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).