Efficacy of vHPvSD Ablation
FPI is an excellent acute indicator of the quality of RF PVI, and it
serves as a reliable surrogate for long term success rates. As such it
is notable that the reported FPI rates with vHPvSD have been modest,
ranging from 18-61% (Table 1). Rates of early PV reconnection during
the waiting period are also disappointingly high. Both of these result
in need for additional RF applications for gap elimination, thereby
offsetting one potential benefit of using vHPvSD, namely reduction in
procedure and RF times. This is paradoxical – if the bench data
demonstrate a better lesion profile, why is this not mirrored in terms
of acute clinical efficacy?
A few hypotheses bear consideration. Firstly, bench studies are
performed under controlled conditions where stable contact for the full
4 second duration of the RF application is virtually guaranteed. This is
far removed from the clinical setting, where a combination of
respiratory and cardiac motion means that contact is likely to be
intermittent. In fact, it can be argued that catheter stability in
vHPvSD is even more critical than with sRF, because even brief loss of
contact is substantial, in relative terms. For example, 2 sec loss of
contact in a standard 20 sec ablation represents just 10% of the
duration, whilst for vHPvSD it presents 50%. To that end, it is
possible that the use of full general anaesthesia, with high frequency,
low tidal volume ventilation, may improve results with vHPvSD by
stabilising catheter contact. Secondly, lesion contiguity is one of the
central tenets of the highly successful CLOSE protocol, which in turn
relies on accurate placement of automated lesion tags (Visitags). At the
moment, vHPvSD is not compatible with the CARTO Visitag software, which
leads to high variability in placement of the auto-tags depending upon
the phase of respiration. This can make accurate tracking of inter-tag
distance extremely challenging. Upcoming software enhancements should
address this issue. Thirdly, the current approach to vHPvSD utilises a
uniform setting of 90W/4sec ablation throughout all areas of the
ablation circle. It makes no allowance for the well-recognised
differences in tissue thickness between the anterior and posterior left
atrial regions, unlike the CLOSE protocol that utilises different
ablation-index targets for these regions. It may be relevant that the
median depth of vHPvSD lesions seen by Takigawa et al. was 2.7mm,
which may not be adequate to produce transmurality across the thick left
atrial appendage ridge in all patients. Some operators, including
ourselves, have tried to get around this limitation by clustering
lesions closer together on the anterior segments. It may be no
coincidence that the one group that reported high efficacy with vHPvSD
had targeted an inter-tag distance of 3-4mm on the anterior wall.
Perhaps that is what may be needed for vHPvSD in place of the standard
6mm spacing that was validated on entirely different conventional RF
settings.
In summary, whilst more data are needed, it looks likely that the
efficacy of vHPvSD ablation may be improved by use of general
anaesthesia, and by shortening the inter-tag distance, especially on the
anterior segments (Figure 1).