Cardiology

QRS Axis Calculator (Electrical Axis of the Heart)

Q: Why assess the axis in pulmonary embolism?

Acute RV overload can cause abrupt right axis deviation (S1Q3T3 pattern). This supports the diagnosis but is neither a sensitive nor specific finding.

Determines the QRS axis from amplitudes in leads I and aVF

Primary source: Bayés de Luna A. Clinical Electrocardiography, 4th ed. 2012

Aleksandr A. Aulov, MD, internal medicine Last clinical review: April 28, 2026

Online calculation

Algebraic sum of QRS in lead I

R wave height minus S and Q depth (in mm or mV, as long as units are consistent). A positive number means R predominates

Algebraic sum of QRS in lead aVF

R wave height minus S and Q depth in lead aVF

About this score

The electrical axis of the heart (QRS axis) is the direction of the mean ventricular depolarization vector in the frontal plane. It is determined from the polarity of the QRS complexes in leads I and aVF (or I and III) on a standard ECG.

Normal range: -30° to +90° (normogram). Left axis deviation (-30° to -90°) occurs with LVH, left anterior fascicular block, age-related changes, and obesity. Right axis deviation (+90° to +180°) occurs with RVH (cor pulmonale, pulmonary embolism, congenital heart disease), left posterior fascicular block. Extreme deviation (the `northwest axis`) is very rare.

Simple method: positive QRS in leads I and II indicates a normal axis; positive in I, negative in II, III, and aVF indicates left axis deviation; negative in I and positive in aVF indicates right axis deviation.

When to use

Routine ECG interpretation.
Suspected ventricular hypertrophy, fascicular block, or pulmonary embolism.
Differential diagnosis of a wide QRS complex (bundle branch block vs ventricular rhythm).
Teaching practice: axis determination is a basic skill for physicians of any specialty.

Clinical example

Case

A 58-year-old patient with COPD and chronic cor pulmonale. ECG: QRS negative (rS pattern) in lead I, markedly positive (Rs pattern) in leads III and aVF.

Calculation

I (-) + aVF (+) -> right axis deviation (+90° to +180°).

Interpretation and management

Right axis deviation. In a patient with COPD, this is a typical finding of chronic cor pulmonale, reflecting RV hypertrophy and overload. Additional checks: P-pulmonale in lead II, R/S ratio > 1 in V1, T-wave inversion in V1-V3. Echocardiography to assess pulmonary artery pressure and RV function.

Limitations and cautions

The I/aVF method gives a rough estimate; an exact value in degrees requires calculation using the perpendicular method.
In incomplete bundle branch block, the axis may be distorted.
In children and adolescents, right axis deviation is physiologic.
Recording artifacts (reversed electrode placement) can mimic extreme axis deviation.

Frequently asked questions

At what angle does 'axis deviation' begin?

Left deviation starts at -30°. Right deviation starts at +90°. Between -30° and +90° is normal.

What does an isoelectric complex in lead I mean?

The axis is perpendicular to lead I, i.e., close to +90° or -90°. Lead aVF determines the direction: (+) -> +90°, (-) -> -90°.

How to distinguish left anterior fascicular block from LVH?

Left anterior fascicular block: axis more negative than -45°, qR in I and aVL, rS in II, III, aVF; QRS usually < 120 ms. LVH: Sokolow-Lyon and Cornell criteria.

Why assess the axis in pulmonary embolism?

Acute RV overload can cause abrupt right axis deviation (S1Q3T3 pattern). This supports the diagnosis but is neither a sensitive nor specific finding.

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Aleksandr A. Aulov Physician, internal medicine, Sechenov School — independent medical education platform. Calculators are compiled from the original publications and current clinical guidelines. Interpretation thresholds follow the source study unless stated otherwise.

This calculator is intended for healthcare professionals. Do not use it for self-diagnosis or self-treatment. Management decisions are made by the treating clinician based on the full clinical picture of the individual patient.