Supraventricular tachycardia with simultaneous atrial and ventricular activation: What is the mechanism?

a i A a w c ase report 50-year-old man underwent an electrophysiologic tudy for very frequent episodes of supraventricular achycardia. He had a left bundle branch block with a aseline sinus cycle length, atrial-His (AH) interval, and is-ventricular (HV) interval of 670, 100, and 65 ms, espectively. Atrial pacing induced a regular supravenricular tachycardia with a cycle length of 540 ms. Venricular pacing at a cycle length of 500 ms was performed uring the tachycardia (Figure 1). A premature atrial timulus was introduced in the high right atrium during achycardia (Figure 2). What is the mechanism of the achycardia?