Identifying sites for catheter ablation of ventricular tachycardia.
The approach to localizing sites for catheter ablation of ventricular tachycardia foci depends on the type of tachycardia. In large reentry circuits such as those arising from infarct scars, areas of slow conduction in and around the scar should be targeted. During sinus rhythm, these can be suspected from the presence of fractionated electrograms and, at some sites, long stimulus to QRS delays during pacing. Slow conduction areas can be classified as: 1. central slow conduction zone sites, 2. exits from the slow conduction zone, 3. entrances to the slow conduction zone, and 4. bystander areas which are not involved in the tachycardia circuit. In the central slow conduction zone stimulation entrains or resets tachycardia with a long stimulus to QRS (S-QRS) delay (40 to greater than 300 ms) without altering the QRS morphology (entrainment with concealed fusion). At slow conduction zone exits, presystolic electrograms are recorded during VT, the pacemap matches the VT QRS morphology, and with pacing during VT the S-QRS interval is relatively short and VT may or may not be entrained. At entrances to the slow conduction zone electrogram timing is variable but early diastolic electrograms are expected and the pace-map QRS may differ from the VT QRS morphology. Relatively late stimuli or slow trains of stimuli entrain VT with concealed fusion with a relatively longer S-QRS interval than observed in the central slow conduction zone. Early stimuli may entrain VT while altering the QRS morphology due to propagation of the stimulated antidromic wavefront out of the scar from a site other than the tachycardia exit. At bystander sites electrogram timing, pace-mapping, and the effects of programmed stimulation are variable but may occasionally mimic reentry circuit sites. Relatively late stimuli are likely to capture the site without altering the VT. If discrete electrograms are present, analysis of these during pacing may provide further evidence that the site is not in the reentry circuit. Catheter ablation will probably be most effective at central slow conduction zone sites. When VT originates from a small focus surrounded by normal myocardium, such as is likely for idiopathic RV outflow tract and some idiopathic left ventricular tachycardias, presystolic electrical activity and pacemapping are likely to identify the tachycardia focus. For macroreentry involving the bundle branches, the right bundle branch can be easily targeted.