This document outlines the steps for systematically reading a 12-lead ECG: 1. Calculate the heart rate 2. Determine the rhythm 3. Determine if the QRS axis is normal or shows left/right axis deviation 4. Calculate intervals between waves 5. Assess for signs of cardiac hypertrophy 6. Look for evidence of a myocardial infarction It provides guidance on analyzing each component, such as how to identify normal vs. abnormal QRS axis using leads I and II.

1. ECG Rhythm Interpretation
Reading 12-Lead ECG’s

2. Course Objective
• To systematically analyze a 12-lead
ECG.

3. Learning Modules
• ECG Basics
• How to Analyze a Rhythm
• Normal Sinus Rhythm
• Heart Arrhythmias
• Diagnosing a Myocardial Infarction
• Advanced 12-Lead Interpretation
• Reading 12-Lead ECG’s

4. Reading 12-Lead ECGs
• The 12-Lead ECG contains information that will assist
you in making diagnostic and treatment decisions in your
clinical practice. In previous modules you learned how to
read and interpret parts of the ECG. Now, we will bring all
that you have learned together so that you can
systematically read and interpret a 12-lead ECG.
• The information will be divided into two modules, VII a
and VII b.

5. Reading 12-Lead ECGs
The best way to read 12-lead ECGs is to develop a step-
by-step approach (just as we did for analyzing a rhythm
strip). In these modules we present a 6-step approach:
1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS
4. Calculate INTERVALS
5. Assess for HYPERTROPHY
6. Look for evidence of INFARCTION

6. Rate Rhythm Axis Intervals Hypertrophy Infarct
• In Module II you learned how to calculate the
rate. If you need a refresher return to that module.
• There is one new thing to keep in mind when
determining the rate in a 12-lead ECG…

7. Rate Rhythm Axis Intervals Hypertrophy Infarct
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. Rate? 12 (R waves) x 6 = 72 bpm

8. Rate Rhythm Axis Intervals Hypertrophy Infarct
• In Module II you learned how to systematically
analyze a rhythm by looking at the rate, regularity,
P waves, PR interval and QRS complexes.
• In Modules III, IV and V you learned how to
recognize Normal Sinus Rhythm and the 13 most
common rhythm disturbances.
• If you need a refresher return to these modules.

9. Rate Rhythm Axis Intervals Hypertrophy Infarct
Tip: the rhythm strip portion of the 12-lead ECG is a good
place to look at when trying to determine the rhythm
because the 12 leads only capture a few beats.
Lead II
Rhythm?
Atrial
fibrillation
Rhythm strip
1 of 12 leads

10. Rate Rhythm Axis Intervals Hypertrophy Infarct
Axis refers to the mean QRS axis (or vector) during ventricular
depolarization. As you recall when the ventricles depolarize (in a
normal heart) the direction of current flows leftward and downward
because most of the ventricular mass is in the left ventricle. We like
to know the QRS axis because an abnormal axis can suggest
disease such as pulmonary hypertension from a pulmonary
embolism.

11. Rate Rhythm Axis Intervals Hypertrophy Infarct
The QRS axis is determined by overlying a circle, in the frontal
plane. By convention, the degrees of the circle are as shown.
The normal QRS axis lies between -30o
and +90o
.
0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
A QRS axis that falls between -30o
and -90o
is abnormal and called left
axis deviation.
A QRS axis that falls between +90o
and +150o
is abnormal and called
right axis deviation.
A QRS axis that falls between +150o
and -90o
is abnormal and called
superior right axis deviation.

12. Rate Rhythm Axis Intervals Hypertrophy Infarct
• Causes of left axis deviation
include:
– Left ventricular hypertrophy
– Inferior wall MI
– Left bundle branch block
– Left anterior fascicular block
– Horizontal heart
0o
-90o
90o
180o
• Causes of right axis deviation
include:
– Right ventricular hypertrophy
– Lateral wall MI
– Right bundle branch block
– Pulmonary hypertension
– Vertical heart

13. Rate Rhythm Axis Intervals Hypertrophy Infarct
We can quickly determine whether the QRS axis is
normal by looking at leads I and II.
If the QRS complex is
overall positive (R > Q+S)
in leads I and II, the QRS
axis is normal.
QRS negative (R < Q+S)
In this ECG what leads
have QRS complexes
that are negative?
equivocal?
QRS equivocal (R = Q+S)

14. Rate Rhythm Axis Intervals Hypertrophy Infarct
How do we know the axis is normal when the QRS
complexes are positive in leads I and II?

15. Rate Rhythm Axis Intervals Hypertrophy Infarct
The answer lies in the fact that each frontal lead
corresponds to a location on the circle.
0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
I
II
avF
avLavR
Limb leads
I = +0o
II = +60o
III = +120o
Augmented leads
avL = -30o
avF = +90o
avR = -150o
I
IIIII

16. 0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
Rate Rhythm Axis Intervals Hypertrophy Infarct
Since lead I is orientated at 0o
a wave of depolarization directed towards it
will result in a positive QRS axis. Therefore any mean QRS vector
between -90o
and +90o
will be positive.
I

17. 0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
Rate Rhythm Axis Intervals Hypertrophy Infarct
Since lead I is orientated at 0o
a wave of depolarization directed towards it
will result in a positive QRS axis. Therefore any mean QRS vector
between -90o
and +90o
will be positive.
Similarly, since lead II is orientated at 60o
a wave of depolarization directed towards
it will result in a positive QRS axis.
Therefore any mean QRS vector between
-30o
and +150o
will be positive.
I
II

18. 0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
Rate Rhythm Axis Intervals Hypertrophy Infarct
Since lead I is orientated at 0o
a wave of depolarization directed towards it
will result in a positive QRS axis. Therefore any mean QRS vector
between -90o
and +90o
will be positive.
Similarly, since lead II is orientated at 60o
a wave of depolarization directed towards
it will result in a positive QRS axis.
Therefore any mean QRS vector between
-30o
and +150o
will be positive.
Therefore, if the QRS complex is positive
in both leads I and II the QRS axis must
be between -30o
and 90o
(where leads I
and II overlap) and, as a result, the axis
must be normal.
I
II

19. 0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
Rate Rhythm Axis Intervals Hypertrophy Infarct
Now using what you just learned fill in the following table. For example, if
the QRS is positive in lead I and negative in lead II what is the QRS axis?
(normal, left, right or right superior axis deviation)
QRS Complexes
I
AxisI II
+ +
+ -
normal
left axis deviation
II

20. 0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
Rate Rhythm Axis Intervals Hypertrophy Infarct
… if the QRS is negative in lead I and positive in lead II what is the QRS
axis? (normal, left, right or right superior axis deviation)
QRS Complexes
I
AxisI II
+ +
+ -
- +
normal
left axis deviation
right axis deviation
II

21. 0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
Rate Rhythm Axis Intervals Hypertrophy Infarct
… if the QRS is negative in lead I and negative in lead II what is the QRS
axis? (normal, left, right or right superior axis deviation)
QRS Complexes
I
AxisI II
+ +
+ -
- +
- -
normal
left axis deviation
right axis deviation
right superior
axis deviation
0o
30o
-30o
60o
-60o
-90o
-120o
90o120o
150o
180o
-150o
II

22. Rate Rhythm Axis Intervals Hypertrophy Infarct
Is the QRS axis normal in this ECG? No, there is left axis
deviation.
The QRS is
positive in I
and negative
in II.

23. Rate Rhythm Axis Intervals Hypertrophy Infarct
To summarize:
– The normal QRS axis falls between -30o
and +90o
because ventricular
depolarization is leftward and downward.
– Left axis deviation occurs when the axis falls between -30o
and -90o
.
– Right axis deviation occurs when the axis falls between +90o
and +150o
.
– Right superior axis deviation occurs when the axis falls between between
+150o
and -90o
.
QRS Complexes
AxisI II
+ +
+ -
- +
- -
normal
left axis deviation
right axis deviation
right superior
axis deviation
– A quick way to determine
the QRS
axis is to look at the QRS
complexes in leads I and II.

24. SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct
To summarize VII a:
1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS
– Normal
– Left axis deviation
– Right axis deviation
– Right superior axis deviation

25. SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct
In VII b we will cover the next 3 steps:
1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS
4. Calculate INTERVALS
5. Assess for HYPERTROPHY
6. Look for evidence of INFARCTION