Radiofrequency Lesion Formation Research Articles

Correlation between Lesions Size Index (LSI) and impedance dynamics during radiofrequency ablation for atrial fibrillation: impact of lesion duration and contact force

Abstract Introduction Impedance drop has been employed as a surrogate marker for lesion formation with its magnitude correlated to lesion size in in-vitro studies. The Lesion Size Index (LSI), and algorithm from Abbott Medical, serves as a predictor for lesion formation during radiofrequency (RF) ablation. While this formula integrates contact force (CF), lesion duration and power, it currently leaks impedance information. The new module of Ensite Cardiac Mapping system (Abbot) is capable of measuring the "filtered" impedance during each RF lesion, effectively eliminating artifacts from cardiac and respiratory oscillations. The objective of this study is to examine the correlation between the LSI and the average impedance drop during RF lesion formation. Methods We included a total of 18 consecutive patients who underwent catheter ablation for atrial fibrillation (AF) with Ensite Mapping System (Abbot). In all cases, 40 watts (W) were applied and the target LSI was set as 5 for the posterior wall and 6 for the anterior wall. For each lesion, the software automatically annotated the initial impedance, final impedance, average impedance drop (filtered), lesion duration and average contact forced (CF). The correlation between LSI, impedance drop and CF was assessed. Results A total of 1300 RF lesions delivered during AF ablation were analyzed. Mean LSI was 5.3 ± 0.8, mean average impedance drop was 10.9± 5 ohms and mean CF 12.3±6 grams (g). The LSI showed a weak but statistically significant correlation with the impedance drop (Person correlation=0.3; p<0.001). When ablation lesions were grouped by duration (<20 seconds, 21-40, 41-60, >60 seconds), the correlation between LSI and impedance drop remained weak but statistically significant for lesions shorter than 20 seconds (p<0.001) and lesions between 21 and 40 seconds (p<0.001). However, there was no longer correlation for lesions between 40-60 seconds and those longer than 60 seconds (Figure1). Mean CF for each group (from <20 to >60 seconds) was 15 ±7 g vs 11 ±5 g vs 9 ± 3 g vs 7± 3g respectively (p<0.001) (Figure 2), suggesting that the CF is key to improve the predictive value of LSI. Conclusions The LSI shows a weak correlation with the average impedance drop during AF ablation. This correlation is influenced by lesion duration, which is ultimately dependent on CF. These findings emphasize the ongoing need to optimize predictive models for RF lesion formation in the ever-evolving landscape of AF treatment.Figure 1Table 1

The influence of electrode-tissue-coverage on RF lesion formation and local impedance: Insights from an ex vivo model.

The influence of power, duration and contact force (CF) on radiofrequency (RF) lesion formation is well known, whereas data on local impedance (LI) and electrode-tissue-coverage (ETC) is scarce. The objective was to investigate their effect on lesion formation in an ex vivo model. An ex vivo model was developed utilizing cross-sections of porcine heart preparations and a force-sensing, LI-measuring catheter. N=72 lesion were created systematically varying ETC (minor/full), CF (1-5g, 10-15g, 20-25g) and power (20W, 30W, 40W, 50W). In minor ETC, the distal tip of the catheter was in electric contact with the tissue, in full ETC the whole catheter tip was embedded within the tissue. Lesion size and all parameters were measured once per second (n=3320). LI correlated strongly with lesion depth (r=-0.742 for ΔLI; r=0.781 for %LI-drop). Lesions in full ETC were significantly wider and deeper compared to minor ETC (p<.001) and steam pops were more likely. Baseline LI, ΔLI, and %LI-drop were significantly higher in full ETC (p<.001). In lesions resulting in steam pops, baseline LI, and ΔLI were significantly higher. The influence of CF on lesion size was higher in minor ETC than in full ETC. ETC is a main determinant of lesion size and occurrence of steam pops. Baseline LI and LI-drop are useful surrogate parameters for real-time assessment of ETC and ΔLI correlates strongly with lesion size.

Time dependency in the radiofrequency lesion formation for a local impedance guided catheter in an ex vivo experimental model.

The local impedance (LI) is an emerging technology that monitors tissue-catheter coupling during radiofrequency (RF) ablation. The relationships between the LI, RF delivery time, and lesion formation remain unclear. Using an LI-enabled RF catheter in an ex vivo experimental model, RF lesions were created combined with various steps in the power (40 and 50 W), CF (10g, 30 g, and 50 g), and time (10s, 20s, 30s, 40s, 50s, and 60s at 40 W and 5s, 10s, 20s, 30s, 40s, 50s, and 60s at 50 W). The correlations between the LI drop, lesion size, and RF delivery time were evaluated. The rate of change in the time-dependent gain in the LI, depth, and diameter and the time to reach 90% decay of the peak dY/dT (time to 90% decay) were assessed. The correlation between the LI drop and ablation time revealed non-linear changes. The time to a 90% decay in the LI drop differed depending on the RF ablation setting and was always shorter with the 50 W setting than 40 W setting. The LI drop always correlated with the lesion formation under all ablation power settings. Deeper or wider lesions were predominantly created within the time to 90% decay of the LI drop. The LI drop was useful for predicting lesion sizes. Deeper or wider lesions cannot be obtained with a longer ablation than the 90% decay time of the LI drop. A shorter ablation than the 90% decay time of the LI drop would be preferable for an effective ablation.

Acute efficacy of contiguous versus temporally discontiguous point-by-point radiofrequency pulmonary vein isolation in patients with paroxysmal atrial fibrillation: a randomized study.

Durable pulmonary vein (PV) isolation (PVI) determines the clinical success of catheter ablation for atrial fibrillation. In this randomized study, we investigated whether the temporally discontiguous deployment of ablation lesions adversely affected the acute efficacy of PVI. Thirty-six consecutive patients with drug-refractory paroxysmal atrial fibrillation (aged 59 ± 11, 58% males) were randomized 1:1 to either discontiguous (D-PVI) or contiguous (C-PVI) encircling radiofrequency (RF) lesions around ipsilateral PVs. A contact force-sensing catheter was used targeting a final interlesion distance < 6mm and the ablation index of 400-450 (anterior wall) and 300-350 (posterior wall). The study endpoint was defined as failure of first-pass PVI or acute PV reconnection during a waiting time (> 30min) followed by adenosine challenge. The total RF time, number of RF lesions, and mean interlesion distance were comparable in both groups. Total endpoint rates were 1/36 (3%) in the D-PVI vs 4/36 (11%) in the C-PVI groups; P = 0.34 for superiority, P = 0.008 for non-inferiority. Adenosine-induced reconnection of right PVs was the only endpoint in the D-PVI group. In the C-PVI group, first-pass PVI failed in 2 right PVs and spontaneous reconnection occurred in 2 other circles (left and right PVs). Temporally discontiguous deployment of RF lesions is not associated with lower procedural PVI efficacy when strict criteria for interlesion distance and ablation index are applied. The development of local edema around each ablation site does not prevent effective RF lesion formation at adjacent positions. clinicaltrials.gov (NCT03332862).

Radiofrequency ablation to achieve durable pulmonary vein isolation.

Pulmonary vein isolation (PVI) by radiofrequency (RF) ablation is an important alternative to antiarrhythmic drugs in the treatment of symptomatic atrial fibrillation. However, the inability to consistently achieve durable isolation of the pulmonary veins hampers the long-term efficacy of PVI procedures. The large number of factors involved in RF lesion formation and the complex interplay of these factors complicate reliable creation of durable and transmural ablation lesions. Various surrogate markers of ablation lesion formation have been proposed that may provide information on RF lesion completeness. Real-time assessment of these surrogates may aid in the creation of transmural ablation lesions, and therefore, holds potential to decrease the risk of PV reconnection and consequent post-PVI arrhythmia recurrence. Moreover, titration of energy delivery until lesions is transmural may prevent unnecessary ablation and subsequent adverse events. Whereas several surrogate markers of ablation lesion formation have been described over the past decades, a 'gold standard' is currently lacking. This review provides a state-of-the-art overview of ablation strategies that aim to enhance durability of RF-PVI, with special focus on real-time available surrogates of RF lesion formation in light of the biophysical basis of RF ablation.

Local impedance for the optimization of radiofrequency lesion delivery: A review of bench and clinical data.

Radiofrequency catheter ablation is a cornerstone of treatment for many cardiac arrhythmias. Progression in three-dimensional mapping and contact-force sensing technologies have improved our capability to achieve success, but challenges still remain. In this article, we discuss the importance of overall circuit impedance in radiofrequency lesion formation. This is followed by a review of the literature regarding recently developed "local impedance" technology and its current and future potential applications and limitations, in the context of established surrogate markers currently used to infer effective ablation. We discuss the role of local impedance in assessing myocardial substrate, as well as its role in clinical studies of ablation. We also discuss safety considerations, limitations and ongoing research. Local impedance is a novel tool which has the potential to tailor ablation in a manner distinct from other established metrics.

Lesion Sequence and Catheter Spatial Stability Affect Lesion Quality Markers in Atrial Fibrillation Ablation.

This study sought to analyze high-frequency catheter excursion in relation to lesion quality markers in 20 consecutive patients undergoing first-time radiofrequency (RF) ablation for paroxysmal atrial fibrillation (AF). Ablation therapy for AF requires the delivery of durable lesions. The extent to which lesion sequence, catheter spatial stability, and anatomic location influence lesion formation during RF ablation of AF is not well understood. Three-dimensional spatial excursion of the ablation catheter sampled at 60Hz during pre-specified pairs of RF lesions was extracted from the CARTO3 System (Biosense Webster Inc., Irvine, California) and analyzed by using custom-developed MATLAB software (MathWorks, Natick, Massachusetts) to define precise catheter spatial stability during RF ablation. Ablation parameters including bipolar electrogram amplitude reduction, impedance decline and transmurality-associated unipolar electrogram (TUE) as evidence of lesion transmurality during lesion placement were recorded and analyzed. We collected 437,760 position data points during lesion placement. Ablation catheter spatial stability and lesion formation parameters varied considerably by anatomic location. Lesions placed immediately had similar bipolar electrogram amplitude reduction, smaller impedance decline, but higher likelihood of achieving TUE compared to delayed lesions. Greater catheter spatial stability correlated with lesser impedance decline. Lesion sequence, ablation catheter spatial stability, and anatomic location are important modifiers of RF lesion formation. Lesions placed immediately are more likely to exhibit TUE. Greater ablation catheter stability is associated with lesser impedance decline but greater likelihood of TUE.

Computer simulation study on the effect of electrode–tissue contact force on thermal lesion size in cardiac radiofrequency ablation

Purpose In cardiac radiofrequency (RF) ablation, RF energy is often used to create a series of transmural lesions for blocking accessory conduction pathways. Electrode–tissue contact force (CF) is one of the key determinants of lesion formation during RF ablation. Low electrode–tissue CF is associated with ineffective RF lesion formation, whereas excessive CF may increase the risk of steam pop and perforation. By using finite element analysis, we studied lesion size and features at different values of electrode–tissue CF in cardiac RF ablation. Materials and methods A computer-model-coupled electrode–tissue CF field, RF electric field, and thermal field were developed to study temperature distribution and lesion dimensions in cardiac tissue subjected to CF of 2, 5, 10, 20, 30, and 40 g with identical RF voltage and duration. Results Increasing CF was associated with an increase in lesion depth, width, and cross-section area. The lesion cross-section area exhibited a linear increase, and the lesion width was significantly greater than lesion depth under the identical ablation condition. The relationship between CF value and lesion size is a power function: Lesion Size = a × CFb (Lesion Depth = 3.17 × CF0.14 and Lesion Width = 5.17 × CF0.14). Conclusions This study confirmed that CF is a major determinant of RF lesion size and that electrode–tissue CF affects the amount of power dissipated in tissue. At a constant RF voltage and application time, RF lesion size increases as CF increases.

Beyond catheter tip and radiofrequency lesion delivery: the role of robotics in ablation of ventricular tachycardia.

Catheter ablation of ventricular tachycardia (VT) has become an important and increasingly performed treatment [1]. The use of remote magnetic navigation (RMN) for this procedure has been shown to have several advantages compared with manual catheter navigation (MCN) [2]. Since RMN was introduced in 2003 the most obvious advantage of RMN when compared with MCN is safety. Increased safety for both operators and patients is achieved by shortening fluoroscopy time and by reducing complications due to catheter flexibility [3]. So far, there has not been a single report of myocardial perforation. Additionally, after an initial learning curve has been completed, the use of RMN most likely shortens procedure time [4], may prevent operator fatigue and helps to reach the parts of the heart that are less easily accessible during mapping. In the current issue of the Netherlands Heart Journal, Wu et al. [5] present their meta-analysis on RMN vs. MCN for the ablation of VT. They included four non-randomised studies and conclude that acute and long-term success rates for VT ablation are equal between RMN and MCN. However, complication rates, procedure and fluoroscopy times are favourable for RMN. Their conclusion adds to the aforementioned findings for improved safety and at least a non-inferior success rate. In fact, the only limitation of RMN that remains in experienced teams is the cost of the equipment which can be up to 2 million € [6]. However, the long-term benefits of this installation should be taken into account when assessing these expenses [6]. Looking at success rate, the authors have been cautious in their interpretation. Several studies have in fact demonstrated a better outcome compared with MCN in VT ablation [4, 7]. What could be a possible explanation for this variable outcome? As RMN is a relatively new and evolving method, procedural outcomes can be further improved when sufficient experience is obtained. Only a limited group of operators could acquire the appropriate level of skill due to the cost of the system and a consequently limited availability. This could potentially result in an operator-dependent success rate variability and also brings up another point related to the financial aspect. Most of the centres able to afford the installation are academic centres, dealing with a very specific patient population. This means the procedures performed with RMN will be those in patients with a more complex pathology, which in turn may result in an underestimation of treatment success. Finally, because of the limited availability of RMN technology, present studies are mostly non-randomised. Non-significant trends in success rates favourable to RMN, as seen in Dinov et al. [8], may in the future prove significant in randomised trials. Notably, the papers by Bauernfeind et al. and Szili-Torok et al., both used in the current report, showed superiority to MCN in acute success rates in structurally normal hearts but equal results in structural heart disease [4, 7]. This raises the issue of how RMN can improve success rates. Is it related to the improved manoeuvrability of the catheter tip: tip delivery, or due to the constant type of tissue tip contact which improves radiofrequency lesion formation: lesion delivery? The data from of the above-mentioned manuscripts as well as this meta-analysis indirectly suggest that tip delivery efficiency may be superior using RMN while lesion delivery should be equally good when compared with MCN. RMN is theoretically more suitable for reaching difficult locations because of the flexible nature of the catheter in combination with the possibility of more accurate positioning. What are the future perspectives? An important step that could definitively make RMN favourable to MCN will be the utilisation of contact force measuring ablation catheters for magnetic navigation and ablation. Optimal electrode—tissue contact has been previously demonstrated to be of great importance for lesion formation [9] and has already been shown to improve clinical outcome in the treatment of atrial fibrillation [10]. When this technique becomes available for RMN and reaches its full potential (new combined parameters, involvement of cardiac imaging for real time visualisation of lesion formation) it might revolutionise treatment of ventricular and supraventricular arrhythmias with RMN and might prove pivotal in achieving long-term success in VT ablation.

The biophysics of renal sympathetic denervation using radiofrequency energy

Renal sympathetic denervation is currently performed in the treatment of resistant hypertension by interventionists who otherwise do not typically use radiofrequency (RF) energy ablation in their clinical practice. Adequate RF lesion formation is dependent upon good electrode-tissue contact, power delivery, electrode-tissue interface temperature, target-tissue impedance and the size of the catheter's active electrode. There is significant interplay between these variables and hence an appreciation of the biophysical determinants of RF lesion formation is required to provide effective and safe clinical care to our patients. In this review article, we summarize the biophysics of RF ablation and explain why and how complications of renal sympathetic denervation may occur and discuss methods to minimise them.

Lower catheter tip temperatures are associated with inhomogeneous lesion formation during radiofrequency catheter ablation in a canine thigh muscle model

Purpose: Previous studies on open irrigation radiofrequency (RF) ablation catheters focused on created lesion volumes, and did not evaluate lesion quality. The aim of this study was to evaluate the effect of different irrigation levels on lesion quality and volume. Methods: A canine thigh model was used. A skin incision was made and a cradle was created and filled with blood (37 °C, 250 ml/min). The ablation catheter was positioned parallel or perpendicular to the muscle surface using a constant force of 10 g. RF lesions were created using a gold tip electrode catheter with 12 irrigation holes and a PtIr catheter with 6 irrigation holes (Biotronik, Berlin, Germany). Both catheters were 7F and had a 3.5 mm electrode tip. RF current was delivered for 60 s at either 50 W (n=18) or 30 W (n=39). Temperature probes were inserted immediately below the surface and at 3.5 and 7 mm depth. Histological examination was performed to evaluate lesion size as well as the homogeneity of the RF lesions. Inhomogeneity was defined as a visual observed multi-band lesion pattern indicating different histological characteristics. Electrode temperature and lesion dimensions were measured. Results: Data of 57 lesions, created in a mongrel dog model, were analyzed. A total number of 47 inhomogeneous and 10 homogeneous lesions were detected. Inhomogeneous lesions were deeper (7.5±1.4 vs. 6.3±2.1 mm, p=0.036), however lesion volume was comparable between the two groups (652±487 vs. 834±396, p=0.249). Lesions with inhomogeneous histological characteristics had lower maximum electrode temperatures during ablation (42.9±6.6 vs. 48.2±8.6 °C, p=0.049). The mean temperatures at 3,5 and 7 mm below the muscle surface were lower for inhomogeneous lesions (41.2±8.1 vs. 50.2±21.4 °C, p=0.035; 38.7±4.0 vs. 45.2±19.0 °C, p=0.034). Inhomogeneity of the lesion was not associated with catheter type (p=0.164), orientation (p=0.185), amount of saline infusion (p=0.305) and power output (p=0.213). Conclusions: Our data suggest that lower catheter tip and tissue temperatures are associated with the development of inhomogeneous acute RF lesion formation. The presence of inhomogeneous RF lesions can theoretically be responsible for the differences between acute and chronic ablation success rates.

How to achieve durable pulmonary vein isolation: Use the force

How to achieve durable pulmonary vein isolation: Use the force

Abstract 9867: Insulating an Open Irrigated Ablation Electrode Improves Electrode Temperature Feedback for Radiofrequency Lesion Formation and Steam Pop

Introduction: Open irrigation (Irg) cools electrode and electrode-tissue interface (ETI), and prevents thrombus while producing large RF lesions. However, electrode temperature (ET) does not predict excessive tissue heating and steam pop. The purpose of this study was to compare the relationship of ET to lesion size and steam pop between a novel thermally insulated Irg electrode (Gen2, St Jude Medical) and a conventional, non-insulated 4 mm Irg electrode (St Jude Medical) using a canine thigh muscle preparation. Methods: In 4 dogs, the skin over the thigh muscle was incised and raised to form a cradle, which was superfused with heparinized blood at 37°C. Gen2 has a 7F, 2.5 mm tip electrode with 6 proximal and 1 distal thermally insulated Irg channels to provide only external electrode cooling ( Fig A ). Gen2 and a conventional 7F, 4mm Irg catheter were held perpendicular or parallel to thigh muscle. Catheter-tissue contact force was applied at 5, 20 or 40 g. RF was applied for 60 sec at 20 W for Gen2 (Irg 17 ml/min) and 40 W for the 4mm electrode (Irg 17 ml/min). ET, ETI, and tissue temperatures (3 and 7mm depths) were measured. Dogs were sacrificed at 2 hours after ablation and lesion size was measured. Results: Table and Fig . Conclusions: Gen2 had a wider range of peak ET than 4 mm. Gen2 had better correlation between peak ET and lesion depth (R=0.71 vs. R=0.26), and between peak ET and steam pop (ET&gt;54°C vs. ET &gt;34°C).

Direct visualization of radiofrequency lesion formation and gap in humans

Currently, few tools are available for determining or monitoring lesion formation in real time. A new catheter (IRIS, Voyage Medical, Redwood City, CA, USA) that allows direct visualization of endocardial cardiac structure as well as radiofrequency (RF) delivery has recently been developed (Online Supplement Movie 1).1

Integration of cardiac imaging and electrophysiology during catheter ablation procedures for atrial fibrillation

As nonpharmacologic therapies for atrial fibrillation expand, the complexity of the anatomical and electrical substrates of atrial fibrillation requires integration of multiple imaging modalities for successful treatment. Combining chamber-specific imaging and electrophysiologic data points during ablation therapy has improved pulmonary vein isolation accuracy while diminishing risk. Merging 3-dimensional computed tomography left atrial renditions, intracardiac echocardiography, and electroanatomical mapping during pulmonary vein isolation is a reality that relates the complex anatomy of the left atrium to its often variable electrical targets. Direct visualization of radiofrequency lesion formation as well as electrode-tissue contact during current delivery has been accomplished in animal models using ultrasound and infrared imaging tools. The combination of these early integrative stepping stones may lead to imaging with real-time feedback relating tissue desiccation to electrical effect and lesion transmurality. Atrial fibrillation ablation outcome will improve as the electrophysiology laboratory continues to integrate real-time 3-dimensional cardiac chamber and tissue images before, during, and after radiofrequency or cryoablation of specific electrophysiologic targets.

Intramural coronary vasculature prevents transmural radiofrequency lesion formation: implications for linear ablation.

Blood flow near a radiofrequency (RF) lesion can reduce lesion size by convective cooling. It is unknown whether blood flow through small vasculature within an RF lesion can prevent transmural lesion formation. In 40 rabbit right ventricle preparations, 2 epicardial RF lesions were created straddling a selectively perfused (0 to 12 mL/min) marginal artery (diameter, 0.34+/-0.1 mm). RF lesions were created at either 60 degrees C or 80 degrees C and delivered either sequentially or simultaneously. Conduction through the lesion area was measured. The lesions were analyzed histologically. At a perfusion rate of 0 mL/min, all RF lesions were transmural and without conduction. As little as 1 mL of flow through the artery during RF delivery could prevent transmural lesion formation by preserving a cuff of tissue along the length of the vessel. High-energy delivery (45 W) and very high tissue temperatures (93 degrees C) were needed to overcome the protective effect of vascular perfusion at 12 mL/min. The volume of preserved myocardium was related to arterial perfusion rate, artery diameter, and lesion temperature but not to the sequence of RF delivery (sequential versus simultaneous). Conduction persisted through the RF lesion in 20 experiments. Conduction through the lesion was related to the arterial perfusion rate and volume or cross-sectional area of preserved myocardium. Flow through even small intramyocardial vessels can prevent transmural lesion formation and preserve conduction through an RF lesion. These findings may represent an unrecognized obstacle to the creation of linear RF lesions in the clinical setting.

Acute and chronic electrophysiologic changes surrounding radiofrequency lesions.

Arrhythmia recurrences may occur after acutely successful radiofrequency (RF) ablation, suggesting reversible electrophysiologic effects on myocardial tissue. The aim of this study was to examine the electrophysiologic changes occurring in myocardium surrounding acute and chronic RF lesions. RF lesions (60 degrees C for 30 sec) were produced on the epicardial left ventricular surface of 35 isolated perfused rabbit hearts. Microelectrode transmembrane action potential recordings were made at 0.25-mm intervals along the epicardial (n = 20) or intramural (n = 6) aspects of the lesions for up to 3.5 mm from the lesion edge at baseline and after acute RF lesion formation. In nine hearts, chronic lesions were studied with epicardial recordings made 22 +/- 13 days after RF lesion formation. In the acute hearts at 600-msec paced cycle length, action potential duration at 50% repolarization (APD50) and 90% repolarization (APD90) were reduced by up to 41% and 19%, respectively, within 2.5 mm from the lesion edge (all P < 0.05). Maximal action potential amplitude (APDmax) was reduced by 16% at 0.25 mm from the lesion edge (P = 0.009) but dV/dt was not changed. Conduction time (CT) from the outermost recording site to the lesion edge (3.5-mm distance) was reduced from 16 +/- 3 msec at baseline to 13 +/- 4 msec post ablation (P = 0.006). Qualitatively similar findings occurred at 200-msec cycle lengths. There were no changes in APD, CT, APDmax, or dV/dt surrounding the chronic lesions at 22 +/- 13 days after lesion formation. Acutely following RF energy delivery, APD, APDmax, and CT are reduced in the tissue surrounding the lesion. These electrophysiologic changes resolve within 22 +/- 13 days of lesion formation.

Biophysical characteristics of radiofrequency lesion formation in vivo: Dynamics of catheter tip–tissue contact evaluated by intracardiac echocardiography

During clinical radiofrequency catheter ablation a wide range of delivered power may be necessary to achieve success despite an apparently stable catheter position on fluoroscopy. The purpose of this study was to use intracardiac echocardiography to characterize the relation between catheter tip–tissue contact and the efficiency of heating during applications of radiofrequency energy in vivo and to determine whether intracardiac echocardiography could be used prospectively to improve tissue contact. A closed-loop temperature feedback control system was used during radiofrequency applications at five anatomic regions in the right atrium of 15 anesthetized dogs to ensure achievement of a predetermined temperature (70° C) at the catheter tip thermistor by automatic adjustment of delivered power (maximum 100 W). The efficiency-of-heating index was defined as the ratio of steady-state temperature (degrees Celsius) to power (watts). Two-dimensional intracardiac echocardiography was used to evaluate movement of the catheter tip relative to the endocardium. Perpendicular contact was scored as good, average, or poor and lateral catheter sliding as <2, 2 to 5, or >5 mm. Two groups of animals were included: group 1, in which tissue contact was guided by fluoroscopic and electrographic criteria for stability of contact, with intracardiac echocardiography used simply to observe the application; and group 2, in which tissue contact was guided by intracardiac echocardiography. Of 66 applications, 18 (27.3%) had poor perpendicular contact on echocardiography, and 12 (18.2%) demonstrated lateral sliding of >5 mm even though they had been considered to have good tissue contact by fluoroscopic and electrographic criteria. Perpendicular catheter contact and anatomic location were shown to be independently related to the efficiency-of-heating index. Applications with good perpendicular contact had a significantly higher efficiency-of-heating index and a significantly greater lesion size than those with average or poor contact. The percentage of applications having good perpendicular tissue contact and the lesion size were significantly greater when tissue contact was guided by intracardiac echocardiography compared with fluoroscopic and electrographic guidance. This study demonstrates that variations in catheter tip–tissue contact account for differences in the efficiency of tissue heating, independently of the anatomic site of the application. Poor tissue contact was observed by intracardiac echocardiography and confirmed by indexes of tissue heating in approximately one third of radiofrequency applications despite a fluoroscopic appearance and electrographic morphologic appearance suggestive of good tissue contact. There was a significant correlation between echocardiographic evaluation of tissue contact, parameters of tissue heating (efficiency-of-heating index), and lesion size. In addition, intracardiac echocardiography could be used prospectively to improve the percentage of good contact applications and increase the lesion size. (Am Heart J 1997;133:8-18.)

Early postoperative appearance of radiofrequency lesions on magnetic resonance imaging.

Eleven patients who underwent stereotactic radiofrequency lesions in the central nervous system had magnetic resonance imaging follow-up within 72 hours of surgery to determine the early appearance of their lesions. Eight patients with severe tremor, one with chronic pain, and two with dystonia were analyzed. There were six female patients and five male patients, age 7 to 75 years (mean +/- standard deviation = 42 +/- 21). Magnetic resonance imaging was performed postoperatively at 32 +/- 25 hours (range, 3-72). Postoperative T1-weighted spin echo images demonstrated foci of iso- to hyperintensity surrounded by an edge of hypointensity, and corresponding T2-weighted images showed a lesion with three concentric zones consisting of inner hypointense, middle hyperintense, and outer hypointense zones. Gadolinium increased T1-weighted image lesion visibility, and a ring of enhancement around the zone of hypointensity was observed. Lesions could be seen as early as 3 hours after surgery. The lesions were best shown on gadolinium-enhanced T1-weighted images and on T2-weighted images. The edema surrounding the lesion increased over time, up to the 72 hours studied. These data provide important information on the development of lesion appearance, which may be applied in the development of real-time magnetic resonance imaging monitoring of radiofrequency lesion formation. This technique associated with electrophysiological response and the real-time visualization of the anatomic correlation of the probe may allow for a very precise and selected lesion in the central nervous system for the treatment of functional disorders and brain tumors.

Transesophageal echocardiography during radiofrequency catheter ablation of ventricular tachycardia

Radiofrequency lesion formation requires stable catheter tip/endocardial contact. Energy delivery is limited when temperatures are >100 °C due to coagulum formation at the catheter tip. Transesophageal echocardiographic imaging may be useful for monitoring catheter position and detecting boiling. Transesophageal echocardiographic images were recorded during production of 22 radiofrequency lesions in bovine myocardium in a saline bath. Lesion size, tissue temperature and appearance of echo contrast (bubbles) were assessed. In 11 patients, transesophageal echocardiography was used to guide catheter movement and detect boiling during radiofrequency ablation for ventricular tachycardia. In the tissue bath, the appearance of echo bubbles was associated with visual bubbling at the catheter tip, tissue temperatures >60 °C and larger lesions (284 ± 165 vs 30 ± 54 mm 3; p < 0.001). In humans, transesophageal images easily identified the catheter tip in either ventricle and enabled continuous observation of electrode-tissue contact during radiofrequency application. Transesophageal echocardiographic bubbles appeared in 59 of 217 radiofrequency applications (27%). Continued radiofrequency application after appearance of bubbles was followed by an increase in impedance. Prolonged placement of the probe in heavily sedated patients resulted in a mild sore throat, but no other complications. Transesophageal echocardiographic imaging enables continuous monitoring of catheter position during radiofrequency energy application. The abrupt appearance of echo bubbles indicates boiling and impending coagulum formation at the catheter tip.