Editorial
Catheter ablation for atrial fibrillation (AF) has undergone successive
technical advances that incrementally improved its performance. Such
evolution reached a stage of maturity that made it the most common
procedure performed by electrophysiologists. In the past few years,
radiofrequency (RF) ablation using high-power, short duration (HPSD)
settings (50W for 5 to 10 seconds) has been advocated as a safe and
effective strategy to achieve point-by-point pulmonary vein isolation.
(1,2) But before universally adopting such a strategy,
electrophysiologists should be familiarized with the biophysics of RF
ablation and understand what makes HPSD ablation unique in this setting.
In contrast to conventional ablation, which relies on passive heat
conduction into deeper tissue for transmural lesion formation (thus
requiring longer RF application at lower power), HPSD relies on
resistive heat from the catheter-tissue interface for effective
ablation. (3) This produces much less conductive heat leading to wider,
shallower lesion formation and much less reversible injury
(figure 1 ). In AF ablation, in which injury to deep tissue
beyond the thin left atrial wall is of major concern, the arguments in
favor of HPSD can thus potentially extend beyond faster lesion formation
and shorter procedural time. For those who remain skeptical, however,
the question yet to be answered is: ‘Can HPSD improve efficacy of AF
ablation while maintaining equal or higher safety compared to more
standard energy delivery techniques?’.
In this issue of the Journal of Cardiovascular Electrophysiology ,
Hansom et al. address this question by systematically reporting
outcomes using HPSD ablation – defined as 50W for 6 to 8 seconds
posteriorly and 8 to 10 seconds elsewhere – to low-power, long duration
(LPLD) ablation – defined as up to 25W with force-time integral (FTI)
of at least 300g.s posteriorly and up to 35W with FTI of at least 400g.s
elsewhere. This well-designed, well-executed historical control study
was based on a change in strategy at their center in 2017, allowing a
comparison between 107 consecutive patients who underwent AF ablation
with LPLD settings with 107 consecutive for whom HPSD settings were
used. Although not a randomized controlled trial, patients in both
groups had similar baseline characteristics, including similar
proportion of paroxysmal and persistent AF (respectively, 63% and 37%
in the HPSD group versus 56% and 44% in the LPLD group).
In terms of safety, their findings supported the notion that HPSD
ablation is at least as safe as LPLD ablation. Specifically, there were
no steam pops and no atrioesophageal fistulas in their series; however,
the only case of phrenic nerve palsy happened in the HPSD group.
Although not powered to detect statistically significant differences
with such low event rates, this observation is a reminder that HPSD
ablation could still lead to unintended complications, highlighting the
importance of phrenic nerve pacemapping prior to right pulmonary vein
isolation regardless of the power settings. Their overall low
complication rate is in agreement with prior studies that also indicate
no increase in complication rates when HPSD is compared to LPLD
ablation. (1,4–6)
In regard to efficacy, Hansom et al. showed that HPSD achieved
results comparable to LPLD, but with shorter ablation and procedure
times. Acute procedural success (defined as pulmonary vein isolation
with bidirectional block) was achieved in all patients, with an average
procedural time of 229 minutes and 25.8 minutes of ablation in the HPSD
group compared to 309 and 64.8 minutes, respectively, in the LPLD group
(p < 0.005 for both parameters). Freedom from atrial
arrhythmias after 1 year was similar between groups: 79% in HPSD and
73% in LPLD (p < 0.004 for non-inferiority). The same was
true when AF ablation outcome was further stratified by type. Again,
these findings were consistent with previous reports that suggest at
least equivalent short- and long-term outcomes when HPSD is employed
instead of LPLD. (4,7,8)
The most powerful message from this paper, however, is revealed after a
closer look into how HPSD ablation performed in different segments of
the pulmonary veins. While acute pulmonary vein isolation was
accomplished in all patients, additional right pulmonary vein carinal
lesions were required in almost twice as many patients in the HPSD group
compared to LPLD (32% versus 17%, p = 0.011). Furthermore, among
patients who underwent repeat procedure due to recurrent atrial
arrhythmias, late reconnection was concentrated on right pulmonary vein
carina segments in HPSD patients, while a more even distribution of
reconnected segments was seen in LPLD patients. This higher proportion
of right carinal reconnection (respectively 47% versus 21%, p = 0.035)
was observed in spite of no significant difference in overall number of
reconnected segments or reconnected veins per patient.
This intriguing observation requires an explanation, and the authors
offer some insight as to why this might be the case. An increased tissue
thickness at these sites explains worse performance of HPSD ablation, as
thicker muscle would require more conductive heating for deeper lesion
formation. (7) However, linear ablations in thick tissue often performed
concomitantly during AF ablation (such as cavotricuspid-isthmus
ablation, left atrial roof and anteroseptal mitral annulus lines) would
also perform poorly with HPSD, which is not supported by current
evidence but admittedly not studied in detail. (9,10) The current
authors also previously demonstrated that the right pulmonary vein
carina is more susceptible to catheter instability, (11) largely
explained by the more complex local anatomy as well as more challenging
catheter manipulation given close proximity to transseptal access site.
Therefore, caution should be taken when excellent catheter stability
cannot be achieved. In those instances, longer lesion duration at lower
power might be more effective than much faster energy applications, with
proportionally more time in true contact with tissue and better current
delivery. (12) If HPSD eventually becomes the preferred approach for AF
ablation, care should also be taken when ablating the posterior wall
immediately adjacent to the esophagus or when ablating near the phrenic
nerve, given the potential for less reversible collateral injury with
HPSD. In these regions, high power lesion duration of 2 or 3 seconds
longer may be enough to cause unintended irreversible tissue injury to
adjacent structures and the time limits of ’short duration’ still need
to be better defined. In the future, perhaps a hybrid approach
alternating HPSD and LPLD settings in different sites of the left atrium
would potentially become the optimal approach. Hansom and colleagues are
to be congratulated for a job well done in both confirming the value and
documenting a remaining challenge when using HPSD for pulmonary vein
isolation.