The QRS axis: where it points and why

1. Six limb leads, one clock

The six limb leads look at the heart in the frontal plane, 30° apart: I at 0° (towards the patient’s left), II at +60°, aVF at +90° (towards the feet), III at +120°, aVL at −30° and aVR at −150°. The QRS axis is the average direction of ventricular depolarisation; the yellow arrow shows it on the heart. Drag the arrow on the wheel: the complexes follow.

Surgeon’s corner: Quick method: QRS positive in I and in aVF = normal. Positive in I but negative in aVF: look at II — still positive is normal (0 to −30°), negative is left axis deviation.

2. Why it points down and to the left

Depolarisation runs from the AV node down the septum and out through both ventricles at once. The left ventricular wall is about three times thicker than the right, so its forces win: the net vector points at the left ventricle — down, to the left and back, between about −30° and +90°.

Surgeon’s corner: Rule of thumb: the axis turns towards hypertrophied muscle and towards muscle that is activated late (a blocked fascicle), and away from dead muscle (an infarct).

3. Left axis deviation

Beyond −30°. The commonest cause is left anterior fascicular block: the thin anterior fascicle of the left bundle fails, the anterosuperior wall is activated last and the arrow swings up to between −45° and −90° (qR in I and aVL, rS in II, III and aVF, QRS < 120 ms). Other causes: inferior infarction (the inferior forces are lost), left ventricular hypertrophy, pacing from the right ventricular apex.

Surgeon’s corner: The anterior fascicle depends on the LAD septal branches: new left axis deviation during an anterior infarction suggests a proximal LAD lesion. Right bundle branch block plus left anterior fascicular block is a bifascicular block.

4. Right axis deviation

Beyond +90°. The right ventricle has grown (pulmonary hypertension, COPD, congenital heart disease) or is acutely strained (pulmonary embolism). Other causes: left posterior fascicular block (rare, a diagnosis of exclusion), lateral infarction, dextrocardia. It is normal in children and in tall, thin young adults.

Surgeon’s corner: Before diagnosing right axis deviation, check the electrodes: with the arm leads swapped, lead I is upside down (negative P, QRS and T) and aVR looks upright, while the chest leads are normal. New right axis deviation with tachycardia and low oxygen: think of pulmonary embolism — but a normal ECG does not rule it out.

5. Extreme axis ("north-west")

Between −90° and ±180°: I and aVF both negative. Think of ventricular tachycardia (activation starts at the apex and runs upwards), severe hyperkalaemia, some paced rhythms — and electrode misplacement (a right arm–left leg swap can mimic it).

Surgeon’s corner: Extreme axis with a broad QRS is ventricular tachycardia until proven otherwise. With a narrow QRS, check the potassium and the electrode positions before anything rarer.

6. The isoelectric lead trick

For an exact axis, find the limb lead in which the QRS is equiphasic (as much up as down): the axis is at 90° to it, on the side where the perpendicular lead is positive. Example: QRS equiphasic in aVL (−30°) and positive in II: the axis is +60°. Try it on the wheel: turn the arrow until one lead goes flat.

Surgeon’s corner: Read every ECG the same way: rate, rhythm, axis, intervals, then ST and T. A systematic read catches the fascicular block hidden behind more dramatic ST changes.