Very high‐power short‐duration catheter ablation for treatment of cardiac arrhythmias: Insights from the FAST and FURIOUS study series

The QDOT MICRO™ Catheter is a novel open‐irrigated contact force‐sensing radiofrequency ablation catheter. It offers very high‐power short‐duration (vHPSD) ablation with 90 W for 4 s to improve safety and efficacy of catheter ablation procedures. Although the QDOT MICRO™ Catheter was mainly designed for pulmonary vein isolation (PVI) its versatility to treat atrial fibrillation (AF) and other types of arrhythmias was recently evaluated by the FAST and FURIOUS study series and other studies and will be presented in this article. Available study and registry data as well as case reports concerning utilization of the QDOT MICRO™ Catheter for the treatment of cardiac arrhythmias including AF, focal and macroreentry atrial tachycardia, typical atrial flutter by cavotricuspid isthmus block, premature ventricular contractions, and accessory pathways were reviewed and summarized. In summary, the QDOT MICRO™ Catheter showed safety and efficacy for PVI and is able to treat also other types of arrhythmias as is was recently evaluated by case reports and the FAST and FURIOUS studies.

[13][14] Recently 3D mapping and point-by-point based catheter ablation achieved several improvements by implementing contact force (CF), ablation index (AI), or lesion size index (LSI) guided RF ablation which have been shown to decrease procedure time, improving safety and patient outcomes. 15,16[25] However, it has been shown that real-time subendocardial tissue temperature monitoring during RF energy delivery is a direct indicator of lesion formation but measurements by temperature sensors embedded in the catheter tip have been shown to be not an accurate indicator of actual tissue temperature. 26,27

| QDOT MICRO™ catheter
To overcome those issues a novel ablation catheter was recently introduced.The QDOT MICRO™ Catheter (Biosense Webster, Inc. ) is an open-irrigated CF sensing single-tip catheter (Figure 1).The QDOT MICRO™ Catheter incorporates three microelectrodes and six miniature thermocouples (1 mm in diameter [3 proximal and 3 distal]), at its tip for precise temperature monitoring. 17,28,29The thermocouples are embedded at distinct locations within the outer metal shell of the catheter, just 75 μm underneath the tip surface. 26e reason for this superficial location design was to obtain more accurate local temperature measurements from the catheter tip which is in direct contact with the tissue, in comparison to the conventional tip temperature measurements, that is, altered by the cooler parts of the tip that are not in touch with the tissue. 26It has been developed allowing for real-time catheter-to-tissue interface temperature measurements.Therefore, a real temperaturecontrolled catheter ablation with automatic adjustment of power and irrigation output based on real-time temperature measurement is achieved which allows for safe and effective RF-lesion formation. 29,30The novel "bulls eye" tool of Carto™ offers realtime visualization of the temperature controlled ablation.Furthermore, catheter stability and contact is visualized by the tool (Supporting Information: Video 1).
The QDOT MICRO™ Catheter offers two different ablation modes.
In QMODE (conventional temperature-controlled ablation mode with a maximum of 50 W) the system adjusts (1) the irrigation flow rate and (2) power based on the measured temperature to stabilize the catheter tip temperature within the allowed temperature range avoiding over-heating and steam-pop.Lesions formation is guided by AI in the QMODE and is individualizable concerning power and ablation duration.
In QMODE+ (vHPSD mode, 90 W/4 s) only power is adapted to adjust the target temperature. 30The target temperature of the temperature-controlled ablation is usually 60°C based on the hottest surface thermocouple.The irrigation flow rate delays the energy application for a minimum of 2 s before and 4 s after each RF application.It is always possible to stop the lesion formation, however, in QMODE+ it is only possible to ablate with 90 W for a maximum of 4 s.The default irrigation setting is set at 2 and 8 mL/min with a recommended CF working ranges 5-30 g.
The vHPSD strategy aims to create shallower but wider lesions in a very short time by reducing conductive heating and increasing resistive heating at the same time.Additionally, collateral tissue damage might be reduced. 28,31e three microelectrodes at the catheter tip of the QDOT MICRO™ Catheter offer sharp potentials and a higher resolution during mapping. 29Previous analyses provided evidence for reduced RF ablation time and procedure duration while showing a good safety profile in comparison to conventional power-controlled ablation. 11,30though the QDOT MICRO™ Catheter was mainly designed for PVI its versatility to treat other types of arrhythmias was recently evaluated and will be presented in this article (Tables 1 and 2).| 549 The safety profile has been shown to be similar to those of standard moderate power and moderate duration irrigated RF ablation. 30vHPSD in combination with temperature-controlled ablation enables the ablation to stabilize the catheter tip temperature within the target temperature range and is avoiding over-heating and steam-pops.

| PULMONARY VEIN ISOLATION UTILIZING THE QDOT MICRO™ CATHETER
With the ability to perform temperature-controlled ablation in combination with vHPSD the QDOT MICRO™ Catheter is an ideal tool for thinwalled LA procedures especially for PVI (Figure 2).The vHPSD strategy aims to create shallower but wider lesions in a very short time by reducing conductive heating and increasing resistive heating at the same time. 17Additionally, collateral tissue damage might be reduced. 28Utilizing conventional power-controlled moderate-power long-duration RF applications the "close-protocol" with an interlesion distance (ILD) of 6 mm has been introduced and verified for PVI. 41  p < .0001)and the first pass isolation rate was 74% (vHPSD) and 76% (control, p = .817).Severe adverse events were reported in 2% (vHPSD) and 6% (control, p = .307)and the 12-month recurrence-free survival was 78% (vHPSD) and 64% (control, p = .142).In conclusion, PVI solely utilizing vHPSD via a very close protocol was shown to provide safe and effective PVI with a high rate of first-pass isolations.
No steam pops and no catheter tip charring were detected.An esophageal temperature probe was utilized in all vHPSD patients.A temperature of >38.5°C was detected in 18 (36%) patients solely at the posterior part of the left PVs.The mean maximum esophageal temperature was measured at 42 ± 2°C.No clinical apparent esophageal lesions and no atrio-esophageal fistulas were found. 11e QDOT-FAST trial was a first-in-human, prospective, of cases (41 of 52), PVI was achieved using the QMODE+ only.No severe adverse events occurred in this study. 30e Q-FFICIENCY trial was a prospective, multicenter (n = 22), nonrandomized study that aimed to evaluate the safety and effectiveness of the QDOT MICRO™ Catheter in treating drugrefractory, symptomatic PAF patients (n = 166) utilizing vHPSD only.
The median procedural duration was 132 min and the median RF time was 8.0 min while the primary adverse event rate was 3.6%.The 12-month clinical success rate was 76.7%.Although the findings concerning safety and effectiveness were similar to the FAST and FURIOUS PVI study the procedure time was almost doubled which could be the consequence of its multicenter character with some centers unexperienced in QDOT MICRO™ Catheter procedures. 32e POWER PLUS trial was a multicenter, randomized controlled trial, the authors compared procedural efficiency, efficacy, and safety of PVI using 90-W/4-s ablation to A nonsignificant trend towards lower rates of first-pass isolation was seen in the 90-W group (83.9% vs. 90%; p = .0852).However, no differences in 6-month outcomes were observed.The authors suggest to test a hybrid approach combining QMODE+ for anterior and QMODE for posterior aspects of the LA in future studies. 33though rare atrio-esophageal fistula after catheter ablation of AF is a devastating and potentially lethal complication. 42The fact that vHPSD applications create shallower and wider lesions might be a factor for preventing atrio-esophageal fistulas.A recent study of 90 consecutive patients treated by vHPSD-based PVI underwent postablation esophageal endoscopy.None of the 90 patients demonstrated esophageal ulceration (0%) which might support the abovementioned positive effect of the lesion formation in the prevention of esophageal lesions.Furthermore, no steam pop, cardiac tamponade, stroke, or fistula was reported in this study. 43Although the rate of clinical apparent stroke and TIA is reported to be low initial cerebral magnetic resonance imaging (MRI) data after QMODE+ based PVI showed silent cerebral lesions in 6/23 (26%) patients.Since coagulation on the catheter tip was detected at the end of the procedure suggesting this observation to being related to those events. 19,44

| QDOT MICRO™ catheter for cavotricuspid isthmus (CTI) ablation
Beside the fact that QDOT MICRO™ Catheter QMODE+-based ablation for PVI has been evaluated in several trials and studies, there is only limited data for catheter ablation of the CTI in patients presenting with typical atrial flutter (AFL) (Figure 3).The main concern about vHPSD is the lack of transmurality in regions with thicker tissue like the CTI. 45wever, data utilizing 50 W in studies by Kwon et  procedure (30 ± 4 vs. 34 ± 10 min, p = .5)time was observed. 35sides PVI in the FAST and FURIOUS PVI study CTI block was achieved by Qmode+ only in 13/13 patients.In one patient with a repeat procedure, the CTI was checked and was found to be durable blocked. 48milar findings have been evaluated in the FAST and FURIOUS CTI study.Here complete CTI block using vHPSD ablation was achieved in all 15 patients.A median of 23 (20, 39) RF applications over a median RF ablation time of 92 (78, 154) s were applied and no periprocedural complications, no charring, and no steam pops were observed. 49e preliminary data show that vHPSD ablation might represent an effective and safe strategy to achieve bidirectional CTI block for the treatment of typical AFL.Please also see Supporting Information: Video 2 for CTI-block by vHPSD applications only.

| ABLATION OF ATRIAL TACHYCARDIA BY THE QDOT MICRO™ CATHETER
Although the QDOT MICRO™ Catheter was designed for atrial procedures data on the treatment of atrial tachycardia is limited to only one published case report. 39re a patient with perimitral atrial tachycardia with a critical vHPSD ablation seems to be an interesting option for mapping and ablation of atrial tachycardias (Figure 4). 39Please also see Supporting Information: Video 3 for perimitral AT ablation by vHPSD only.

| ACCESSORY PATHWAY ABLATION
Another case report has been recently published demonstrating the safety and efficacy of vHPSD ablation of a left lateral accessory pathway by a single 90 W/4 s RF application utilizing QDOT MICRO™ Catheter.An immediate loss of pathway conduction was reported.Afterward, three bonus applications of vHPSD have been conducted.The 12-lead ECG confirmed the absence of delta wave even after 12 months of follow-up. 37

| INAPPROPRIATE SINUS TACHYCARDIA
A case report of therapy-refractory inappropriate sinus tachycardia has been recently published.Ablation by a total of 42 vHPSD applications at the sinus nodal area resulted in a significantly decreased in heart rate.Since the sinus nodal area is located close to the phrenic nerve within the thin-walled RA vHPSD applications may result in more effective, broader, and superficial lesion formation potentially avoiding collateral damage to adjacent structures like the esophagus and the phrenic nerve. 40

| ABLATION OF PREMATURE VENTRICULAR CONTRACTIONS UTILIZING QMODE+
Although vHPSD concepts have been evaluated for atrial procedures, data for ablation within the ventricles is very limited.Only one case report and one study for catheter ablation of PVC utilizing the QDOT MICRO™ Catheter have been published in humans up to date. 34,38 the initial case report a patient with frequent monomorphic PVC and severe adverse events (4%, study group, 8%, control group, p = .551). 34though the FAST and FURIOUS PVC study was a single center nonrandomized study it demonstrated the feasibility and safety of the QDOT MICRO™ Catheter for treatment of OVC originated from the RVOT and LVOT.In the thin walled RVOT, the success rate of vHPSD was 92%, while it was 42% for LVOT case.Therefore, it seems to be reasonable to consider vHPSD (QMODE+) for PVC originating from the RVOT (Figure 5) and conventional QMODE for PVC originating from the LVOT.

2 |
BIOPHYSICS OF VHPSD RF ABLATION During vHPSD ablation, myocardial lesions are created by a greater amount of resistive (direct) rather than conductive (indirect) heating when compared with conventional ablation settings.Therefore, vHPSD may result in more effective, broader, and superficial lesion formation potentially avoiding collateral damage to adjacent structures like the esophagus and the phrenic nerve.In preclinical models, vHPSD ablation has shown improved atrial linear lesion contiguity, transmurality, and durability and has substantially reduced RF ablation times.F I G U R E 1 QDOT MICRO™ Catheter.Picture of the QDOT MICRO™ Catheter tip showing microelectrode at the tip.The black arrows highlight one microelectrode, the contact force sensor and the 56 irrigation holes at the catheter tip.Source: Property of C. Heeger.

F
I G U R E 2 QDOT MICRO™ Catheter during pulmonary vein isolation in QMODE+.(A) Three-dimensional electroanatomic reconstruction (CARTO 3, UNIVIEW module, Biosense Webster) of the left atrium in PA view and (B) superior view.Please note the circle depicted through redblack tags created by radiofrequency ablation utilizing the QDOT MICRO™ Catheter in the QMODE+ ablation mode.The data of the current application is depicted in (C) and shows the biophysics parameters of a very-high-power short-duration ablation by 90 W/4 s.The parameters of power (W) Impedance (Ω), temperature (°C), and contact force (g) are shown.Right upper corner depicts the bulls eye view with temperature measurements of the QDOT MICRO™ Catheter tip.Source: Property of C. Heeger.
isthmus on the anterior wall was treated by the QDOT MICRO™ Catheter utilizing QMODE+ only.An ablation of an anterior line was performed.After 10 vHPSD applications the atrial tachycardia terminated and the anterior line was completed with a total of 29 applications with an extremely low RF time of 116 s.No periprocedural complications occurred.Interestingly the microelectrodes of the QDOT MICRO™ Catheter showed sharp and fragmented potentials at the area of the critical isthmus even though the standard electrodes showed no visible signals.This observation demonstrated the applicability of the microelectrodes for identifying potential targets for catheter ablation.The combination of microelectrodes and

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I G U R E 3 QDOT MICRO™ Catheter during cavotricuspid isthmus-ablation in QMODE+.Three-dimensional electroanatomic reconstruction (CARTO 3, UNIVIEW module; Biosense Webster) of the right atrium in left anterior oblique and right anterior oblique during catheter ablation at the cavotricuspid isthmus for the treatment of typical common type atrial flutter.Please note the white-red tags created by radiofrequency ablation utilizing the QDOT MICRO™ Catheter in the QMODE+ ablation mode.Source: Property of C. Heeger.
originating from the right ventricular (RV) outflow tract (OT) (RVOT) received 3D electroanatomic reconstruction of the right ventricle.The earliest activation was detected within the anterolateral RVOT.At this area the microelectrodes detected early fragmented potentials which were not detectable on the standard bipolar electrode of the ablation catheter.A single vHPSD application of 4 s was performed resulting in an immediate loss of PVC.No periprocedural complications occurred and no recurrent PVC was reported during long-term follow-up.38After successful performing the above-mentioned case a pilot study for PVC treatment via QDOT MICRO™ Catheter was conducted.In the prospective single-center FAST and FURIOUS PVC study, we sought to investigate the efficacy, safety, and clinical outcome of vHPSD ablation for the treatment of idiopathic PVCs originating from the right and left ventricular OTs (LVOTs).These data were compared to standard powercontrolled ablation strategy using conventional contact-force sensing ablation catheters.In this study, 24 consecutive patients underwent PVC ablation utilizing vHPSD ablation (study group) and were compared with 24 consecutive patients previously treated with power-controlled ablation (control group).Each group included 12 patients with PVCs F I G U R E 4 QDOT MICRO™ Catheter for ablation of atrial tachycardia by QMODE+.(A) Electroanatomic map of the left atrium utilizing CARTO 3, V7 (Biosense Webster).Left side, left anterior oblique, LAO) view.A local activation time map with evidence of a peri-mitral atrial tachycardia, suggesting the critical isthmus at the anterior wall.Coppery area-zone of slow or no conduction (Coherent module; Biosense Webster).(B) Voltage map with evidence of a large scar area on the anterior and posterior wall.The bipolar voltage reference interval was set between 0.05 and 0.35 mV.(C) QDOT MICRO™ Catheter in QMODE+ and ablation at the anterior wall at the moment of atrial tachycardia termination Left side, right anterior oblique (RAO) view; right side, LAO view.Note the ablation catheter in the anterior wall during delivery of a very high-power short-duration application of 90 W/4 s.The contact force was 18 g and the distance from the previous application was 5.3 mm.The "bullseye" in the left upper corner indicates the temperature of the ablation catheter tip.(D) Final lesion set up with an anterior line depicted by red dots with white points.Source: Property of C. Heeger.originatingfrom the RVOT and 12 patients with PVCs originating from the LVOT.The acute endpoint of PVC elimination was achieved in all patients.In this study, vHPSD was used as the first approach and was switch to conventional QMODE after ineffectiveness.In 16/24 (67%) patients (study group) the acute endpoint was achieved by using vHPSD only (RVOT: 92%, LVOT: 42%).The earliest activation found on the microelectrodes of the QDOT MICRO™ Catheter was significantly earlier than on the standard bipolar electrodes.The median RF delivery time was massively reduced utilizing vHPSD (p < .0001).No difference was observed regarding procedure duration (p = .489),follow-up (p = .712) Studies on QDOT MCRO ablation catheter.
Case reports on QDOT MICRO Ablation catheter.
Lesion formation of vHPSD applications creates wider but shallower lesions.To achieve continuous lesions, we recently adapted the close-protocol to an individualized and tighter "very close-protocol."Utilizing the "very close-protocol" an ILD of 3-4 mm at anterior aspect and ILD of 5-6 mm at the posterior aspect of the LA using vHPSD only is performed and safety, efficacy, and follow-up in comparison to conventional CF sensing AI-guided RF ablation has been For patients of the vHPSD group, the QMODE+ was exclusively used for all procedures.No switch to QMODE was necessary to achieve PVI.No differences were observed between the groups concerning catheter maneuverability and catheter stability.Here a significantly reduced mean RF time of 352 ± 81 s (vHPSD) versus 1657 ± 570 s (control, p < .0001)was observed.Furthermore, the mean procedure duration was 59 ± 13 (vHPSD) and 101 ± 38 (control, T A B L E 2 Abbreviation: vHPSD, very high-power short-duration.
Therefore, the RF generator software has been recently modified aiming to reduce the rate catheter tip coagulation.
The available data of the QDOT MICRO™ Catheter for PVI is promising.While demonstrating a good safety profile, the total ablation time, and procedural duration, were impressively low utilizing vHPSD.The retrospective peQasus study (clinical trials.gov:ID: NCT05710822, vHP-SD ablation utilizing the QDOT MICRO™ Catheter for pulmonary vein isolation-a multicenter study) will evaluate safety and efficacy of the QDOT MICRO™ Catheter for PVI in multiple centers in Europe in a large group of >500 patients.
al. and Yavin et al.