Fusion with the prolonged first postpacing interval in pacing of ventricular tachycardia

In cases of recurrent sustained ventricular tachycardia (VT), constant fusion and progressive fusion in the surface electrogram were observed during overdrive pacing of VT. However, following cessation of pacing, the return cycle of VT (stimulus to the first nonpaced QRS) was longer than the VT cycle length. This phenomenon can be explained by orthodromic capture of the exit site of the VT current with a conduction time (stimulus to QRS) exceeding the VT cycle length. This site can therefore be activated during entrainment of VT during the mid-to-terminal portion of the paced QRS complex, resulting in constant fusion.