Determining QRS Axis
Key facts for finding normal, left, right, and extreme axis using leads I, aVF, and II.
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Questions Covered in This Set
11 cards to master
What is the mean QRS axis?
The sum of all ventricular depolarization vectors — one overall arrow showing the direction ventricular electricity travels, normally down and to the patient's left.
Normal axis range (degrees)
−30° to +90° (some texts extend to +100°).
Lead angles in the hexaxial system
I = 0°, II = +60°, aVF = +90°, III = +120°, aVL = −30°, aVR = −150°.
Golden rule of vectors
If depolarization travels toward a lead's positive electrode, that lead shows an upright QRS; if away, the QRS is negative.
Two-lead quadrant method results
I+/aVF+ = normal; I−/aVF+ = right axis; I+/aVF− = possible left axis; I−/aVF− = extreme axis.
Three-lead algorithm for true LAD
Lead I positive + aVF negative + lead II negative = left axis deviation (−30° to −90°).
If I+ , aVF−, but lead II is positive?
Axis is between 0° and −30° — still normal, not true LAD.
Common causes of left axis deviation
Left anterior fascicular block (most common), LVH, inferior MI, horizontal heart in obesity/pregnancy.
Common causes of right axis deviation
RVH, pulmonary hypertension/COPD, acute PE, lateral MI, left posterior fascicular block, tall thin young people, normal in infants.
Causes of extreme ("northwest") axis
Ventricular tachycardia, hyperkalemia, paced rhythm — or reversed limb leads (upright aVR with inverted lead I).
Isoelectric lead method
Find the most equiphasic limb lead; the axis is perpendicular (90°) to it — choose the direction confirmed by an upright lead.
