About

In electrocardiography (ECG), the electrical axis represents the average direction of the wave of depolarization during ventricular contraction. Determining the Mean Electrical Axis (MEA) is crucial for diagnosing ventricular hypertrophy, conduction blocks (hemiblocks), and identifying the anatomical position of the heart.

This tool utilizes vector analysis of Lead I and Lead aVF to compute the precise angle in degrees. Unlike simple visual estimation methods, this calculator provides a quantitative angle based on the arctangent function. It visualizes the vector on a coordinate plane, instantly categorizing the result into normal or pathological deviations.

Formulas

The calculator resolves the net QRS amplitudes into a single vector v.

The angle θ is calculated using the arctangent of the orthogonal leads:

θ = arctanLead aVFLead I

Note: Since Lead aVF points to +90° and Lead I points to 0°, standard trigonometric quadrant adjustment is applied to place the vector in the correct hexaxial segment.

Reference Data

Axis Classification	Angle Range	Clinical Implications
Normal Axis	-30° to +90°	Physiological standard.
Left Axis Deviation (LAD)	-30° to -90°	LVH, LBBB, Left Anterior Fascicular Block, Inferior MI.
Right Axis Deviation (RAD)	+90° to +180°	RVH, LPFB, Lateral MI, Right ventricular strain (PE).
Extreme Axis Deviation	-90° to -180°	Rare. Ventricular tachycardia, severe electrolyte imbalance.

Frequently Asked Questions

Lead I and Lead aVF are orthogonal (perpendicular) to each other in the frontal plane. Lead I is at 0° (horizontal) and Lead aVF is at +90° (vertical). This makes them the ideal pair for simple Cartesian coordinate calculation of the axis vector.

Measure the height of the positive deflection (R wave) in millimeters and subtract the depth of the negative deflections (Q and S waves). For example, if R = 10mm and S = 4mm, the net amplitude is +6mm.

Physiologic LAD (0° to -30°) is often considered normal, especially in pregnancy or obesity (diaphragm elevation). Pathologic LAD is generally defined as an axis beyond -30°, often indicating conduction defects like Left Anterior Fascicular Block.