This is from 25 yo s/p AV valve and TV valve repair. This is a tricky strip.
Most will call it "in and out" of accelerated junctional rhythm (AJR) or just AJR because you are not seeing the transitory P & QRS separation. Always remember that the AV junction as a pacemaker has a "passive" and an "active" mode. If you have read Chou's Electrocardiography in Clinical practice you will understand what I am saying. Anyway, the AV junction just like the sinoatrial node has an inherent rate as a pacemaker (35-60 bpm). When the supraventricular impulse (e.g. SAN) becomes slower than that of the AVJ, the AVJ becomes the site of impulse formation. This represents the slow or passive type of junctional rhythm. This can be seen in cases of extreme bradycardia or sinus pause/arrest where we see AV junctional escape beats/rhythm. In contrast, there is an active type of AV junctional rhythm resulting in a rhythm generally at a rate > 60 bpm (AJR and junctional tachycardia). This is caused by an abnormal automaticity in the AVJ. In the case presented, this is a regular, narrow QRS rhythm with P waves that cannot be seen in the leads (II and V). However, on close observation in other strips, P waves can be seen gradually separating and fusing with the QRS. This means that the rate of the PP (SAN) and RR(AV junction) remains constant and the same. This created incomplete AV dissociation. When the rates of the dissociated pacemakers are the same, it is called isorhythmic AV dissociation. This was described by the late Dr. H Marriott and Menendez as a "flirtatious relationship" because the two rhythms tend to chase each other.