Some ectopic QRS complexes – especially junctional ectopic complexes – can look very much like the sinus conducted beats or atrial ectopic impulses. It is very easy to confuse a junctional premature complex with an atrial premature complex at times, especially when the atrial premature complex has a very small or nearly isoelectric P’ that is hidden in a normal-appearing T wave. This is usually a problem in the limb leads (frontal plane) more than the precordial leads (horizontal plane).
When you are not certain of your ability to distinguish the origin of the QRS complexes, focus more on the repolarization of those complexes (ST-T). Even when depolarizations (the QRS’s) look virtually the same, repolarization almost always looks different. A change in repolarization is characteristic of supraventricular impulses that are transmitted aberrantly and most impulses that arise below the AV node. The reason for this is that the fibers in the His bundle are already arranged to split into the right and left bundle branches. Therefore, junctional impulses that arise below the AV node usually enter the His bundle eccentrically and excite some fibers before others, while activating some fibers more strongly than others. Some fibers may not be activated at all. This results in a repolarization (ST-T) that is often obviously different than the repolarizations of the sinus-conducted complexes.
This is also the reason that junctional QRS complexes do NOT have to bear any resemblance to the sinus-conducted QRS complexes. I know that you were probably taught in your introductory courses or books that junctional beats look exactly like sinus-conducted beats. Granted, most do! But you were not told the whole truth! Junctional complexes may bear little or no resemblance at all to the sinus-conducted QRS complexes! Aberrancy may be due to the way the junctional impulse enters the His bundle or it may be due to conduction delays within the bundle branches themselves.