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7.3 Calculation ofQRS Axis
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Fig. 7.16 Mean QRS axis is +51°. This is normal QRS axis. The axis is directed towards the positive pole of lead II
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Tips and Tricks
• Do not get confused. Axis determination is not very complicated.
• Only limb leads are studied to calculate the QRS axis.
• There is normal axis if lead I, II and III has R wave and the tallest R wave is pres-
ent in lead II.
• Normal axis means there are dominant R waves in both leads I and aVF.
• Normal QRS axis lies between −30° and +90°.
Self-Assessment Questions
1. QRS axis is the mean direction of depolarization wavefront of the two ventricles.
True or false?
2. The normal QRS axis lies between −60° and +90°. True or false?
3. In normal QRS axis, the R wave is taller in lead III than in lead II.True or false?
4. In normal QRS axis, R wave is seen in leads I, II and III.True or false?
5. In triaxial reference system each angle is separated by 30°. True or false?
6. What is the normal range for the QRS axis on an ECG?
a. −30° to +90° b. −90° to +30° c. 0° to +180° d. −180° to 0°
7. A QRS axis of −60° would be considered:
a. Normal b. Left axis deviation c. Right axis deviation d. Indeterminate axis
8. A QRS axis of +120° would be considered:
a. Normal b. Left axis deviation c. Right axis deviation d. Indeterminate axis
9. QRS axis of −20° would be considered:
a. Normal b. Left axis deviation c. Right axis deviation d. Indeterminate axis
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aVF
I
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7 Electrical Axis
Case Studies
1. Calculate the mean QRS axis, based on the six limb leads given below (Fig.7.17).
2. Calculate the mean QRS axis based on lead I and lead aVF as shown in Fig.7.18.
III III
Fig. 7.17 Calculate the mean QRS axis
–
–
+
aVF
Fig. 7.18 Calculate the mean QRS axis
I
+
aVFaVLaVR
–
7.3 Calculation ofQRS Axis
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99
Answers
1. True 2. False 3. False 4. True 5. False 6. a 7. b 8. c 9. a
Case Studies
1. Let us use method 1and method 3 to calculate the mean QRS axis.
Method 1: There is predominantly positive wave in lead I and completely
positive wave in lead aVF.Hence, the mean QRS axis is normal.
Method 2: By using method 2, it can be seen that the algebraic sum of the
deections in lead I is +2 (+5+[−3]=+2). Similarly, the resultant of the deections in lead aVF is +10 (+10+0=+10). Hence, by plotting the resultant on the
ECG grid, the mean QRS axis calculated is +78° (Fig.7.19).
2. The following steps are to be followed to calculate the QRS axis of the ECG
(lead I and lead aVF shown) in Fig.7.18.
Step I
Plot the lead axis of lead I and lead aVF (Fig.7.20).
Step II
Calculate the total positive and total negative deection of the QRS complex
in lead I.For example, it is +7 (R wave) and −2 (S wave) in the given ECG.So
the net resultant is +5 (+7+[−2]=+5).
Fig. 7.19 Mean QRS axis
is +78°. This is normal
QRS axis
–
Fig. 7.20 Lead axis of
lead I and lead aVF
–
+
aVF
–
aVF
+78°
+
+
I
+
I
100
–
–
–
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7 Electrical Axis
Now plot +5in the lead axis of lead I (Fig.7.21).
Step III
Draw a perpendicular through the plotted point on lead I axis (Fig.7.22).
Step IV
Now similarly calculate the net resultant in lead aVF.In the given ECG, the
net resultant is +7 (+9+[−2]=+7). Now plot +7in the lead axis of lead aVF
(Fig.7.23).
Step V
Draw a perpendicular through the plotted point on lead aVF axis and prolong
it to meet the perpendicular drawn on lead I axis (Fig.7.24).
Step VI
Draw a line joining the point of the intersection of axis of lead I and aVF and
the point of intersection of the two perpendicular lines. The QRS axis is +55°
(Fig.7.25). So, it is normal QRS axis.
Fig. 7.21 Plotting of +5
on axis of lead I
Fig. 7.22 Perpendicular
drawn on lead I axis
through +5
Fig. 7.23 Plotting of +7
on axis of lead aVF
–
+
aVF
–
+
aVF
–
+
aVF
+
I
+
I
+
I
–
–
7.3 Calculation of QRS Axis
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Fig. 7.24 Perpendicular
drawn on axis of lead aVF
through +7
Fig. 7.25 QRS axis
is +55°
101
–
+
aVF
+
I
–
+
aVF
+
+55°
I
Chapter 8
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Left Axis Deviation
Learning Objectives
After studying this chapter, the reader will learn about:
• Left axis deviation
• Calculation of left axis deviation
• Causes of left axis deviation
In left axis deviation (LAD), the QRS axis lies between −30° and −90°. QRS axis
between −90° and −180° is very rare. To calculate QRS axis, let us consider the
QRS tracing of Fig.8.1. At rst glance, it may be possible to say that there is a left
axis deviation by using the rst method as there is dominant R wave in lead I and
dominant S wave in lead aVF.However, to calculate the exact QRS axis, the second
method has to be followed.
Step I
At rst, plot the lead axis of lead I and lead aVF.
Step II
Next calculate the total positive and total negative deection of the QRS complex in
lead I.For example, it is +4 (r wave) and −1 (s wave) in the given ECG.So, the net
resultant is +3 (+4+[−1]=+3).
Now, plot +3in the lead axis of lead I (Fig.8.2).
Step III
Now draw a perpendicular through the plotted point on lead I (Fig.8.3).
Step IV
Similarly, calculate the net resultant in lead aVF.In the given ECG, the net resultant
is −8 (−12+[+4]=−8). Plot −8in the lead axis of lead aVF (Fig.8.4).
Ltd. 2024
T. K. Koley, Rapid Review of ECG,
https://doi.org/10.1007/978-981-99-9116-7_8
103© The Author(s), under exclusive license to Springer Nature Singapore Pte
104
+
+
+
aVF
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8 Left Axis Deviation
III III
Fig. 8.1 ECG tracing of standard leads
Fig. 8.2 Plotting of +3 on
axis of lead I
Fig. 8.3 Perpendicular
drawn on lead I axis
through +3
aVFaVLaVR
–
–
aVF
–
–
+
I
+
I
Fig. 8.4 Plotting of −8 on
axis of lead aVF
aVF
–
–
+
I
+
+
aVF
8 Left Axis Deviation
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Step V
Next draw a perpendicular through the plotted point on lead aVF axis and prolong
it to meet the perpendicular drawn on lead I axis (Fig.8.5).
Step VI
In the last step, draw a line joining the point of the intersection of axis of lead I and
aVF and the point of intersection of the two perpendicular lines. The QRS axis is
−70° (Figs.8.6 and 8.7).
The various causes of left axis deviation are enumerated in Box 8.1.
Box 8.1 Causes of Left Axis Deviation
Left anterior hemiblock
Left bundle branch block
Left ventricular hypertrophy
Inferior wall myocardial infarction
WPW syndrome
Ostium primum atrial septal defect
Hyperkalaemia
Emphysema
Mechanical shift: ascites and pregnancy
Normal variant or physiologic
Fig. 8.5 Perpendicular
drawn on axis of lead aVF
through −8
Fig. 8.6 QRS axis is
−70°. This is left axis
deviation
–
–
aVF
–
–
-70°
+
I
+
I
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Fig. 8.7 Left axis deviation (QRS axis −70°). The axis is directed towards the negative pole of
lead III
8 Left Axis Deviation
Tips and Tricks
• In left axis deviation, QRS axis lies between −30° and −90°.
• Check the dominant waves in lead I and lead aVF.
• If lead I has dominant R wave and lead aVF has dominant S wave, it is left axis
deviation.
• Any doubt, use method 2.
• In the presence of left axis deviation, always rule out LVH and LAHB.Rule out
hypertension in these patients.
• Asymptomatic patients with left axis deviation do not require any treatment.
Self-Assessment Questions
1. Left axis deviation on an ECG may be a normal nding. True or false?
2. Left axis deviation is seen in ostium secundum ASD.True or false?
3. Left axis deviation is typically caused by a blockage in the right bundle branch
of the heart. True or false?
4. Left axis deviation is observed in individuals with long standing uncontrolled
hypertension. True or false?
5. In left axis deviation, the QRS vector is directed upward and to the left. True
or false?
Case Study
1. Based on the six limb leads shown below (Fig.8.8), calculate the mean QRS
axis. If you can detect any axis deviation, name one condition where you will get
such axis deviation.
8 Left Axis Deviation
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Fig. 8.8 Calculate the mean QRS axis
Answers
1. True 2. False 3. False 4. True 5. True
Case Study
1. To calculate the mean QRS axis, let us use method 1 and method 2.
Method 1: There is positive deection in lead I and negative deection in
lead aVF.Hence, there is left axis deviation.
Method 2: By using method 2, it can be seen that the algebraic sum of the
deections in lead I is +5 (+5+0=+5). Similarly, the resultant of the deections
in lead aVF is −4 (−5+1=−4). Hence, by plotting the resultant on the ECG
grid, the mean QRS axis calculated is −41° (Fig.8.9). Thus, the ECG shows left
axis deviation. Because of left axis deviation, there is tall R wave in lead I (mean
depolarization wavefront moving towards the positive pole of lead I), deep S
wave in lead III (mean depolarization wavefront moving away from the positive
pole of lead III, Fig.8.10) and rS complex in lead II (mean depolarization wavefront moving slightly towards, but mainly away from the positive pole of lead II),
where the depth of S wave is more than the height of r wave.
In left anterior hemiblock, there is left axis deviation. Besides this, there is
deep S waves in both lead II and lead III, and the depth of S wave in lead III is
more than that of S wave in lead II.This is an important feature of left anterior
hemiblock, about which you will read in details in subsequent chapter.
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