Figure legends:

Figure 1: Intracardiac electrogram with intervals of tachycardia 1 in Figure 1a and tachycardia 2 in Figure 1b. From top to bottom, I, aVF, V1 represents; leads I, avF, and V1 of 12 lead ECG. RA d represents the distal bipole of the right atrial catheter. His p and His d represent proximal to distal poles of His bundle catheter. CS 9-10 to 1-2 represent proximal to distal poles of a decapolar catheter in the coronary sinus. RV d represents the distal bipole of the catheter in the right ventricular apex. Figure 2: Simultaneous atrial and ventricular pacing from proximal CS and RV apex, respectively. Figure 2a shows the initial atrial electrogram after cessation of pacing for tachycardia 2. Figure 2b shows the initial atrial electrogram after cessation of pacing for tachycardia 1. Figure 2c shows initial His electrogram after cessation of pacing for tachycardia 1. Catheter position and abbreviation same as in Figure 1. H represents His bundle electrogram. RA+RV – simultaneous RA and RV pacing. Figure 3: Ladder diagram explaining the probable mechanism for the response to simultaneous atrial and ventricular pacing. A – Atrium, H – His bundle, LCP – lower common pathway, V – Ventricle, #- refractoriness, green star – pacing, green circle – collision, and dotted lines represent continued A and V pacing for few beats before the last paced beat.