Supraventricular bigeminy originating from the mitral annulus: What is the mechanism?

We present a case of a 67-year-old female with a previous history of pulmonary vein isolation for paroxysmal atrial fibrillation who presented with supraventricular bigeminy with a constant coupling interval. The supraventricular bigeminy originated from the anterior mitral annulus with initial mapping suggestive of a focal mechanism. However detailed mapping using an ultrahigh resolution mapping system (with the manual shifting of the annotation window) revealed very low amplitude potentials connecting the previous sinus beat with continuous activation along the mitral annulus. Our observations were indicative of a re-entry mechanism underlying the supraventricular bigeminy.