The Electrical Axis of the Heart
At any one instant the ventricles begin to depolarize and continue to
conduct, there is a primary direction in which the electrical activity is flow at
that very instant. The mean electrical axis is the average of all the
instantaneous mean electrical vectors occurring sequentially during
depolarization of the ventricles.
1. Conduction down the branch interventricular septum depolarizes from
left to right (Q wave; negative; away from the positive lead II electrode).
2. 20 msec later: Depolarisation toward the apex (vector 2)
3. 20 msec later: Depolarisation toward the left arm (vector 3)
(predominantly right to left movement)
4. S Wave (vector 4) (towards the left)
Figure 1: The sum of electrical activity during
the QRS complex identifies that the major
direction of flow of electrical activity is about
Diagnostic Use of the Heart’s Electrical Axis 60 degrees
Deviation to the Right (above 120 degrees)
o Increased Right Ventricular Mass (Ventricular
hypertrophy) due to something preventing the right ventricle to pump blood
through the pulmonary circuit.
o Chronic obstructive lung disease – hinders breathing out (expiration) and leads to
inefficient gas exchange (leads to heart to compensate by increasing force of
o Pulmonary embolism – blockage of blood flow in the lungs which leads the heart
to force more blood through the lungs (forcing heart to grow)
o Congenital heart defects
o Severe pulmonary hypertension – high blood pressure in pulmonary circuit and
so the right ventricle will compensate by growing so that it can attempt to more
Deviation to the Left (less than 0 degrees)
o Increased left ventricular mass (left ventricular hypertrophy)
o Hypertension - high blood pressure in the systemic circulation
o Aortic stenosis – where the aortic valves doesn’t open properly (i.e. smaller
opening in which blood can flow through)
o Ischemic heart disease Thiruvarangan
Calculating the Mean Electrical Axis
1. Look at the lead I ECG. Calculate the isoelectric line to R distance. This equals “a”
2. Look at the lead I ECG. Calculate the isoelectric line to S distance. This equals “b”
3. Add “a” plus “b”. Note that “b” is a
4. Do the same for the lead II and II
ECG traces to find “c”, “d”, “e” and “f”.
5. Calculate “c + d” and “e + f”
6. Draw an equilateral triangle.
7. Starting at the centre of each line
(which represent leads I, II and II)
measure the distance represented by
“a + b”, “c + d” and “e + f” (right is
8. Draw a perpendicular line from the
end of these vectors into the middle
of the triangle.
9. Determine the centre of the triangle.
10. Draw a line from the centre of the
triangle to the point at which the
perpendicular lines (from the end of a + b, c + d and e + f meet).
11. The line from the centre of the triangle the meeting point of