4 DISCUSSION
The main findings were as follows: (1) HPSD approach was associated with
higher first-pass PVI, and recurrence of atrial arrhythmias; (2) HPSD
approach could significantly reduce procedural time, ablation time and
fluoroscopy time compared with the LPLD approach; and (3) Major
complications and ETI were similar between two groups.
Thermal injury by RF ablation comprises two consecutive phases:
resistive and conductive. The balance between power and duration
parameters in resistive and conductive heating has a significant
influence on lesion creation. The resistive phase has a resistive
component adjacent to the catheter tip, which results in local heating
and dissipation of energy as a heat source. Resistive heating probably
occurs in the first few seconds during the RF application. With
immediate heating, the electrical current is delivered at the
catheter–tissue interface. The tissue necrosis is confined only to the
first 1–1.5 mm from the catheter tip; however, the temperature always
rises above 50°C with a conventional power
setting17. Greater
resistive heating can be achieved using higher-power delivery. In the
conductive phase, the resistive heat source then extends energy
passively to deeper tissues. Conductive heating is time dependent. The
heating of deeper tissues increases with longer-duration RF applications2,
7, 21-23.
The LPLD approach is associated with longer conduction heating.
Low power longer duration time of 10–30 s per site is based on earlierin vivo studies on the ventricular tissue 7.
Simmers et al. showed that the lesion dimensions increased in 30 s using
25 W power; however, the mean lesion depth already reached 7.25 mm in 30
s 24. In the left atrium, where the mean posterior
wall thickness is 1.5–2.5 mm and the esophageal distance from the
posterior wall is as small as 2.5 mm, the LPLD approach might cause
serious injury to the
esophagus7,
18, 24,
25. The incidence of atrio-esophageal
fistula (AEF) is reported as 0.1%–0.25%, and the incidence of
esophageal thermal injury (ETI) is 2%–50% for the LPLD approach18. In addition, longer procedural time, ablation
time, and fluoroscopy time are necessary for the LPLD approach due to
its inherent characteristics. Prolonged procedural time would inevitably
lead to a longer anesthesia duration, which also increases the
procedural risk, especially for elderly patients 26.
One potential approach to optimize the longer ablation time of LPLD is
to modify the relationship between the resistive and the conductive
heating phases by increasing the resistive heating phase and reducing
the conductive hearting phase to deliver immediate heating to the full
thickness of tissues and limit deep tissue
injury2,
17, 27.
To achieve this, high energy must be delivered in a short duration.
Thus, the strategy of HPSD was proposed and applied for AF ablation.
The HPSD approach is largely based on immediate heating during the
resistive phase, however, whether high power with shorter duration might
create trans-mural lesions in the left atrium? The average thickness of
the left atrium is 2.8 ± 1.1 mm in
humans28. For patients
with persistent AF, the mean atrial wall thickness is only 1.89 ± 0.5 mm
and never exceeds 3.5 mm25.The HPSD approach
affects a tissue depth of 3.5–4 mm2, thus, the left atrial
wall thickness is well within the depth for the HPSD approach. Hence,
HPSD is well suited to AF ablation.
Several previous experiments evaluated the utility of HPSD approach.
Bourier et al evaluated the lesion metrics created by HPSD compared with
LPLD application. They found that the HPSD approach created lesions
similar in volume but wider and shallower compared with the LPLD
approach 29. In fresh
killed porcine ventricles, Goyal et al. showed that for a given CF, 20 s
were needed to create a 4 mm deep lesion using 20 W ablation, while only
6–7 s were enough for 50 W ablation22. Bhaskaran et al.
examined several combinations of high-power ablation in vitro and
in a sheep model. The study showed that the use of 50 W for 5 s resulted
in a similar lesion depth compared with the use of 40 W for 30 s. Steam
pops occurred in 8% of the 40 W (30 s) ablation and in none of the 50 W
(5 s) ablation7. So,
HPSD approach could achieve rapid, more controlled, resistive tissue
heating, and avoiding deeper collateral injury.
For clinical studies, Winkle et al showed that HPSD approach using CF
sensing catheter was safe and result in excellent long-term freedom from
AF with short procedure times and delivery of small amounts of total RF
energy. No complications were reported in this
trial27. Other studies
also showed the safety and efficacy of HPSD approach in an abstract
form. Nonetheless, these studies are too small to evaluate the safety
and efficacy of HPSD approach, and the comparison data between HPSD and
LPLD were still limited. So, the present meta-analysis was performed.
The present study showed a higher first-pass PVI in the HPSD group when
compared to the LPLD group. The main reasons may due to greater size,
more uniform and better consistency of the lesions created by HPSD
approach. Catheter-tissue contact stability is an important factor
contributing to lesion creation, and catheter instability in a
constantly moving heart may account for the difficulty to transmit heat
to the tissue17. This
shortening of HPSD approach may mitigate the negative effects of
catheter instability and probably optimizes lesion creation by
increasing the likelihood to keep the catheter stable throughout the
entire application, before stability becomes a
consideration27. In the
LPLD group, the catheter stability was an issue when longer
single-lesion ablation was needed, leading to unevenness of lesions,
tissue edema, and lower rate of first-pass PVI. Moreover, HPSD approach
could reduce the rate of long-term recurrence of atrial arrhythmias.
Complete PVI with trans-mural injury is most important for the freedom
from AF during the long-term
follow-up30,
31. Hence, HPSD approach trend to form
more trans-mural, continuous and permanent lesions.
The pooled analysis showed significant advantages of the HPSD approach.
The approach could shorten procedural time and ablation time compared
with the LPLD approach, thus limiting patient exposure to intravenous
fluids and anesthesia. Additionally, fluoroscopy time was also shorter
in the HPSD group, which had a direct favorable impact on the patient,
operator, and supporting staff. These results were consistent with
previously published
findings4,
16, 20,
22, 23,
32. A significant reduction in the
procedural time was observed in the HPSD group because of shorter
ablation time. The shorter ablation time for HPSD were due to the
shorter time required for lesion creation, higher first-pass PVI, and
fewer acute PV
reconnections2. In the
LPLD group, additional ablations were required for gap ablation in
non-transmural lesions and achieving biphasic block of PVs so as to
achieve completed PVI17. During the
procedure, catheter movement mostly relies on the combination of x-ray
and 3D electro-anatomic mapping system. Hence, longer ablation time in
the LPLD group inevitably led to greater fluoroscopy time to locate and
move the catheter in the left atrium. Additionally, less irrigation
fluid was needed during HPSD due to shorter ablation time, making HPSD
more suitable for patients with impaired left ventricular function.
Previous studies involved an increased power of 40–50 W for AF
ablation. One study using HPSD showed improved outcomes but an increase
in complications, such as ETI, cardiac tamponade, and so forth. The
safety of using high power for AF ablation, especially on the posterior
wall, was a concern33.
However, another study using HPSD ablation reported no increase in
complications 34. According to the principle of HPSD
with increasing resistive and reducing conductive phases, minimizing
damage to collateral tissues has been a crucial consideration in HPSD.
Several animal studies suggested that HPSD was superior to LPLD with
lower complication rate2,
21. Some human studies using HPSD showed
excellent clinical outcomes with fewer complications15,
35, 36.
In the present study, the pooled analysis of included studies also
showed that the rate of complications similar between two groups. The
AEF or ETI was rarely reported because most of the studies were
single-center studies, and none of them were large enough to evaluate
infrequent serious complications. A large observational study focused on
the complications of the HPSD approach. In this study, 13,974 ablations
were performed on 10,284 patients, revealing an extremely low
complication rate in the HPSD group. Only 1 AEF occurred in 11,436
ablations using HPSD; however, 3 AEFs occurred in 2538 ablations using
LPLD32. There were two
studies included in our analysis discussed ETI in patients received
ablation treatment. The results showed low and similar rates of ETI in
the two groups. The subgroup analysis showed that HPSD could reduce mild
ETI. No AEFs were reported in the included studies. Thus, the HPSD
approach was safe enough for AF
ablation18.
Additionally, another study reported by Reddy et al compared very high
power short-duration (vHPSD, a delivery of 90 W for 4 s) approach using
a QDOT microcatheter (Biosense Webster, Inc., CA, USA) with the LPLD
approach. They demonstrated that vHPSD was an efficient, feasible, and
safe strategy for AF ablation. However, this study was not included in
the present meta-analysis due to a completely different setting of
delivery of the vHPSD and HPSD approaches. Nevertheless, vHPSD may be
another promising strategy in the
future23.
Our study has some limitations. First, publication bias could
not be completely excluded, the inclusion of only published data
contributed to bias. Second, the number of included studies was limited
to only seven, and most of the studies were designed as nonrandomized,
thus, more well-designed and large-scale RCTs are required to confirm
the findings. Third, limited collateral tissue damage is one of the
important advantages of HPSD. However, in the present meta-analysis,
this damage was not completely reflected due to limited endpoints
reported from the included studies. Fourth, in some studies, the
catheters applied in the HPSD and LPLD groups were different, thus may
affect the outcomes of the pooled analysis. Fifth, the ablation power
and duration settings in the included studies were not completely
consistent.