Defining the substrate for ventricular tachycardia ablation: The impact of rhythm at the time of mapping

Fig. 1. Example of a change in potential during (a) atrial pacing (b) ventricular pacing. Note during atrial pacing there is a late potential that is fused during ventricular pacing to the local EGM (yellow arrow).

Fig. 2. Change in voltage at specific points during atrial vs. ventricular pacing (a) RAO (b) LAO.

Fig. 3. Example of a case in which initial ablation was done on the atrial paced map. VT (middle left ECG) was still inducible. On the ventricular paced voltage map (right hand map) there were areas of late potentials not obvious during atrial pacing (left hand map), pacing at which showed similarity to the inducible VT. Ablation here rendered the patient subsequently non-inducible.

Fig. 4. Impact of wavefront directionality on electrogram. Shown is a schematic of how orientation of the wavefront to a region of myocardial abnormality may impact appearance of an electrogram. In the case of a wavefront that is perpendicular to an area of substrate, in which the wave must propagate slowly through channels in the substrate region, one may see a fractionated potential. If parallel, however, the electrogram may appear singular without fractionation if velocity is similar on both sides.

Fig. 5. Impact of normal His purkinje system conduction versus RV pacing on electrogram appearance in region of substrate. Demonstrated is one potential mechanism for why electrograms may appear different depending on the wavefront of activation. In Fig. 5A–B, there are two wavefronts from fascicular activation with entry into the substrate zone approaching the cathode from 2 angles – one through normal tissue and the other via the scar (5B). As a result, wavefronts all collide in the area of scar simultaneously resulting in appearance of a single signal. In Fig. 5C–F, there is a singular wavefront during RV pacing which results in a single electrogram as it reaches the cathode (5D). Then it slowly traverses the scar zone, eventually passing the bipolar recording electrode again resulting in a second (or late) potential.