Correspondence & Reprint Requests:
Chirag R. Barbhaiya, MD
Chirag.Barbhaiya@nyulangone.org
403 E 34th Street, 4th floor
New York, NY 10016
Disclosures/COI: Dr. Maidman has no relationships to disclose.
Dr. Barbhaiya has received consulting fees/honoraria from Abbott, Inc.,
Biosense Webster, Inc., and Zoll, Inc. His relationships to industry are
all modest (<$10,000).
Keywords : atrial fibrillation, radiofrequency ablation, very high
power short duration
High power short duration (HPSD) radiofrequency (RF) ablation, utilizing
45-50W for durations of 5-15 seconds per lesion, is increasingly
accepted as a safe and effective technique to efficiently achieve
pulmonary vein isolation to prevent atrial fibrillation
(AF).1 The next contender in the progression of hotter
and faster RF technology is very high power short duration (vHPSD) RF
ablation – 90W power for a duration of 4 seconds per lesion – using
the QDOT-Micro ablation catheter (Biosense Webster, CA, USA). The
catheter is designed to mitigate risks of vHPSD RF ablation by
monitoring temperature at the catheter-tissue interface using six
thermocouples embedded in the electrode’s tip which allows for power
modulation to maintain a target temperature during RF
delivery.2 Relative to HPSD ablation utilizing 45-50W,
vHPSD ablation with 90W may further maximize shallow, resistive heating,
while the shorter duration may additionally minimize deep, conductive
heating.2-4 The frequency and risk factors for the
feared complications related to ablation, like steam pop, cardiac
perforation, or esophageal injury, are not well understood for
vHPSD.5 Early clinical studies of vHPSD ablation
reported first pass PVI in approximately 50% of cases, while first pass
PVI was achieved in greater frequency with both conventional and HPSD RF
ablation using recently described approaches such as the “CLOSE
Protocol”.5-7 First pass isolation has been shown to
be a powerful predictor of procedural efficacy,3,8,9thus a greater understanding of the underlying biophysics of lesions
created with vHPSD RF ablation may inform optimization of timing and
spacing to further improve outcomes for this new technology.
In this issue of the Journal, Yamaguchi and
colleagues10 undertook a detailed analysis of 480 RF
ablation lesions created with the QDOT-Micro ablation catheter at 90W
over 4 seconds in an in-vitro porcine myocardial model. They
attempted to better elucidate the biophysics of single and multiple
point-by-point RF ablation applications to inform creation of lesion
sets using a vHPSD approach. Single application (SA) vHPSD ablation was
compared to double repetitive application (DRA), in which a second vHPSD
RF application was performed as soon as the mandatory four seconds RF
lockout expired, and double non-repetitive RF application (DNRA), in
which a second RF application was performed approximately one minute
after initial ablation. Additional variables for lesion creation that
were systematically evaluated include contact force (CF), temperature
limit, and catheter orientation.10
The primary analysis evaluated lesion size (depth, volume) and surface
areas as well as rates of steam pop. DRA resulted in the deepest and
largest lesions followed by DNRA then SA (depth: 3.8 mm vs 3.3 mm vs 2.6
mm; volume: 177 mm3 vs 145 mm3 vs 97
mm3, respectively). Similar trends were found
regardless of catheter orientation (perpendicular versus parallel), with
a perpendicular catheter orientation associated with slightly deeper
lesions, but similar surface areas. Lesions created with target
temperature of 60oC, compared to
55oC, were slightly larger, without increased risk of
steam pop. Steam pop occurred significantly more frequently in the DRA
arm (16%), compared to 7% and 4% in DNRA (7%) and SA (4%),
respectively. In the double application group (DRA and DNRA arms
combined), the authors found that lesions made with the DRA approach
and, counterintuitively, CF< 15g were found to be
significant predictors of steam pop by both univariate and multivariate
analyses.10 Traditionally, higher contact forces have
been associated with increased risk of steam pop with conventional and
HPSD RF ablations.11 Presumably, lower contact force
during vHPSD ablation results in less robust ascertainment of tissue
temperature and thus a lower ability to reduce power in response to
significant tissue heating. Further investigation is required to better
understand this unique relationship between vHPSD RF ablation and
contact force.
The findings in the present study add to the growing body of evidence
supporting the conclusion that inter-lesion spacing and timing
significantly influence transmural lesion creation during point-by-point
RF ablation. A prior study by Jankelson and colleagues using
conventional power and duration RF ablation demonstrated that
consecutively placed lesions resulted in a higher likelihood of
transmural lesion creation compared to time-spaced
lesions.12 The present study by Yamaguchi and
colleagues confirms the presence of a similar phenomenon with vHPSD
ablation, likely related to a heat stacking effect which facilitates
increased conductive heating of deeper tissue. This occurs despite
vHPSD’s inherent property of a time-limited ablation and a four second
lockout period between lesions, both of which are meant to minimize deep
heating.
While utilization of DRA to create deeper lesions may address the
suboptimal first-pass isolation rate observed with vHPSD ablation, the
risk of injury to collateral structures, such as the esophagus, requires
further evaluation. In-vivo and in-vitro studies have
shown significant, additive esophageal heating occurs with consecutive,
adjacent, short duration, 50W RF applications compared to time-spaced
lesions.13,14 Given that esophageal temperature has
been found to remain elevated for approximately 60 seconds after a
single HPSD RF application,14 the four seconds lockout
period between vHPSD lesions in the DRA arm is unlikely to fully
mitigate esophageal heat stacking. Furthermore, the in-vitromodel utilized in the present study is not designed to evaluate the risk
of injury to collateral structures during catheter ablation.
As RF ablation powers escalate and the potential to utilize repetitive
RF applications to create deeper lesions in thicker tissue is better
appreciated, an important question remains: what is the optimal delay
between adjacent lesions? A one minute long delay, as utilized in the
present study for the DNRA strategy, may be reasonable for ablating
tissue overlying the esophagus, while DRA may be better suited in areas
of thicker walls where there is minimal risk of collateral injury to
surrounding structures. Currently espoused ablation strategies, such as
the “CLOSE Protocol” specify inter-lesion spacing, but not
inter-lesion timing.5-7 Similarly, current
electro-anatomic mapping systems are well suited for the ascertainment
of numerous parameters for a single RF application and inter-lesion
spacing, but do not easily allow consideration of inter-lesion timing
for either clinical or investigational purposes. Better tools are needed
to understand and optimize spacing and timing of lesions for the
creation of complex ablation lesion sets.
Yamaguchi and colleagues should be commended for their innovative study
design and execution of this in-vitro evaluation of vHPSD
strategies. They highlighted the critical importance of optimizing
inter-lesion timing while also elucidating a counterintuitive
relationship between lower contact forces and increased risk of steam
pop with vHPSD ablation. While single vHPSD RF applications appear to
consistently produce safe and shallow ablation lesions, successful AF
ablation requires numerous RF applications at anatomic locations with
varied tissue thickness and risk of collateral injury. The present study
highlights that understanding the biophysics of lesion creation of a
single vHPSD lesion is a start, but it, in fact, takes two (or more) to
make a thing go right.