Once we’ve located the EARLY complexes on this strip that’s running at 25mm/s, we want to look at the width of those early complexes.

If it’s greater than or equal to 0.12 seconds, its likely ventricular in origin.

A narrow complex QRS would indicate the impulse originated above and was conducted normally through the bifurcation of the His-purkinje system. In this case we want to determine if it’s atrial or junctional.

With premature atrial complexes, we expect to see a P wave preceding. The P wave MAY be buried in the previous T wave depending on the rate of the underlying rhythm and just how SOON the early complex is. In this case, we’re looking for disturbances in the typically smooth contour of the T wave.