The cardiac axis represents the average direction of electrical depolarization through the ventricles. It is normally between -30 and 90 degrees but can deviate right or left. Right axis deviation occurs when the axis swings right of normal, seen as a negative deflection in lead I. Left axis deviation occurs when the axis swings left of normal, seen as a negative deflection in lead II. Specific conduction defects or conditions can cause axis deviation by altering the pathways of ventricular depolarization.

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Simplified

2. Cardiac Axis
● It is the average direction of spread
of the depolarization wave through
the ventricles as seen from the
front.
● Because the left ventricle contains
more muscle than the right, it has
more influence on the cardiac axis.
● The depolarization wave normally
spreads through the ventricles from
11 to 5 o'clock.
● The direction of Cardiac Axis can be
derived from the QRS complex in
leads I, II and III.

3. General Principles
● When the depolarization wave is
moving towards a lead, the QRS
complex is predominantly upward
(positive), the R wave is greater
than the S wave.
● When the depolarization is moving
away from a lead, the QRS complex
is predominantly downward
(negative), the S wave is greater
than the R wave.
● When the depolarization wave is
moving at right angles to the lead,
the R and S waves are of equal size
(equiphasic).
X
lead
X
lead
X
lead

4. Normal Cardiac Axis
● Normally, the depolarizing
wave is spreading away
from lead VR towards leads
I, II and III.
● Normally, there is a
predominantly upward
deflection in these 3 leads.
● On the contrary, there is a
predominantly downward
deflection in the VR lead.

5. Right Axis Deviation
(RAD)
● In RAD the average
depolarization wave (the
axis) will swing towards
the right.
● The deflection in lead I
becomes negative, and
the deflection in lead III
becomes more positive.

6. Left Axis Deviation
(LAD)
● In LAD, the cardiac axis
swing to the left.
● The QRS becomes
predominantly negative in
lead III, and positive in lead
I.
● LAD is not significant until
the QRS deflection is also
predominantly negative in
lead II.

7. The Cardiac Axis
in Degrees
● The cardiac axis is sometimes
measured in degrees, though
this is not clinically useful.
● Lead I is taken as looking at
the heart from 0°; lead II from
+60°; lead VF from +90°; and
lead III from +120°. Leads VL
and VR look from-30° and-
150°, respectively.
● The normal cardiac axis is in
the range -30° to +90°.

8. LAD in Degrees
● If in lead II the S wave is
greater than the R wave,
the axis must be more
than 90° away from
lead II.
● In other words, it must
be at a greater angle
than -30°, and LAD is
present.

9. RAD in Degrees
● If the size of the R wave
equals that of the S wave
in lead I, the axis is at right
angles to lead I or at +90°.
● If the S wave is greater
than the R wave in lead I,
the axis is at an angle of
greater than +90°, and
RAD is present.

11. Anatomy
of Conduction System
● The left bundle branch
has two divisions: the
anterior and posterior
'fascicles’.
● The right bundle
branch has no
divisions.
● The depolarization
wave therefore spreads
into the ventricles by
three pathways.

12. left Anterior Fascicular Block
● If the anterior fascicle of the LBB
fails to conduct, the left ventricle
depolarized through the posterior
fascicle, so the cardiac axis
rotates upwards.
● LAD is therefore due to left
anterior fascicular block, or 'left
anterior hemiblock’.
● If the posterior fascicle blocked
(rare), the ECG shows RAD.

13. Right Bundle Branch Block
● In RBBB the cardiac axis
usually remains normal,
because there is normal
depolarization of the left
ventricle with its large muscle
mass.
● If both the RBB and the left
anterior fascicle are blocked
(bifascicular block), the ECG
shows RBBB and LAD.

15. Causes of Right Axis Deviation
• Normal variation (e.g., children, young adults)
• Limb-lead reversal (left- and right-arm electrodes)
• Right ventricular hypertrophy
• Conduction defects: left posterior fascicular block
• Lateral wall myocardial infarction
• Wolff-Parkinson-White syndrome
• Ventricular ectopic rhythms (e.g., ventricular tachycardia)
• Congenital heart disease (e.g., secundum atrial septal defect)
• Dextrocardia
• Left pneumothorax
• Conditions that cause right ventricular strain (e.g., pulmonary embolism,
pulmonary stenosis, pulmonary hypertension, chronic lung disease, and
resultant cor pulmonale)

16. Causes of Left Axis Deviation
● Normal variation
● Left ventricular hypertrophy
● Conduction defects: left anterior fascicular block, bifascicular block
● Inferior wall myocardial infarction
● Wolff-Parkinson-White syndrome
● Ventricular ectopic rhythms (e.g., ventricular tachycardia)
● Congenital heart disease (e.g., primum atrial septal defect)
● Hyperkalemia
● Mechanical shift, such as with raised diaphragm (e.g., pregnancy,
ascites, abdominal tumor, organomegaly)
● Pacemaker-generated rhythm or paced rhythm