
Standard 12-lead electrocardiogram
The ECG Complex
Component
Physiologic Correlation
P wave
PR interval
PR segment
Atrial depolarization
AV node refractory period
Atrial repolarization
QRS complex
J point
ST segment
T wave
U wave
TP Segment
Ventricular depolarization
End of ventricular depolarization
Blood flow from the ventricles
Ventricular repolarization
Uncertain
Rest prior to next heartbeat cycle
Heart Excitation Related to ECG
SA node generates impulse;
atrial excitation begins
SA node
Impulse delayed
at AV node
AV node
Impulse passes to
heart apex; ventricular
excitation begins
Bundle
branches
Ventricular excitation
complete
Purkinje
fibers
Normal Criteria for the ECG
P wave
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≤ 110 msec. <0.25 mvolts in the limb leads.
Upright in 1, 2, aVF, V4-6.
Inverted in aVR.
Leads 1-3: single peak, or two peaks if they are < 40
msec apart.
Second half in V1 positive.
PR interval
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120 - 200 msec.
Usually measure in lead 2.
Shorter at faster rates and younger persons.
Longer in larger hearts and older persons.
PR segment
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Usually isoelectric
QRS complex
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< 120 msec. And > 0.5 mV in all the limb leads.
> 1.0 mV in all of the precordial leads
Measure in the lead with the longest QRS duration.
Any initial upward deflection means that an R wave,
not a Q wave, is present.
R wave increases in amplitude across the precordial
leads, usually becoming larger in voltage than the S
wave in V2 - V4 (transition zone) in adult
Limb lead QRS axis
 Net electrical vector of ventricular
contraction
 -30o to +105o
 Overweight: more leftward, Thin: more
rightward.
ST segment
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Usually within 1 mm of the isoelectric baseline.
Horizontal, with a smooth entry to the T wave.
Limb leads: isoelectric in 75% of normal persons; in the
remaining 25% slight elevation is more common.
Precordial leads: slight elevation in 90% of normal
persons, more common in men, usually in V2 and V3,
where it can reach 3 mm.
ST depression is rare in 1, 2, and aVF in normal
persons, and is considered abnormal in the precordial
lead
T wave
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< 0.5 mvolts in the limb leads.
< 1.0 mvolt in the chest leads.
More gradual slope in the first half than the second
half.
Upright in 1, 2 and V4-6 , inverted in aVR, variable in
all other leads.
Upright, flat or slightly inverted in 3.
Upright in aVL, aVF if QRS > 0.5 mV.
Can be inverted in V1-2 in adult men, V1-3 in adult
women (shallowly in V3).
Always upright in V5-V6.
QT interval
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Varies with the heart rate, age and gender.
Usually less than half the RR interval at normal sinus
rates (65-90 bpm).
TP segment
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Usually isoelectric
Can be used as a baseline reference like the PR
segment.
U wave
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Usually 5-25% the amplitude of the T wave.
Usually same polarity as the T wave.
The Normal EKG (ECG)
A Systematic Approach to
Electrocardiogram Interpretation.
Step 1
To know the age, gender and history of the patient
Step 2
Standardization.
 Y-axis (voltage, normally one millivolt per cm
 X-axis (rate of recording, normally 25 mm/sec,
or 200 msec/5 mm).
Step 3
Rate.
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Calculate the rate by knowing that the Y-axis
standardization is 25 mm/sec.
One small square (1 mm) is 40 msec in duration.
Therefore 2.5 cm represents one second.
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Count the number of complexes across the 25
cm page (10 seconds), multiply by 6 and you get
the average rate per one minute.
This is a useful way to estimate rate for irregular
features.
Count the large squares between P waves for
atrial rate and R waves for ventricular rate
300 ÷ number of large squares = number of
beats/min
Step 4
Rhythm
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Identify P waves and QRS complexes.
Is the RR interval regular or irregular?
Note whether the QRS complex is wide (> 3
mm or 120 msec) or narrow.
Observe for -- rate, P wave and QRS complex
synchrony, RR interval regularity, and width of
the QRS complex – you can begin to sort out
the rhythm.
Look for extra complexes such as premature
atrial, junctional and ventricular contractions.
Normal sinus rhythm
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Rate 60 – 100 bpm
Rhythm Regular
P wave Upright, normal
P-R 0.12 – 0.20 sec
QRS 0.06 – 0.12 sec
P : QRS Rasio 1: 1
12 lead ECG
Step 5
Axis
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Each of the 12 leads on the ECG has a different
pattern because each lead views the hearts electrical axis
from a different position
Atrial and ventricular depolarization and repolarization
generate an electric current known as an electrical axis
or vector (different from the axis of a lead)
The electrical current of a heartbeat normally flows
from the SA node into the ventricles, roughly toward
the positive pole of lead 2.
The normal range -30o to 105o but varies with age.
Limb lead axis.
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Left axis deviation (LAD) any age group.
-30o to -90o.
Right axis deviation (RAD) in adults.
+90o to +180o.
Extreme axis deviation (EAD) any age
group. -90o to -180o.
Step 6
P Waves.
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Note the width, height and shape of the P waves.
PR interval
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Duration of PR interval
Dose PR segment is isoelectric
QRS Complex.
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Note the width, height, shape and pattern of the
complex.
Always assess Q waves relative to other clinical
information
If Q waves are not clearly diagnostic, then call them
“nondiagnostic”.
ST Segments
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Study the ST segments separately from the T waves
first, then together with the T waves.
Look for deviation from the baseline, and shape of the
ST segment. Shapes include flat, upsloping,
downsloping, curved and straight.
Note where the ST segment is relative to baseline 2 mm
(80 msec) after the J point.
Note carefully how the ST segment enters the T wave:
is it smooth, or does it have an ominous abrupt entry?
Measuring ST Segments
ST measurement = vertical difference between the
isoelectric line + end of QRS complex, the “J”
point”
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ST segment elevation = >1 mm (>0.1 mV) above
baseline after the J point
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ST segment elevation due to severe injury temporary
until ischemia resolved or injured heart tissue heals or
dies
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ST segments elevate in leads facing the injury
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ST segments depress in leads opposite (reciprocal )
leads
T Waves
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Note height and shape of the T waves. Shapes include
smooth symmetry, asymmetry sharp peaking, inversion,
and biphasic.
Check again for the way the ST segment enters the T
wave.
QT Interval
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Calculate the QT interval to spot unusually short or
long duration.
Sinus Tachycardia
Causes
 Fever
 Pain
 Anxiety
 Hypoxia
 CHF
 Fright or stress
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Rate 100-160
Rhythm regular
P wave upright, normal
P-R 0.12- 0.20 sec
QRS 0.06- 0.12 sec
P: QRS ratio 1: 1
Sinus Bradycardia
Causes
 Normal in athletes
 Increased vagal tone
 Sinus node disease
 Hypothermia
 Acute MI
 Drug effect (digoxin, CCB , BB)
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Rate less than 60 bpm
Rhythm regular
P wave upright, normal
P-R 0.12- 0.20 sec
QRS 0.06- 0.12 sec
P: QRS ratio 1: 1
Junctional Rhythm
Causes
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Digitalis toxicity
Acute MI
Ischemia
Hypoxia
Post valve surgery
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Sinus node and atria fail to pace the heart.
AV junction paces at → 40-60/min
No P wave or PR interval < 0.12, QRS normal
Idioventricular Rhythm
Causes
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SA node and AV node fail to initiate rhythm.
MI
Digoxin toxicity
Metabolic imbalance
Dying heart
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Sinus node, atria, and AV junction fail to pace. Ectopic
pacemaker in the ventricles paces at → 20-40/min
No P wave, QRS wide, ST & T waves often abnormal
AV Blocks
First degree AV block
Causes
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May be normal findings especially in athletes
Myocardial ischemia injuring the AV node or junction
Medication :quinidine, BB, CHB, digoxin, and
amiodaron
Acute MI
Rheumatic heart disease
Increased vagal tone
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Usually normal rate , regular
PR interval is prolonged > 0.20
Normal QRS
P: QRS Ratio 1:1
Second degree AV block
Causes
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Ischemic damage below the AV node, involving
the bundle of his or the bundle branch
Drugs , digoxin ,BB
Increased vagal tone
Mobitz type I (Wenckebach)
 Progressive prolongation of PR interval until P wave
fails to conduct to the ventricle
 Progressive shortening of RR interval until P wave is
not conducted
 The RR interval containing the nonconducted
P wave is shorter than the sum of two PP intervals
Acute inferior myocardial infarction with atrioventricular
block, second degree–Mobitz type I (Wenckebach),
AV block, second degree–Mobitz type II
 There are intermittent nonconducted P waves
with no evidence of atrial prematurity
 In conducted beats, PR intervals stay constant
 The RR interval containing the nonconducted P
wave is equal to two PP intervals
Sinus rhythm with atrioventricular block, second degree–Mobitz
type II, and right bundle branch block.
Third degree AV block
Causes
 Ischemic damage
 Post cardiac surgery
 Chronic degenerative changes of the conduction
system
 Drugs, digoxin ,BB
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Independent atrial and ventricular activities
Atrial rate faster than ventricular rate
Ventricular rhythm maintained by a junctional
or idioventricular escape rhythm or ventricular
pacemaker
When the ventricular rate is faster than the atrial
rate, AV dissociation (not AV block)
Superventricular Tachycardia
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Ectopic focus in atria or AV junction paces the
heart
or Abnormal conduction thru AV node
or Accessory pathway
P wave or no P wave, QRS narrow or wide,
rate > 150/min
Ventricular Tachycardia
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Ectopic pacemaker in ventricles paces the heart
No P wave, QRS wide and bizarre
Premature Ventricular Contractions
QRS Duration
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QRS duration - depolarization of right and left
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Normal QRS duration - 0.06-0.12 sec
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QRS duration > 0.12 sec, a conduction delay exists in
the bundle branches, Purkinjie network or ventricular
myocardium, or ventricular ectopic conduction exists
ventricles, from the endocardium to epicardium
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PVC’s, premature ventricular complexes:
the premature beat originates in an ectopic
focus in one ventricle, it depolarizes that
ventricle, then the other
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No P wave, QRS wide & bizarre, ST often abnormal, T
wave often opposite the rhythm
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Multifocal PVC’s come from more than one ectopic
focus, each foci has a different shape
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1 PVC
2 PVC’s
3 PVC’s
4 PVC’s
=
=
=
=
a PVC
couplet
triplet
ventricular tachycardia
Every 2nd PVC = bigeminy
Every 3rd PVC = trigeminy
Bigeminy or trigeminy can refer to any ectopic beat so clarify eg. bigeminal PVC’s or bigeminal PAC’s, etc.
ST Segments
Other Common Causes of
ST Segment Elevation
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Coronary artery vasospasm
Acute pericarditis
Ventricular aneursym
Hyperkalemia
Non-specific ST-T wave changes
ST Segment Depression
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ST segment depression = > 1 mm below
baseline after the J point
ST segment depression due to severe ischemia
temporary until ischemia resolved or heart tissue
heals
ST segments depress in leads facing the ischemia
ST segments elevate in opposite (reciprocal)
leads
Other Common Causes of
ST Segment Depression
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Left and right ventricular hypertrophy
Left and right bundle branch block
Digitalis in therapeutic and toxic doses
Acute MI Facing Leads Opposite Leads
Anterior
Septal
Anterior
Lateral
Inferior
V1-V2
V3-V4
I, aVL, & V5 or V6
II, III, & aVF
Posterior
V7,V8, V9 on 18 lead
Right Ventricle
V4R
None
None
II, III, & aVF
I & aVL
V1-V4
None
Thank you