Supraventricular tachycardia with a variable RP relationship but without a change in the cycle length: What is the mechanism?

A 70-year-old male with a history of hypertension, hyperlipidemia, coronary artery disease, and CABG, presented with symptoms of moderate-intensity palpitations that terminated with Valsalva maneuvers. He was noted to have supraventricular tachycardia (SVT) on an event monitor. He had persistent symptoms despite treatment with beta-blockers. The ECGs recorded in the EP lab during the symptoms is shown in Figures 1A and 1B. What is the most likely etiology of the SVT?