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Aims: Irreversible electroporation (IRE) ablation is a non-thermal ablation method based on the application of direct current between a multi-electrode catheter and skin electrode. The delivery of current through blood leads to electrolysis. Some studies suggest that gaseous (micro)emboli might be associated with myocardial damage and/or (a)symptomatic cerebral ischaemic events. The aim of this study was to compare the amount of gas generated during IRE ablation and during radiofrequency (RF) ablation., Methods and Results: In six 60-75 kg pigs, an extracorporeal femoral shunt was outfitted with a bubble-counter to detect the size and total volume of gas bubbles. Anodal and cathodal 200 J IRE applications were delivered in the left atrium (LA) using a 14-electrode circular catheter. The 30 and 60 s 40 W RF point-by-point ablations were performed. Using transoesophageal echocardiography (TOE), gas formation was visualized. Average gas volumes were 0.6 ± 0.6 and 56.9 ± 19.1 μL (P < 0.01) for each anodal and cathodal IRE application, respectively. Also, qualitative TOE imaging showed significantly less LA bubble contrast with anodal than with cathodal applications. Radiofrequency ablations produced 1.7 ± 2.9 and 6.7 ± 7.4 μL of gas, for 30 and 60 s ablation time, respectively., Conclusion: Anodal IRE applications result in significantly less gas formation than both cathodal IRE applications and RF applications. This finding is supported by TOE observations., (© The Author(s) 2020. Published by Oxford University Press on behalf of the European Society of Cardiology.)

Background: Irreversible electroporation (IRE) by inserting needles around the tumor as treatment for locally advanced pancreatic cancer entails several disadvantages, such as incomplete ablation due to field inhomogeneity, technical difficulties in needle placement and a risk of pancreatic fistula development. This experimental study evaluates outcomes of IRE using paddles in a porcine model., Methods: Six healthy pigs underwent laparotomy and were treated with 2 separate ablations (in head and tail of the pancreas). Follow-up consisted of clinical and laboratory parameters and contrast-enhanced computed tomography (ceCT) imaging. After 2 weeks, pancreatoduodenectomy was performed for histology and the pigs were terminated., Results: All animals survived 14 days. None of the animals developed signs of infection or significant abdominal distention. Serum amylase and lipase peaked at day 1 postoperatively in all pigs, but normalized without signs of pancreatitis. On ceCT-imaging the ablation zone was visible as an ill-defined, hypodense lesion. No abscesses, cysts or ascites were seen. Histology showed a homogenous fibrotic lesion in all pigs., Conclusion: IRE ablation of healthy porcine pancreatic tissue using two plate electrodes is feasible and safe and creates a homogeneous fibrotic lesion. IRE-paddles should be tested on pancreatic adenocarcinoma to determine the effect in cancer tissue., (Copyright © 2017. Published by Elsevier Ltd.)

Background: Over the past decade, radiofrequency catheter ablation (RFCA) of atrial fibrillation (AF) has evolved into a frequently performed procedure. The aim of this study was to monitor changes in patient characteristics, procedural characteristics, outcomes and complications over the past 10 years., Methods: All consecutive patients who underwent primary RFCA treatment of AF in the University Medical Center Utrecht from 2005-2015 were included. In all patients, the primary ablation strategy was pulmonary vein (PV) antrum isolation without additional substrate modification. Baseline patient and procedure characteristics, and 1‑year follow-up data of 975 patients were prospectively collected., Results: In 2005, 73.4% of patients suffered from paroxysmal AF, which decreased to 45.3% in 2014. Mean age increased from 54 ± 9 to 61 ± 10 years and CHA 2 DS 2 -VASc score ≥2 from 18 to 40.6%. History of AF decreased significantly from 7 to 4 years. Mean procedure duration was 237 ± 53 min in 2005 and 163 ± 41 min in 2014. Fluoroscopy time significantly decreased from 41 ± 17 to 19 ± 8 min and total radiation exposure from 465 (263-687) to 210 (118-376) mGy. One-year success remained similar (2005: 55.6%, 2014: 54.8%), as did the amount of PV reconnection observed during redo procedures. Due to a marked reduction in vascular complications and moderate PV stenosis, the total complication rate decreased significantly., Conclusion: Over the past decade, AF ablation has increasingly been performed in older patients with persistent AF and more comorbidity. Moreover, it has been performed earlier after AF diagnosis. Although several performance parameters, such as procedure duration and complication rate, improved, 1‑year single procedure success remained unchanged.

Background: Irreversible electroporation (IRE) with needle electrodes is being explored as treatment option in locally advanced pancreatic cancer. Several studies have shown promising results with IRE needles, positioned around the tumor to achieve tumor ablation. Disadvantages are the technical difficulties for needle placement, the time needed to achieve tumor ablation, the risk of needle track seeding and most important the possible occurrence of postoperative pancreatic fistula via the needle tracks. The aim of this experimental study was to evaluate the feasibility of a new IRE-technique using two parallel plate electrodes, in a porcine model., Methods: Twelve healthy pigs underwent laparotomy. The pancreas was mobilized to enable positioning of the paddles. A standard monophasic external cardiac defibrillator was used to perform an ablation in 3 separate parts of the pancreas; either a single application of 50 or 100J or a serial application of 4x50J. After 6 hours, pancreatectomy was performed for histology and pigs were terminated., Results: Histology showed necrosis of pancreatic parenchyma with neutrophil influx in 5/12, 11/12 and 12/12 of the ablated areas at 50, 100, and 4x50J respectively. The electric current density threshold to achieve necrosis was 4.3, 5.1 and 3.4 A/cm2 respectively. The ablation threshold was significantly lower for the serial compared to the single applications (p = 0.003). The content of the ablated areas differed between the applications: areas treated with a single application of 50 J often contained vital areas without obvious necrosis, whereas half of the sections treated with 100 J showed small islands of normal looking cells surrounded by necrosis, while all sections receiving 4x 50 J showed a homogeneous necrotic lesion., Conclusion: Pancreatic tissue can be successfully ablated using two parallel paddles around the tissue. A serial application of 4x50J was most effective in creating a homogeneous necrotic lesion., Competing Interests: The authors have declared that no competing interests exist.

Background: Circular electroporation ablation is a novel ablation modality for electrical pulmonary vein isolation. With a single 200-J application, deep circular myocardial lesions can be created. However, the acute and chronic effects of this energy source on phrenic nerve (PN) function are unknown., Objective: The purpose of this study was to analyze nerve vulnerability to electroporation ablation in a porcine model., Methods: In 20 animals (60-75 kg), the course of the right PN was pace-mapped inside the superior caval vein (SCV). Thereafter, a single 200-J circular electroporation ablation was performed via a multipolar circular catheter in firm contact with the inner SCV wall., Results: In 19 of 20 animals, the PN could be captured along an estimated 6-8 cm trajectory above the right atrial contour. Directly after the application, the PN could be captured above the ablation level in 17 of 19 animals and after maximally 30 minutes in all animals. Fifteen animals were restudied after 3-13 weeks, and PN functionality was unaffected in all. Histological analysis in 5 animals in which the application had been delivered in the muscular sleeve just above the right atrium showed a transmural circular lesion. However, no lesion was found in the other animals in which the application had been delivered in the fibrous section more cranial in the SCV., Conclusions: Electroporation ablation at an energy level that may create deep myocardial lesions may spare the targeted right PN. These animal data suggest that electroporation may be a safe ablation modality near the right PN., (Copyright © 2015 Heart Rhythm Society. Published by Elsevier Inc. All rights reserved.)

Background: Radiofrequency ablation inside pulmonary vein (PV) ostia can cause PV stenosis. A novel alternative method of ablation is irreversible electroporation, but the long-term response of PVs to electroporation ablation is unknown., Methods and Results: In ten 6-month-old pigs (60-75 kg), the response of PVs to circular electroporation and radiofrequency ablation was compared. Ten consecutive, nonarcing, electroporation applications of 200 J were delivered 5 to 10 mm inside 1 of the 2 main PVs, using a custom-deflectable, 18-mm circular decapolar catheter. Inside the other PV, circular radiofrequency ablation was performed using 30 W radiofrequency applications via an irrigated 4-mm ablation catheter. PV angiograms were made before ablation, immediately after ablation, and after 3-month survival. PV diameters and heart size were measured. With electroporation ablation, PV ostial diameter decreased 11±10% directly after ablation, but had increased 19±11% after 3 months. With radiofrequency ablation, PV ostial diameter decreased 23±15% directly after ablation and remained 7±17% smaller after 3 months compared with preablation diameter despite a 21±7% increase in heart size during aging from 6 to 9 months., Conclusions: In this porcine model, multiple circumferential 200-J electroporation applications inside the PV ostia do not affect PV diameter at 3-month follow-up. Radiofrequency ablation inside PV ostia causes considerable PV stenosis directly after ablation, which persists after 3 months., (© 2014 American Heart Association, Inc.)

Despite the advent of non-fluoroscopic technology, fluoroscopy remains the cornerstone of imaging in most interventional electrophysiological procedures, from diagnostic studies over ablation interventions to device implantation. Moreover, many patients receive additional X-ray imaging, such as cardiac computed tomography and others. More and more complex procedures have the risk to increase the radiation exposure, both for the patients and the operators. The professional lifetime attributable excess cancer risk may be around 1 in 100 for the operators, the same as for a patient undergoing repetitive complex procedures. Moreover, recent reports have also hinted at an excess risk of brain tumours among interventional cardiologists. Apart from evaluating the need for and justifying the use of radiation to assist their procedures, physicians have to continuously explore ways to reduce the radiation exposure. After an introduction on how to quantify the radiation exposure and defining its current magnitude in electrophysiology compared with the other sources of radiation, this position paper wants to offer some very practical advice on how to reduce exposure to patients and staff. The text describes how customization of the X-ray system, workflow adaptations, and shielding measures can be implemented in the cath lab. The potential and the pitfalls of different non-fluoroscopic guiding technologies are discussed. Finally, we suggest further improvements that can be implemented by both the physicians and the industry in the future. We are confident that these suggestions are able to reduce patient and operator exposure by more than an order of magnitude, and therefore think that these recommendations are worth reading and implementing by any electrophysiological operator in the field., (Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2014. For permissions please email: journals.permissions@oup.com.)

Purpose: The success of PVAI in eliminating AF has been proven; however, its impact on the LA remains uncertain. This study aimed to determine the impact of pulmonary vein antrum isolation (PVAI) on left atrial (LA) size and function in patients with atrial fibrillation (AF)., Methods: Consecutive patients with AF were included (n = 206). Magnetic resonance imaging (MRI) was performed before and after PVAI in all patients. A subgroup (n = 52) underwent delayed enhancement MRI. Maximal LA volume (LAVmax) and minimal LA volume (LAVmin) were assessed by Simpson's rule. LA function was determined by calculating LA ejection fraction (LA EF). LA fibrosis was manually encircled and summed in the region of interest., Results: Single procedure success rate was 64 %. LAVmax decreased post-ablation in all patients (125.1 to 111.9 ml, p < 0.001). LAVmin only decreased in patients with a successful outcome post-ablation (65.6 to 58.8 ml, p < 0.001). As a result, LA EF only showed a marked reduction in patients with AF recurrences (42.7 % to 37.9 %, p < 0.001). Post-ablation LA fibrosis could be visualized in 77 % of patients who underwent delayed enhancement MRI (mean amount 1.4 cm(3)). LA fibrosis showed no correlation with the decrease in LAVmax or LA EF., Conclusions: PVAI resulted in a reduction of LAVmax in all patients, indicating an effect of ablation induced fibrosis. LAVmin only decreased in patients with a successful outcome, indicating an effect of reverse atrial remodeling. As a result, LA function post-ablation was preserved in patients with a successful outcome and decreased in patients with AF recurrence.

Aims: Radiofrequency catheter ablation is a successful treatment for cardiac arrhythmias, but may lead to major complications such as permanent coronary damage. Irreversible electroporation (IRE) is a new non-thermal ablation modality, but its effect on coronary arteries is still unknown., Methods and Results: In a porcine model, epicardial IRE lesions were created at the base of the left ventricle in four hearts (group A) and directly on the left anterior descending artery (LAD) in five hearts (group B). After 3 weeks, coronary arteries inside IRE lesions and in apparently undamaged myocardium next to the lesions were (immuno-)histologically studied. Two untreated hearts served as controls. Coronary damage was defined as intimal hyperplasia. Left anterior descending artery angiograms were obtained before ablation, directly after ablation, and before termination in group B. In group A, 103 arterial branches were studied. Of these, 5 of 56 arterial branches inside lesions and 1 of 47 outside lesions showed intimal hyperplasia, but all had <50% area stenosis. Targeted LADs (group B) did not reveal intimal hyperplasia and angiograms showed no signs of stenosis. Expression of connective tissue growth factor was observed in the scar tissue, but not in the fibrotic tissue directly around the arteries, confirming that the arteries are indeed spared from tissue damage and remodelling., Conclusion: Coronary arteries remain free of clinically relevant damage 3 weeks after epicardial IRE ablation, even amid very large myocardial lesions. This suggests that IRE ablation can be applied safely near or even on coronary arteries. With IRE ablation, arterial blood flow does not appear to affect lesion formation.

Background: Complex fractionated atrial electrograms (CFAEs) are supposed to be related to structural and electrical remodeling. Animal studies suggest a role of the autonomic nervous system (ANS). However, this has never been studied in humans., Objective: The goal of this study was to investigate the influence of ANS on CFAEs in patients with idiopathic atrial fibrillation (AF)., Methods: Thirty-six patients (28 men, 55 ± 9 years) were included before undergoing catheter ablation. In the 24 hours preceding the procedure, 20 patients were in AF (group 1) and 16 were in sinus rhythm (SR, group 2). With 2 decapolar catheters, 1 in the right atrium (RA) and 1 in the left atrium (LA), 20 unipolar electrograms were simultaneously recorded during a 100-second AF-period (in group 2 after induction of AF). After atropine and metoprolol administration, a second 100-second AF-period was recorded 30 minutes later. Five patients of group 2 served as controls and did not receive atropine and metoprolol prior to the second recording. CFAEs were assessed and the prevalence of CFAEs was expressed as percentage of the recording time., Results: The prevalence of CFAEs was greater in group 1 than in group 2 in both RA and LA (P = 0.026, P < 0.001, respectively). Atropine and metoprolol significantly reduced CFAEs in group 1 (P < 0.001) and prevented the time-dependent increase of CFAEs in group 2., Conclusion: The prevalence of CFAEs is greater in long-lasting AF episodes. Atropine and metoprolol administration reduces CFAEs in both atria. Thus, CFAEs are at least partly influenced by the ANS., (© 2011 Wiley Periodicals, Inc.)

Aims: Pulmonary vein antrum isolation (PVAI) is an effective treatment for atrial fibrillation (AF); however, its impact on left atrial (LA) size is unknown. This study evaluates the impact of PVAI on LA size, and whether LA size differs between patients with a successful outcome and patients with AF recurrences., Methods and Results: Seventy-nine patients (76% male, mean age 56 ± 8 years) with symptomatic, drug refractory AF (70% paroxysmal, 30% persistent/permanent) underwent radiofrequency PVAI. Ablation lesions were created encircling right and left pulmonary venous ostia in pairs. The endpoint was complete isolation of all pulmonary veins. Magnetic resonance imaging was performed before and 4 months after PVAI and LA volume was measured by manually tracing the LA area. Clinical follow-up was at 1, 3, 6, 12, and 24 months. Rhythm status was determined by history, electrocardiogram, and 48 h Holter monitoring. After a mean follow-up of 12 ± 5 months, 62 patients (78%) were free of AF (72% without antiarrhythmic drugs). In the total group, LA volume decreased from 104 ± 27 mL to 91 ± 25 mL, P < 0.001. Patients with a successful outcome showed a decrease in LA volume of 103 ± 27 mL to 89 ± 24 mL, P < 0.001. Among patients with AF recurrences, LA volume decreased from 105 ± 29 mL to 95 ± 27 mL, P = 0.012. No significant difference was seen between the change in LA volume in both subgroups, P = 0.27., Conclusion: Pulmonary vein antrum isolation in patients with AF resulted in a significant decrease of LA size. There was no relation between the decrease in LA size and the recurrence of AF after PVAI.

Introduction: There is an obvious need for a better energy source for pulmonary vein (PV) antrum isolation., Objective: We investigated the feasibility and safety of electroporation for the creation of PV ostial lesions., Methods: After transseptal puncture, a custom 7F decapolar 20 mm circular ablation catheter was placed in the PV ostia of 10 pigs. Ablation was performed with a nonarcing, 200 J application delivered between the catheter and an indifferent patch electrode on the lower back. A single pulse was applied for each catheter position, with a maximum of 4 per ostium. Local PV electrogram amplitude and stimulation threshold were measured at multiple locations in both ostia before and directly after ablation, and after 3 weeks survival, using a regular 4 mm mapping catheter. All PV ostia were sectioned, stained, and histologically investigated., Results: The 3-week survival period was uneventful. PV ostial electrogram amplitude decreased and stimulation threshold increased significantly in most ostia. PV angiograms did not show any stenosis during this short follow-up. Histologically, up to 3.5-mm-deep lesions were found., Conclusion: Data suggest that electroporation can safely be used to create lesions in a sensitive environment like PV ostia., (© 2010 Wiley Periodicals, Inc.)

Unlabelled: Left Atrial Volume and Function Assessment. , Introduction: In patients with atrial fibrillation undergoing catheter ablation, magnetic resonance imaging (MRI) can determine left atrial (LA) volume and function before and after ablation. The most accurate, but time consuming, method to determine LA volume is the multiple slice method (MSM), which involves manual tracing of LA area on each slice. The area length method (ALM) offers a simplified, but unvalidated, alternative for LA volume assessment by MRI. The aim of this study was to compare LA volume and function assessment by ALM with MSM., Methods and Results: MRI was performed before and after catheter ablation in 40 patients with atrial fibrillation (30 male, mean age 57 years). All patients had sinus rhythm during imaging. In total, 72 MRI scans were available. LA end-diastolic and end-systolic volumes (EDV, respectively ESV) were measured by both methods. LA function was determined by calculating LA ejection fraction (EF = (EDV-ESV)/EDV). Measured by ALM, mean LA EDV and ESV were significantly lower than using MSM (102 mL and 49 mL vs 111 mL and 65 mL, respectively, P < 0.001) with a larger difference in mean ESV than EDV (16 mL vs 9 mL). This resulted in an overestimation of LA EF by ALM with a mean of 11% (54% by ALM and 42% by MSM, P < 0.001). ALM correlated well with MSM for LA EDV and ESV (r = 0.77, respectively 0.85), and showed no significant difference in intraobserver and interobserver variability., Conclusion: ALM significantly underestimates LA volumes and overestimates LA function, but correlates well with the more accurate MSM., (© 2010 Wiley Periodicals, Inc.)

In atrioventricular nodal and atrioventricular reentrant tachycardia, the relative timing of atrial and ventricular activation may sometimes be very similar, even during electrophysiological studies, and this may lead to an erroneous diagnosis and inappropriate treatment. As examples, we describe two cases that were recently referred to our hospital for a second opinion and treatment of paroxysmal supraventricular tachycardia. In both, the original diagnosis of the referring centres was commontype atrioventricular nodal reentrant tachycardia. Catheter ablation in those centres was unsuccessful. During our electrophysiological studies, however, an atrioventricular reentrant tachycardia was demonstrated, using a concealed accessory pathway for retrograde conduction in both patients. The accessory atrioventricular connection was successfully ablated and on follow-up both patients remained free of symptoms without medication. These findings illustrate the importance of complete electrophysiological analysis even for apparently simple supraventricular arrhythmias. (Neth Heart J 2010;18:78-84.).

The present treatment of atrial fibrillation by radiofrequency catheter ablation requires long continuous lesions in the thin walled left atrium where side effects may lead to serious complications. Better understanding of the physical processes that take place during ablation may help to improve the quality, safety, and outcome of these procedures. These processes include the distribution of power between blood, tissue, and patient; the mechanisms of tissue heating and coagulum formation; the relation between tissue and electrode temperatures; and the effects of increased electrode size and internal and external electrode cooling. With normal electrode-tissue contact, only a fraction of all power is effectively delivered to the tissue. Due to the variability of blood flow cooling, applied power and electrode temperature rise are poor indicators of lesion formation. With a longer electrode, the efficiency of tissue heating is decreased and the greater variation in tissue contact caused by electrode orientation makes lesion formation even more unpredictable. The absence of impedance rise during ablation does not guarantee the absence of blood clot formation on the tissue contact site. Blood clots may unnoticeably be created on the lesion surface and are caused by thermal denaturization of blood proteins, independent of heparinization. Irrigated ablation with external flush may prevent blood clot formation. Irrigation minimally affects lesion size by cooling the tissue surface. Larger lesions may only be created by the application of higher power levels. Electrode cooling, however, impedes electrode temperature feed back and blinds the operator for excessive tissue heating. External cooling alone with preservation of temperature feed back is a promising concept that may lead to improved procedural safety and success.

Aims: A linear lesion between the left inferior pulmonary vein orifice and mitral annulus, the so-called mitral isthmus, may improve the success of catheter ablation for atrial fibrillation. Gaps in the lesion line, however, may facilitate left atrial flutter. The aim of the study was to determine the optimal location of the lesion line by serial sectioning of the isthmus area., Methods and Results: In a post-mortem study of 16 patients with normal left atria, serial sections of the isthmus area from 10 mm superior to and 30 mm inferior to the isthmus were studied by light microscopy. The length of the isthmus was 35+/-7 mm. On average, the muscle sleeve around the coronary sinus ended 10 mm inferior to the isthmus. The prevalence of a ramus circumflexus <5 mm from the endocardial surface, decreased from 60% in the most superior section to 0% in the most inferior section. Atrial arteries were frequently present in all sections., Conclusions: The thickness of atrial myocardium, the ramus circumflexus sometimes very close to the endocardium, a myocardial sleeve around the coronary sinus, and local cooling by atrial arteries and veins may complicate the creation of conduction block in the mitral isthmus.

Introduction: Open flush, irrigated ablation electrodes may improve the safety of radiofrequency catheter ablation by preventing protein aggregation and coagulum formation. This is particularly important in left-sided procedures like catheter ablation of atrial fibrillation. Electrode cooling and the inherent loss of temperature feedback, however, grossly reduce the ability to monitor tissue heating. Intimate contact may not be recognized and the delivery of nominal RF power levels may then lead to excessive tissue heating, steam explosions, and even tamponade., Methods and Results: Standard, open flush, irrigated catheters (Sprinklr, Medtronic Inc, Minneapolis, MN) were modified by thermally insulating the irrigation channels inside the ablation electrode. Using the thigh muscle preparation, multiple lesions were created with standard and modified catheters using 60 s, 20-50 Watt applications and a constant saline flush rate of 20 cc/min. A total of 57 lesions were created on five thigh muscles of three dogs. Lesion dimensions were not significantly different between both types of catheter, but the maximum electrode temperature rise during ablation was significantly higher with the modified catheter. Insulation of the irrigation channels improved the correlation coefficient between maximum electrode temperature rise and lesion volume from 0.38 (ns) to 0.62 (P < 0.001)., Conclusion: Thermal insulation of the irrigation channels facilitates temperature feedback during radiofrequency ablation and controllability of lesion formation.

Aims: In young patients, slow pathway ablation for treatment of atrioventricular nodal reentrant tachycardia (AVNRT) carries a small but definite risk of permanent AV block. The aim was to assess the efficacy of slow pathway ablation aided by the LocaLisa mapping system., Patients and Methods: Radiofrequency (RF) modification of the slow AV nodal pathway was performed in 26 children < 19 years of age (median age 9.8 years, range 3-18.9). Three measures to limit the risk of AV block were applied: (1) use of LocaLisa, a non-fluoroscopic mapping system, to determine and mark the location of the AV node/His bundle axis, and monitor ablation catheter position, (2) continuous atrial stimulation during RF delivery to monitor AV conduction, and (3) gradual increase of RF power during RF ablation., Results: AVNRT was rendered non-inducible in all patients. Dual AV physiology was abolished in 24/26 patients; 2 patients had single atrial echoes at the end of the procedure. At follow-up, AVNRT recurred in 3 patients (including the above 2), necessitating a second procedure. The median number of RF applications was 4 (3-8); median fluoroscopy time was 16 (7-33)min. One patient developed transient second-degree AV block, with full recovery within 6 weeks of the procedure., Conclusions: RF modification of the slow AV nodal pathway in children can be safely accomplished, achieving the ideal end-point of abolishing dual AV physiology, aided by use of the LocaLisa mapping system., (Copyright 2004 The European Society of Cardiology)

The authors hypothesized that during RF ablation, the electrode to tissue interface temperature may significantly exceed electrode temperature in the presence of cooling blood flow and produce thrombus. In 12 anesthetized dogs, the skin over the thigh muscle was incised and raised to form a cradle that was superfused with heparinized canine blood (ACT > 350 s) at 37 degrees C. A 7 Fr, 4-mm or 8-mm ablation electrode containing a thermocouple was held perpendicular to the thigh muscle at 10-g contact weight. Interface temperature was measured at opposite sides of the electrode using tiny optical probes. RF applications (n = 157) were delivered at an electrode temperature of 45 degrees C, 55 degrees C, 65 degrees C, and 75 degrees C for 60 seconds, with or without pulsatile blood flow (150 mL/min). Without blood flow, the interface temperature was similar to the electrode temperature. With blood flow, the interface temperature (side opposite blood flow) was up to 36 degrees C and 57 degrees C higher than the electrode temperature using the 4- and 8-mm electrodes, respectively. After each RF, the cradle was emptied and the electrode and interface were examined. Thrombus developed without impedance rise at an interface temperature as low as 73 degrees C without blood flow and 80 degrees C with blood flow (11/16 RFs at 65 degrees C electrode temperature using 4 mm and 13/13 RFs at an electrode temperature of 55 degrees C using an 8-mm electrode with blood flow). With blood flow, interface temperature markedly exceeded the electrode temperature and the difference was greater with an 8-mm electrode (due to greater electrode cooling). In the presence of blood flow, thrombus occurred without an impedance rise at an electrode temperature as low as 65 degrees C with a 4-mm electrode and 55 degrees C with an 8-mm electrode.

Junctional ectopic tachycardia is a relatively rare disorder, frequently refractory to drug therapy, and with a poor prognosis in childhood. This report describes a successful radiofrequency catheter ablation of the focus of this arrhythmia in a 9-year-old girl with preservation of normal atrioventricular conduction, using precise catheter navigation with the LocaLisa system and carefully titrated RF delivery.

Background: During a catheter ablation procedure for selective electrical isolation of pulmonary vein (PV) ostia, the size of these ostia is usually estimated using fluoroscopic angiography. This measurement may be misleading, however, because only the projected supero/inferior ostium diameters can be measured. In this study, we analyzed 3-dimensional magnetic resonance angiographic (MRA) images to measure the minimal and maximal cross-sectional diameter of PV ostia in relation to the diameter that would have been projected on fluoroscopic angiograms during a catheter ablation procedure., Methods and Results: In 42 patients with idiopathic atrial fibrillation who were scheduled for selective electrical isolation of PV ostia, the minimal and maximal diameters of these ostia were measured from 3-dimensional MRA images. Thereafter, these images were oriented in a 45 degrees right or left anterior oblique direction and the projected diameter of the PV ostia were measured again. The average ratio between maximal and minimal diameter was 1.5+/-0.4 for the left and 1.2+/-0.1 for the right pulmonary vein ostia. Because of the orientation and oval shape of especially the left pulmonary vein ostia, their minimal diameters were significantly smaller than the projected diameters., Conclusion: Pulmonary vein ostia, especially those at the left, are oval with the short axis oriented approximately in the antero/posterior direction. Consequently, PV ostia may sometimes be very narrow despite a rather normal appearance on angiographic images obtained during a catheter ablation procedure.

In pulmonary vein isolation as a treatment for atrial fibrillation the proximal part of the pulmonary veins is catheterized. A protocol for preinterventional assessment of pulmonary vein anatomy was developed, based on contrast-enhanced magnetic resonance angiography (MRA) in combination with three-dimensional visualization to tailor periprocedural angiography. The results allow for assessment of the number, morphology, and location of the ostia of the pulmonary veins, as well as complicating anatomical variations, such as common trunks and aberrant courses.

Inappropriate sinus tachycardia is an unusual arrhythmia that is difficult to treat. To date, catheter ablation has concentrated on modifying the sinus node to attain rate control. We describe a patient where sinoatrial block was created by radiofrequency ablation for the treatment of inappropriate sinus tachycardia.

During RF catheter ablation, local temperature elevation can result in coagulum formation on the ablation electrode, resulting in impedance rise. A recent study has also demonstrated the formation of a so-called soft thrombus during experimental ablations. This deposit poorly adhered to the catheter tip and did not cause an impedance rise. The mechanism of soft thrombus formation and the role of the natural coagulation system are unknown. The formation of a soft thrombus was investigated experimentally by temperature-controlled RF delivery in heparinized blood at different heparin concentrations and in serum. After 60 seconds of RF delivery in blood with an electrode target temperature of 80 degrees C, a semisolidified mass had formed around the ablation electrode at all heparin concentrations. A smaller but structurally similar deposit had formed after RF delivery in serum. Scanning electron microscopy analysis revealed that these deposits consist of denaturized and aggregated proteins, and not of a classical thrombus. The formation of the so-called soft thrombus resultsfrom heat induced protein denaturation and aggregation and occurs independent of heparin concentration and also in serum. The formation of such deposits may occur at temperatures well below 100 degrees C, which may have important consequences for further development of ablation technologies.

Objectives: The objective was to assess the effect ofverapamil on atrial fibrillation (AF) cycle length and spatial dispersion of refractoriness in patients with chronic AF., Background: Previous studies have suggested that verapamil prevents acute remodeling by AF. The effects of verapamil in chronic AF are unknown., Methods: During electrophysiologic study in 15 patients with chronic AF (duration >1 year), 12 unipolar electrograms were recorded from right atrial free wall, right atrial appendage and coronary sinus, along with monophasic action potential recordings from the right atrial appendage. The mean fibrillatory interval at each atrial recording site was used as an index for local refractoriness. Dispersion of refractoriness was calculated as the standard deviation of all local mean fibrillatory intervals expressed as a percentage of the overall mean fibrillatory interval. After baseline measurements, verapamil (0.075 mg/kg intravenous in 10 min) was infused and the measurements were repeated., Results: After administration ofverapamil, mean fibrillatory intervals shortened by a mean of 16.6 +/- 3.3 ms (p < 0.001) at the right free wall, 15.0 +/- 3.5 ms (p < 0.001) at the appendage and 17.1 +/- 3.2 ms (p < 0.01) in the coronary sinus. Monophasic action potential duration decreased by 15.9 +/- 4.0 ms (p < 0.01). Dispersion of refractoriness increased in all patients from 3.8 +/- 0.8 to 5.1 +/- 1.8 (p < 0.001). A strong correlation between mean fibrillatory intervals and action potential duration was found, both before and after verapamil., Conclusions: Verapamil caused shortening of refractoriness and increase in spatial dispersion of refractoriness in patients with chronic AF. This implies that verapamil is not useful in reversing the remodeling process in these patients.

The purpose of this study was to determine the effects of various protective measures on patient and operator radiation dose levels in catheter ablation procedures. Catheter ablation procedures are associated with significant radiation levels. The patient's skin and operator radiation levels were measured (1) at baseline, (2) after primary beam filtration by 0.3-mm copper sheet and 2-mm aluminium plate and implementation of the LocaLisa system, and (3) after reduction of the left anterior oblique fluoroscopic pulse rate and installation of a lead glass screen. Additionally, a comparative analysis of radiation exposure levels was performed in the seven Dutch catheter ablation centers. Filtration of both primary beams resulted in a more than two-fold reduction in patient skin dose. Together with the LocaLisa system, this resulted in a six-fold reduction in patient and operator dose. As expected, lowering of the left anterior oblique pulse rate from 25 to 12.5 Hz reduced the corresponding patient skin dose with a factor 2 while the lead-glass protection caused an extra factor 2 reduction for the operator. Large differences were observed between fluoroscopy systems used for catheter ablation in the Netherlands. Depending on patient body mass and fluoroscopy system, patient skin dose varied between 0.2 and 8.4 Gy/hour. Proper measures may allow for a significant reduction of patient and operator radiation exposure in catheter ablation procedures. The large influence of body mass and equipment on patient's skin dose requires a more direct monitoring of skin dose than total fluoroscopy time.

A 61-year-old woman with dilated cardiomyopathy, who previously underwent successful radiofrequency catheter ablation for atrial flutter, developed monomorphic ventricular tachycardia (VT). The site of VT origin was the inferobasal right ventricle adjacent to the previous atrial isthmus ablation area. The most likely mechanism for the VT was scar-related reentry, the scar being the result of previous radiofrequency lesions in the atrial isthmus. The VT was successfully ablated.

Background: Experimental studies have shown that atrial fibrillation (AF) causes remodeling, which facilitates AF perpetuation. AF may also, however, occur in patients without remodeling and underlying structural cardiac disease. The substrate for enhanced vulnerability in these patients is unknown., Methods and Results: We studied 43 patients without structural heart disease: 18 patients with documented sporadic paroxysmal AF and 25 control patients without AF. In each patient, a decapolar catheter was positioned against the right atrial free wall, and a quadripolar catheter was positioned in the right atrial appendage. Unipolar electrograms were recorded. Atrial vulnerability was assessed according to an increasingly aggressive stimulation protocol. Mean local fibrillatory interval (FI) was used as an index of local refractoriness. Spatial dispersion of refractoriness was assessed through the calculation of the coefficient of dispersion (CD), which was defined as the SD of mean local FI expressed as a percentage of the mean FI. In the AF group, AF was induced with a single extrastimulus in 16 of 18 patients; the CD was 5.4+/-2.6, and the mean FI was 164+/-29 ms. In the control group, AF could be induced only with more aggressive pacing in 23 of the 25 patients; the CD was 1.4+/-0.7 (P<0.0001), and the mean FI was 175+/-26 ms (NS)., Conclusions: Patients with idiopathic AF showed increased dispersion of refractoriness, which may be the substrate for the observed enhanced inducibility and spontaneous occurrence of AF.

Background: Estimation of the 3-dimensional (3D) position of ablation electrodes from fluoroscopic images is inadequate if a systematic lesion pattern is required in the treatment of complex arrhythmogenic substrates., Methods and Results: We developed a new technique for online 3D localization of intracardiac electrodes. Regular catheter electrodes are used as sensors for a high-frequency transthoracic electrical field, which is applied via standard skin electrodes. We investigated localization accuracy within the right atrium, right ventricle, and left ventricle by comparing measured and true interelectrode distances of a decapolar catheter. Long-term stability was analyzed by localization of the most proximal His bundle before and after slow pathway ablation. Electrogram recordings were unaffected by the applied electrical field. Localization data from 3 catheter positions, widely distributed within the right atrium, right ventricle, or left ventricle, were analyzed in 10 patients per group. The relationship between measured and true electrode positions was highly linear, with an average correlation coefficient of 0.996, 0.997, and 0.999 for the right atrium, right ventricle, and left ventricle, respectively. Localization accuracy was better than 2 mm, with an additional scaling error of 8% to 14%. After 2 hours, localization of the proximal His bundle was reproducible within 1.4+/-1.1 mm., Conclusions: This new technique enables accurate and reproducible real-time localization of electrode positions in cardiac mapping and ablation procedures. Its application does not distort the quality of electrograms and can be applied to any electrode catheter.

Estimation of the 3-dimensional (3D) position of ablation electrodes from fluoroscopic images is inadequate in the ablation of complex arrhythmogenic substrates. We developed a new technique for real-time 3D localization of intracardiac electrodes. Regular catheter electrodes are used as sensors for a high-frequency transthoracic electrical field, which is applied via standard skin electrodes. We investigated localization accuracy by comparing measured and true interelectrode distances between the tip and the 10th electrode of a decapolar catheter, and the tip and the 4th electrode of a quadripolar catheter during catheter ablation procedures. Long-term stability was analyzed by localization of the proximal His bundle before and after slow pathway ablation. Accuracy achieved with the 54-mm distance between the two outer electrodes of the decapolar catheters was 101% +/- 15%, 95% +/- 10%, and 97% +/- 8% in the right atrium, right ventricle, and left ventricle, respectively. During catheter ablation procedures, the measured distance between the tip and 4th electrode of the mapping catheter was 100% +/- 15% in atrial flutter, 100% +/- 12% in slow pathway ablation, and 100% +/- 14% in ablations for left ventricular tachycardia. After 2 hours, localization of the proximal His bundle was reproducible within 1.4 +/- 1.1 mm. The LocaLisa technique allows for reproducible, real-time nonfluoroscopic 3D visualization of standard mapping and ablation catheters and is sufficiently accurate for the creation of linear radiofrequency lesions. The freedom of catheter choice makes the LocaLisa system an invaluable tool in catheter mapping and ablation procedures.

Introduction: Stable electrode-tissue contact is crucial for successful radiofrequency ablation of cardiac tachyarrhythmias. In this in vitro study, a custom-made radiofrequency generator was used to evaluate the correlation between tip temperature response to a minimal radiofrequency power delivery (Low Energy Temperature Response: LETR-Principle) and electrode-tissue contact as well as lesion size., Methods and Results: A battery-powered radiofrequency generator (LETR-Box, 500 kHz, 0.1 to 0.3 W) could measure the temperature increase at the tip electrode with 0.01 degrees C accuracy. The device was tested in vitro using isolated porcine ventricular tissue. For various electrode-tissue settings (i.e., 0 to 0.89 N contact force), the temperature increase (deltaT) due to 0.1-W power delivery for 10 seconds was recorded. Subsequently, for the same electrode-tissue contact, a temperature-controlled radiofrequency ablation was performed (70 degrees C target temperature, 50-W maximum output, 30 sec). Thereafter, the lesion size was measured histologically. To prove the safety of the applied LETR-Principle, the tissue was inspected microscopically after continuous radiofrequency power delivery of 0.3 W for 1 hour with high contact pressure (1.33 N). The delivery of 0.1-W radiofrequency power resulted in an average deltaT of 0.18 degrees +/- 0.13 degrees C. During temperature-controlled radiofrequency ablation, the tip temperature was 59 degrees +/- 8.5 degrees C, resulting in a lesion depth of 4.8+/-0.6 mm. The correlation coefficient between deltaT and contact force was 0.97 and 0.81, respectively, for lesion depth. No lesion was microscopically visible after power delivery of 0.3 W for 1 hour with 1.33 N contact pressure., Conclusion: The LETR-Principle safely indicates electrode-tissue contact and lesion depth under in vitro conditions and can be useful for catheter positioning during radiofrequency ablation procedures.

Background: Clinical efficacy has driven the use of larger electrodes (7F, length > or =4 mm) for radiofrequency ablation, which reduces electrogram resolution and causes variability in tissue contact depending on electrode orientation. With active cooling, ablation electrode size may be reduced. The purpose of this study was to examine the effect of electrode length on tissue temperature and lesion size with saline irrigation used for active cooling., Methods and Results: In 11 anesthetized dogs, the thigh muscle was exposed and bathed with heparinized canine blood. A 7F ablation catheter with a 2- or 5-mm irrigated tip electrode was positioned perpendicular or parallel to the thigh muscle. Radiofrequency current was delivered at constant voltage (50 V) for 30 seconds during saline irrigation (20 mL/min) to 148 sites. Tissue temperature at depths of 3.5 and 7 mm and lesion size were measured. In the perpendicular electrode-tissue orientation, radiofrequency applications at 50 V with the 2-mm electrode compared with the 5-mm electrode resulted in lower power at 50 V (26 versus 36 W) but higher tissue temperatures, larger lesion depth (8.0 versus 5.4 mm), and greater diameter (12.4 mm versus 8.4 mm). Also, in the parallel orientation, overall power was lower with the 2-mm electrode (25 versus 33 W), but tissue temperatures were higher and lesions were deeper (7.3 versus 6.9 mm). Lesion diameter was similar (11.1 versus 11.3 mm) for both electrodes., Conclusions: The smaller electrode resulted in transmission of a greater fraction of the radiofrequency power to the tissue and resulted in higher tissue temperature, larger lesions, and lower dependency of lesion size on the electrode orientation.

Introduction: The application of high-frequency current to the AV junctional area results in a temperature rise in the myocardium and may cause accelerated junctional rhythm (AJR). The aim of the study was to characterize heat-induced AJR in an in vitro animal model., Methods and Results: Studies were performed in isolated perfused pig and rabbit hearts. Using a small heating probe, we could induce AJR from a discrete area located in the middle of the triangle of Koch, which was smaller than the area from which RF energy application could elicit AJR. Histology showed that the heat-sensitive area was located over, or close to, the compact AV node. It did not correspond with the areas where double potentials were found or with the site(s) of earliest atrial activation during VA conduction. Microelectrode recordings revealed that AJR arose in nodal-type cells. Heat increased the slope of the phase 4 depolarization and shortened the action potential duration. Two types of AJR were observed: the first one was regular and the second one showed irregularity in the intervals. Interaction of multiple foci and the presence of conduction block between the foci and the His bundle caused the irregularity of the His-His intervals during the second type of AJR., Conclusion: AJR observed during heat and RF application in the AV nodal area results from the effect of heat on AV nodal cells with underlying pacemaker activity. The heat-sensitive area is located over, or very close to, the compact AV node.

Introduction: Increasing electrode size allows an increase in radiofrequency lesion depth. The purpose of this study was to examine the roles of added electrode cooling and electrode-tissue interface area in producing deeper lesions., Methods and Results: In 10 dogs, the thigh muscle was exposed and superfused with heparinized blood. An 8-French catheter with 4- or 8-mm tip electrode was positioned against the muscle with a blood flow of 350 mL/min directed around the electrode. Radiofrequency current was delivered using four methods: (1) electrode perpendicular to the muscle, using variable voltage to maintain the electrode-tissue interface temperature at 60 degrees C; (2) same except the surrounding blood was stationary; (3) perpendicular electrode position, maintaining tissue temperature (3.5-mm depth) at 90 degrees C; and (4) electrode parallel to the muscle, maintaining tissue temperature at 90 degrees C. Electrode-tissue interface temperature, tissue temperature (3.5- and 7.0-mm depths), and lesion size were compared between the 4- and 8-mm electrodes in each method. In Methods 1 and 2, the tissue temperatures and lesion depth were greater with the 8-mm electrode. These differences were smaller without blood flow, suggesting the improved convective cooling of the larger electrode resulted in greater power delivered to the tissue at the same electrode-tissue interface temperature. In Method 3 (same tissue current density), the electrode-tissue interface temperature was significantly lower with the 8-mm electrode. With parallel orientation and same tissue temperature at 3.5-mm depth (Method 4), the tissue temperature at 7.0-mm depth and lesion depth were greater with the 8-mm electrode, suggesting increased conductive heating due to larger volume of resistive heating because of the larger electrode-tissue interface area., Conclusion: With a larger electrode, both increased cooling and increased electrode-tissue interface area increase volume of resistive heating and lesion depth.

Objectives: We sought to quantify the effects of electrode-target distance and intracavitary blood flow on radiofrequency (RF) power required to induce transient conduction block, using a Langendorff-perfused canine ablation model., Background: Given the thermally mediated nature of RF catheter ablation, cooling effects of intracavitary blood flow and electrode-target distance will influence lesion extension and geometry and electrophysiologic effects., Methods: In eight Langendorff-perfused canine hearts, the right ventricular free wall was opened, and the right bundle branch (RBB) carefully localized by multielectrode activation mapping. The right atrium was paced at cycle length of 500 ms. Proximal and distal electrodes were attached at the endocardial aspect of the RBB, and the perfused heart was submerged in heparinized blood at 37 degrees C. A standard 4-mm tip ablation electrode was positioned at a constant contact pressure of 5 g between the two electrodes at the site of maximal RBB potential (0 mm) and 2 and 4 mm distant from this site along a line perpendicular to the RBB. RF pulses (500 kHz) were delivered for 30 s at 0.5-W increments until transient bundle branch block. In four hearts, intracavitary flow was simulated by directing a 30-cm/s jet of blood parallel to the septum at the ablation site, and the protocol was repeated to assess the effects on power required for block. In one heart, the effect of variable flow was assessed (0, 15 and 30 cm/s)., Results: An exponential distance-related increase was seen in power required for block, from 1.8 +/- 0.9 W (mean +/- SD) at 0 mm to 5.4 +/- 1.1 W at 4 mm. In the presence of 30-cm/s flow, an increase to 3.9 +/- 0.8 W at 0 mm and 13.1 +/- 2.4 W at 2 mm was seen. At 4 mm, coagulum formation invariably occurred before block could be induced. For 15-cm/s flow, less power was required: 3 and 7 W at 0 and 2 mm, respectively., Conclusions: Increasing the ablation electrode-target distance causes an exponential increase in power required for conduction block; this relation is profoundly influenced by intracavitary flow. Given the geometry of endomyocardial RF lesions, these findings are particularly relevant for directly subendocardial ablation targets.

Background: Progression of unintentionally induced atrioventricular delay is occasionally observed directly after termination of radiofrequency delivery in the vicinity of the atrioventricular node. We postulated that the application of a radiofrequency pulse may result in a tissue temperature rise that continues after the pulse., Methods and Results: Using the thigh muscle preparation, 5-, 10-, 20-, and 30-second pulses were applied as 30 to 40 W via a standard 4-mm tip electrode with 10-g contact pressure. Forty-one undisturbed pulses were delivered while recording intramural temperatures at 2-, 4-, and 7-mm depth. Maximal "thermal latency" was observed with the shortest pulse duration and at greatest depth. With 5-second applications, tissue temperature at 7-mm depth peaked 11.6 seconds after termination of radiofrequency delivery and stayed above end-of-pulse value as long as 34.5 seconds after the pulse. The additional rise in tissue temperature was 2.9 degrees C. If only recording within the lesion border zone were considered, the duration of latency was maximal with 10-second pulses: an additional gain in tissue temperature of 3.4 degrees C was observed 6.4 seconds after the pulse while tissue temperature stayed above end-of-pulse value during 18.3 seconds., Conclusions: With relatively short applications, tissue temperature continues to rise after termination of radiofrequency delivery. This "thermal latency" may result in lesion growth after the pulse and may so explain the incidentally observed progression of conduction block after short pulses in the vicinity of the atrioventricular node. It also may explain the apparent discrepancy between lesion growth rate and intramural temperature rise studies.

Introduction: Although it is generally accepted that radiofrequency (RF) ablation causes exclusively thermally mediated effects, it has never been proved., Methods and Results: In a previous report, temperatures required to induce conduction block in superfused canine epicardial ventricular myocardium were identified by exposure to heated superfusate: 50.3 degrees +/- 1.1 degrees C and 53.6 degrees +/- 0.6 degree C for transient and permanent block, respectively. In the present study, heating was performed using RF power in an otherwise identical model. Nine preparations from four dogs were used. A 1-cm diameter electrode was placed beneath the center of each preparation for RF delivery. Incisions were made to create a conductive isthmus over the ablation electrode. Preparations were paced to one side of the isthmus and electrograms recorded from the center of the isthmus and to either side. Temperature was measured using a miniature thermocouple located just below the epicardial surface, adjacent to the recording electrode in the heated zone. RF was delivered for 30 seconds at 5-minute intervals with increments in power per episode causing increments in temperature of approximately 2 degrees C. Temperature during pulses at which transient block occurred was 50.7 degrees +/- 3.0 degrees C; temperature at 30 seconds of heating in pulses leading to permanent block was 58.0 degrees +/- 3.4 degrees C., Conclusion: These findings provide evidence suggesting that the electrophysiologic effects of RF ablation are exclusively thermally mediated and are otherwise unrelated to the dissipation of high-frequency current.

The tension strength of mandibular halves of beagle dogs, with two different types of segmental resections, was studied. It was found that the radius of the posterior resection corner is of utmost importance in reducing the risk of mandibular fractures.

Objectives: The goal of the study was to quantify the response of myocardial impulse propagation to hyperthermia and identify the temperatures required for transient and permanent block in conduction., Background: Although it is generally accepted that the effects of radiofrequency ablation are thermally mediated, the precise response of myocardial impulse conduction to heating remains to be quantified., Methods: Twenty-three preparations of ventricular myocardium from 10 beagle dogs were superfused at 36.5 to 37.5 degrees C and paced at a cycle length of 600 ms. Heating was performed for 30 s at 5-min intervals by an independent flow of heated superfusate. A 16-electrode grid was used to record extracellular electrograms directly before each heating episode (control value) and at 10, 20 and 30 s., Results: Between 38.5 and 45.4 degrees C, conduction velocity was higher than that at the directly preceding control value (p < 0.05), reaching a maximum of 114% between 41.5 and 42.5 degrees C. Above 45.4 degrees C, a gradual decrease occurred, with transient block (absence of impulse conduction for < or = 5 min) after heating to 49.5 to 51.5 degrees C. This was followed by tachycardia in 69% of all cases immediately after cessation of heating. Permanent block occurred after a significantly higher temperature of 51.7 to 54.4 degrees C had been reached. Pacing at sites allowing preferential conduction either parallel or perpendicular to fiber orientation caused no difference in reaction to heating. Repeated heating of some preparations to 47.0 to 50.5 degrees C revealed no cumulative effects on conduction velocity., Conclusions: Transient and permanent block in impulse conduction occurred at 49.5 to 51.5 degrees C and 51.7 to 54.4 degrees C, respectively, in superfused canine myocardium, the former frequently being followed directly by tachycardia. Reaction of conduction velocity to hyperthermia was independent of myocardial fiber orientation and number of preceding heating episodes. Results may contribute to a better understanding of electrophysiologic phenomena observed during radiofrequency ablation procedures.

Objective: To monitor atrioventricular conduction after radiofrequency ablation for atrioventricular nodal re-entry tachycardia., Design: Measurement of PR interval from 12 lead surface electrocardiograms before; at 0, 24, 48, 72, and 96 hours; and at 1 and 6 months after radiofrequency ablation., Patients: 40 consecutive patients with atrioventricular nodal re-entry tachycardia. The anterior approach was used in 23 patients, the posterior approach in 17., Results: With the anterior approach the PR interval increased significantly and progressively until 48 hours after ablation (maximum 282 (SD 62.2) ms, before ablation 142 (29.5) ms; P < 0.0001). Up to 96 hours no further change was observed, but one month after ablation the PR interval had decreased to a value not significantly different from that 24 hours after the procedure (231 (51.2) ms). In one patient total atrioventricular block developed 24 hours after an uncomplicated procedure and a permanent pacemaker was implanted. With the posterior approach the PR interval increased slightly in the first 24 hours (156 (22.7) ms, before ablation 144 (21.2) ms P = 0.004), but it had returned to preablation values at 1 month. One patient developed second degree atrioventricular block during the first 24 hours after ablation, despite delivery of all radiofrequency pulses posterior to Koch's triangle at sites without His bundle deflection. PR intervals at 6 months did not differ significantly from the values at 1 month., Conclusion: After the anterior approach the progressive delay in atrioventricular conduction up to 48 hours after radiofrequency ablation for atrioventricular nodal re-entry tachycardia warrants continuous in hospital monitoring of patients for at least two days after the procedure.

During radiofrequency catheter ablation, steady-state electrode-tissue interface temperatures are reached within 5 seconds. Within the myocardium, however, a much slower temperature rise has been observed in vitro with stabilization after approximately 2 minutes. The discrepancy suggests that tissue temperature rise time depends on distance from the ablation electrode and, thus, that temperature rise measured at the electrode-tissue interface does not correspond with temperature rise within the myocardium. In five beagles, closed-chest radiofrequency catheter ablation was performed in the vicinity of intramural thermocouples. Sequences of 60 seconds, 10- and 25-watt pulses were delivered in the unipolar mode via the 4-mm distal electrode of a 7 French steerable catheter. At all distances > 3 mm from the ablation electrode, the rate of myocardial temperature rise was low: relative rise after 5, 10, 20, and 30 seconds was 22%, 32%, 48%, and 63% of that achieved at 60 seconds, and even then steady-state temperatures had not yet been reached. Temperature rise was faster at sites closer to the ablation electrode. There was no difference in rate of rise between first and second pulses at the same site. A 6% higher myocardial temperature was reached with a second identical pulse at the same site. Tissue temperatures achieved with 25 watts were 2.4 times higher than with a preceding 10-watt pulse at the same ablation site.

While radiofrequency catheter ablation has proved highly effective in the treatment of various supraventricular tachyarrhythmias, results in the treatment of ventricular tachycardia invite improvement. Knowledge of lesion growth in vivo might improve understanding of this discrepancy. So far only information from in vitro and in vivo studies using a small 2 mm tip electrode is available. Growth of ventricular radiofrequency lesions created with a 4 mm ablation electrode was studied in 11 closed-chest dogs. Endocardial ablations were performed at 31 left and 15 right ventricular sites at a power setting of 25 Watts and 5, 10, 20, 30 or 60 seconds pulse duration. Macroscopic and histopathologic lesion examination were performed after one week survival. Mean lesion volume increased from 52 mm3 after 5 seconds pulse duration to a maximum 388 mm3 and approximately 7 mm depth after 30 seconds. Lesions were prolate spheroid in form, with a sparing of subendocardial myocardium and maximum lesion diameter at some millimeters depth. Results indicate that catheter positioning at no more than 7 mm from the target is required for successful ablation. Due to lesion geometry, subendocardial targets demand even more exact catheter positioning, while subepicardial substrates may not be ammenable to ablation if ventricular wall thickness exceeds 7 mm at the ablation site. Repeated pulses at adjacent sites may be required for ablation of extended arrhythmogenic areas. Volume at 5 seconds was only approximately 15% of mature lesions. Therefore, the use of a short 'test pulse' after careful mapping may be useful to pinpoint the most appropriate site for ablation in discrete pathways.