The cardiac electrical axis !
- Measurement
- Exam priority: medium
The axis is the average direction in which ventricular depolarization travels; you work it out from the QRS in the limb leads.
Updated 2026-11-01
Also called: QRS axis
See it in 3D: Left ventricle · Right ventricle · Bundle branches · Interventricular septum
What it is
Left ventricular mass dominates depolarization, so the mean QRS vector points down and to the left. If one ventricle grows or a conduction fascicle is blocked, the vector swings. The axis is not a diagnosis by itself, but it points toward hypertrophy, fascicular blocks or acute right ventricular strain.
Key facts
- Hexaxial system: I 0°, II +60°, III +120°, aVF +90°, aVL −30°, aVR −150°.
- The lead where the QRS is equiphasic (as positive as negative) is perpendicular to the axis.
- The lead with the most positive QRS lies closest to the axis.
- People living at high altitude tend toward a slightly more rightward axis from the extra right ventricular load.
Electrocardiogram: a normal beat in lead II
HR 75 /minRR 0.80 sPR 0.16 sQRS 0.09 sQT 0.37 sQTc 0.41 s
Tap a wave (P, QRS, T, U), an interval (PR, ST, QT) or the RR.
- ECG trace (lead II)
- Small box: 0.04 s · 0.1 mV
- Big box: 0.20 s · 0.5 mV
- Selected part
QRS axis ranges (frontal plane)
| Parameter | Value | Note |
|---|---|---|
| Normal (adult) | −30° to +90° | |
| Left axis deviation | < −30° | |
| Right axis deviation | > +90° | |
| Extreme axis ("no man's land") | −90° to ±180° | ventricular tachycardia, lead error |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Quadrants from leads I and aVF
| Lead I | aVF | Axis |
|---|---|---|
| Positive | Positive | Normal (0° to +90°) |
| Positive | Negative | Leftward: normal if II is positive; left deviation if II is negative |
| Negative | Positive | Right axis deviation |
| Negative | Negative | Extreme axis |
Clinical notes
- A new rightward shift with tachycardia and an S in I, Q and inverted T in III suggests acute RV strain (pulmonary embolism), though the pattern is insensitive.
- Long-standing hypertension causes LV hypertrophy and pushes the axis left.
- Swapping the arm electrodes inverts lead I and mimics right axis deviation.
Test yourself
Think of the answer, then tap the question to see it.
Why does the QRS axis normally point down and to the left?
Because the left ventricle has far more mass and its depolarization dominates the mean vector, which heads toward the apex.
A patient's QRS is positive in I, negative in aVF and negative in II. What is the axis and which conduction problem often explains it?
Left axis deviation (more negative than −30°); a typical cause is left anterior fascicular block.
Why can acute pulmonary embolism swing the axis to the right?
The clot abruptly raises right ventricular afterload; the RV dilates and gains weight in the mean vector.
What does an equiphasic QRS in lead I tell you?
That the axis is perpendicular to lead I, near +90° or −90°; aVF tells you which.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- ECG basics: waves, intervals and leads
- The cardiac conduction system
- The ECG in ischemia and infarction
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 19.2 Cardiac Muscle and Electrical Activity (OpenStax (Rice University), 2022); Acute Pulmonary Embolism (StatPearls (NCBI Bookshelf), 2024); Hypertensive Heart Disease (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 12). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.