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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_2818_Библиотеки_им_академика_М_И_Перельмана
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7.1 Axial Reference System
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Fig. 7.3 Diagram showing triaxial reference system formed by axes of standard limb leads
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7 Electrical Axis
Fig. 7.4 Diagram showing triaxial reference system formed by axes of augmented limb leads
7.2 Basic Concept ofQRS Axis
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7.1.2 Hexaxial Reference System
The hexaxial reference system is constituted and the lead axes are separated from
each other by 30° (Fig.7.5) by combining and superimposing the two triaxial reference system. In this reference system, all the lead axes maintain their polarity and
direction. By convention, all the degrees in the lower half are labelled as positive (0°
to +180°) and all the degrees in the upper half are labelled negative degrees (0° to
−180°). It is very important to understand some basic facts before we move onto the
actual calculation of the QRS axis.
7.2 Basic Concept ofQRS Axis
• The QRS axis is expressed in the form of degrees on the hexaxial reference sys-
tem and it represents the mean direction of the electrical force on the fron-
tal plane.
• An electrical force traveling perpendicular to a lead produces nil or equiphasic
deection in that lead and when the electrical force travels parallel to a lead it
produces maximum deection in that lead. For example, if the QRS axis is 0°,
then the maximum deection will be seen in lead I and least deection is recorded
in lead aVF which is at right angle to lead I (Fig.7.5). Keep in mind that lead I
and lead aVF are perpendicular to each other. Similarly, lead II and lead aVL are
perpendicular to each other, and lead III and lead aVR are perpendicular to
each other.
• In the lead showing the maximum deection, the axis corresponds to the major
deection in that lead. For example, if in lead II the major deection is positive,
then the axis is towards +60°. See Fig.7.5 and note that the positive pole of the
lead II axis points to +60°. Similarly if the major deection in lead aVF is
−7mm, then the axis is −90°. See Fig.7.5 and note that the negative pole of the
lead aVF axis points to −90°.
• The net or resultant deection in any lead is the algebraic sum of the positive and
negative deection in that lead. For example, if in lead I the height of R wave is
10mm (+10) and the depth of S wave is 4mm (−4), then the net deection is
+10+(−4)=+6.
The mean QRS vector points downwards and to the left. It is aligned to the direction
of current ow, from base of heart towards the apex. Thus, it can be observed that
the direction is towards +40° to +60° if we take into account the hexaxial reference
system. This is towards the positive pole of lead II (Fig.7.6). This in turn means the
deection in lead II will be upward (positive) and R wave will be recorded in lead II.
(Standard limb leads)
(Augmented unipolar leads)
L
-90°
L
-90°
L
-90°
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I
I
aV
aV
aV
-30°
-30°
-30°
-60°
-60°
-60°
-120°
-120°
-120°
-150°
-150°
-150°
aVR
aVR
aVR
I
0°
0°
0°
180°
180°
180°
+
+
+
+30°
+30°
+30°
+150°
+150°
+150°
+60°
+60°
+60°
+90°
+90°
+90°
+120°
+120°
+120°
II
II
II
aVF
aVF
aVF
Hexaxial reference system
III
III
III
7 Electrical Axis
aVL
aVR
60°
I
+
60°
II
III
aVF
Triaxial reference system
Triaxial reference system
Fig. 7.5 Diagram showing hexaxial reference system formed by combination of two triaxial reference systems
7.3 Calculation ofQRS Axis
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Fig. 7.6 Mean QRS axis is directed towards positive pole of lead II (+60°)
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7.3 Calculation ofQRS Axis
No unanimity exists on the range of normal QRS axes. It has been described as
between 0° and +90° by many authors, while some others have taken the view that
it ranges from −30° to +110°. In this book, the normal range of QRS axis will be
considered as −30° to +90° (Fig.7.7). In most of the cases, the normal axis lies
between +40° and +60°.
QRS axis beyond the normal range indicates deviation of electrical axis. QRS
axis beyond −30° (more negative, anticlockwise) is called left axis deviation (LAD),
and if it lies beyond +90° (more positive, clockwise), it is called right axis deviation
(RAD) (Fig. 7.8). If the QRS axis happens to fall between −90° and 180°, this
would be referred to as extreme axis deviation or northwest axis whereby the ventricular vector is directed upward and to the right. Axis in northwest region is very
rare. It may be a manifestation of extreme right or left axis deviation.
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Moving clockwise
(--) Left axis deviation more than -30 degrees
(+) Right axis deviation more than +90 degree
(-)90
0
Mean QRS axis lies between -30 to +90 degrees
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7 Electrical Axis
Fig. 7.7 Normal QRS
axis. The area shaded light
green is the normal QRS
axis, which is directed
between −30° and +90°
180
aVR
-150
0
0
0
0
(-)60
0
(+) 60
II
aVL
(-)30
0
0
(+) 30
0
I
0
(+)
0
0
(-)
120
aVR
0
(-)
150
0
-
( )180
+
0
(+)150
0
(+)
120
III
0
-120
0
-90
0
-600
(+)
90
aVF
Moving anticlockwise
aVL
0
-30
I(+)
0
150
+
0
+
120
III
+90
aVF
s
Fig. 7.8 Axis deviation. In right axis deviation QRS axis is in between +90° and +180° and in left
axis deviation the QRS axis is in between −30° and −90°
Tips and Tricks
• Minor degrees of axis deviation is common in tall thin persons as well as in short
obese persons.
• One should look for hypertrophy of ventricles, conduction defect and pulmonary
embolism if an axis deviation is present.
Hexaxial reference system is used for calculation of QRS axis. Out of several
methods described in various books, some of which are confusing for beginners,
only two simple methods will be described here.
0
+
30
0
60
+
0
II
III aVF
Normal
Normal
Normal
Left axis deviation
Right axis deviation
North west axis
7.3 Calculation ofQRS Axis
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7.3.1 Method 1
The rst method is the quadrant approach or two-lead method. A rough estimation
of the QRS axis can be obtained by this rapid method. This method is for the beginners, for whom it is important to determine whether the QRS axis is in normal range
or not. In this method, the dominant QRS deections in lead I and lead aVF are
considered. After this the interpretation is as follows (Fig.7.9):
Lead I Lead aVF QRS axis
Positive (R wave) Positive (R wave) Normal (−30° to +90°)
Negative (S wave) Positive (R wave) Right axis (+90° to +180°)
Positive (R wave) Negative (S wave) Left axis (−30° to −90°)
Negative (S wave) Negative (S wave) Northwest axis (−90° to −180°)
The main drawback of this method is it only gives a close approximation to the
true axis. It narrows the normal axis range. This can result in an inaccurate interpretation of the true electrical axis. For example, if using this approach with a
positive lead I and negative lead aVF, the axis would be interpreted as left axis
deviation. However, if the true axis were −10°, it would still be within the normal
axis range. In spite of this drawback, this method is easy to learn and sufcient in
most cases.
Fig. 7.9 Method 1 of QRS
axis determination
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7 Electrical Axis
7.3.2 Method 2
This is a more precise method of calculation of QRS axis. In the methods described
above, axis calculation is an estimate or near approximation. Generally, further
accuracy is not clinically signicant, yet, it is always important to know the precise
calculation to gain complete understanding about QRS axis calculation.
The following steps are to be followed to calculate the QRS axis of the ECG
shown in Fig.7.10.
IV1
II
III aVF
aVR
aVL
V2
V3
V4
V5
V6
Fig. 7.10 12-Lead ECG
7.3 Calculation ofQRS Axis
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Step I
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 +9 (R wave) and −1 (s wave) in the given ECG.So, the net
resultant is +8 (+9+[−1]=+8).
Now, plot +8in the lead axis of lead I (Fig.7.11).
Step III
Next draw a perpendicular through the plotted point on lead I axis (Fig.7.12).
Step IV
Now calculate the net resultant in lead aVF.In the given ECG, the net resultant is
+10 (+10+0)=+10). Plot +10in the lead axis of lead aVF (Fig.7.13).
Fig. 7.11 Plotting of +8
on axis of lead I
–
Fig. 7.12 Perpendicular
drawn on lead I axis
through +8
–
+
aVF
–
–
+
aVF
+
I
+
I
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7 Electrical Axis
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.7.14).
Step VI
Next draw a line joining the point of the intersection of axis of lead I and lead aVF
and the point of intersection of the two perpendicular lines. The QRS axis is +51°
(Figs.7.15 and 7.16). This is normal QRS axis.
Fig. 7.13 Plotting of +10
on axis of lead aVF
–
Fig. 7.14 Perpendicular
drawn on axis of lead aVF
through +10
Fig. 7.15 QRS axis
is +51°
–
+
aVF
–
–
+
aVF
–
+
I
+
I
–
+
aVF
+51°
+
I
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