The QRS complex of a PAC can occasionally differ from the sinus QRS if one of the bundle branches (more frequently the right) is partially refractory. This type of PAC has a wider QRS and is called an aberrantly conducted PAC; the QRS pattern is more commonly right bundle branch block than left bundle branch block. An aberrant PAC is usually benign.
An aberrant PAC can be mistaken for a premature ventricular complex (PVC), which also has a wide QRS, particularly if the premature P wave is not obvious; PVC’s are not preceded by a premature P wave. If the premature P wave is not obvious, check to see if the next P wave is on time. The absence of interference with the sinus node (that is, a “compensatory pause”) is typical of a PVC, whereas sinus node resetting is more typical of a PAC. The following animation illustrates what we have just described.