Biophysics and pathology of catheter energy delivery systems

Thermal and Non-Thermal Energies for Atrial Fibrillation Ablation

The cornerstone of ablative therapy for atrial fibrillation (AF) is pulmonary vein isolation (PVI). Whether PVI should be added with additional lesions in persistent atrial fibrillation (PerAF) or for any post-ablative recurrent AF is a matter of debate. Whatever the ablative strategy, it must determine the choice of energy source to achieve the most durable lesion sets with the least likelihood of complications. Radiofrequency (RF) is the most studied thermal ablation technique. It can be combined with high-density electroanatomic mapping and can be used for both pulmonary and extrapulmonary atrial ablation. Cryoenergy is at least as effective as radiofrequency for PVI; it is rapid, relatively safe, and has a steep learning curve. Therefore, it has been proposed as a first-line approach for PVI-only procedures. More recently, a non-thermal technique based on the application of pulsed direct current (Pulsed Field Ablation-PFA) has been introduced. PFA causes cell death by opening cell membrane pores (electroporation) without a significant increase in tissue temperature. It is fast and does not alter the extracellular matrix as thermal techniques do, although it ends up causing long-lasting, transmural lesions. Most importantly, it is relatively selective on cardiac myocytes and therefore potentially safer than thermal techniques. Some PFA systems can be combined with electroanatomic mapping systems. However, as of now, it appears that these ablation technologies should be considered complementary rather than alternative for a number of practical and theoretical reasons.

Radiofrequency and Cryoablation as Energy Sources in the Cox-Maze Procedure: A Meta-Analysis of Rhythm Outcomes

AIM

Cox-maze IV is the most effective surgical procedure for atrial fibrillation (AF) treatment to date; however, few studies have compared the outcomes of the different energy sources applied to achieve transmurality. This study aimed to analyse the impact of the different energy sources on Cox-maze IV results in terms of sinus rhythm restoration.

METHOD

A systematic review and meta-analysis was conducted by including studies reporting rhythm outcomes on biatrial Cox-maze AF ablation with bipolar radio-frequency (BRF), cryoenergy (Cryo), or both (BRF+Cryo). The primary endpoints were the early and late rhythm outcomes of AF ablation using the different energy sources. Late AF recurrences were evaluated through timepoint analysis, and freedom from AF from Kaplan-derived data. Sixty articles including 8,293 patients were selected (3,364 patients Cryo, 1,937 BRF, and 2,992 BRF+Cryo).

RESULTS

At 6 months, AF incidence was significantly lower in the Cryo group at 6.73%; it was 25.52% in the BRF and 16.79% in the BRF+Cryo groups (p=0.0112). At the 4-year timepoint, AF incidence was lower in the Cryo group compared with the BRF and BRF+Cryo: 6.14% vs 51.59% vs 16.09%, respectively (p=0.0392). Freedom from AF was 76.7%±2.2%, 60.9%±2.2%, and 66.3%±1.6% for Cryo, BRF, and BRF+Cryo at 4 years, respectively (p<0.001). At meta-regression, mean left atrial diameter was positively associated with higher AF recurrences (OR 1.04, 95% CI 1.01-1.08; p=0.0159).

CONCLUSION

When performing this procedure, cryoablation seems to be associated with improved rhythm outcomes when compared with bipolar radiofrequency ablation.

In vitro characterization of radiofrequency ablation lesions in equine and swine myocardial tissue

Radiofrequency ablation is a promising technique for arrhythmia treatment in horses. Due to the thicker myocardial wall and higher blood flow in horses, it is unknown if conventional radiofrequency settings used in human medicine can be extrapolated to horses. The study aim is to describe the effect of ablation settings on lesion dimensions in equine myocardium. To study species dependent effects, results were compared to swine myocardium. Right ventricular and right and left atrial equine myocardium and right ventricular swine myocardium were suspended in a bath with circulating isotonic saline at 37 °C. The ablation catheter delivered radiofrequency energy at different-power-duration combinations with a contact force of 20 g. Lesion depth and width were measured and lesion volume was calculated. Higher power or longer duration of radiofrequency energy delivery increased lesion size significantly in the equine atrial myocardium and in equine and swine ventricular myocardium (P < 0.001). Mean lesion depth in equine atrial myocardium ranged from 2.9 to 5.5 mm with a diameter ranging from 6.9 to 10.1 mm. Lesion diameter was significantly larger in equine tissue compared to swine tissue (P = 0.020). Obtained data in combination with estimated wall thickness can improve lesion transmurality which might reduce arrhythmia recurrence. Optimal ablation settings may differ between species.

Radiofrequency ablation: technological trends, challenges, and opportunities

More than three decades have passed since utilization of radiofrequency (RF) ablation in the treatment of cardiac arrhythmias. Although several limitations and challenges still exist, with improvements in catheter designs and delivery of energy the way we do RF ablation now is much safer and more efficient. This review article aims to give an overview on historical advances on RF ablation and challenges in performing safe and efficient ablation.

Energy Sources for the Surgical Treatment of Atrial Fibrillation

The surgical treatment of atrial fibrillation has evolved over the past 2 decades due to the advent of ablation technology, and the introduction of less invasive surgical approaches. Current devices produce ablation lines that aim to replace the incisions of traditional surgical ablation strategies, such as the Cox-Maze procedure. This has helped to simplify and shorten surgical ablation procedures and has allowed for the development of minimally invasive surgical techniques. This review discusses surgical ablation energy sources and devices, providing background on device characteristics, mechanism of tissue injury, and success in creating transmural lesions.

Foramen ovale cannulation guided by intraoperative computed tomography with magnetic resonance image fusion plays a role in improving the long-term outcome of percutaneous radiofrequency trigeminal rhizotomy

Background

Percutaneous radiofrequency trigeminal rhizotomy (RF-TR) is a well-established treatment for patients suffering from trigeminal neuralgia (TN) as a primary modality or for those refractory to medical treatment. However, few existing studies have identified intraoperative parameter or navigation technique that can be used to predict the rates of short-term or long-term pain relief. In this study, we analyzed patient characteristics, intraoperative parameters and technical factors, and postoperative changes in relation to immediate and long-term pain relief.

Method

This study included a total 252 patients in which 340 RF-TR were performed under the guidance of intraoperative computed tomography (iCT) alone or with magnetic resonance image (MRI) and iCT fusion imaging.

Result

The immediate pain relief of RF-TR with iCT alone and iCT with MR image guidance with or without cerebrospinal fluid (CSF) outflow were all above 90.4%. The 2-year pain relief rate of RF-TR using iCT alone and iCT with MR images guidance with or without CSF outflow were 47.8%, 39.8%, 71.7%, and 53.9% respectively. Significant factors for 2-year pain relief were CSF outflow, iCT with MR image fusion, non-recurrent TN, and presence of postoperative facial numbness.

Conclusion

This preliminary study demonstrated foramen ovale cannulation under the aid of iCT with MR image guidance could improve 2-year pain relief.

Influence of energy source on early atrial fibrillation recurrences: a comparison of cryoballoon vs. radiofrequency current energy ablation with the endpoint of unexcitability in pulmonary vein isolation

Introduction

Comparative data of early recurrence rates of atrial fibrillation (ERAF) following second-generation cryoballoon (CB-G2) and radiofrequency current (RFC) ablation for pulmonary vein isolation (PVI) in paroxysmal AF (PAF) are rare. We randomized PAF patients into either PVI with CB-G2 (group 1) or PVI with a combined RFC-approach applying contact force (CF) with the endpoint of unexcitability (group 2) to investigate ERAF.

Methods and results

In group 1 (n = 30), CB-G2-PVI was performed. After CF-PVI in group 2 (n = 30), bipolar pacing on the ablation line and additional ablation until unexcitability was conducted. Follow-up included 48 h of in-hospital monitoring followed by 5-day Holter ECGs 1, 2, 3, 6, 12 months postablation to evaluate ERAF. Acute PVI was reached in 100% of group 2 and in 99% of group 1. Shorter procedure durations (98.0 ± 21.9 vs. 114.3 ± 18.7 min, P < 0.05) but extended fluoroscopy times (15.4 ± 3.9 vs. 10.0 ± 4.3 min, P < 0.05) were found in the CB-G2 group. Ten non-severe complications occurred (6 vs. 4 in group 1 and 2, P = 0.73). In group 2, five patients suffered from ERAF vs. seven patients in group 1 (P = 0.67). The time until the occurrence of ERAF was shorter in group 2 (1 day (q1-q3: 1-4.5)) when compared with group 1 (22 (q1-q3: 6-54) days, P = 0.025).

Conclusion

ERAF rates were equal among groups; however, they occurred earlier in the initial phase after RFC ablation when compared with CB-G2. PVI utilizing cryoablation is associated with shorter procedure durations but extended fluoroscopy time while being similarly secure.

Repeat ablation for paroxysmal atrial fibrillation - Does adenosine play a role in predicting pulmonary vein reconnection patterns?

BACKGROUND

Pulmonary vein (PV) reconduction after PV isolation (PVI) unmasked by adenosine is associated with a higher risk for paroxysmal atrial fibrillation (PAF) recurrence. It is unknown if the reconnected PVs after adenosine testing and immediate re-ablation can predict reconnection and reconnection patterns of PVs at repeat procedures. We assessed reconnection of PVs with and without dormant-conduction (DC) during the first and the repeat procedure.

METHODS

We included 67 patients undergoing PVI for PAF and a second procedure for PAF recurrence. DC during adenosine administration at first procedure was seen in 31 patients (46%). 264 PVs were tested with adenosine; DC was found in 48 PVs (18%) and re-ablated during first procedure. During the second procedure, all PVs where checked for reconnection.

RESULTS

Fifty-eight patients (87%) showed PV reconnection during the second procedure. Reconnection was found in 152/264 PVs (58%). Of 216 PVs without reconnection during adenosine testing at the first ablation, 116 PVs (53.7%) showed reconnection at the repeat procedure. Overall, 14.9% of patients showed the same PV reconnection pattern in the first and second procedure, expected statistical probability of encountering the same reconnection pattern was only 6.6%(p = 0.012).

CONCLUSIONS

In repeat procedures PVs showed significantly more often the same reconnection pattern as during first procedure than statistically expected. More than 50% of initial isolated PVs without reconnection during adenosine testing showed a reconnection during repeat ablation. Techniques to detect susceptibility for PV re-connection like prolonged waiting-period should be applied. Elimination of DC should be expanded from segmental to circumferential re-isolation or vaster RF application.

Ablation Index for Catheter Ablation of Atrial Fibrillation - Clinical Applicability and Comparison With Force-Time Integral

BACKGROUND

Key determinants for lesion formation in catheter ablation are contact force, radiofrequency (RF) power and time. The aim of this study was to evaluate the clinical applicability of ablation index (AI), a novel non-linear formula based on these components, and to compare AI with the conventional linear force-time interval (FTI) in pulmonary vein isolation (PVI).

METHODS AND RESULTS

Target AI ranges were defined for anatomical segments of the ipsilateral pulmonary veins. The operator was blinded to AI during PVI for the initial 11 patients (group A), and was unblinded for the remaining 23 patients (group B). We assessed (1) the clinical value of AI to avoid excessively high and low values with an operator blinded vs. non-blinded to AI; and (2) the relation of AI and FTI in predefined ranges. In group A, 235/564 lesions (41.7%) were in the predefined target range as compared with 1,171/1,412 lesions (82.9%) in group B (P<0.001). A given AI may correspond to a wide range of FTI, as reflected by a quartile coefficient of dispersion for AI of 0.11 vs. a quartile coefficient of dispersion for FTI of 0.36.

CONCLUSIONS

Incorporating RF current power, the non-linear AI provides more comprehensive information during PVI compared with FTI. Given that the FTI for a given AI varies widely, the value of FTI in clinical practice is questionable.

Cardiac Lesion Mapping In Vivo Using Intracardiac Myocardial Elastography

Radio frequency (RF) ablation of the myocardium is used to treat various cardiac arrhythmias. The size, spacing, and transmurality of lesions have been shown to affect the success of the ablation procedure; however, there is currently no method to directly image the size and formation of ablation lesions in real time. Intracardiac myocardial elastography (ME) has been previously used to image the decrease in cardiac strain during systole in the ablated region as a result of the lesion formation. However, the feasibility of imaging multiple lesions and identifying the presence of gaps between lesions has not yet been investigated. In this paper, RF ablation lesions ( ) were generated in the left ventricular epicardium in three anesthetized canines. Two sets of two lesions each were created in close proximity to one another with small gaps (1.5 and 4 cm), while one set of two lesions was created directly next to each other with no gap. A clinical intracardiac echocardiography system was programmed to transmit a custom diverging beam sequence at 600 Hz and used to image the ablation site before and after the induction of ablation lesions. Cumulative strains were estimated over systole using a normalized cross-correlational displacement algorithm and a least-squares strain kernel. Afterward, lesions were excised and subjected to tetrazolium chloride staining. Results indicate that intracardiac ME was capable of imaging the reduction in systolic strain associated with the formation of an ablation lesion. Furthermore, lesion sets containing gaps were able to be distinguished from lesion sets created with no gaps. These results indicate that the end-systolic strain measured using intracardiac ME may be used to image the formation of lesions induced during an RF ablation procedure, in order to provide critical assessment of lesion viability during the interventional procedure.

Hyperthermia dependence of cardiac conduction velocity in rat myocardium: Optical mapping and cardiac near field measurements

Hyperthermia during radiofrequency ablation causes reversible and irreversible changes of the electrophysiological properties of cardiac tissue. However, the mechanisms are incompletely understood. We studied changes of conduction velocity (CV) in rat myocardium under hyperthermic conditions from macroscopic to microscopic scale by using simultaneous optical mapping and a miniaturized electrode array. Atrial preparations from five rats were superfused at tissue bath temperatures between 36.7°C and 43.8°C. Optical mapping data showed an elevated median CV by 21% when increasing the temperature from 36.7°C to 42.0°C. CV did not increase above 42.0°C. Electrical measurements revealed a similar temperature dependence of CV between 36.7°C and 42.0°C, i.e. an increase of median CV by 26%. The consolidation of optical and electrical data in this study allowed investigation of excitation during global hyperthermia. Macroscopic optical mapping and microscopic electrical measurements demonstrated that hyperthermia strongly influenced electrical propagation at a microscopic scale.

Determinants of Acute and Late Pulmonary Vein Reconnection in Contact Force–Guided Pulmonary Vein Isolation

Background—

Pulmonary vein reconnection (PVR) still determines recurrences of atrial fibrillation after contact force (CF)–guided pulmonary vein isolation. We studied whether acute PVR (adenosine and waiting time) and late PVR (at repeat) are explained by incomplete transmurality and contiguity within the deployed radiofrequency circle.

Methods and Results—

We analyzed 42 CF-guided ipsilateral pulmonary vein isolation procedures. For each radiofrequency tag within the circle, we collected data reflecting lesion depth (time of application, power, impedance drop [Δ-Imp], CF, force–time integral [FTI], and ablation index [AI]) and contiguity (automated interlesion distance [ILD]). Ablation line contiguity index (ALCI) was developed as a novel automated algorithm combining depth and contiguity into one single criterion. Each circle was subdivided into 10 segments. For each segment, we determined its weakest link by annotating time min , power min , Δ-Imp min , CF min , FTI min , AI min , ILD max , and ALCI min . Compared with segments without PVR (n=758), PVR segments (n=44) were characterized by lower Δ-Imp min (4.8 versus 7.4 Ω), CF min (8.5 versus 11.8 g), FTI min (351 versus 473 gs), AI min (367 versus 408 arbitrary unit [au]), and higher ILD max (6.8 versus 5.5 mm). ALCI min was significantly lower in segments with PVR (74% versus 104%; P <0.001) and was associated with the highest accuracy to predict durable segments (area under the curve=0.73).

Conclusions—

In CF-guided pulmonary vein isolation, PVR is explained by lack of both lesion depth and contiguity within the deployed radiofrequency circle. ALCI, a novel measure combining contiguity and depth, is the most accurate predictor for durable segments. By avoiding weak links in the ablation chain, ALCI-guided ablation is expected to improve success rate of point-by-point radiofrequency ablation.

Effect of parenchymal uptake of perfluorobutane microbubbles (Sonazoid(®) ) on radiofrequency ablation of the liver: in vivo experimental study

BACKGROUND & AIMS

To investigate the differences in mechanical effects and ablation zone between radiofrequency (RF) ablation with and without Sonazoid uptake in an in vivo rabbit liver model.

METHODS

Our study was approved by the Institutional Animal Care and Use Committee. Twenty-five rabbits were randomly allotted to one of five ablation durations (i.e., 1, 2, 3, 6 and 12 min). For each rabbit, RF ablation was performed twice, before and 10 min after Sonazoid administration (i.e. control group vs. Sonazoid group), using a 1-cm internally cooled electrode (40W) equipped with a parallelly fixed pressure-monitoring device. During ablation, a 'popping' sound was perceived and recorded along with tissue pressure changes and RF ablation parameters. Then, the ablation volume and microscopic changes were compared.

RESULTS

Popping sounds were more frequently perceived in the control group (21/25 vs. 1/25, P < 0.001). The time to first pressure peak was shorter in the Sonazoid group (22.3 ± 1.1 s vs. 46.3 ± 4.4 s, P < 0.001) with similar pressures (39.8 ± 4.2 mmHg vs. 35.6 ± 4.1 mmHg, P = 0.350). Time to first roll-off and mean power output were significantly less in the Sonazoid group (17.6 ± 1.2 s vs. 71.2 ± 8.2 s, P < 0.001; 9.4 ± 0.3 W vs. 12.8 ± 0.5 W, P < 0.001). Consequently, the Sonazoid group had lower total energy and ablation volumes for all durations. Microscopically, the control group showed larger conflu-ent disruptions, whereas the Sonazoid group showed many smaller disruptions scattered throughout the ablation zones.

CONCLUSIONS

Radiofrequency ablation after Sonazoid uptake induces a smaller ablation zone than conventional RF ablation. However, it appears to ablate the liver tissue with less mechanical effects.

Combined Therapeutic and Monitoring Ultrasonic Catheter for Cardiac Ablation Therapies

This study evaluated the feasibility of a combined therapeutic and diagnostic ultrasonic catheter for cardiac ablation therapies. Ultrasound can be used to determine when diseased cardiac tissues have become fully coagulated through a method known as local harmonic motion imaging (LHMI). LHMI is an imaging modality for treatment monitoring that uses acoustic radiation force, displacement tracking and the different mechanical properties of viable and ablated tissues. In this study, we developed catheters that are capable of LHMI measurements. Experiments were conducted in phantoms, ex vivo cardiac samples and the in vivo beating hearts of healthy porcine subjects. In vivo experiments revealed that four of four epicardial sonications revealed a decrease in measured displacements from LHMI experiments and that when lower power was used, no lesions formed and there was no corresponding decrease in measured displacement amplitudes. In addition, two of three endocardial lesions were confirmed and corresponded to a decrease in the measured displacement amplitude.

Unmasking the dormant pulmonary vein conduction with adenosine administration after pulmonary vein isolation with laser energy

AIMS

The isolation of the pulmonary veins (PVs) is the mainstay of atrial fibrillation (AF) ablation, which with current ablation techniques can be achieved in almost all cases. Reconnection of PVs constitutes the most frequent cause of AF recurrence. Visually guided laser balloon ablation (VGLA) is a novel system with very high rate of persistence of pulmonary vein isolation (PVI) three months after the first procedure shown in preclinical and clinical studies. We aimed to determine the acute efficiency of the laser energy during PVI with the help of adenosine provocation.

METHODS AND RESULTS

Twenty-six patients (19 male; mean age 64 ± 9 years) with symptomatic paroxysmal AF were included in the study. Pulmonary vein isolation was performed using the VGLA system. After successful PVI, we studied the effects of intravenous adenosine (18 mg) on activation of each PV at least 20 min after PVI. A total of 104 PVs were targeted. The balloon catheter could not be placed in two PVs. Of the remaining 102 PVs 99 (97% of the ablated PVs) could be successfully isolated. Adenosine was administered for each isolated PV in 25 patients. Only six PVs (6.7%) in five patients (20%) showed a PV reconnection during adenosine provocation.

CONCLUSION

Pulmonary vein isolation with VGLA is a feasible technique for PVI with a very effective acute lesion formation. The clinical significance of this low reconnection rate has to be determined.

Intracardiac myocardial elastography in canines and humans in vivo

Intracardiac echocardiography (ICE) is a useful imaging modality which is used during RF ablation procedures to identify anatomical structures. Utilizing ICE in conjunction with myocardial elastography (ME) can provide additional information on the mechanical properties of cardiac tissue and provide information on mechanical changes caused by ablation. The objective of this study was to demonstrate that ICE can be used at high frame rate using a diverging beam transmit sequence to image myocardial strain and differentiate myocardial tissue properties before, during, and after ablation for a clinical ablation procedure. In this feasibility study, three normal canines and eight patients with atrial fibrillation (AF) were studied in vivo. A 5.8-MHz ICE transducer was used to image the heart with a diverging beam transmit method achieving 1200 frames per second (fps). Cumulative axial displacement estimation was performed using 1-D cross-correlation with a window size of 2.7 mm and 95% overlap. Axial cumulative strains were estimated in the left atrium (LA) and right atrium (RA) using a least-squares estimator with a kernel of 2 mm on the axial displacements. In the canine case, radial thickening was detected in the lateral wall and in the interatrial septum during LA emptying. For AF patients, the mean absolute strain in the ablated region was lower (6.7 ± 3.1%) than before the ablation (17.4 ± 9.3%) in LA at the end of the LA emptying phase. In the cavotricuspid isthmus (CTI) region, mean absolute strain magnitude at the end of the RA emptying phase was found to be higher during ablation (43.0 ± 18.1%) compared with after ablation (33.7 ± 15.8%). Myocardial strains in the LA of an AF patient were approximately 2.6 times lower in the ablated region than before ablation. This initial feasibility indicates that ME can be used as a new imaging modality in conjunction with ICE in RF ablation guidance and lesion monitoring.

Development of a novel shock wave catheter ablation system--the first feasibility study in pigs

Introduction

Radio-frequency catheter ablation (RFCA) using Joule heat has two fundamental weaknesses: the limited depth of treatment and the risk of thrombus formation. In contrast, focused shock wave (SW) therapy could damage tissues at arbitrary depths without heat generation. Thus, we aimed to develop a SW catheter ablation (SWCA) system that could compensate for the weaknesses of RFCA therapy.

Methods and Results

We developed a SWCA system where the SW generated by a Q-switched Holmium: yttrium aluminum garnet (YAG) laser beam was reflected by a reflector attached to 14-Fr catheter tip and then was converged onto the focus. We examined the feasibility of our system on pigs in vivo. When applied using the epicardial approach, the SWCA caused persistent spheroidal lesions with mild superficial injury than the RFCA. The lesions were created to a depth based on the focal length (2.0 mm) [2.36 ± 0.45 (SD) mm immediately after procedure, n = 16]. When applied to the atrioventricular (AV) node using the endocardial approach, the SWCA caused junctional escape rhythms in 2 pigs and AV block in 12 pigs (complete AV block in 9) in acute phase (n = 14). Nine of the 14 pigs survived with pacemakers for the long-term study, and the AV block persisted for 12.6 ± 3.9 (SD) days in all surviving pigs. Histological examination showed AV nodal cell body atrophy in the acute phase and fibrotic lesions in the chronic phase. Importantly, no acute or chronic fatal complications were noted.

Conclusions

Our novel SWCA system could be a promising modality as a non-thermal ablation method to compensate for the weaknesses of RFCA therapy. However, further research and development will be necessary as the current prototype still exhibited the presence of micro-thrombus formation in the animal studies.

Characterization of radiofrequency ablation lesion development based on simulated and measured intracardiac electrograms

Radiofrequency ablation (RFA) therapy is the gold standard in interventional treatment of many cardiac arrhythmias. A major obstacle is nontransmural lesions, leading to recurrence of arrhythmias. Recent clinical studies have suggested intracardiac electrogram (EGM) criteria as a promising marker to evaluate lesion development. Seeking for a deeper understanding of underlying mechanisms, we established a simulation approach for acute RFA lesions. Ablation lesions were modeled by a passive necrotic core surrounded by a borderzone with properties of heated myocardium. Herein, conduction velocity and electrophysiological properties were altered. We simulated EGMs during RFA to study the relation between lesion formation and EGM changes using the bidomain model. Simulations were performed on a three-dimensional setup including a geometrically detailed representation of the catheter with highly conductive electrodes. For validation, EGMs recorded during RFA procedures in five patients were analyzed and compared to simulation results. Clinical data showed major changes in the distal unipolar EGM. During RFA, the negative peak amplitude decreased up to 104% and maximum negative deflection was up to 88% smaller at the end of the ablation sequence. These changes mainly occurred in the first 10 s after ablation onset. Simulated unipolar EGMs reproduced the clinical changes, reaching up to 83% negative peak amplitude reduction and 80% decrease in maximum negative deflection for transmural lesions. In future studies, the established model may enable the development of further EGM criteria for transmural lesions even for complex geometries in order to support clinical therapy.

The evaluation of steerable ultrasonic catheters for minimally invasive MRI-guided cardiac ablation

PURPOSE

The purpose of this study was to develop steerable MR-compatible ultrasound catheters suitable for minimally invasive MRI-guided cardiac ablation therapies.

METHODS

MRI-compatible ultrasound steerable catheters were developed and tested for their overall tissue heating performance and safety. Ultrasound transducers were mounted on a monodirectional deflectable catheter tip that was made to be MRI-compatible. Catheter safety was assessed on the potential to form hot spots at the distal end of the catheter throughout fast spin echo and thermometry scans. Heating experiments were performed on phantoms and ex vivo porcine cardiac samples.

RESULTS

During catheter safety experiments, a maximum temperature increase of 11.35 ± 0.83°C was evident after a 12-min, 40-s fast spin echo scan with a whole body specific absorption rate (SAR) of 1.9 W/kg and 1.07 ± 0.22°C during thermometry scans (flip angle = 90°; scan time = 12 min, 41 s; whole body SAR = 0.34 W/kg). Temperature elevations induced by the sonication were shown to be on the order of 38.1 ± 5.2°C for phantom experiments and 49.3 ± 9.7°C for ex vivo cardiac samples.

CONCLUSION

Steerable ultrasound catheters have the potential to be safely placed in an MR system with little concern of catheter self-heating and driven to heat surrounding structures to cause ablations. In addition, these catheters have the added benefit of a deflectable tip that allows the treatment of multiple targets from within the bore of the MR scanner.

Radiofrequency ablation in the management of Barrett's esophagus: present role and future perspective

Esophageal adenocarcinoma is the most rapidly increasing gastrointestinal cancer. Barrett's esophagus has been identified as a precancerous condition and major risk factor for esophageal cancer. Radiofrequency ablation has been shown to be a highly efficient in promoting remission of intestinal metaplasia. This technology has seen widespread clinical use since 2005. Radiofrequency ablation is common with all other ablative techniques; the concern that sound oncological principles are not being adhered to, that is, appropriate pathological staging, followed by appropriate definitive therapy. Endoscopic mucosal excision techniques are technically demanding; however, they are more attractive from an oncological perspective. Future research endeavors focusing on facilitation of large population screening, the identification of high risk phenotypes, endoscopic mucosal resection techniques will combat the esophageal adenocarcinoma epidemic.

Lumped Element Electrical Model based on Three Resistors for Electrical Impedance in Radiofrequency Cardiac Ablation: Estimations from Analytical Calculations and Clinical Data

The electrical impedance measured during radiofrequency cardiac ablation (RFCA) is widely used in clinical studies to predict the heating evolution and hence the success of the procedure. We hypothesized that a model based on three resistors in series can mimic the total electrical impedance measured during RFCA. The three resistors or impedances are given by: impedance associated with the tissue around the active electrode (myocardium and circulating blood) (Z-A), that associated with the tissue around the dispersive electrode (Z-DE) and that associated with the rest of the body (Z-B). Our objective was to quantify the values associated with these three impedance types by an analytical method, after which the values obtained would be compared to those estimated from clinical data from previous studies. The results suggest that an RFCA using a 7 Fr 4-mm electrode would give a Z-A of around 75 ohms, a Z-DE around 20 ohms, and Z-B would be 15±10 ohms (for body surface area variations between 1.5 and 2.5 m^2). Finally, adaptations of the proposed model were used to explain the results of previous clinical studies using a different electrode arrangement, such as in bipolar ablation of the ventricular septum.

Tissue temperature feedback control of power: the key to successful ablation

Multiple ablation technologies are used to treat atrial fibrillation during cardiac operations. All such ablation technologies use locally induced temperature extremes (>50°C or <-20°C) to kill tissue and create a lesion pattern in the atria which blocks activation pathways that initiate and sustain atrial fibrillation. The technologies used to heat tissue have included radiofrequency (RF), microwave, high-intensity focused ultrasound, and infrared laser. RF accounts for more than 95% of the heating-based ablation technology used by cardiac surgeons. Energy delivery with RF is easier to control than with some other technologies, the heating produced by the energy source is well understood, and manufacturing costs are not excessive. Whichever heating technology is used, control of energy delivery is required to ensure both safe and effective heating of the targeted tissue. All targeted tissue needs to be heated above 50°C to achieve cell death. However, the targeted tissue should not be heated above 100°C, as this can cause perforation due to a steam pop. In addition, adjacent noncardiac tissues must not be damaged during the ablation procedure. The best method to achieve this control uses direct measurement of tissue temperature, because the tissue temperature defines both the safe and effective limits for the ablative process.

Bimodal electric tissue ablation (BETA) compared with the Cool-Tip RFA system

BACKGROUND

Bimodal electric tissue ablation (BETA) incorporates the process of electrolysis into radiofrequency ablation (RFA) to increase the size of tissue ablation. This study investigated whether BETA could increase the efficacy of the Cool-Tip RF system (Covidien, Boulder, CO, USA) to produce larger ablations. It also investigated whether applying electrolysis only during the pretreatment phase (called electrochemical treatment (ECT)/RFA group) is as effective as BETA (where electrolysis was used during both the pretreatment and RFA phases).

METHODS

A Cool-Tip RF system (Covidien) was used to test three types of ablations (RFA, BETA, and ECT/RFA) in a pig liver model. In BETA, 9 V of direct current was provided for 10 min, after which the RF generator was started and both electrical circuits were allowed to run concurrently. In ECT/RFA, however, the direct current circuit was switched off after 10 min of pretreatment and only RFA was performed as described above. Ablation sizes were measured in three dimensions.

RESULTS

The size of ablations (transverse diameter A and B) produced by BETA and ECT/RFA was significantly larger compared with standard RFA (P < 0/001). BETA also created larger ablations compared with ECT/RFA (P < 0.001).

CONCLUSION

BETA could improve the efficacy of the Cool-Tip RF system (Covidien) to achieve larger ablations. The increased tissue hydration improved delivery of electrical energy to the tissues and delayed the process of desiccation, thus allowing the ablation process to continue for longer periods of time to produce larger ablations. BETA could be used to treat larger liver tumours more effectively than standard RFA.

Distant infusion of saline may enlarge coagulation volume during radiofrequency ablation of liver tissue using cool-tip electrodes without impairing predictability

OBJECTIVE

Our aim was to evaluate the capability of a Cool-tip electrode to create larger coagulation volumes combined with a low-flow (0.1 mL/min) perfusion of hypertonic saline at a distance of 2 mm (hybrid applicator) without reducing either predictability or sphericity of the coagulation zone.

MATERIALS AND METHODS

A total of 48 radiofrequency ablations were performed on a total of 12 adult pigs: 24 with the Cool-tip (group 1) and 24 with the hybrid applicator (group 2). Volumes and diameters were assessed both macroscopically and with imaging techniques (ultrasound and MRI). Digital reconstruction techniques were also used. Reproducibility of the coagulations was assessed by means of the coefficient of variation.

RESULTS

The macroscopic assessment showed a significantly larger coagulation zone in group 2 than in group 1, both with (19.40 ± 11.38 cm(3) vs 9.16 ± 5.62 cm(3); p < 0.001) and without (19.54 ± 11.39 cm(3) vs 9.21 ± 5.74 cm(3); p < 0.001) digital reconstruction. Differences were also significant in the MRI assessment. The minimum transverse diameter was also significantly (p < 0.01) larger in group 2 than group 1: 2.46 ± 0.61 versus 1.86 ± 0.55 cm for macroscopic assessment, 2.33 ± 0.96 versus 1.69 ± 0.53 cm for ultrasound, and 2.41 ± 0.58 versus 1.8 ± 0.52 cm for MRI. The coefficient of variation was similar in both groups.

CONCLUSION

The results suggest that low-flow perfusion of hypertonic saline at 2 mm from a Cool-tip electrode could increase coagulation zone volume without reducing predictability.

Does adenosine response predict clinical recurrence of atrial fibrillation after pulmonary vein isolation?

BACKGROUND

Approximately 30% of patients undergoing pulmonary vein isolation (PVI) for atrial fibrillation (AF) have clinical recurrence of AF, and a great majority of these patients have recovery of vein conduction. Adenosine can be associated with acute recovery of conduction to the pulmonary veins immediately after isolation. However, it is not known whether this is prognostic for permanent recovery of conduction or recurrence of AF.

METHODS

Patients with paroxysmal AF underwent PVI, with administration of adenosine after electrical isolation. Those with transient conduction recovery (TCR+) underwent no further ablation and were compared to those without (TCR-) for clinical AF recurrence and conduction recovery at second procedure.

RESULTS

Seventy-two consecutive PVI patients were studied (mean age 56.7 ± 9.2, 61 male). Twenty-five (35%) patients had transient recovery of conduction with adenosine. After 1 year, 18 patients (25%) had symptomatic recurrence of AF. In this group of 18 patients, 6 were TCR+ at initial ablation (sensitivity 33%, NPV = 74%). In the remaining group of 54 patients free from AF recurrence, 35 patients (65%) were TCR- at initial ablation (specificity 65%, PPV = 24%). All 18 patients with recurrent AF underwent repeat procedure and had at least 1 pulmonary vein with recurrent conduction. The initial adenosine test correctly predicted 13 out of 36 (36%) veins, with positive predictive value 90% and negative predictive value 15%.

CONCLUSIONS

Adenosine testing for TCR does not appear to predict recurrence of clinical AF. TCR- veins remain susceptible to conduction recovery, as determined at the follow-up procedure. 

The role of radiofrequency ablation in the management of Barrett's esophagus

Studies in the last several years have consistently shown radiofrequency ablation (RFA) to be effective, safe, and well tolerated in the treatment of nondysplastic and dysplastic Barrett's esophagus (BE). The results found at academic medical centers have been reproduced in the community setting. RFA provides a safe and cost-effective alternative to surgery or surveillance in the management of high-grade dysplasia (HGD). RFA should be given serious consideration as first-line therapy for HGD. This article reviews the evidence behind RFA to differentiate it from other management strategies in terms of efficacy, durability, safety, tolerability, and cost-effectiveness. The role of RFA in the management of BE is described, including endoscopic resection. Future directions are identified for research that will help to better define the role of RFA in the management of BE.

Increased ablation zones using a cryo-based internally cooled bipolar RF applicator in ex vivo bovine liver

PURPOSE

To evaluate the feasibility of ex vivo ablation implementing a cryo-based internally cooled bipolar radiofrequency (RF) applicator and to determine the influence of power and gas pressure on the size and shape of the resulting ablation zones.

MATERIALS AND METHODS

Two hundred twenty-five ablations were performed using a custom-built internally CO2-cooled bipolar cryo RF applicator in ex vivo bovine livers. The active tip of the applicator was 55 mm long. RF power (32-50 watts) and gas pressure of cooling medium (500-600 psi) were varied independently. Power was applied in continuous mode. Control group experiments were carried out solely using the RF function at 32, 40, and 50 watts. Ablation duration was 15 minutes for all applications. Experiments were repeated 5 times for all parameter combinations. Short and long axes of the induced white ablation zone were macroscopically assessed. The ablation zone was referred to as homogeneous if complete ablation was observed without spots of untreated tissue. The short axis diameters for the simultaneous application of cryo and RF function were analyzed using a multiple linear regression analysis. An unpaired Mann-Whitney U test was used to analyze the differences between the short axes with RF alone and RF using cryo cooling.

RESULTS

All ablation zones were homogeneous. Using simultaneous RF ablation function and gas cooling with a single applicator, the long axes of the ablation zones ranged between 42 +/- 2 mm (mean +/- SD) and 59 +/- 5 mm, the short axes between 24 +/- 1 and 44 +/- 1 mm, depending on the parameter combination. At a stable gas pressure level, short axes increased with rising power levels and decreased after reaching a maximum. The maxima of the short axis increased with higher gas pressure levels and were shifted to higher power values. Optimal parameter settings were 46 to 50 watts and 600 psi gas pressure, resulting in a short axis of 44 +/- 1 mm. Short axis weakly correlated with gas pressure (r2 = 0.10) and power (r2 = 0.34) alone, whereas the correlation was r2 = 0.76 for the combined factors. Without cooling, short axis diameters were significantly shorter (P < 0.05), ranging between 13 +/- 2 mm at 50 watts and 15 +/- 2 mm at 32 watts.

CONCLUSION

The results of this initial ex vivo study show that the combined cryo RF ablation device allows for large ablation volumes using a single needle, which is superior to RF ablation alone.

The significance of repetitive ventricular responses induced by radiofrequency energy application for idiopathic left ventricular tachycardia

In radiofrequency (RF) ablation for idiopathic left ventricular tachycardia (ILVT), the termination of tachycardia during RF ablation is considered a hallmark of success. However, in cases of patients with difficulty of induction of ventricular tachycardia (VT), the evaluation of procedural success can be problematic. We have observed thermal responses reflected as ventricular rhythm change to RF energy delivered on sinus rhythm for ILVT. We therefore describe the significance of repetitive ventricular responses. The study subjects were 11 ILVT patients for whom RF energy was delivered during sinus rhythm because of difficulty in re-induction of tachycardia. During each energy delivery, we focused on the occurrence of repetitive ventricular responses especially exhibiting a similar morphology to clinical VT. The repetitive ventricular responses were noted in 10 of 11 patients. Two patients received a second procedure due to the recurrence of ILVT. The mean follow-up period was 36.2+/-12.8 months. The clinical course of the remaining patients was favorable and without recurrence of ILVT. Based on the favorable clinical outcomes, ablation-induced repetitive ventricular responses with similar QRS morphology to clinical ILVT are useful markers for selecting an ablation site and could be used as an additional mapping method, termed as "thermal mapping".

Irrigated radiofrequency-biophysics and application to surgical ablation

Surgical ablation has become a routine procedure in many institutions and is a nearing standard of care for certain conditions. A variety of energy sources are available to the surgeon to induce cell death and create a line of conduction block. This article provides a brief review of the biophysics of irrigated radiofrequency and its application to monopolar and bipolar ablation.

The change in dimension of the masseter muscle in rabbits after radiofrequency therapy

PURPOSE

The objective of this study was to evaluate the change in muscle dimension after administering radiofrequency (RF) therapy.

MATERIALS AND METHODS

This study used 6 male New Zealand rabbits. Groups were divided by number of applications of RF (eg, 1 to 4 points). The dimension of the masseter muscle was measured using a computerized tomogram scan before operation, and at 1, 2, 3, and 4 weeks after RF therapy was administered under the same conditions. Two horizontal cuts were selected for measurement.

RESULTS

The size of the measured areas for each group at 1 week after RF therapy was significantly increased compared with the preoperative value (P< .05). When the measurements of each group at 3 and 4 weeks after RF therapy were compared with the preoperative value, they were significantly decreased (P< .05). The dimensional change was significantly different among groups at 1 and 3 weeks post RF therapy (P< .05). The swelling at 1 week after RF therapy was increased in terms of the number of RF applications. The ratio of dimension was decreased at 3 weeks after RF therapy in terms of the number of RF applications.

CONCLUSION

There was an increase in muscle dimension because of swelling in the early stages of RF therapy. However, this dimension decreased at 3 weeks post-RF administration compared with the preoperative value. Therefore, it can be concluded that the change in the masseter muscle dimension was dependent on the number of RF applications.

Radiofrequency hepatic ablation with internally cooled electrodes and hybrid applicators with distant saline infusion using an in vivo porcine model

AIMS

Radiofrequency ablation (RFA) of tumors by means of internally cooled (ICE) or multitined expandable electrodes combined with infusion of saline into the tissue may improve results. Our aim was to determine the efficacy of a previously optimized hybrid ICE system (ICE combined with infusion of saline into the tissue at a distance of 2mm) in comparison with a conventional ICE cluster electrode in porcine liver in vivo.

METHODS

A total of 32 RFA were performed on a total of 10 farm pigs using two RFA systems: Group A (n=16): Cluster electrode. Group B (n=16): Hybrid system (with continuous infusion of 100ml/h of 20% NaCl at 2mm distance from the electrode shaft by an independent isolated needle). Livers were removed for macroscopic and histological assessment after the procedure. Coagulation volume, coagulation diameters, coefficient of variability (CV) of coagulation volume, sphericity ratio (SR), deposited power (DP), deposited energy (DE), deposited energy per coagulation volume (DEV) and rise of animal temperature during the procedure were compared and correlated among groups. Additionally, linear regression analysis was modeled to study the relationship between deposited energy and either coagulation volume and rise of animal temperature during the procedure in both groups.

RESULTS

Both coagulation volume and short diameter of coagulation were significantly greater (p<0.05) in group B compared to group A (22.7+/-11.0 cm(3) and 3.1+/-0.7 cm vs. 13.5+/-7.7 cm(3) and 2.5+/-0.5 cm, respectively). A similar CV and SR was observed among groups (57.1% and 1.4+/-0.3 vs. 48.6% and 1.3+/-0.2 for groups B and A, respectively). In group B, DE and DP were more than double group A, but DEV was nearly twice as high (9782 J/cm(3) vs. 5342 J/cm(3), for groups B and A, respectively). No significant relationship between DE and coagulation volume was encountered.

CONCLUSION

Efficacy of a single ICE may be improved with continuous infusion of saline at around 2 mm from the electrode shaft. Coagulation volume obtained with this improved system may be even greater than that obtained with a cluster electrode.

RF tumor ablation with internally cooled electrodes and saline infusion: what is the optimal location of the saline infusion?

Background

Radiofrequency ablation (RFA) of tumors by means of internally cooled electrodes (ICE) combined with interstitial infusion of saline may improve clinical results. To date, infusion has been conducted through outlets placed on the surface of the cooled electrode. However, the effect of infusion at a distance from the electrode surface is unknown. Our aim was to assess the effect of perfusion distance (PD) on the coagulation geometry and deposited power during RFA using ICE.

Methods

Experiments were performed on excised bovine livers. Perfusion distance (PD) was defined as the shortest distance between the infusion outlet and the surface of the ICE. We considered three values of PD: 0, 2 and 4 mm. Two sets of experiments were considered: 1) 15 ablations of 10 minutes (n ≥ 4 for each PD), in order to evaluate the effect of PD on volume and diameters of coagulation; and 2) 20 additional ablations of 20 minutes. The effect of PD on deposited power and relative frequency of uncontrolled impedance rises (roll-off) was evaluated using the results from the two sets of experiments (n ≥ 7 for each PD). Comparisons between PD were performed by analysis of variance or Kruskal-Wallis test. Additionally, non-linear regression models were performed to elucidate the best PD in terms of coagulation volume and diameter, and the occurrence of uncontrolled impedance rises.

Results

The best-fit least square functions were always obtained with quadratic curves where volume and diameters of coagulation were maximum for a PD of 2 mm. A thirty per cent increase in volume coagulation was observed for this PD value compared to other values (P < 0.05). Likewise, the short coagulation diameter was nearly twenty five per cent larger for a 2 mm PD than for 0 mm. Regarding deposited power, the best-fit least square function was obtained by a quadratic curve with a 2 mm PD peak. This matched well with the higher relative frequency of uncontrolled impedance rises for PD of 0 and 4 mm.

Conclusion

Saline perfusion at around 2 mm from the electrode surface while using an ICE in RFA improves deposition of energy and enlarges coagulation volume.

Thermal mapping of right ventricular outflow tract tachycardia

BACKGROUND

Acute and long-term success of catheter ablation of right ventricular outflow tract tachycardia (RVOT VT) may be limited by the inability to reproduce the arrhythmia at the time of activation (AM) and pace mapping (PM). We have observed early initiation of the clinical VT when subtherapeutic radiofrequency (RF) energy was applied to the target area (TA), defined as a 2-cm(2) area around a pace match. We describe a novel approach using thermal mapping (TM) to guide the ablation of RVOT VT.

METHODS

Thirteen patients (10 female, mean age 46.2 +/- 13.7 years) with symptomatic VT of left bundle branch block (LBBB) inferior axis morphology and no structural heart disease underwent standard electrophysiologic evaluation with PM (n = 13), AM (n = 13), and 3D noncontact mapping (n = 4). Thermal mapping was performed after standard techniques failed to induce stable sustained VT for mapping in all 13 patients: RF was applied for 5-10 seconds in the TA to achieve a tip temperature of 45-50 degrees C. At sites where morphologically consistent with the clinical VT was induced, RF was applied at target temperature between 50 and 60 degrees C for 30-60 seconds. TM was repeated before and after intravenous Isoproterenol infusion until no further VT could be induced by low temperature application.

RESULTS

Noninducibility was achieved in all 13 patients. During a mean follow-up of 29 months (9-69 months), all patients remain arrhythmia-free, off antiarrhythmic medications.

CONCLUSION

Thermal mapping is a safe and effective adjunctive technique for the mapping and ablation of RVOT VT when sustained tolerated clinical VT cannot be induced.

Bipolar radiofrequency ablation of liver metastases during laparotomy. First clinical experiences with a new multipolar ablation concept

BACKGROUND AND OBJECTIVE

Radiofrequency ablation (RFA) is a promising method for local treatment of liver malignancies. Currently available systems for radiofrequency ablation use monopolar current, which carries the risk of uncontrolled electrical current paths, collateral damages and limited effectiveness. To overcome this problem, we used a newly developed internally cooled bipolar application system in patients with irresectable liver metastases undergoing laparotomy. The aim of this study was to clinically evaluate the safety, feasibility and effectiveness of this new system with a novel multipolar application concept.

PATIENTS AND METHODS

Patients with a maximum of five liver metastases having a maximum diameter of 5 cm underwent laparotomy and abdominal exploration to control resectability. In cases of irresectability, RFA with the newly developed bipolar application system was performed. Treatment was carried out under ultrasound guidance. Depending on tumour size, shape and location, up to three applicators were simultaneously inserted in or closely around the tumour, never exceeding a maximum probe distance of 3 cm. In the multipolar ablation concept, the current runs alternating between all possible pairs of consecutively activated electrodes with up to 15 possible electrode combinations. Post-operative follow-up was evaluated by CT or MRI controls 24-48 h after RFA and every 3 months.

RESULTS

In a total of six patients (four male, two female; 61-68 years), ten metastases (1.0-5.5 cm) were treated with a total of 14 RF applications. In four metastases three probes were used, and in another four and two metastases, two and one probes were used, respectively. During a mean ablation time of 18.8 min (10-31), a mean energy of 48.8 kJ (12-116) for each metastases was applied. No procedure-related complications occurred. The patients were released from the hospital between 7 and 12 days post-intervention (median 9 days). The post-interventional control showed complete tumour ablation in all cases.

CONCLUSIONS

Bipolar radiofrequency using the novel multipolar ablation concept permits a safe and effective therapy for the induction of large volumes of coagulation in the local treatment of liver metastases.

A review of the general aspects of radiofrequency ablation

As an alternative to standard surgical resection for the treatment of malignant tumors, radiofrequency ablation (RFA) has rapidly evolved into the most popular minimally invasive therapy. To help readers gain the relevant background knowledge and to better understand the other reviews in this Feature Section on the clinical applications of RFA in different abdominal organs, the present report covers the general aspects of RFA. After an introduction, we present a simple definition of the energy applied during RFA, a brief historical review of its technical evolution, and an explanation of the mechanism of action of RFA. These basic discussions are substantiated with descriptions of RFA equipment including those commercially available and those under preclinical development. The size and geometry of induced lesions in relation to RFA efficacy and side effects are discussed. The unique pathophysiologic process of thermal tissue damage and the corresponding histomorphologic manifestations after RFA are detailed and cross-referenced with the findings in the current literature. The crucial role of imaging technology during and after RFA is also addressed, including some promising new developments. This report finishes with a summary of the key messages and a perspective on further technologic refinements and identifies some specific priorities.