ECG axis: the quick I and aVF method
The electrical axis of the heart refers to the net direction of ventricular depolarization in the frontal plane. Normal axis is +0° to +90° (sometimes defined as -30° to +90°).
Quick determination using leads I and aVF: (1) Lead I positive + aVF positive = normal axis (+0° to +90°). (2) Lead I positive + aVF negative = left axis deviation (LAD, more negative than -30°). (3) Lead I negative + aVF positive = right axis deviation (RAD, > +90°). (4) Lead I negative + aVF negative = extreme axis (northwest axis, -90° to ±180°).
More precise determination: if both I and aVF are positive, the axis is normal. For borderline cases between normal and LAD, check lead II — if lead II is also positive, axis is normal. If lead II is negative, axis is LAD (> -30°).
Clinical causes of axis deviation
Left axis deviation (LAD, more negative than -30°): most common causes — left anterior fascicular block (LAFB), inferior MI (loss of inferior electrical forces pulls axis leftward), LBBB, ventricular pacing, WPW (type B pattern). LAFB: LAD in a narrow-QRS rhythm without other cause = LAFB until proven otherwise.
Right axis deviation (RAD, > +90°): most common causes — right ventricular hypertrophy (RVH — cor pulmonale, pulmonary hypertension, congenital heart disease), left posterior fascicular block (LPFB), lateral MI, RBBB, normal variant in young slender adults (vertical heart position). New RAD in a patient with dyspnea warrants echocardiography to assess RV pressure.
Extreme axis (northwest, -90° to ±180°): consider lead reversal FIRST before diagnosing pathologic extreme axis. Lead reversal — particularly right arm/left arm swap — produces pseudo-extreme axis. Check lead aVR: it should normally be negative. If aVR is positive, suspect lead reversal.