Sinus rythm. This is specifically common in Type 1 second degree heart block.
I don't believe this is complete bundle branch block, because the QRS is a narrow complex.
This is 2nd Deg, Type 2, 2:1 heart block. Type 1 is Wenckebock, has progressing lengths of PR interval until the dropped beat. Here, the P waves are regular, but every other QRS is dropped, hence the 2:1. 1st Deg AV block has increased PR interval.