Safety of vHPvSD Ablation
It is important to note that all published case series utilising vHPvSD
are extremely small, with patients ranging from 28 to 90. Given the
relative infrequency of serious complications such as clinical stroke
and atrio-oesophageal complications with AF ablation, these studies are
grossly underpowered to provide conclusive safety data. As such, we need
to look for surrogate markers of complications, such as oesophageal
lesions on endoscopy and asymptomatic cerebral lesions (ACLs) on cranial
MRIs.
The incidence of post-procedure oesophageal injury is reassuringly low
in the 3 studies that have evaluated this systematically with endoscopy
(Table 2). In QDOT-FAST, a haemorrhaging ulcer was seen in just 1 of the
52 patients, and healed with medical therapy. The other two studies,
comprising 134 patients, showed no evidence of oesophageal injury in any
patient. This reassuring observation is in keeping with the findings of
the bench studies; vHPvSD lesions tend to be wider but shallower thereby
reducing the potential for extracardiac damage.
However, more worrying are the reports of coagulum formation on the
catheter tip and high rates of ACLs which likely represent associated
thromboembolic events from this charring (Table 2). Rates of
post-procedure ACL in vHPvSD studies have varied from 11.8% to 26%,
which are higher than seen with sRF. These have occurred in spite of
appropriate intra-procedural anticoagulation, and even after recent
software modifications to the nGEN RF generator. Whilst these ACLs were
not associated with clinical stroke events, and most (but not all)
resolved on follow-up MRI a few months later, recent prospective data
suggest that even silent ACLs can be associated with cognitive decline
over a relatively short timeframe. As such, it is clearly preferable to
minimise – or prevent entirely – the risk of ACL occurrence. How can
we do so?
One possible solution is suggested by Mueller et al. themselves.
While they found catheter tip coagulum in almost a third of patients
initially (6 out of the first 19 patients), this stopped happening
entirely when the baseline circuit impedance was increased – via
repositioning of the neutral electrode – from 90Ω to 110Ω. This
interesting observation lends credence to the theory that coagulum
formation results from excessively high current flow with lower
impedance. Bourier et al. recently demonstrated the critical
impact of circuit impedance on ablation, emphasising that it is current
delivery, rather than power input, which determines lesion size, and
that current delivery can vary widely due to fluctuations in impedance.
This effect may be particularly magnified in vHPvSD due to the short
duration of current delivery. More research is needed to find the
optimal balance of current delivery by modulation of impedance and
power, perhaps by development of a ‘constant current mode’ as suggested
by Bourier et al. .
vHPvSD ablation represents a quantum leap from the RF settings that have
traditionally been used in electrophysiology. If used to its full
potential, it may improve procedural efficiency by reducing ablation and
procedure times. However, with great power comes great responsibility;
we need to ensure that we use it judiciously and safely (Figure 1). To
that end, we are grateful to Mueller and colleagues for highlighting
these issues for us to work on.