This document provides guidance on interpreting 12-lead electrocardiograms (ECGs). It discusses calculating the heart rate and determining the rhythm from the ECG. It outlines the steps to assess the electrical axis, measure intervals like PR and QT, evaluate for hypertrophy, and identify signs of ischemia or infarction. Common abnormalities are described, including left and right axis deviation, bundle branch blocks, and left ventricular hypertrophy. The summary emphasizes interpreting the ECG involves calculating the rate, assessing the rhythm, axis, intervals, hypertrophy, and looking for evidence of heart disease.

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The 12-lead can provide a computer generated
interpretation.
 The computer is right about 98% of the
time.
 But if the computer isn’t absolutely sure, it
will not say anything about it.
 So YOU are the primary interpreter, the
computer is your backup.

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Arrangement of 12-Lead on ECG

4. Normal ECG RULES
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RULE 1:-PR interval should be 3 to 5 little squares
RULE 2:-The width of the QRS should 2 -3 little squares
RULE 3:-The QRS complex should be dominantly upright in
leads I and II
RULE 4:-QRS and T waves tend to have the same general
direction in the limb leads
RULE 5:- All waves are negative in lead aVR

5. Cont.…
RULE 6
The R wave must grow from V1 to at least V4
The S wave must grow from V1 to at least V3
and disappear in V6
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6. Cont..
RULE 7:-The ST segment should start isoelectric
except in V1 and V2 where it may be elevated
RULE 8:-The P waves should be upright in I, II, and V2
to V6.
RULE 9:-There should be no Q wave or only a small q
less than 0.04 seconds in width in I, II, V2 to V6.
RULE 10:-The T wave must be upright in I, II, V2 to
V6.
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7. Step wise approach in Reading 12-Lead ECGs
1. Calculate RATE
2. Determine RHYTHM
3. Determine AXIS
4. Calculate INTERVALS
5. Assess for HYPERTROPHY
6. Look for evidence of INFARCTION
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8. 1. Calculate Rate
If you use the rhythm strip
portion of the 12-lead
ECG the total length of it
is always 10 seconds long.
So you can count the
number of R waves in the
rhythm strip and multiply
by 6 to determine the beats
per minute.
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Rate? 12 (R waves) x 6 = 72 bpm
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9. 2. Determine RHYTHM
 Look at the R-R distances
 Regular (are they equidistant apart)? Occasionally irregular?
Regularly irregular? Irregularly irregular?
 Interpretation? Regular
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10. 3. DETERMINE AXIS
• The QRS axis represents the net overall direction of
the heart’s electrical activity.
Determine QRS AXIS
Normal
Left axis deviation
Right axis deviation
Right superior axis deviation
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11. Cont. ..
 Normal = -30° to +90°.
 -30° to -90° =LAD
 +90° to +180° =RAD
 -90°to +180 = Right
extrime axis deviation
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12. Determining the axis..cont
Limb leads Precordial leads
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13. Determining the axis..cont
 Examine the QRS complex in lead I ,II and aVF to
determine if they are predominantly positive or
predominantly negative.
 The combination should place the axis into one of
the 4 quadrants below.
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14. Cont.… axis deviation
Is the QRS axis normal in this ECG?
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No, there is left axis
deviation.
The QRS is positive in I and negative in II.
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15. COMMON CAUSES OF LAD
 May be normal in the elderly and very obese
 Inferior wall MI
 left Ventricular Hypertrophy
 Left Bundle Branch Block
 Emphysema
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16. COMMON CAUSES OF RAD
 Normal variant
 Right Ventricular Hypertrophy
 Anterior MI
 Right Bundle Branch Block
 Left Posterior Hemiblock
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17. Causes Right extreme axis deviation
• Lateral myocardial infarction.
• Right ventricular hypertrophy.
• Ventricular tachycardia.
• Left ventricular atrophy.
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18. 4. Calculate INTERVALS
Intervals refers to the length of:
PR
QT intervals and
the width of the QRS complexes.
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19. PR interval
< 0.12 s 0.12-0.20 s > 0.20 s
High catecholamine
states
Wolff-Parkinson-
White
Normal AV nodal blocks
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Wolff-Parkinson-White 1st Degree AV Block
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20. QRS complex
< 0.10 s 0.10-0.12 s > 0.12 s
Normal
Incomplete bundle
branch block
Bundle branch block
Ventricular rhythm
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3rd degree AV block
with ventricular
escape rhythm
Incomplete bundle
branch block
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The duration of the QT interval is ~ HR.
The faster the HR, the faster the ventricles
repolarize so the shorter the QT interval.
For each heart rate you need to calculate an
adjusted QT interval, called the “corrected
QT” (QTc):
QTc = QT / square root of RR interval
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QT interval

22. QTc interval
0.38-0.44 s > 0.44 s
Normal Long QT
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A prolonged QT can be very dangerous. It may
predispose an individual to a type of ventricular
tachycardia called Torsades de Pointes. Causes
include drugs, electrolyte abnormalities, CNS
disease, post-MI, and congenital heart disease.
Torsades de Pointes
Long QT
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23. Intervals cont..
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PR interval? QRS width? QTc interval?
0.08 sec
0.16 sec 0.49 sec
QT = 0.40 s
RR = 0.68 s
Square root of
RR = 0.82
QTc = 0.40/0.82
= 0.49 s
Interpretation of intervals? Normal PR and QRS, long QT
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24. Intervals cont..
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Tip: Instead of calculating the QTc, a quick way to estimate
if the QT interval long is to use the following rule:
A QT > half of the RR interval is probably long.
Normal QT Long QT
QT
RR
10 boxes
23 boxes 17 boxes
13 boxes
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25. 5. Assess for HYPERTROPHY
Three things can happen to a wave on the ECG when a
chamber hypertrophies or enlarges:
1. The chamber can take longer to depolarize. The ECG
wave may therefore increase in duration.
2. The chamber can generate more current and thus a
larger voltage. The wave may therefore increase in
amplitude.
3. A larger percentage of the total electrical current can
move through the expanded chamber.
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26.  Right atrial enlargement (RAE)
 Left atrial enlargement (LAE)
 Right ventricular hypertrophy (RVH)
 Left ventricular hypertrophy (LVH)
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In this step, we use the ECG to
determine

27. a. Right atrial enlargement is diagnosed by
 P waves with an amplitude >2.5 mm in at
least one of the inferior leads II, III, and aVF
or >1.5mm in chest leads.
 No change in the duration of the P wave
 Possible right axis deviation of the P wave
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28. Cont..
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A cause of RAE is RVH from pulmonary hypertension.

29. b. To diagnose Left atrial enlargement
you can use the following criteria:
II > 0.04 s (1 box) between notched peaks,
or
V1 Neg. deflection > 1 box wide x 1 box deep
The duration of the P wave is increased.
No significant axis deviation is seen because the
left atrium is normally electrically dominant.
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30. LAE
Normal
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A common cause of LAE is LVH from hypertension

31. c. Right ventricular hypertrophy
– Take a look at this ECG. What do you notice
about the axis and QRS complexes over the right
ventricle (V1, V2)?
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There is right axis deviation (negative in I, positive in
II) and there are tall R waves in V1, V2.

32. Cont. …RVH
To diagnose RVH you can use the following
criteria:
Right axis deviation, and
V1 R wave > 7mm tall
In lead V1, the R wave is larger than the S
wave.
In lead V6, the S wave is larger than the R
wave.
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33. Cont. …RVH
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A common cause of RVH is left heart failure.

34. d. Left ventricular hypertrophy
Take a look at this ECG. What do you notice about the axis
and QRS complexes over the left ventricle (V5, V6) and right
ventricle (V1, V2)?
There is left axis deviation (positive in I, negative in II) and
there are tall R waves in V5, V6 and deep S waves in V1, V2.
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35. LVH… cont
• The deep S waves seen in the leads
over the right ventricle are created
because the heart is depolarizing left,
superior and posterior (away from
leads V1, V2).
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36. LVH… cont
To diagnose LVH you can use the following
criteria*:
 S in V1+ R in V5 or V6 > 35 mm
 The R-wave amplitude in lead V5 exceeds 26 mm.
 The R-wave amplitude in lead V6 exceeds 20 mm.
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37. LVH… cont
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A common cause of LVH is hypertension.
* There are several
other criteria for the
diagnosis of LVH.

38. 6. Look for evidence of ischemia
and INFARCTION
ST depression T waves inversion
(ischemia)
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39. ST elevation(infraction)
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40. Q Waves
Pathologic Q waves present in II, III and aVF suggest necrosis
has occurred in the inferior region of the left ventricle.
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41. Summary of ischemia and infraction
 A normal ECG does NOT rule out Acute
Coronary Syndrome(ACS).
 ST segment depression represents ischemia
-Possible infarct
 ST segment elevation is evidence of AMI
 Q wave MI may follow ST elevation or
depression
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42. Practical example
A 63 years old man has longstanding, uncontrolled
hypertension. Is there evidence of heart disease from
his hypertension? (Hint: There a 3 abnormalities)
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Yes, there is left axis deviation (positive in I,
negative in II), left atrial enlargement (> 1 x 1
boxes in V1) and LVH (R in V5 = 27 + S in V2 = 10
 > 35 mm).

44. Summary to 12 lead ECG interpretation
1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS
– Normal
– Left axis deviation
– Right axis deviation
– Right superior axis
deviation
4. Calculate INTERVALS
– PR
– QRS
– QT
5.Assess for HYPERTROPHY
– Right and left atrial
enlargement
– Right and left ventricular
hypertrophy
6.Look for evidence of
INFARCTION
– Abnormal Q waves
– ST elevation or
depression
– Peaked, flat or inverted T
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45. Reading assignment
Bundle branch block
Left Bundle branch block
Right Bundle branch block
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Question ?
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