Hexaxial reference system
Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
The hexaxial reference system is diagram based on the first six leads of the 12 lead ECG. It is used to help determine the heart's electrical axis in the frontal plane. To use the hexaxial reference system, locate the most isoelectric (or equiphasic) lead (I, II, III, aVR, aVL, or aVF) on a diagnostic quality ECG with proper lead placement. Then find the corresponding spoke on the hexaxial reference system. The perpendicular spoke will point to the heart's electrical axis. To determine which numerical value should be used, observe the polarity of the perpendicular lead on the ECG. For example, if the most isoelectric (or equiphasic) lead is aVL, the perpendicular lead on the hexaxial reference system is lead II. If lead II is positively deflected on the ECG, the heart's electrical axis in the frontal plane will be approximately +60o.
- Normal axis: -30o to +90o
- Left axis deviation: -30o to -90o
- Right axis deviation: +90o to +180o
- Extreme axis deviation: -90o to -180o