Two planes, twelve cameras
The twelve views split into two groups. The six limb leads (called I, II, III, aVR, aVL and aVF) look at the heart in the vertical plane — roughly head-to-foot and left-to-right. The six chest leads (V1 through V6) wrap around the front and side of the chest and look at the heart in the horizontal plane, like a ring of cameras at the same height. Each lead has a fixed angle, so naming a lead names a direction of view.
This is why the same beat looks different lead to lead, and why that is a feature, not a flaw. Leads that sit over the same region of muscle form natural neighborhoods. The chest leads V1 and V2 sit over the front wall fed by the left anterior descending artery; leads I and aVL sit over the side of the left ventricle; leads II, III and aVF look up from below at the bottom wall. Knowing these neighborhoods is what lets the next guide say not just that the heart is injured but where.
The cardiac axis: which way does the heart point?
If you add up all the little electrical vectors of the ventricles depolarizing, they point in one overall direction. That summed direction is the cardiac axis. In a normal adult it aims down and to the patient's left, because the big muscular left ventricle dominates the signal. The axis is just an arrow describing the average heading of the QRS.
- Look at lead I: is its main QRS mostly up or mostly down?
- Look at lead aVF: is its main QRS mostly up or mostly down?
- Both up = normal axis (the arrow points down-left).
- I up, aVF down = left axis deviation; I down, aVF up = right axis deviation.