Abbreviations:
AF Atrial fibrillation
GP Ganglionated plexus
LSPV Left superior pulmonary vein
LIPV Left inferior pulmonary vein
PFA Pulsed field ablation
PVI Pulmonary vein isolation
RSPV Right superior pulmonary vein
RIPV Right inferior pulmonary vein
TJVS Transjugular vagal stimulation
Introduction:
Atrial Fibrillation (AF), the most common sustained cardiac arrhythmia,
presents a significant challenge in cardiac rhythm
management1,2. Pulmonary vein isolation (PVI) stands
as the cornerstone of AF ablation strategies, predominantly due to the
pulmonary veins’ role as a major source of ectopic beats initiating AF3,4. Catheter ablation is being increasingly performed
as first line therapy or following antiarrhythmic drug failure.
Different energies and techniques are available to perform PVI, however,
the quest for an optimal ablation technique continues, particularly one
that minimizes collateral damage while maximizing efficacy.
Pulsed Field Ablation (PFA) is a relatively novel approach in this
landscape5. The technique utilizes electric fields to
create electroporation of cell membranes, primarily affecting myocardial
tissue while theoretically sparing adjacent structures such as nerves,
blood vessels and the esophagus6. This selectivity
offers a potential reduction in procedural complications commonly
associated with thermal ablation techniques7,8.
Clinical and pre-clinical data have begun to shed light on the neural
implications of PFA9-14. Studies suggest that PFA,
compared to traditional ablation techniques, results in less cardiac
denervation15. This is evidenced by a lower degree of
damage to neurons and axons, which in theory should translate to fewer
autonomic complications post-ablation. However, recent clinical
observations challenge this assumption. Reports of reversible and
irreversible phrenic palsy, a complication associated with collateral
nerve damage, indicate that PFA may not be entirely benign in its neural
implications16.
One of the intriguing phenomena observed following PFA applications is
the occurrence of asystolic pauses. These episodes, while frequently
transient, raise concerns about the direct stimulation of cardiac
autonomic ganglia. The potential for PFA to activate parasympathetic
ganglia adjacent to the left atrium’s epicardial surface, thereby
inducing asystole, is a hypothesis of significant clinical relevance.
Understanding the mechanisms behind these asystolic pauses is crucial
for refining PFA and mitigating its risks.
Methods:
Patient inclusion
We identified 24 patients with paroxysmal AF, as defined by the
guidelines17, that had not undergone ablation.
Pulmonary vein isolation by PFA (Farapulse, Boston Scientific Inc.,
Marlborough, Massachusetts, USA) was scheduled at the CHU Haut Leveque
(CHU Haut-Leveque, Avenue de Magellan, 33604 PESSAC CEDEX) between june
2023 and december 2023. The procedures were performed by two experienced
operators (BB, FS). This study was approved by the institutional ethics
committee. Written patient consent was collected prior to the procedure.
The order of pulmonary veins chosen for electroporation was randomized
to avoid cumulative electroporation effects. With four veins in each
patient (LSPV, LIPV, RSPV, RIPV), this resulted in 24 possible
permutations, that were assigned to patients in randomized order.
Electrophysiological study and ablation
Left atrial (LA) thrombus was ruled out via pre-operative computed
tomography scan. All procedures were performed under general anesthesia.
Oral anticoagulation was uninterrupted and complemented during the
procedure by intravenous heparin, administered to reach an ACT of over
300. A quadripolar catheter was positioned in the coronary sinus and
used for atrial pacing. Pulmonary vein isolation was performed through
transeptal access using a PFA ablation catheter (Farawave, Farapulse)
with five splines, deployed in either a flower or a basket
configuration. The catheter was advanced over a guidewire such that the
splines achieved circumferential contact/proximity with the PV antra.
Twelve applications were performed on the superior pulmonary veins (3
applications in 4 different positions), and eight at the inferior
pulmonary veins (2 applications in 4 different positions). The order in
which the veins were treated was predefined by randomization before the
procedure. Following each PFA application, potential sinus pauses or
atrioventricular block episodes were recorded and measured (figure.1a).
Transjugular vagal stimulation (TJVS)
TJVS was performed using a quadripolar catheter inserted in the internal
right jugular vein, and the anteromedial aspect of the internal jugular
vein, at the level of the upper wisdom tooth. Stimulation was conducted
using a dedicated voltage-controled neurostimulator with a pacing
frequency of 50Hz, pulse duration of 0.05ms, pacing duration of 5
seconds, and a pulse amplitude of up to 1 V/kg (max 70V). TJVS was
performed before PVI and after isolation of each pulmonary vein. At each
step, stimulation was performed in sinus rhythm to assess potential
sinus pauses and repeated during atrial pacing to assess the effect on
atrioventricular conduction. Sinus pause durations were measured by
taking the longest A-A interval recorded after TJVS (figure.1b), and
atrioventricular pauses were measured by taking the longest V-V interval
recorded after TJVS and simultaneous atrial pacing with a cycle length
of 600ms.
Data acquisition for analysis
Electrogram recording and measurements were performed on using a
dedicated digital electrophysiology recording system (LabSystem Pro,
Boston Scientific).
Statistical analysis
Continuous data are represented as mean + standard deviation (SD) or
median and interquartile range (IQR) as appropriate. Normality was
evaluated using the Kolmogorov-Smirnov test. Comparisons between two
groups were made with Student’s t-tests and summarized with means and
standard deviations for independent samples if normally distributed, or
if not normally distributed, with the Mann-Whitney U test and summarized
with medians and quartiles. Nominal values were expressed as n (%) and
compared with chi-square tests, the Fisher exact test when expected cell
frequency was < 5. A probability of < 0.05 was
considered statistically significant.
Results:
Demographics
Table.1 shows patient demographics. The patients were all paroxysmal AF
patients, with no prior ablation.
Vagal stimulation after PFA of each individual vein – sinus node
block
Before the procedure vagal stimulation led to sinunodal pauses of 6.2 ±
3.5 seconds. Sinunodal block over 3 seconds was present in 23 out of 24
patients. Post-PFA, the Right Superior Pulmonary Vein (RSPV) showed the
highest decrease of TJVS-induced sinus pauses (RSPV: before 8.41 ± 4.53
vs after 3.27 ± 3.53 sec, p<0.001, RIPV: before 6.76 ± 4.54
sec vs. 6.89 ± 5.07 sec, p=0.90; LSPV: before 6.76 ± 5.25 sec vs. after
6.93 ± 4.29 sec, p=0.61; LIPV: before 7.80 ± 4.06 sec vs. after 7.88 ±
3.84 sec, p=0.91). Notably, sinus blocks over 3 seconds decreased
significantly after RSPV ablation (19 before PFA, 10 after PFA,
p<0.01), with less dramatic changes in other veins (RIPV:
before 19, after 16, p=0.33; LSPV: before 14, after 19, p=0.11; LIPV:
before 21, after 21, p=1.00).
Vagal stimulation after PFA of each individual vein –
atrioventricular block
Before the procedure vagal stimulation led to atrioventricular pauses of
3.8 ± 2.2 seconds. Atrioventricular block over 3 seconds was present in
21 out of 24 patients. RSPV PFA also had the strongest impact on
TJVS-induced AV block duration compared to the remaining veins (RSPV:
before 6.49 ± 3.48 vs after 4.07 ± 3.27 sec, p<0.01, RIPV:
before 6.00 ± 3.29 sec vs. 4.58 ± 3.99 sec, p=0.08; LSPV: before 5.15 ±
3.94 sec vs. after 5.14 ± 3.48 sec, p=0.93; LIPV: before 6.06 ± 3.98 sec
vs. after 5.83 ± 3.44 sec, p=0.38). The incidence of AV blocks over 3
seconds was reduced, albeit non-significantly, after-RSPV and post RIPV
ablation (RSPV: before:19 vs. after: 14, p=0.11, RIPV: before:19 vs.
after: 14, p=0.11), with minor changes in other veins (LSPV: before 14,
after 16, p=0.55; LIPV: before 17, after 18, p=0.77).
Pauses after electroporation
To assess the acute effect of electroporation we measured the duration
between PFA application and the following atrial or ventricular
activation. During PFA the Left Superior Pulmonary Vein showed the
longest pauses after PFA application followed by the remaining pulmonary
veins (RSPV: 1.89 ± 0.64 sec, RIPV: 1.81 ± 1.70 sec, LSPV: 1.95 ± 1.52
sec, LIPV: 1.72 ± 0.85 sec). Figure.2 shows the duration of pauses as an
average after each PFA complication. PFA causes relatively long pauses
during the first six applications, which shorten for the remaining six
applications (1.99 ± 0.35 sec vs. 1.56 ± 0.22 seconds,
p<0.01).
Pause duration after electroporation in dependence of first ablated
vein
There was a difference between the measured pause durations in
dependence of the vein that was targeted for ablation first. The results
are presented figure.3 and the corresponding table.2. Targeting one of
the left veins first led to relatively long pauses measured during PFA
of the following veins, while ablation of the right pulmonary veins led
to shorter pauses. Starting at the left superior pulmonary vein showed
the longest pauses. Starting with the right superior pulmonary vein led
to significantly shorter pauses during the ablation of subsequent veins
in comparison with starting on the left pulmonary veins
(p<0.03).
Discussion:
Effect of PFA on the parasympathetic nervous system
This study demonstrates the direct influence of PFA on the
parasympathetic nervous system. Our findings confirm this by showing a
notable reduction in sinus and atrioventricular blocks post PFA,
particularly after ablating the right superior pulmonary vein (RSPV).
The RSPV ablation resulted in a significant decrease in sinus block
duration (from 8.41 ± 4.53 to 3.27 ± 3.53 sec, p<0.001) and
atrioventricular block (from 6.49 ± 3.48 to 4.07 ± 3.27 sec,
p<0.01). Sinus blocks over 3 seconds decreased significantly
after RSPV ablation (19 before PFA, 10 after PFA, p<0.01),
whereas there were no significant changes in other veins. PFA at the
right superior pulmonary vein having the greatest effect on the
autonomous nervous system suggests the PFA application at this location
affects the right superior and or posterior nervous plexi, mainly
responsible for sinunodal block18. These findings
suggest a potent acute impact of PFA on the parasympathetic nervous
system.
Are pauses during PFA linked to parasympathetic activation?
The pauses observed during PFA, particularly after RSPV ablation, were
significantly shorter compared to other pulmonary veins. Ablation at the
RSPV first showed a significant reduction in overall pause duration
compared to ablation strategies targeting the left superior (p=0.01) or
inferior(p=0.03) pulmonary vein first. There was a tendency towards
shorter pauses compared with strategies starting at the right inferior
pulmonary veins. These finding‘s resemblance to the effect of PFA on
parasympathetic nervous system support the hypothesis that PFA-induced
pauses are predominantly parasympathetic in nature. Studies that showed
atropine reduced the pause duration are in concordance with our results
as they also imply the parasympathetic nature of these
pauses19.
Is there permanent nerve damage or is it acute?
Although the pauses during PFA are linked to parasympathetic activation
and PFA affects the parasympathetic nervous system acutely, the question
whether there are long-term effects has not yet been clarified. Studies
evaluating biomarkers of neuronal detriment, which was similar before
and after PFA procedures, suggest there is no or little durable damage
to the GPs on the left atrium15,20.
Optimizing Ablation Strategy
This is the first paper to assess different ablation strategies to
minimize vagal reactions during PFA. The data we present advocate for
initiating ablation at the right superior pulmonary vein, as this
approach significantly reduces pause duration, a likely manifestation of
parasympathetic activation. Subsequent targeting of the left pulmonary
veins, where pauses were longer, and concluding with the right inferior
pulmonary vein, appears to be the most effective sequence.
Limitations:
In our study several limitations were encountered. The study involved a
small cohort, with only 24 patients. Larger cohorts are required to
evaluate the findings of this study in the future. Additionally, the
methodology included transjugular vagal stimulation (TJVS) between each
PFA application, which extended the time between interventions. This
approach might have influenced the observed effects of PFA on the sinus
and atrioventricular node. Another critical factor to consider is the
variability in autonomous nervous system innervation. Our study used
TJVS from the right side. The right vagus nerve somewhat selectively
innervates the right superior and posterior ganglionated plexuses, which
then project to the sinus and AV node. In contrast, left vagus nerve
inputs mostly connect to the left superior and left posteromedial GP,
which then project to the AV node. If this study had been performed
using left sided TJVS, denervation resulting from left sided GP damage
(virtually absent in our study) may have been more apparent.
Conclusions:
PFA applications during PVI have acute effect on cardiac GPs, as
evidenced by the decrease in TJVS-induced sinus and atrioventricular
block at the level of the right superior pulmonary vein. PFA-induced
pauses are more frequent during applications on the LSPV, and less
frequent when prior isolation of the RSPV has been performed, suggesting
a vagally-mediated mechanism involving the right superior and/or right
posterior ganglionated plexi. To avoid vagal pauses during PFA ablation
we suggest beginning the ablation by the right superior pulmonary vein.