Unusual response to entrainment of ventricular tachycardia: In or out?

FEATURED ARRHYTHMIA Unusual response to entrainment of ventricular tachycardia: In or out? Roderick Tung, MD, FHRS, Kalyanam Shivkumar, MD, PhD, FHRS From the UCLA Cardiac Arrhythmia Center, David Geffen School of Medicine at UCLA, Los Angeles, California. Case presentation A 70-year-old man with history of inferior myocardial infarction with an ejection fraction of 25% was urgently transferred for ablation of incessant slow ventricular tachycardia (VT). Electroanatomic mapping revealed extensive inferolateral scar extending to the inferior septum. Entrainment mapping demonstrated an entrance in periannular inferolateral scar with an exit anterior to the aortomitral continuity. Entrain- ment mapping was performed in the scar at a site in between these 2 regions. Is this response in or out of the circuit? (Figure 1) Commentary Entrainment mapping, or continuous resetting of VT, is the gold standard technique for characterizing the components within a reentrant circuit. Demonstration of an isthmus requires the presence of (1) concealed fusion, (2) postpacing interval within 30 ms of the tachycardia cycle length, and (3) stimulus-QRS (S-QRS) interval equal to electrogram-QRS (EGM-QRS) interval. 1 The presence of these responses is highly predictive of termination with ablation. 2,3 In this case, overdrive pacing of the slow VT (680 ms) is performed at 620 ms (10 mA @ 2 ms). Two distinct diastolic potentials are seen during VT, and it is important to distinguish the near-ﬁeld EGM captured from far-ﬁeld components in order to accurately measure the postpacing interval. 4 Disappearance of a local EGM during pacing represents either direct capture or saturation of the channel from pacing. Two features conﬁrm that the early diastolic EGM is not captured during entrainment: (1) the S-QRS interval would be longer as the EGM-QRS occurs in early diastole and (2) analysis of the onset of pacing reveals KEYWORDS Ventricular tachycardia; Entrainment ABBREVIATIONS EGM-QRS ¼ electrogram-QRS; MES ¼ multiple exit site; S-QRS ¼ stimulus-QRS; VT ¼ ventricular tachycardia (Heart Rhythm Address reprint requests and correspondence: Dr Roderick Tung, UCLA Cardiac Arrhythmia Center, David Geffen School of Medicine at UCLA, 100 Medical Plaza, Suite 660, Los Angeles, CA 90095. E-mail address: rtung@mednet.ucla.edu. 1547-5271/$-see front matter B 2014 Heart Rhythm Society. All rights reserved. saturation (Figure 2, dashed circle) from the pacing current in the absence of local and QRS capture. The last entrained beat demonstrates overt or manifest fusion. The postpacing interval measured to the peak component of the return sensed EGM is within 30 ms of the tachycardia cycle length (680 ms). The S-QRS interval appears to match the EGM-QRS interval when measuring the peak deﬂection of the late diastolic EGM. These responses suggest entrainment from an outer loop site on the periphery of the reentrant circuit with antidromic fusion outside the circuit. However, careful analysis of the ﬁrst 2 paced beats on the tracing demonstrates a QRS morphology with an exact 12-lead match for the VT. The possibility of transient loss of capture during these 2 complexes is ruled out by acceleration of the VT to the pacing cycle length (620 ms). These 2 beats represent concealed fusion with a perfect match for the VT, and the S-QRS interval (145 ms) matches the EGM-QRS interval when the EGM is timed to the onset of the return diastolic potential rather than the peak. Analysis of the diastolic EGM before pacing is useful because the EGM is not yet saturated by the pacing channel, which allows more detailed analysis of multicomponent local EGMs (Figure 2). Closer analysis of this EGM conﬁrms a fractionated compo- nent of the late diastolic potential 35 ms before the peak deﬂection (EGM-QRS interval 145 ms). Note that the ﬁrst entrained beat after the onset of pacing has the same morphology as the last entrained beat. Interpretation of this entrainment response using the ﬁrst 2 complexes in Figure 1 suggests a critical isthmus site. Ablation at this site resulted in the abrupt termination of VT within 4 seconds (Figure 3). We have previously reported the observation of multiple exit site (MES) morphologies when pace mapping within scar during sinus rhythm. 5 These sites are likely speciﬁc for reentrant isthmuses and importantly can be identiﬁed as surrogates for critical sites in sinus rhythm. However, pacing during sinus rhythm from a point-like source is likely to have a radial spread of activation in contrast to a broad reentrant wave front with a preferential exit during VT. Therefore, activation through MESs out of scar is more likely to be seen during pace mapping. The entrainment response in this case likely represents an alternate exit site from a common http://dx.doi.org/10.1016/j.hrthm.2013.09.006