Introduction
Programmed electrical stimulation (PES) of the ventricle is an important tool for initiation of ventricular tachycardia (VT) during ablation procedures. It is used to confirm the diagnosis, guide the procedural approach, and assess the effect of ablation.1 The absence of inducible VT following VT ablation is associated with lower risk of recurrence.2 Some observations, however, call into question the reliability of programmed stimulation for these purposes. Some VTs are not inducible in the electrophysiology laboratory despite aggressive PES. When this occurs, ablation is typically guided by targeting the presumptive VT substrate based on electroanatomic voltage mapping, electrogram characteristics, and/or pace-mapping. Secondly, it is well recognized that a clinical VT may be rendered no longer inducible immediately after ablation, and then be inducible again a few days later. In one study this occurred in 18.2% of patients for whom ablation was deemed acutely successful.3
Inducibility of VT is related to the aggressiveness of the PES. Using multiple drive cycle lengths and up to 3 or 4 extrastimuli, PES has a high sensitivity for inducing VT in patients who have had this arrhythmia spontaneously.4 The site of stimulation also plays a role. The initial ventricular stimulation site is typically the RV apex (RVA) and then the RV outflow tract (RVOT) if VT is not inducible from the RVA. This approach is widely adopted after earlier studies demonstrated the incremental yield of programmed electric stimulation (PES) from two RV sites.5 Before the era of catheter ablation, Josephson and colleagues reported that up to 11% of patients studied in the electrophysiology laboratory for VT had VTs that could be induced only with left ventricular (LV) and not with right ventricular stimulation.6 While their series did not find any electrocardiographic predictors of the need for LV stimulation, it is likely that site-specific inducibility varies with the location of the reentry circuit as well as with tissue conduction properties.
We describe here a case series of patients in whom inducibility was exquisitely sensitive to the site of stimulation and discuss the implications for VT ablation studies and patient management.