On how to determine the QRS axis I would like to suggest the following:
1. The QRS axis is normal if positive in both leads DI and DII. A positive QRS in DI limits the axis to the left semicircle (between -90º and +90º) and a positive QRS in DII narrows this down to between -30º and +90º.
2. If the former (a positive QRS in both DI and DII) is not present, the next easy step is to look for an iso-electric QRS. The QRS axis will be perpendicular to the lead with the iso-electric QRS, and parallel to the complimentary lead (the lead perpendicular to the first lead, DI-aVF, DII-aVL, DIII-aVR). The QRS polarity on this last lead will determine the axis.
3. If the QRS is not positive in both DI and DII, and there is no iso-electric QRS, use this last method:
a) Determine if the QRS axis is in the left or right semicircle looking at DI. Determine if the QRS axis is on the upper or lower semicircle looking at aVF. This will locate the QRS axis in one of the four quadrants.
b) Examine the QRS in the 2 leads located within the two quadrants that are adjacent to the one determined in the former step. The QRS polarity in these 2 leads will narrow down the QRS axis to a 30º arc within the quadrant.
I can explain further if necessary or (maybe better) send in a simple video, if you wish.
Daniel Alvarez, MD