When you have both narrow and wide complexes in the same rhythm strip

This is the one that I hate the most, but it's really not that hard.

The pearl is to look at the coupling interval between two narrow complex beats. And then to look at the coupling interval between a narrow complex beat and a wide complex beat. If it's smaller, then the wide complex is aberrant conduction. If it's larger, then you have an accelerated idioventricular rhythm that is ignoring the sinus or atrial pacemaker that was originally driving the ventricle.

That's it. The only two possibilities.