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.