Introduction to the
12-lead ECG
Gustav Schellack
SASOCP Workshop
10 March 2012
© G Schellack; 2012
Electrocardiogram (ECG)
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The ECG is a graphic recording of the electrical
activity of the heart, at a specific moment in
time.
Note: ‘Electrical activity’ refers to the
depolarisation and repolarisation of the atria
and ventricles.
Note: The ECG cannot tell us anything about
the ‘mechanical activity’ (i.e. contraction and
relaxation) of the myocardium.
Cardiac Conduction System
Anatomical orientation:
Figure 1:
(Marquette Electronics, 1996)
(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the
Internet]. c2012. Available from: http://library.med.utah.edu/kw/ecg/index.html.)
The Cardiac Impulse
Originates from the
SA-node (in the RA)
Spreads through
both atria [virtually]
simultaneously
RA
Spreads through
both atria [virtually]
simultaneously
LA
Activates both ventricles
via the His-Purkinje
network; first the septum
from (L) to (R), then the
RV, then the LV, and
finally the apex
Reaches the AVnode (where it is
momentarily
delayed)
Enters the
Bundle of His
and the right
and left bundle
branches
RBB
Enters the
Bundle of His
and the right
and left bundle
branches
LBB
Repolarisation of the ventricles (note that
the repolarisation of the atria is obscured
by the depolarisation of the ventricles)
The Cardiac Impulse
SA-node
Bachmann’s
bundle
Ca2+
AV-node
Internodal
pathways
Ca2+
Right atrium
(RA)
Bundle
of His
Bundle
branches
Figure 2: Diagram of the functional SA- and AV-nodes
© G Schellack, 2012
The Standard 12-lead ECG
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ECG…
•Patient identifiers (e.g. name, hospital number,
treating physician, etc.)
•Date and time that the ECG was obtained
•Vital signs, including BP and HR
•Comment on any relevant symptoms (e.g.
chest pain), cardiovascular drugs and special
lead placements…
The Standard 12-lead ECG
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The 12 leads are:
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Six limb leads (I, II, III, aVR, aVL and aVF)
Six precordial (chest) leads: V1 to V6
The 12 leads are displayed at a standardised
tracing speed of 25 mm per second, and
with 1 cm representing 1.0 mV on the
vertical axis.
 Check the calibration mark.
The Standard 12-lead ECG
Criteria:
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Standardisation: 10 mm in height = 1 mV; 25 mm per second.
Double and half-standardisation may be required.
Sinus rhythm:
 Each P-wave must be followed by a QRS-complex, and vice versa.
 RR-intervals must be constant (or ≤ than 0.12 s of physiological
variance).
 Normal heart rate (60-100 beats per minute).
 Normal PR-interval (0.12 to 0.2 s in duration).
P-waves: Examine for normality in Lead I, II and V1.
QRS-complexes: Must be ‘positive’ in Lead I, II, III, aVF and aVL, and
‘negative’ in aVR.
Narrow QRS-complexes (0.08 to 0.12 s in duration); Q-waves in V5 and
V6 are < 0.04 s and < 3 mm deep.
R-S-wave progression from V1 to V6; QTc-interval is normal.
Normal cardiac axis: Positive QRS-complexes in Lead I, III and aVF.
Normal ST-segments and T-waves.
Limb Lead Placement and
Einthoven’s Triangle
RA
-
Lead I
aVR (+)
aVL (+)
+
-
Lead III
Lead II
aVF
(+)
+ +
LL
LA
Limb Lead Placement and
Einthoven’s Triangle
(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the
Internet]. c2012. Available from: http://library.med.utah.edu/kw/ecg/index.html.)
Limb Lead Placement and
Einthoven’s Triangle
(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the Internet]. c2012.
Available from: http://library.med.utah.edu/kw/ecg/index.html.)
Anterolateral view
Anterior axillary
Midaxillary
Midclavicular
Placement of the Six
Chest Leads
Transverse
plane
Important clinical considerations
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Correct lead-placement and good skin contact are
essential.
Avoid electrical interference (machine to be earthed).
Compare serial tracings, if available.
Relate any changes to age, gender, clinical history, etc.
Consider co-morbidities and intercurrent illnesses that
may have an effect on the ECG.
For thermal paper: Obtain a photocopy for future
reference.
Interpret the ECG systematically to avoid errors.
The Standard 12-lead ECG
3 Standard
(bipolar) limb
leads
[I, II and III]
3 Augmented
(unipolar)
limb leads
[aVR, aVL and
a VF]
[5/5]
What/where are 6the
12 leads?
Precordial (chest) leads
[V1 to V6]
Calibration mark
ECG Graph Paper
Voltage
(millivolts; mV)
0.04 s
y
(1  1) mm2
x’
x
0.1 mV
Time (seconds; s)
Question: What would the
bigger square, i.e. the (5  5)
mm2, represent?
y’
Answer: 0.2 s on the x-axis; 0.5
mV on the y-axis.
Deflections on the ECG
R
P-wave
QRScomplex
(qRs)
T-wave
U-wave
Q (q)
S (s)
QRSduration
Segments and
Intervals on the
ECG
STsegment
PRinterval
QT-interval
J-point
Isoelectric
(base) line
[0 mV]
The P-wave
P-wave
PR-interval
Conduction through the Right Atrium (RA)
 Conduction through the Left Atrium (LA)
 Conduction through the His-Purkinje network

PR-interval: Represents the depolarisation of the
atria and the time it takes for the impulse to spread
from the SA-node, through the AV-junction, the
bundle of His, and to the bundle branches.
Drawing an Isoelectric Baseline
Why do we need to do this?
[A portion of the rhythm strip.]
Drawing an Isoelectric Baseline
Rate Determination:
First Rwave
Next Rwave
21
Example:
What is the heart rate ?
•Are the RR-intervals constant?
•RR-intervals
are approximately
3
Count the number
of squares
‘big’ squares
apart.
between two
consecutive Rwaves…
•Thus, the heart
rate is: 300 ÷ 3 =
100 beats per minute.
22
Calculating Heart Rate (HR)
‘Counting
squares’
‘Accurate
measurement’
Calculating Heart Rate (HR)
Calculating Heart Rate (HR)
Measure the shortest and the longest distance in mm,
and divide 1,500 by each measurement, for example:
•Shortest distance measured = 10 mm
•Longest distance measured = 15 mm
Then 1,500  10 = 150 and 1,500  15 = 100
Thus, the HR = 100-150 beats per minute
Correct answer:
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Shortest measurement = 16 mm
Longest measurement = 19 mm
Remember: Each small square represents 0.04 s, and
1,500 small squares represent one minute on the
horizontal axis.
Therefore:
 1,500  16 = 93.75 (~ 94 beats per min.)
 1,500  19 = 78.95 (~ 79 beats per min.)
Thus, this patient’s heart rate (HR) is 79 to 94 beats per
minute.
Calculating Variance in HR
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Following from the previous example,
subtract the shortest distance measured,
from the longest distance measured (i.e. 150
mm – 100 mm = 50 mm).
Then, multiply the answer by 0.04 s (i.e. 50
 0.04 = 2.0 seconds).
Normal physiological variance should be 
0.12 seconds.
Correct answer:
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Shortest measurement (RR-interval) = 16 mm
Longest measurement (RR-interval) = 19 mm
Therefore:
 19 mm – 16 mm = 3 mm
 3 mm  0.04 s = 0.12 s (i.e. the upper limit of
normal)
Normal physiological variance:  0.12 s.
The ‘PQRSTU-complex’ Summary:
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P-wave and PR-interval:
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Represents atrial depolarisation – Normal: ~ 0.10 s
in duration, with an amplitude of 0.50 to 2.50 mm.
PR-interval is measured from the beginning of Pwave to the beginning of the QRS-complex
(whether this a Q-wave or an R-wave) – Normal:
0.12 to 0.20 s in duration.
Digoxin prolongs the PR-interval, effectively causing
a 1st-degree AV-block, which will worsen in the
event of digitalis toxicity.
(Cont’d):
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QRS-complex and ST-segment:
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Represents depolarisation of the ventricles
Ventricular contraction – Normal: 0.08 to 0.12 s in
duration; amplitude of 5 to 15 mm.
ST-segment is the interval between the end of the
QRS-complex and the beginning of the T-wave –
Normal: on the isoelectric line (‘electric silence’);
deviation of < 1 mm on either side.
T-wave:
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Should be a positive deflection (except in aVR, V1
and V2).
Represents repolarisation of the ventricles. Normal:
0.10 to 0.25 s in duration; amplitude < 5 mm.
(Cont’d):
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QT-interval:
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Is measured from the beginning of QRScomplex to the end of T-wave; requires
rate-correction for heart rate (QTc).
Certain drugs may prolong the QTc (i.e.
cardiotoxicity) and elicit torsades de pointes.
Example of QT-interval correction: Bazett’s
formula.
(Cont’d):
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U-wave: ? (not necessarily present on all
normal ECGs).
RR-interval:
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Interval between two consecutive R-waves. Each Rwave should immediately precede a contraction of
the ventricles.
We use the RR-interval to calculate HR (heart rate);
heart monitors “beep” whenever R-waves are
recognised.
Remember: Electromechanical dissociation (EMD)…
Rate Correction of the QT-interval
Bazett’s Formula:
[Measured]
QT-interval (s)
QTc (s) =
RR-interval (s)
Correct answer:
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Measurement of the QT-interval = 9.5 mm
Therefore: QTm (s) = 9.5 mm  0.04 s = 0.38 s
Using Bazett’s formula:
 Shortest RR-interval: 16 mm  0.04 s = 0.64 s
 Longest RR-interval: 19 mm  0.04 s = 0.76 s
Thus:
 QTc (s) = 0.38 s 
0.64 s = 0.475 s (~ 0.48 s)
QTc (s) = 0.38 s 
0.76 s = 0.437 s (~ 0.44 s)
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A normal QT-interval (corrected for HR) should measure
between: 0.35 and 0.45 s.
Cardiac Axis Determination
Complexes
moving
towards one
another
‘Positive’
complexes
Complexes
deflecting
away from
one another
Normal axis
Right axis
deviation
Left axis
deviation
Cardiac Axis Determination
Atrial P-wave Variants
Normal
II
V1
Right Axis
Deviation
Left Axis Deviation
Questions?
Back-up Slides…
Ventricular Hypertrophy
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Ventricular muscle
hypertrophy
Increased QRS-voltages in
V1 and V6, Lead I and aVL
May need to change the
tracing to half of the
normal standardisation
T-wave changes in the
opposite direction to the
QRS-complexes
Associated axis-deviation
Associated atrial
hypertrophy
Blood Supply to the Heart
RCA
LCX
LAD
RCA
LCA
Blood Supply to the Heart
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The heart has four surfaces:
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Anterior surface: LAD, Left Circumflex (LCx)
Left-lateral surface: LCx, and partly via LAD
Inferior surface: RCA, terminal portion of the LAD
Posterior surface: RCA, branches of the LCx.
(R) and (L) coronary arteries arise from the
root of the aorta (just above the aortic valve
opening).
ST-segment Abnormalities
Source: Bing Images, at http://www.bing.com/images
Lead aVR
Flip the ECG
vertically
Lead aVL
?
?
?