Image 1 - Long lead II
This is best appreciated in a ladder diagram.
Image 2 Full disclosure and ladder diagram
The rhythm is sinus at a rate of about 50 bpm. Every other sinus beat is followed by an inverted (negative) P wave. This negative P wave is atrial activation from a premature junctional beat (in bigeminy). However, the PJC's are not conducted to the ventricles.