The beats marked as J are junctional escape beats, occurring at a slow rate. They are followed by inverted P waves in inferior leads prior to the next QRS complex (marked by blue up arrows). These beats could be either beats originating from the low atrium (coronary sinus rhythm) or from the high junction.
Inverted P waves preceding QRS complexes can be seen in high junctional rhythm as the conduction into the atria are from below upwards, away from the positive electrodes of inferior leads. In low junctional rhythm, inverted P waves occur after the QRS complex as the ventricular activation occurs before the atrial activation. In mid junctional rhythm, P waves are not visible as there is simultaneous activation of the ventricles and the atria and P waves are obscured by the QRS complexes. Beats marked as J can be considered as mid junctional. The bigeminal rhythm is suggestive of an escape capture bigeminy. But typically, the second beat in an escape capture bigeminy is a sinus beat. It appears that here escape capture bigeminy occurs in the setting of a low atrial rhythm. Overall features suggest sick sinus syndrome.
Thanks to my Twitter colleague Sergio Pinski for enlightening me on the possibility of Escape-echo bigeminy in this case .