Answer
PAC with aberrant conduction
Discussion
The QRSs are wider than and different from other QRSs, raising a possibility of PVC. But they are clearly preceded by a prematurely occurring P wave and so are PAC with aberrant conduction.
Aberrant conduction occurs simply because the right and left bundle branches have different lengths of refractory periods. In Figure 2, the shaded area is the refractory period of the left and right bundle branches. As can be seen, one bundle branch, usually the right, has a longer refractory period than the other.
If a P wave occurs at point "c" when both bundle branches have recovered from the refractory period, then the atrial impulse will conduct to the ventricles normally (the QRS complex labeled c in Figure 1 and tracing c in Figure 2).
If a P wave occurs at point "a" when both bundle branches are refractory, the atrial impulse cannot conduct to the ventricles and a nonconducted atrial impulse results (the P waves indicated by arrows in Figure 1 and tracing "a" in Figure 2).
If a P wave occurs at point "b" when one bundle branch is still refractory while the other has recovered from the refractory period, the impulse will conduct through the available bundle branch, bypassing the refractory bundle branch. The result is an aberrant conduction (the QRSs being questioned in Figure 1 and tracing "b" in Figure 2) Because it is usually the right bundle branch that has a longer refractory period, the aberrantly conducted QRS complex more often has right bundle branch block morphology, as demonstrated in this case.
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Cite this: Aberrant Conduction or Premature Ventricular Complexes? - Medscape - Mar 07, 2012.
