What is the Diagnosis?

An 83-year-old female implanted a dual-chamber pacemaker (Adapta DR) one year before threating a sinus node disease with symptomatic sinus arrest (up to 4.2 s). In the last month, the patient complained of palpitation and dizziness. She has a previous history of hypertension and nondialytic chronic renal failure. The long-term medical therapy consisted daily of losartan 100 mg, atorvastatin 10 mg and aspirin 100 mg. Pacemaker telemetry showed a 90% battery, normal function of pacing leads and short episodes of atrial fibrillation. Statistical pacing was 92% in the atrium and 26% in the ventricle. The pacemaker was programmed in managed ventricular pacing (MVP) mode with sensor (AAIR and DDDR) and basic heart rate of 65 cpm. A 24 h-Holter monitoring was used in the investigation (Fig. 1). What is the mechanism?Figure 1. ECG was recorded during moderate efforts with complain of dizziness and palpitation.