ECG interpretation · Interactive physiology
One vector.
Six limb-lead views.
Rotate a mean frontal QRS vector and see its signed projection into each limb lead. Toward the positive pole gives a positive projection; away gives a negative projection; perpendicular gives zero.
Set the mean QRS axis
+60° · Normal adult axis
Use arrow keys for 1° steps. +180° and −180° are the same direction. This demo assigns −30° through +90° to normal, −90° through <−30° to left deviation, >+90° to <+180° to right deviation, and −180° through <−90° to extreme axis. Exact cutoffs are conventions; inspect neighboring angles.
Frontal plane · facing the patient
Labels mark each lead’s positive pole; the opposite end is negative. The red arrow is the mean vector, not the direction of every instant of ventricular activation.
Signed lead deflections
Up = positive · down = negative. Stems show scalar projection, not QRS morphology. All rows use one common arbitrary scale.
Zero means zero net projection. A real QRS can be biphasic or equiphasic with a small net value; it need not be a flat line.
Show the projection equations
For mean axis θ, lead orientation α, and sensitivity s: projection = s × cos(θ − α). Angles: I 0°, II +60°, III +120°, aVR −150°, aVL −30°, aVF +90°.
The shared scale preserves II = I + III, aVR = −(I + II)/2, aVL = I − II/2, and aVF = II − I/2. Normalizing each lead independently would preserve signs but lose these amplitude relationships.
Boundary note: an exactly perpendicular vector has zero projection. At −30°, II is zero; at +90°, I is zero. Small shifts across those boundaries change the sign.