The document discusses the cardiac axis and methods for determining it from an electrocardiogram (ECG). The cardiac axis represents the overall direction of electrical conduction through the heart. It is normally between -30° and +90° but can deviate left or right. Left axis deviation occurs between -30° and -90° and may indicate conduction abnormalities, while right axis deviation is between +90° and +180° and can be caused by right ventricular hypertrophy. The hexaxial reference system and other methods are described for calculating the axis from the ECG readings.

1. JAMIA MILLIA ISLAMIA
Centre for Physiotherapy and Rehabilitation Sciences
Subject: Physiotherapy in Cardiopulmonary conditions (BPT 402)
Submitted to: Dr. Jamal Moiz
Submitted by: Hamda Furqan
BPT 4th Year
Roll no. 17BPT012
Enrolment no. 17-7395
Serial no. 16
Topic: Cardiac Axis

2. Cardiac Axis
• The electrical activity of the heart starts at the sinoatrial node then spreads
to the atrioventricular (AV) node.
• It then spreads down the bundle of His and then Purkinje fibres to cause
ventricular contraction.
• Whenever the direction of electrical activity is towards a lead you get a
positive deflection in that lead.
• Whenever the direction of electrical activity is away from a lead you get a
negative deflection in that lead.
• The cardiac axis gives us an idea of the overall direction of electrical
activity.
• Axis interpretation using the schematic illustration demonstrates the
relationship between QRS axis and the frontal leads of the ECG.

4. Hexaxial reference system
• The hexaxial reference system is diagram based on the first six leads of the 12 lead ECG.
It is used to help determine the heart's electrical axis in the frontal plane. To use the
hexaxial reference system, locate the most isoelectric (or equiphasic) lead (I, II, III, aVR,
aVL, or aVF) on a diagnostic quality ECG with proper lead placement. Then find the
corresponding spoke on the hexaxial reference system. The perpendicular spoke will point
to the heart's electrical axis. To determine which numerical value should be used, observe
the polarity of the perpendicular lead on the ECG. For example, if the most isoelectric (or
equiphasic) lead is aVL, the perpendicular lead on the hexaxial reference system is lead
II. If lead II is positively deflected on the ECG, the heart's electrical axis in the frontal
plane will be approximately +60o.
• Normal axis: -30o to +90o
• Left axis deviation: -30o to -90o
• Right axis deviation: +90o to +180o
• Extreme axis deviation: -90o to -180o

5. Method 2:
• Find the net deflection in lead I and aVF, which are perpendicular to each other
• Plot the net deflection in these leads onto their respective axes, on a scale of 0 to 10
• Drop perpendicular lines from these points and plot a point where these points intersect
• Join the centre of the circle to the intersection point and extend it to the circumference
• The point on the circumference where the line intersect is the QRS axis
• Example: net deflection in L1 is +5, Net deflection in Avf is 0, axis is 0
Method 3:
For rapid and easy estimation of QRS axis, just scan the direction of the dominant deflection in the
leads L1 and Avf whether positive or negative. This gives us quadrant in which the QRS axis is located

7. Normal Cardiac Axis
• In healthy individuals, you would expect the axis to lie between -30°
and +90º.
• The overall direction of electrical activity is towards leads I, II and III
(the yellow arrow below).
• As a result, you see a positive deflection in all of these leads, with lead
II showing the most positive deflection as it is the most closely aligned
to the overall direction of electrical spread.
• You would expect to see the most negative deflection in aVR. This is
due to aVR looking at the heart in the opposite direction.

9. Right Axis Deviation
• Right axis deviation (RAD) involves the direction of depolarisation
being distorted to the right (between +90º and +180º).
• The most common cause of RAD is right ventricular hypertrophy.
• Extra right ventricular tissue results in a stronger electrical signal
being generated by the right side of the heart.
• This causes the deflection in lead I to become negative and the
deflection in lead aVF/III to be more positive.
• RAD is commonly associated with conditions such as pulmonary
hypertension, as they cause right ventricular hypertrophy.
• RAD can, however, be a normal finding in very tall individuals.

11. Left Axis Deviation
• Left axis deviation (LAD) involves the direction of depolarisation
being distorted to the left (between -30° and -90°).
• This results in the deflection of lead III becoming negative (this is only
considered significant if the deflection of lead II also becomes
negative).
• LAD is usually caused by conduction abnormalities.

14. References
• Author: Michael Rosengarten BEng, MD.McGill – Right axis
deviation – via Wikimedia Commons – Licence: CC BY-SA 3.0
• Author: Michael Rosengarten BEng, MD.McGill – Left axis deviation
– via Wikimedia Commons – Licence: CC BY-SA 3.0