Concealed conduction

No clinical hx. Why is there sudden prolongation in PRI of QRS#6 and non-conducted P wave after QRS#7?

Image 1 - Image Case

The strip shows sinus rhythm (~50 bpm) with pri of 160 ms but the PRI in QRS #6 is prolonged at 280 ms (7 small squares) and the P wave after QRS # 7  is not conducted.

Something is happening in the AV node not seen on surface ecg or something is concealed. 

Image 2 - Ladder Diagram

The PVC's (QRS #5 and 7) were retrogradely conducted reaching the AV node. The P wave before QRS # 6 arrived at the AV node while it is still in relative refractory period prolonging the conduction (prolonging the PRI) and the P wave after QRS 7 was not conducted because the AV node is in the absolute refractory period.

Interpretation: sinus rhythm with concealed conduction (prolonging a PRI and causing non-conducted P).