Learn / Walkthroughs
Learn / Walkthroughs
Read an ECG, step by step
The quadrant method on leads I and aVF, then the isoelectric lead, then a number.
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.
aVF looks up from the feet at +90 degrees. A net positive QRS means the vector points downward, between 0 and +180.
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.
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.
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.
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.