An In Vitro Assessment of Acoustic Radiation Force Impulse Imaging for Visualizing Cardiac Radiofrequency Ablation Lesions

Ablation Lesion Quantification with ARFI Imaging . Introduction: Lesion placement and transmurality are critical factors in the success of cardiac transcatheter radiofrequency ablation (RFA) treatments for supraventricular arrhythmias. This study investigated the capabilities of catheter transducer based acoustic radiation force impulse (ARFI) ultrasound imaging for quantifying ablation lesion dimensions. Methods and Results: RFA lesions were created in vitro in porcine ventricular myocardium and imaged with an intracardiac ultrasound catheter transducer capable of acquiring spatially registered B‐mode and ARFI images. The myocardium was sliced along the imaging plane and photographed. The maximum ARFI‐induced displacement images of the lesion were normalized and spatially registered with the photograph by matching the surfaces of the tissue in the B‐mode and photographic images. The lesion dimensions determined by a manual segmentation of the photographed lesion based on the visible discoloration of the tissue were compared to automatic segmentations of the ARFI image using 2 different calculated thresholds. ARFI imaging accurately localized and sized the lesions within the myocardium. Differences in the maximum lateral and axial dimensions were statistically below 2 mm and 1 mm, respectively, for the 2 thresholding methods, with mean percent overlap of 68.7 ± 5.21% and 66.3 ± 8.4% for the 2 thresholds used. Conclusion: ARFI imaging is capable of visualizing myocardial RFA lesion dimensions to within 2 mm in vitro. Visualizing lesions during transcatheter cardiac ablation procedures could improve the success of the treatment by imaging lesion line discontinuity and potentially reducing the required number of ablation lesions and procedure time. (J Cardiovasc Electrophysiol, Vol. 21, pp. 557‐563, May 2010)     

Insights into the Electrophysiology of Atrial Arrhythmias Gained by the Catheter Ablation Experience:

Catheter Ablation of Atrial Arrhythmias. Although the development and wide-scale application of catheter ablative techniques has drastically changed the practice of electrophysiology, catheter ablation does not preclude physiologic investigation. On the contrary, given the precise and directed nature of this technique and the increased attention to detailed cardiac mapping that it requires, catheter ablation may be viewed as a tool to provide unique information about arrhythmia substrates. In this article, the insights provided by the catheter ablation experience into the pathophysiology of the focal atrial arrhythmias, atrial tachycardia, sinus node reentrant tachycardia, and inappropriate sinus tachycardia will be reviewed. Atrial arrhythmias were initially difficult to treat with ablative therapy, particularly because they can occur anywhere within the atria and the experience with mapping for surgical ablation was quite limited. A number of novel approaches to atrial mapping have been developed in response to this challenge, and presently, catheter ablation provides effective therapy for the majority of patients with focal atrial arrhythmias. In addition, deliberate attempts at "learning while burning" have already begun to enhance our understanding of the interaction of the structural and electrophysiologic aspects of the substrate for atrial arrhythmias.     

Efficacy of a Microwave Antenna for Ablation of the Tricuspid Valve—Inferior Vena Cava Isthmus in Dogs as a Treatment for Type 1 Atrial Flutter

INTRODUCTION: Radiofrequency catheter ablation of the tricuspid valve-inferior vena cava (TV-IVC) isthmus for treatment of atrial flutter (AFL), may in some cases require a large number of energy applications and a long procedure and fluoroscopy time. AIMS OF STUDY: Therefore, we studied the safety and efficacy of a 4 cm long microwave antenna mounted on a steerable 9Fr catheter for linear ablation of the TV-IVC isthmus. METHODS: In 6 anesthetized dogs, multi-electrode catheters were positioned in the coronary sinus (decapolar), at the His bundle (quadripolar) and around the TV annulus (decapolar) for pacing and recording atrial activation sequences before and after ablation. The microwave antenna was then positioned across the TV-IVC isthmus from the TV annulus (identified by equal A and V potentials) to the inferior vena cava with slight traction on the catheter to ensure adequate endocardial contact. Microwave energy was then applied at a fixed power for 120 seconds during each ablation attempt. Ablation was repeated until bi-directional isthmus block was demonstrated during pacing from the coronary sinus ostium and low lateral right atrium, respectively. RESULTS: Linear microwave ablation of the TV-IVC isthmus was completed in all ten dogs using a total of 2.6 +/- 1.17 energy applications per dog. Power was applied in a range of 45-50 watts. There were no acute procedural complications. Bi-directional TV-IVC isthmus block was achieved in all ten dogs, as demonstrated by a strictly descending activation wavefront in the ipsilateral atrial wall, during pacing from the CSO and LLRA respectively. In addition, after ablation conduction time to the LLRA during pacing from the CSO increased from 52 +/- 16.62 before to 87 +/- 12.74 msec (p <.05), and to the CSO during pacing from the LLRA from 51 +/- 12.43 before to 79.50 +/- 9.85 msec (p <.05). Gross and histological examination of the TV-IVC isthmus after ablation revealed continuous transmural lesions, ranging from 3-5 mm in width, spanning the entire TV-IVC isthmus in all ten dogs. CONCLUSIONS: (1) Microwave ablation of the TV-IVC isthmus was safe and effective in this study. (2) Ablation of the entire width and thickness of the TV-IVC isthmus can be rapidly achieved using a long microwave antenna in a fixed trans-isthmus position.     

Homogeneity and Diameter of Linear Lesions Induced With Multipolar Ablation Catheters: In Vitro and In Vivo Comparison of Pulsed Versus Continuous Radiofrequency Energy Delivery

Background: For invasive treatment of atrial fibrillation, linear lesions induced with multipolar ablation catheters (MAC) are needed to prevent recurrence. The aim of the study was to compare the efficacy of pulsed versus continuous radiofrequency (RF)-energy delivery using MAC. Methods: In vitro tests were performed using endomyocardial preparations of fresh pig hearts in a 10-liter-bath of physiologic saline solution (37°C) at constant flow conditions (1.5[emsp4 ]l/min). The MAC were placed with a constant pressure of 20 ponds onto the endocardium. The energy (generator: Osypka HAT 200 S) was delivered either pulsed (4 electrodes simultaneously, 5[emsp4 ]ms duty-cycle) or continuously (each electrode separately). In vivo experiments were performed in 6 anesthetized pigs using fluoroscopic positioning of MAC at 40 different intracardial positions and with similar conditions as in vitro experiments. Lesion volume (LV) was calculated after measuring lesion diameter with a microcaliper. The homogeneity of the lesions (LH) was classified from 1–4; with 1 as highest homogeneity. Results: Pulsed energy delivery produced more homogeneous linear lesions in significantly less time. There was no difference in electrode temperature values (50.2±0.8 and 51.3±1.4°C) in vitro and in vivo. In the in vivo experiments, lesion depth and calculated lesion volume were less in both modes of energy delivery but pulsed energy delivery was superior regarding lesion depth and homogeneity. Conclusion: With pulsed energy delivery it is possible to create linear lesions of significantly greater homogeneity. Moreover, larger lesions are induced in less time by pulsed energy delivery in vitro and in vivo.     

Comparison of a Saline Irrigated Cooled-Tip Catheter to Large Electrode Catheters with Single and Multiple Temperature Sensors for Creation of Large Radiofrequency Lesions

Background: Several catheter technologies for creating large radiofrequency (RF) lesions are used in clinical practice, but direct comparisons of the pathological lesions created by these technologies are unavailable. The purpose of this study was to compare the safety and efficacy of lesions created by three different large lesion RF ablation technologies. Methods and Results: RF lesions were created in all four chambers of 15 mongrel dogs using 10 mm-tip multiple temperature sensor catheters, 10 mm tip single temperature sensor catheters, and 4 mm cooled-tip catheters. Pathological lesions were bisected, and measured after viability staining. A total of 242 (79 large-tip single sensor, 82 large-tip multiple sensor, and 81 cooled-tip) lesions were created. All atrial lesions were transmural but tended to have larger surface areas with the single thermistor large-tip catheter (73.4 ± 24.8 mm²) compared to either the multithermistor large-tip (60.9 ± 28.3 mm²) or the cooled-tip (61.9 ± 28.5 mm²) catheters (p = 0.07), especially those in the IVC-TA isthmus. Depths and volumes of ventricular lesions created by the multiple-thermistor catheter (5.0 ± 1.5 mm; 260 ± 168 mm³) were smaller than either the single thermistor (5.7 ± 1.5 mm; 428 ± 290 mm³) or cooled-tip (6.1 ± 1.8 mm; 403 ± 217 mm³) catheters (p < 0.05). The difference in the depth and volume of lesions made by large-tip single thermistor and cooled-tip catheters was not significant. Char formation occurred during 11% of ablation with the single thermistor catheter, 6% with multithermistor and 8% of cooled-tip catheter (p = NS). There were no complications of ablation. Conclusions: All three catheters reliably created full thickness atrial lesions. For ventricular lesions, depths and volumes were similar for 10 mm-tip single thermistor and cooled-tip catheters. The multithermistor catheter lesions were smaller due to more precise temperature regulated power control. Safety was similar in all 3 groups.     

Steroids Prevent Late Extension of Radiofrequency Lesions in the Thigh Muscle of Infant Rats: Implications for Pediatric Ablation

Introduction: Marked late enlargement of radiofrequency (RF) lesions may occur in immature myocardium, suggesting that late proarrhythmic effects may occur in infants and small children undergoing RF ablation. Because late lesion extension may be involved in this phenomenon, we evaluated the impact of corticosteroids on the healing of RF lesions created in the thigh muscle of 29 infant Wistar rats (30 days; 55 g). Methods: Lesion dimensions and histological characteristics were assessed acutely (n = 11), and at 30 days in controls (n = 11, 183 g) and rats (n = 7, 173 g) receiving hydrocortisone after ablation and betametasone for 29 days. Acute (n = 16) and chronic (30 days; n = 5) lesions were also evaluated in adult Wistar rats (300 g). Results: Acutely, lesions in adults and infants were well demarcated from the surrounding tissue. In adults, chronic lesions did not increase in size and were well demarcated histologically. Controls and treated infant rats did not differ with respect to the gross appearance of chronic lesions. Late lesions doubled in size (20 mm in diameter) and were poorly demarcated from the surrounding tissue, exhibiting multiple collagen strands extending from the lesion into normal muscular tissue. In the treatment group, healing was markedly delayed and the extent of collagen proliferation was significantly less than controls. Conclusion: RF lesions created in the thigh muscle of infant rats reveal late enlargement and invasion of normal muscle by intense collagen proliferation. Steroids seem to limit late extension of RF lesions. These findings may have implications for RF ablation procedures in pediatric populations.     

Determinants of Lesion Size During Radiofrequency Catheter Ablation: The Role of Electrode-Tissue Contact Pressure and Duration of Energy Delivery

Determinants of Radiofrequency Lesion Size. Introduction: Radiofrequency (RF) catheter ablation is a promising modality in the management of cardiac arrhythmias, but the optimum protocol for energy delivery has not yet been determined. The purpose of this study was to examine the effects of varying electrode-tissue contact pressure and varying duration of RF energy delivery on the size of the resultant lesion in an in vitro model of isolated perfused and superfused canine myocardium. Methods and Results: RF power (500 kHz) was delivered through the tip of a specially-designed thermistor-tipped 1.6-mm diameter electrode mounted on an adjustable fulcrum so that contact force could be varied. RF power was adjusted to maintain a constant electrode-tissue interface temperature of 80°C during lesion production. Seventy-nine lesions were created with a 90-second RF energy delivery, and a contact force that was varied between 0 and 400 newtons (N) (0-41 g). Lesions produced with a small contact force (10 N) were significantly larger than those with a contact force of zero (width 5.5 mm vs 3.8 mm, P <0.0002), but not significantly different from those produced with the maximum contact force of 400 N (width 6.5 mm, P = NS). However, the greater contact force significantly decreased the power required to maintain a constant electrode-tissue interface temperature. Ninety-six lesions were then created using a constant contact force, but duration of energy delivery was varied from 10 to 600 seconds. Lesion size grew monoexponentially with time. The t_1/2 of lesion growth was 7.6 and 9.6 seconds for depth and width, respectively. Conclusion: Thus, close electrode-tissue contact is essential for adequate lesion formation during RF ablation of myocardium, but increasing contact force does not significantly increase lesion size if power is adjusted to maintain a constant electrode-tissue interface temperature. In order to approach steady-state and maximize lesion size, duration of RF energy delivery should be at least 40 seconds.     

Catheter Mounted Coaxially Moveable Ablation Electrode for the Creation of Linear Transmural Endocardial Lesions

INTRODUCTION: Use of a novel ablation catheter for the creation of linear transmural endocardial lesions, which uses a coaxially moving ablation electrode mounted on the terminal portion of a catheter shaft and able to move axially for a distance of up to 4 cm, is reported. METHODS AND RESULTS: The coaxially moving ablation electrode is moved by a sliding mechanism in the catheter handle. The distal portion of the catheter shaft is steerable. Bipolar or unipolar electrograms can be recorded from electrodes on the catheter tip and the coaxially moving ablation. Radiofrequency (RF) current is delivered to the coaxially moving ablation electrode with thermocouple temperature control. This ablation catheter was evaluated in five (30 to 65 kg) anesthetized pigs and introduced via the venous/arterial systems into the right and left atrium (1 lesion) (using the retrograde aortic approach). The catheter was maneuvered to bring the slide range into apposition with atrial endocardium. The coaxially moving ablation electrode was deployed to the terminal portion of the catheter's slide range and then withdrawn in 2-mm steps. RF current was delivered to the coaxially moving ablation electrode at each point (maximum temperature 70 degrees C). Postmortem examination of eight endocardial linear lesions (2.2 to 4.1 cm length) was made 1 to 3 hours after creation. Histopathologic examination confirmed transmural myocyte necrosis along the length of the lesion, that included the trabeculated right atrium. CONCLUSION: We conclude that a catheter using a moveable electrode creates continuous linear transmural lesions and could find clinical application in the therapy of a variety of reentry tachycardia mechanisms.     

Importance of Catheter Contact Force During Irrigated Radiofrequency Ablation: Evaluation in a Porcine Ex Vivo Model Using a Force‐Sensing Catheter

Effect of Ablation Electrode Contact Force. Introduction: Ablation electrode–tissue contact has been shown to be an important determinant of lesion size and safety during nonirrigated ablation but little data are available during irrigated ablation. We aimed to determine the importance of contact force during irrigated‐tip ablation. Methods and Results: Freshly excised hearts from 11 male pigs were perfused and superfused using fresh, heparinized, oxygenated swine blood in an ex vivo model. One‐minute ablations were placed using one of 3 different power control strategies (impedance control—15 Ω target impedance drop, and 20 W or 30 W fixed power) and 3 different contact forces (2 g, 20 g, and 60 g) to give a grid of 9 ablation groups. The force sensing catheter (Tacticath?, Endosense SA) was irrigated at 17 mL/min for all of the ablations. Of a total 101 ablations, no thrombus formation was noted but popping was seen in 17 lesions. The lesion depth and incidence of pops was 5.0 ± 1.3 mm /0%, 5.0 ± 1.6 mm /10% and 6.7 ± 2.5 mm /45% for the 15 Ω, 20 W, and 30 W groups (P < 0.01), respectively, and 4.4 ± 1.8 mm /3%, 5.8 ± 1.6 mm /17% and 6.6 ± 2.0 mm /37% for the 2 g, 20 g, and 60 g groups, respectively (P < 0.01). The impedance drop in the first 5 seconds was significantly correlated to catheter contact force: 9.7 ± 9.9 Ω, 22.3 ± 11.0 Ω, and 41.7 ± 22.1 Ω, respectively, for the 2 g, 20 g, and 60 g groups (Pearson's r = 0.65, P < 0.01). Conclusion: Catheter contact force has an important impact on both ablation lesion size and the incidence of pops. (J Cardiovasc Electrophysiol, Vol. pp. 806‐811, July 2010)     

Use of Three-Dimensional Mapping Systems in the Catheter Ablation of Atrial Fibrillation

The increasing clinical experience with remarkable advancement in the technology has enabled the catheter ablation of atrial fibrillation (AF) to become more effective and safe. Widespread utilization of three-dimensional (3D) mapping systems has facilitated the improvement in the outcomes after catheter ablation of AF. The purpose of this article is to review the current status, clinical role, and future directions of various 3D mapping systems in catheter ablation of AF.     

Ablation of Ventricular Tachycardia with a Saline-Cooled Radiofrequency Catheter:

INTRODUCTION: In animal models, active cooling of the electrode during radiofrequency (RF) ablation allows creation of larger lesions, presumably by increasing the power that can be delivered without coagulum formation. These RF lesions have not been characterized in human myocardium in regions of infarction and scarring. METHODS AND RESULTS: Cooled-tip RF catheter ablation of ventricular tachycardias (VTs) was performed in two patients who had severe congestive heart failure and subsequently underwent cardiac transplantation. The first patient had four different monomorphic VTs. RF applications along the inferoseptal margin of a scarred region abolished all inducible VTs. The second patient had sarcoidosis involving the myocardium and four different inducible VTs. RF current applied at an inferobasal VT exit and at the right and left septa failed to abolish the VTs. The explanted hearts were examined at the time of cardiac transplantation 18 and 21 days later, respectively. Lesions extended to depths up to 7 mm, reaching clusters of myocardial cells deep to regions of fibrosis. Microscopically, the ablation sites contained coagulation necrosis with hemorrhage, surrounded by a rim of granulation tissue. CONCLUSION: Saline-irrigated RF catheter ablation produces relatively large lesions capable of penetrating deep into scarred myocardium.     

Temperature-Controlled Radiofrequency Catheter Ablation with a 10-mm Tip Electrode Creates Larger Lesions without Charring in the Porcine Heart

Background: Radiofrequency catheter ablation of atrial flutter, atrial fibrillation or ventricular tachycardia may be favoured by large lesions. We compared lesions created in unipolar mode using 10-mm/8 F electrodes with those of 4-mm/7 F catheters.Methods: Ablations were first performed in porcine hearts in vitro (70°C, 60 s, tangential catheter tip-tissue orientation). Anaesthetized pigs were thereafter ablated with 10- or 4-mm catheters in the right atrial free wall (RAFW), inferior vena cava-tricuspid valve (IVC-TV) isthmus and left ventricle (LV).Results: In vitro, lesion length doubled and lesion volume tripled using the 10-mm catheter. Average power supply was 69 (SD12) (10-mm tip) versus 26 (SD7) W (4-mm tip). In vivo, lesion length increased by 50% and lesion volume fivefold. Charring at the lesion surface or sudden impedance rises were not observed in vivo. Histologically, coagulation necrosis and minor haemorrhages were found. One RAFW lesion (10-mm) showed a dissection approaching the epicardium. Fibrinous platelet clots or overt thromboses covered the endocardial surface in half of all lesions. Three 10-mm electrode isthmus lesions extended to the right descending posterior artery and one LV lesion to the left anterior descending artery, but there was no damage to the arterial walls. Following six ablations with the 10-mm electrode and two with the 4-mm tip, injury to the adjacent lung tissue of 0.5 to 6.0 mm depth was found (p = 0.22).Conclusion: RF ablation using 10-mm/8 F electrodes created significantly larger lesions. 10-mm electrodes appeared safe in the porcine IVC-TV isthmus and LV, but not in the RAFW.     

Perception and Documentation of Arrhythmias after Successful Radiofrequency Catheter Ablation of Accessory Pathways

Background: Some patients continue to have palpitations in spite of successful ablation of Wolff‐Parkinson‐White (WPW) syndrome. Recurrence of accessory pathways as well as unrelated arrhythmias may explain the symptoms. Methods: We followed 194 consecutive patients after successful radiofrequency catheter ablation of overt (147) or concealed (47) WPW syndrome. The mean duration of symptoms was 16 ±; 13 years. Atrial fibrillation was documented in 54 patients (24%) prior to ablation. 185 patients responded to a questionnaire 24 ±; 12 months after ablation. Results: The physical well‐being was improved in 94%, unchanged in 5%, and deteriorated in 1%. However, 76 patients (39%) reported arrhythmia symptoms, in 40 patients causing pharmacological treatment (14 patients) and/or continued contact with their doctor. The underlying arrhythmias were orthodromic tachycardia (10), atrial fibrillation (12), premature beats (12), atrial flutter (1), and ventricular tachycardia (1), while in four patients no explanation was found. Minor symptoms in the other 36 patients were explained by premature beats in 29, while unexplained in 7. All patients with atrial fibrillation after ablation had atrial fibrillation before ablation. Ten relapses of WPW syndrome occurred: eight were known before the time of the questionnaire, two were confirmed at transesophageal atrial stimulation. Conclusion: 94% patients with a long history of tachyarrhythmias due to the WPW syndrome reported improved physical well‐being after ablation, but palpitations were common during a 2‐year follow‐up period; 8% continued to use pharmacological antiarrhythmic treatment. Five percent had symptomatic relapses and in 6% atrial fibrillation episodes reoccurred, i.e., in half of those who had atrial fibrillation before ablation. A.N.E. 2001; 6(3):216–221