Electrophysiology study (EPS) tracings showing concealed entrainment

(A) Pacing from the distal electrode pair of the mapping/ablation catheter during mapping of sustained ventricular tachycardia. On the left side of the figure are the last three stimuli during a train of pacing at a cycle length of 410 milliseconds. Following pacing, the tachycardia (cycle length 430 milliseconds) resumes. The interval from the last paced beat to resumption of tachycardia, measured in the distal ablation catheter (Abl d), is just 10 milliseconds greater than the tachycardia cycle length. Note that the interval from the pacing stimulus to onset of the QRS complex is the same as the interval from the local electrogram to the QRS onset when VT resumes after pacing. Note also that the paced VT morphology replicates the morphology of the spontaneous tachycardia. This is an example of "concealed entrainment" and implies the catheter is within a critical part of a reentrant circuit.
(B) Same site on the ablation catheter (Abl d) during sinus rhythm as shown during VT in figure A. During sinus rhythm, a late potential is present. The vertical line denotes the end of the QRS complex. Delivery of radiofrequency energy at this site resulted in termination of VT. The tachycardia was no longer inducible after ablation. Sites such as this displaying late potentials may be targeted during "substrate-guided ablation."