Axis

The quadrant method on leads I and aVF, then the isoelectric lead, then a number.

Understand the measurements

Read lead I

Lead I looks from the left arm at 0 degrees. A net positive QRS means the ventricular vector points leftward, somewhere between -90 and +90.

Read lead aVF

aVF looks up from the feet at +90 degrees. A net positive QRS means the vector points downward, between 0 and +180.

Place the quadrant

Positive in both I and aVF is the normal quadrant, 0 to +90. Positive in I and negative in aVF is left, 0 to -90. Negative in I and positive in aVF is right. Negative in both is extreme, the no man's land between -90 and -180.

Find the isoelectric lead

A lead sees nothing from a vector at right angles to it. Once you have the quadrant, the isoelectric lead pins the axis to within 15 degrees.

Set the axis

Fifteen degrees is the resolution the six-lead method gives you, because the leads are 30 degrees apart. Machines report to the degree, readers report to the nearest 15.

Classify the axis

Human normal runs from -30 to +90. Canine normal runs from +40 to +100, so a dog with an axis of +20 has left axis deviation even though a person would not. Right axis deviation in a dog often means right ventricular enlargement.