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The cardiac (QRS) axis is the mean direction of ventricular depolarization in the frontal plane. It is derived from the limb leads using the hexaxial reference system, where each lead is assigned an angle in degrees. Instead of memorizing exact numbers, it is often more practical to recognize which quadrant the axis falls into (normal, leftward, rightward, or extreme).
Conceptually, the axis follows where the dominant ventricular mass depolarizes. Pathology that removes or adds electrical forces (e.g., infarction, hypertrophy) or that reroutes conduction (e.g., fascicular block, bundle-branch block, pacing) will shift the mean axis.
Key idea: Axis is a vector summary of many QRS complexes, not a single lead. It is most useful for pattern recognition: "Does this ECG behave like LVH? RVH? LAFB? VT?" rather than for calculating an exact degree.
Hexaxial reference system showing limb leads (I, II, III, aVR, aVL, aVF) with corresponding degree angles
Axis deviation chart with shaded regions for normal axis, LAD, RAD, and extreme axis
Swipe to see more
| Axis Classification | Degree Range | Lead I | Lead aVF | Common Associations |
|---|---|---|---|---|
| Normal Axis | -30° to +90° | Positive (↑) | Positive (↑) | Physiologic; no pathology |
| Left Axis Deviation (LAD) | -30° to -90° | Positive (↑) | Negative (↓) | LAFB, LVH, inferior MI, LBBB |
| Right Axis Deviation (RAD) | +90° to +180° | Negative (↓) | Positive (↑) | RVH, PE, LPFB, normal variant in youth |
| Extreme Axis | -90° to -180° | Negative (↓) | Negative (↓) | VT, hyperkalemia, lead misplacement, unusual pacing |
Clinical tip: Most emergency providers use the simple "Lead I and aVF" method. If both are positive, the axis is normal. If they differ, you have LAD or RAD based on which is negative.
1. Two-Lead Method (Fast and Practical)
Look at Lead I and Lead aVF:
- I up / aVF up → normal axis
- I up / aVF down → left axis deviation (LAD)
- I down / aVF up → right axis deviation (RAD)
- I down / aVF down → extreme axis
2. Refine with Lead II
- If I is positive and aVF is negative but Lead II is still positive, the axis is likely at the upper end of normal (~0° to -30°) rather than true LAD.
- This helps avoid over-calling LAD in patients with only mild leftward shift.
3. Isoelectric Lead Method (More Precise)
- Identify the limb lead where the QRS is most isoelectric (R and S nearly equal in amplitude).
- On the hexaxial diagram, the axis lies approximately 90° perpendicular to that lead.
- Choose between the two perpendicular options by seeing which neighboring lead shows a predominantly positive QRS.
In emergency practice, the two-lead method plus a quick look at Lead II will answer most clinical questions about axis. Use the full hexaxial/isoelectric method when you are teasing out fascicular blocks or borderline cases.
ECG Examples by Axis Type
Normal Axis: Both Lead I and aVF are upright (positive)
Left Axis Deviation (LAD): Lead I upright, aVF negative (downward)
Right Axis Deviation (RAD): Lead I negative (downward), aVF upright
Extreme Axis: Both Lead I and aVF are negative (downward) - highly abnormal
Left Axis Deviation (LAD)
Axis between -30° and -90°.
Common causes:
- Left anterior fascicular block (LAFB): classic cause of marked LAD. Look for small q in I/aVL and rS in II, III, aVF.
- Left bundle branch block (LBBB): wide QRS with typical LBBB morphology often accompanied by LAD.
- Left ventricular hypertrophy (LVH): increased LV mass shifts the mean vector leftward and superiorly.
- Inferior MI: loss of inferior forces causes the axis to rotate superiorly.
- Ventricular pacing from the RV apex: paced beats often show leftward axis.
Right Axis Deviation (RAD)
Axis between +90° and +180°.
Think about:
- Right ventricular hypertrophy (RVH): chronic lung disease (COPD), pulmonary hypertension, congenital heart lesions.
- Acute right heart strain: large pulmonary embolus with tachycardia, S1Q3T3, and RAD.
- Left posterior fascicular block (LPFB): uncommon; produces RAD with characteristic frontal plane pattern.
- Normal variant in youth: thin adolescents and young adults can have a relatively vertical heart with RAD and no pathology.
Extreme Axis ("Northwest")
Axis between -90° and -180° is almost never normal.
- Ventricular tachycardia: ectopic ventricular focus, particularly in scar-related VT.
- Severe hyperkalemia: bizarre QRS morphologies and marked axis shifts.
- Paced rhythms from unusual sites: e.g., RV outflow tract pacing.
- Major lead misplacement: limb lead reversal can mimic extreme axis — always check the basics first.
Red flag: new extreme axis in a sick patient should prompt an immediate check of lead placement and a focused search for VT, severe hyperkalemia, or major structural disease.
Because axis is calculated from limb leads, it is very sensitive to electrode misplacement. Before over-interpreting a strange axis, verify that the ECG was recorded correctly.
- Right/left arm reversal: Lead I becomes inverted and aVR may look unexpectedly normal/upright. This can mimic RAD or extreme axis.
- Arm/leg lead swaps: create "impossible" combinations of P wave and QRS orientation that do not match physiology.
- Check sinus orientation: in true sinus rhythm, P waves should be upright in I and II, and negative in aVR.
If the axis and P wave orientation do not make physiologic sense, suspect a technical problem until proven otherwise.
Quick Recognition Tool: The KISS Principle
A simple method for identifying which leads are affected by limb electrode reversal uses clockwise or counterclockwise rotation around the triangle of electrodes. The diagram below provides a quick visual reference:
- Normal QRS axis: -30° to +90° — both Lead I and aVF predominantly upright.
- LAD: -30° to -90° — Lead I up, aVF down. Think LAFB, LVH, inferior MI, paced beats.
- RAD: +90° to +180° — Lead I down, aVF up. Think RVH, PE/right heart strain, LPFB, congenital disease.
- Extreme axis ("northwest"): -90° to -180° — both I and aVF down. Think VT, severe hyperkalemia, major lead error.
- Fast method: Start with I and aVF → confirm borderline cases with Lead II or an isoelectric limb lead.
- Always compare to prior ECGs: new or abrupt axis change is more important than a stable, chronic deviation.
- Axis is supportive, not diagnostic: use it to refine your differential alongside rate, rhythm, ST–T changes, and clinical context.
LAD + RBBB = LAFB until proven otherwise: the combination of left axis and right bundle branch block strongly suggests left anterior fascicular block (a bifascicular pattern).
Trend matters: a slow drift toward LAD over years in a hypertensive patient often reflects progressive LV remodeling; a sudden step change in axis is more concerning for acute ischemia or new conduction disease.
RAD in youth: isolated RAD on a screening ECG in an asymptomatic, thin teenager with a normal exam is often a normal variant, not an emergency.
PE and axis: S1Q3T3 and RAD are classic exam findings but are neither sensitive nor specific. Use them as supporting data alongside symptoms, imaging, and bedside ultrasound.
Body habitus and lung volume: pregnancy, obesity, and diaphragmatic elevation push the heart horizontally (more leftward axis), whereas hyperinflated lungs and COPD pull it vertically (more rightward axis). Know your patient before over-calling pathology.
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- ECG Library • LITFL • ECG Library Basics. (2018). Life in the Fast Lane • LITFL • Medical Blog. https://litfl.com/ecg-library/