Histopathology and transmurality of acute microwave lesions on the beating human atrium

Microwave energy allows thoracoscopic beating-heart ablation for the treatment of atrial fibrillation. However, there is a paucity of data on the histologic effects of microwave energy on the beating human heart. This study aims to histopathologically characterize microwave lesions on the beating human atrium. Microwave energy was applied prior to cardiectomy on the beating native right atrium in eight patients undergoing heart transplantation and as a circumferential left atrial 'box' lesion in one patient undergoing heart-lung transplantation. Lesions were applied following heparinization and cannulation, but before initiation of cardiopulmonary bypass. Following cardiectomy, specimens were resected, fixed and subjected to histologic preparation. Grossly, all atrial lesions were 'comma-shaped' with an area of maximum injury on the surface. Microscopically, myocyte injury manifested as acute coagulation necrosis with hypereosinophilic myocytes with both nuclear loss and pyknosis. Contraction bands were noted at the periphery of lesions. The injury was transmural in all right atrial lesions. The left atrial sample contained a circumferential lesion ranging from 0.1 to 0.8 cm in width. The cut edge demonstrated lesion depths of 0.2-0.6 cm, maximum (transmural) in the inferior margin. Microwave ablation represents an acceptable energy source to create characteristic lesions on the beating human atrium.     

Clinical histopathology and ultrastructural analysis of myocardium following microwave energy ablation.

OBJECTIVE: Due to weaknesses of conventional modes for treating atrial fibrillation (AF), surgical energy ablation methods and tools to cure AF have been under rapid development. One of these methods, microwave energy, is beginning to be applied clinically. The purpose of this study was to examine histology and ultrastructure of lesions produced by microwave energy in the myocardium. METHODS: Fifteen consecutive patients underwent surgical microwave energy ablation (Microwave Ablation System with FLEX 4 probe, AFx Inc., Fremont, CA) concomitant to a valve procedure. Epicardial ablation was carried out on the beating normothermic heart prior to performing the valve procedure. Two tissue specimens (1cm(2)) were obtained from each patient; one from the lesion site (right appendage) and the other from an adjacent, non-ablated site, which was used as control. Tissue samples were fixed and stained as appropriate for histological and ultrastructural analysis. RESULTS: All ablated samples revealed observable microscopic alteration, including loss of nuclei, foci of coagulative necrosis or induced irregular bands of contraction. Ultrastructurally, ablated cells demonstrated architectural disarray, loss of contractile filaments, mitochondrial swelling and focal interruption of plasma membrane. CONCLUSIONS: Histologic appearance of lesions created by epicardial microwave energy ablation was consistent over tissue samples, although acute findings demonstrated differences from cryoablation. In most of the cases, lesions were transmural, as was demonstrated by loss of cellular viability throughout the depth of tissue specimens.     

Comparison of epicardial and endocardial linear ablation using handheld probes

BACKGROUND: The optimal technique for producing linear radiofrequency thermal lesions in myocardial tissue is unclear. We compared epicardial ablation on the beating heart with endocardial ablation after cardioplegia. METHODS: Radiofrequency lesions were produced using a multielectrode malleable handheld probe in ovine myocardium with three wall thicknesses. Detailed analysis of lesion dimensions was used to assess the effects of site of ablation, muscle thickness, and duration of ablation. RESULTS: After epicardial atrial ablation, myocardial lesions were detected in all sections without macroscopically visible epicardial fat (n = 10), but only 43% (6/14) of sections with epicardial fat. Three of 24 atrial epicardial sections (13%) and 92% (23/25) of endocardial atrial lesion sections were clearly transmural. In thicker tissues lesion depth was independent of endocardial (right ventricle: 3.9 +/- 1.1 mm, left ventricle: 3.8 +/- 0.7 mm) or epicardial (right ventricle: 3.4 +/- 0.6 mm, left ventricle: 4.3 +/- 0.9 mm) ablation site. Epicardial lesions are less deep in thinner areas of myocardium (p = 0.003). Lesions were all wider than they were deep. There was no significant increase in lesion depth with the increase in ablation duration from 1 to 2 minutes. CONCLUSIONS: Lesions were unlikely to be transmural with either technique when the wall thickness was greater than about 4 mm. Epicardial fat has an important negative effect on epicardial lesion formation. Where epicardial fat is absent epicardially produced lesions penetrate less deeply when the wall thickness is small, possibly due to endocardial cooling by circulating blood. Prolongation of the duration of ablation from 1 to 2 minutes does not significantly increase lesion depth.     

Comparison of bipolar and unipolar radiofrequency ablation in an in vivo experimental model.

OBJECTIVE: Linear atrial radiofrequency lesions have been used effectively for the treatment of atrial fibrillation. In most cases an endocardial approach has been suggested. A method for epicardial placement of lesions would reduce the complexity of these procedures. We compared lesions created in ovine hearts in vivo using irrigated bipolar or unipolar handheld radiofrequency ablation devices. METHODS: Radiofrequency lesions were produced around a left pulmonary vein, around the left atrial appendage and in the free wall of the right ventricle in ovine hearts. All lesions were created in the beating heart. A bipolar clamping device (n = 7) or a handheld unipolar device (n = 6) was used. Measurements of local electrograms and pacing thresholds were performed before and after ablation at each site to assess the electrical integrity of lesions. Tetrazolium and digital image analysis were used to assess lesion geometry. Results: In atrial tissue continuous transmural lesions were achieved more often with the bipolar than with the unipolar device (92.3 vs. 33.3%, P < 0.02). In atrial tissue the reduction in signal amplitude caused by the lesions was significantly larger with the bipolar than the unipolar device (87.6+/-9.4% vs. 60.6+/-23.7% reduction, P < 0.01). There was a significant relationship between loss of pacing capture and lesion transmurality (P < 0.05). The bipolar device created narrower lesions than the unipolar device (4.1+/-0.9 mm vs. 5.9+/-2.1 mm, P < 0.001). CONCLUSIONS: The bipolar clamping device produces narrower lesions which are more likely to be transmural and lead to electrical isolation of ablated tissue than those produced by the unipolar device. However, both devices failed to consistently produce transmural lesions using the epicardial beating heart technique studied, particularly in thicker tissues. High output pacing within the ablated tissue partially predicts lesion transmurality and be a guide to the need for further ablation. However, endocardial ablation or transmural bipolar ablation are likely to remain the techniques of choice for linear radiofrequency ablation in the atria until improved techniques are developed.     

Electrophysiological and Histological Assessment of Transmurality after Epicardial Ablation Using Unipolar Radiofrequency Energy

Abstract   Background: The transmurality of the ablation lesions created on a beating heart has not been examined histologically or electrophysiologically. This study aimed to assess the feasibility of an atrial epicardial or endocardial ablation in an off-pump setting using unipolar radiofrequency (RF) energy. Methods: A linear ablation lesion of 5 cm was made in the lateral left atrium using unipolar RF energy with a temperature-controlled algorithm in 16 canines either epicardially (n = 8) or endocardially (n = 8) on the beating heart without any cardiopulmonary bypass. The ablation depth and transmurality were examined histologically two hours after the ablation. A conduction block across the linear ablation lesion was tested by epicardial mapping in two animals four weeks after each epicardial and endocardial ablation. Results: There was no significant difference in the ablation depth between the epicardial and endocardial ablations (2.5 ± 0.7 mm vs. 3.0 ± 1.4 mm, p = 0.055) in the histological examination. However, the ablation lesion was transmural in only 14 of 30 (46.7%) evaluated points after the epicardial ablation, while in 28 of 30 (93.3%) after the endocardial ablation (p < 0.0001). The thin atrial tissue adjacent to the endocardium survived after the epicardial ablation. The activation maps demonstrated a complete linear conduction block in all animals after the endocardial ablation, but in none after the epicardial ablation. Conclusions: Epicardial unipolar radiofrequency ablation on the beating heart does not necessarily create a complete linear conduction block. An alternative ablation device that creates a transmural lesion is needed, and intraoperative electrophysiologic assessment of the lesion should be crucial in off-pump AF surgery.     

Chronic transmural atrial ablation by using bipolar radiofrequency energy on the beating heart

BACKGROUND: Recently, there has been renewed interest in the development of minimally invasive procedures to treat atrial fibrillation. Unipolar radiofrequency catheters are plagued by poor results, in part because of their inability to produce transmural lesions. This study tested the ability of bipolar radiofrequency energy to create chronic transmural lesions on the beating heart that isolated atrial myocardium. METHODS: Five sheep underwent a right thoracotomy. Baseline pacing was performed from the following targeted areas: right atrial appendage, superior vena cava, inferior vena cava, and right pulmonary veins. A cuff of atrial myocardium around the targeted tissue was clamped between the 2 arms of the device. Radiofrequency energy was delivered at 750 mA and continued until the tissue conductance between the electrodes reached a stable minimum level. After ablation, pacing was used to document tissue isolation. The animals survived for 30 days. RESULTS: Twenty circumferential lesions were produced at the initial operation. The mean ablation time was 9.3 +/- 4.0 seconds, and the mean peak temperature was 48.4 degrees C +/- 6.4 degrees C. All lesions acutely and chronically isolated the targeted tissue. Trichrome staining showed that all lesions were transmural. There were no instances of pulmonary vein stenosis or thrombosis. CONCLUSIONS: Bipolar radiofrequency energy can produce permanent transmural linear lesions on the beating heart. Online measurement of tissue conductance reliably predicted lesion transmurality. This new technology may enable surgeons to perform a curative minimally invasive operation for atrial fibrillation on the beating heart.     

Support adapter for radiofrequency ablation probe: experimental study

BACKGROUND: The purpose of the present study was to experimentally evaluate a new support adapter, which was created in order to insure a more complete contact between the ablation probe and a beating heart. METHODS: Support sleeve adapters for the Cobra Cooled probe were fashioned using a semi-rigid plastic. Forty epicardial radiofrequency ablation (RFA) lesions were created on both atria under the condition of a beating heart in six pigs weighing 26-30 kg. A Cobra Cooled surgical probe was used with continuous internal irrigation of a saline solution (500 mL/h). The ablation temperature was fixed at 80 degrees C and the duration of the RFA in each case was 20, 30 and 60 s. RESULTS: A support adapter ensured a more complete epicardial contact between the probe and a beating heart. An RFA probe supported by the adapter delivered greater power to the tissue. The histopathological transmural coagulation was obtained in 82% and 72% in RFA lesions with and without the adaptor, respectively. CONCLUSION: A new adapter was effective in the epicardial RFA under the condition of a beating heart.     

Physiological consequences of bipolar radiofrequency energy on the atria and pulmonary veins: a chronic animal study

BACKGROUND: Alternative energy sources have been proposed for the transvenous and surgical treatment of atrial fibrillation. This study examined the physiologic consequences of a novel energy source, bipolar radiofrequency energy, in a chronic animal model in order to determine its ability to produce transmural lesions on the beating heart. METHODS: Five dogs underwent baseline pacing from the following target areas: right and left atrial appendage, superior and inferior vena cavae, and right and left pulmonary veins. A cuff of atrial myocardium, proximal to the target tissue was clamped and ablated between the arms of the bipolar radiofrequency energy device. Tissue conductance was used as a transmural indicator. After ablation, the pacing protocol was repeated. Baseline and postablation pulmonary vein flows were measured. Animals were survived for 30 days, and permanent electrical isolation was evaluated by pacing, epicardial mapping, and histology. RESULTS: Mean ablation time was 5.0 +/- 1.8 seconds and mean peak tissue temperature was 46.7 degrees C +/- 2.8 degrees C. All lesions (30/30) acutely and permanently isolated atrial tissue. There was no change in pulmonary vein flow. Mapping studies with pacing of atrial tissue on both sides of the lesion confirmed isolation. Histology demonstrated that all lesions were linear, continuous, and transmural with no thrombus formation or stenosis. CONCLUSIONS: Bipolar radiofrequency energy rapidly produced permanent transmural linear lesions on the beating heart. Measurement of tissue conductance reliably predicted transmural lesions. This new technology may enable the development of a less invasive, surgical approach to atrial fibrillation.     

Application of microwave energy in cardiac tissue ablation: from in vitro analyses to clinical use

BACKGROUND: Recently, there has been renewed interest in simplifying a surgical cure for atrial fibrillation. Microwave energy ablation provides an easier and faster surgical technique, in lieu of traditional "cut and sew," which can also be applied on the beating heart. METHODS: Specific absorption rate (SAR) and thermal profiles were determined after microwave energy ablation intended for clinical application. Lesion geometry measures obtained in vitro with tissue equivalent media were compared with values collected from animal myocardium. RESULTS: Thermal profile analysis demonstrated lesion penetration depths of 2.5, 3, and 5 mm after 8, 16, and 24 seconds, respectively, with microwave power application of 40 W. Dosimetric studies performed using animal myocardium corresponded to the thermal profile analysis and showed that lesion depth was controlled by the output power and the ablation time. CONCLUSIONS: Lesion geometry in tissue using microwave energy is similar to that predicted from in vitro analysis. The ablation depths and thermal profile of microwave ablation is favorable for performing atrial ablation, and this is corroborated by favorable early clinical results.     

Effect of epicardial fat on ablation performance: a three-energy source comparison

OBJECTIVES: To investigate the effect of epicardial fat on surgical atrial fibrillation ablation performance using an in vitro model. Two tissue models were employed to investigate standard penetration and maximal lesion depth performance of bipolar radiofrequency (RF), microwave, and laser energy sources. METHODS: Ventricular veal tissue was utilized in various thicknesses (3 mm, 5 mm, 7 mm, 15 mm). Epicardial fat was modeled by layering porcine fat (1 mm, 2 mm and 4 mm) on moistened tissue. In each group, 8 to 10 lesions were created. Post ablation, tissue samples were sectioned and ablation depth of each myocardial section measured using 1% tetrazolium tetrachloride dye solution. RESULTS: The laser energy source produced nearly 100% transmural lesions in almost all study groups irrespective of myocardium thickness and fat thickness. The microwave device maintained transmurality in all 3-mm and most 5-mm myocardium trials but fell to near zero with all 7-mm myocardium trials. The bipolar RF maintained transmurality only when no fat was applied. In the maximal lesion depth models, the laser was capable of producing lesions >8 mm with no fat and >6 mm with either 2 mm or 4 mm of fat present. The microwave produced lesions in the no fat (>6 mm) and 2-mm (>4 mm) fat group. The bipolar RF produced 83% transmurality with no fat and zero percent transmurality with 2 mm of fat present. CONCLUSIONS: Epicardial fat can severely limit transmurality in energy sources that utilize conductive heating. Laser energy was uniformly superior at producing both transmural and deep lesions irrespective of the presence of fat.     

Chronic histological transmurality of high-intensity focused ultrasound ablation

We evaluated histologic results for surgical left maze with an high-intensity focused ultrasound (HIFU) energy source. Two patients came to our attention 6 and 48 months, respectively, after ablation concomitant to a valve procedure. Tissue specimens, obtained from the lesion site on the mitral isthmus and from the "box lesion" around the pulmonary veins were analyzed histologically. A complete transmural lesion was found in all specimens. Chronic lesions exhibited replacement of the muscular band with connective tissue. The atrial wall maintained normal thickness and vascularization. HIFU ablation represents an acceptable energy source to create transmural lesions on the beating human left atrium.     

A new device for beating heart bipolar radiofrequency atrial ablation

OBJECTIVE: A technique for mimicking left atrial atriotomies using an ablation device that can be deployed without cardiopulmonary bypass has been developed. METHODS: In 12 healthy large (35-50 kg) adult pigs, maze-like ablation lesions were directly applied to the left atrial epicardium on the beating heart. The ablation device is irrigated, with a bipolar "hemostat" morphology, utilizing radiofrequency energy. Prior to and after ablation, left atrial electromechanical properties were measured during sinus rhythm in the latest 5 pigs using percutaneous endocardial catheter electromechanical mapping and intracardiac echocardiography. Pathologic analysis was performed acutely. RESULTS: All ablation lesions demonstrated conduction block along their entire course. Global left atrial conduction time (49.4 +/- 8.8 milliseconds before vs 58.8 +/- 9 milliseconds after) and pattern were not significantly altered. Although a significant amount (17.12% +/- 9%) of myocardium was either ablated or electrically isolated, ablation was not associated with significant alterations in global left atrial mechanics (left atrium ejection fraction 19% before vs 17% after; pulmonary vein peak flow velocity 1.22 m/s before vs 1.38 m/s after; peak mitral inflow velocity 2.34 m/s before vs 2.64 m/s after), mitral valve function, nor left ventricular function. There was no evidence of atrial thrombus formation. Transmurality was achieved in most lesions with no evidence of charring or barotrauma. CONCLUSIONS: Utilizing this ablation device, atrial lesions similar to the left component of the Maze procedure were deployed with uniform success in a beating heart without cardiopulmonary bypass or atriotomy and without adverse effects on left atrial electromechanics.     

Surgical treatment of ventricular tachycardia by balloon electric shock ablation. Potential effects on the mitral valve apparatus.

We have previously described a new surgical technique for control of arrhythmogenic foci in patients with recurrent ventricular tachycardia that we call balloon electric shock ablation. With this method sequential shocks are delivered to a grid of electrodes on a balloon that can be introduced across the mitral valve into the intact ventricle. A series of experiments was undertaken to investigate possible deleterious effects of balloon electric shock ablation when shocks are delivered directly to the mitral valve apparatus. In six animals shocks totaling 1200 joules were given through a closely spaced electrode grid applied to the area of the mitral valve. Nine to 12 weeks later, left ventricular and mitral valve function were assessed. Balloon electric shock ablation in the basilar portion of the ventricle was associated with decreased myocardial performance, as evidenced by ejection phase indices. In five of six animals balloon ablation led to minor thickening of the valve leaflets and chordal attachments plus necrosis of adjacent myocardium, including papillary muscles. In these animals there was no significant dysfunction of the valve observed. In the remaining animal, however, ablation was centered on the posterior papillary muscle and resulted not only in necrosis of the base of the papillary muscle but also in full-thickness scarring and thinning of the adjacent left ventricular wall. In this dog, mitral regurgitation was seen on long-term follow-up. We conclude that when balloon electric shock ablation is used to destroy a localized area of myocardium in the basilar portion of the intact ventricle, the procedure results in decreased myocardial performance. When shocks were directly applied to the mitral valve apparatus in five of six animals, ablation did not result in significant negative effects on the structure and function of the valve. In the sixth dog, however, shock delivery resulted in transmural necrosis and thinning at the site of papillary muscle insertion and was associated with severe mitral regurgitation with volume loading. Therefore caution should be used when considering clinical application of this technique if the area to be ablated is in the basal portion of the heart.     

Histological findings induced by different energy sources in experimental atrial ablation in sheep

The aim of this study was a detailed comparative investigation of acute cardiac alterations induced by different energy sources and approaches in a sheep model. Experiments were performed on 39 sheep. Circular lesions were created endo- or epicardially in the left atrium and at the pulmonary veins using different energy sources: cryo, microwave, laser and unipolar or bipolar radiofrequency (RF). Electrophysiological examinations were performed immediately post treatment and 2 h after ablation to prove conduction block. Altered areas of the atria and pulmonary veins were investigated histopathologically. Endocardial ablation resulted in transmural lesions, confirmed by electrophysiological examinations. However, endocardial microwave and laser induced intensive thrombus formation, whereas radiofrequency and cryoablation induced more circumscribed necrosis and led to little endocardial thrombi. Epicardial cryoablation and microwave energy were not successful in acute phase in 8 of 9 animals. In contrast, epicardial bipolar RF was efficient and resulted in well demarcated slim lesion lines but induced marked thrombus formation. It can be summarized that surgical ablation techniques using different energy sources and approaches in this acute animal model resulted in different electrophysiological effectiveness and histomorphological lesions. Further mid and long term studies are necessary to confirm these results.     

Epicardial radiofrequency ablation on a beating heart: an experimental study.

PURPOSE: The effect of epicardial radiofrequency ablation (RFA) during normal heart beating was experimentally studied in order to establish safe and effective procedures for RFA. METHODS: Seven pigs weighing approximately 30 to 50 kg were used in this study. Fifty-one epicardial RFA lesions were created on both atria using a Cobra Cooled probe with continuous internal irrigation of a saline solution. The ablation temperature was fixed at 80 degrees C and the duration of the RFA in each case was 20, 30, 60 and 120 seconds. RESULTS: There was significant positive correlation between the right and left atria in wall thickness. Transmural coagulation was obtained in 69% of the total specimens, which decreased according to the increase of wall thickness especially over 3 mm. Transmural coagulation was seen in 64% of the specimens after RFA of less than 30 seconds, and 86% after ablation of >or=60 seconds. Occurence of 90% or deeper coagulation was higher in the right atrium than in the left one (97% vs. 78%). Right atrial rupture occurred in a region of 1 mm in thickness after ablation of 60 seconds. CONCLUSION: Further technical improvements associated with new instruments are indispensable to complete epicardial RFA procedures on a beating heart.     

Open-heart endocardial radiofrequency ablation: an alternative to incisions in Maze surgery

BACKGROUND: Radiofrequency (RF) ablation produces transmural atrial lesions in vitro, and may provide advantages over incisions currently used in maze surgery. This study examines the feasibility, safety, and efficacy of open-heart endocardial RF ablation. METHODS: Eighteen sheep (42.8 +/- 4.4 kg, age < 2 years) underwent left thoracotomy with placement of pacing leads on a pulmonary vein and the left atrial dome. On cardiopulmonary bypass, lesions were made using incision and suture or a novel RF ablation device in three sites: PVC = circle excluding pulmonary veins, IAB = line across the interatrial bundle, SVC = line from the superior to the inferior vena cava. Pacing across the PVC lesion was attempted to assess the completeness of each lesion. Preselected animals (incision n = 4, RF n = 5) were recovered and pacing attempts were repeated at 1 month. After sacrifice, hearts were sectioned and measured for lesion size and completeness. RESULTS: RF ablation lesions took less time to create (total bypass time: RF 51.8 min vs incision 106 min, P < 0.001). No evidence of thromboembolism, atrial rupture, or coronary sinus thrombosis was seen. All PVC lesions were complete as demonstrated by the inability to pace across them. Stained sections demonstrated that acutely studied incision lesions were thinner than RF lesions; however, all lesions were transmural and similar in width at 1 month. CONCLUSIONS: RF ablation consistently created transmural lesions more quickly than the incision and suture method and without additional complications. Endocardial RF ablation appears to be a simple and effective alternative to surgical incisions during open-heart atrial Maze procedures.     

Surgical treatment of atrial fibrillation with diathermy: an in vitro study.

OBJECTIVE: The utilization of diathermy (electrocautery) as an energy source in the treatment of chronic atrial fibrillation has generated positive early clinical results. Although this technology is available and affordable, it has not been well studied for this indication. The objectives of this study were: (1) to characterize atrial lesions created by diathermy, (2) to determine relationships between power setting, tissue contact time, and lesion depth and (3) to histologically compare diathermy and unipolar radiofrequency lesions. METHODS: Fresh bovine atrial tissue samples were used to create endocardial lesions using a unipolar diathermy system with a blade tip. A total of 120 lesions were created at varying power settings and tissue contact times. Subendocardial temperatures were recorded. All lesions were examined grossly, then fixed, sectioned and evaluated histologically by a blinded pathologist. Comparisons were made with saline irrigated unipolar radiofrequency lesions. RESULTS: Gross examination revealed extensive tissue destruction of the endocardial surface at the point of contact. Histological examination showed minimal penetrance of the lesions beyond the destroyed tissue margin of the endocardium. This was corroborated by the finding of minimal thermal penetration beyond the endocardium and superficial myocardium. There was a linear relationship between the power setting (15-55 watts), depth of penetrance (2-15 mm) at varying contact times (1-5s/cm). CONCLUSIONS: In this in vitro model, lesions created by diathermy were not transmural, even with high power settings and prolonged contact times. At these settings, significant tissue destruction was observed that may predispose to atrial perforation without achieving penetration. Diathermy did not constitute an effective energy source in the creation of transmural lesions for atrial fibrillation ablation.     

Comparison of early and late dimensions and arrhythmogenicity of cryolesions in the normothermic canine heart

Little is known about myocardial cryoablation at normothermia and the effect of cryoprobe head size and duration of freeze on final lesion volume. In the present study, cryolesions were created with a carbon dioxide cryoprobe with two head sizes (cylindrical head 6 mm diameter and large circular head 18 mm diameter) in the normothermic canine heart during cardiopulmonary bypass. The duration of freeze (exposure time) varied from 2 to 3 or 4 minutes and the effects on immediate and chronic lesion size were evaluated. Lesions produced by epicardial exposures were compared with intramyocardial lesions created by placing the cylindrical head in a 6 mm stab incision. A minimum of four lesions were created in each dog. Lesion size was evaluated at 0 minutes (iceball) and 24 hours (two dogs), 7 days (one dog), or 4 weeks (five dogs). Iceball diameter was approximately 5 mm larger than chronic lesion diameter regardless of head size or exposure time. Prolongation of exposure time from 2 to 3 minutes resulted in significant increases in the volume of epicardial lesions (cylindrical head: 280 +/- 100 mm3 versus 740 +/- 200 mm3, p = 0.001; circular head: 1200 +/- 100 mm3 versus 2300 +/- 500 mm3, p = 0.007) because of increases in diameter and depth. No further increase in lesion size was observed when exposure time was prolonged from 3 to 4 minutes. A 3-minute intramyocardial exposure with the cylindrical head placed in a stab incision enabled production of transmural lesions (16 +/- 2 mm deep). Two and 4 weeks postoperatively, dogs underwent electrophysiologic study from the right and left ventricular apices. No animals had inducible ventricular tachycardia despite the heterogeneous configuration of the multiple cryolesions. In conclusion, it is possible to produce rapid and predictable ablation of clinically useful volumes of myocardium during normothermic bypass with the use of currently available equipment. Under these conditions, an exposure time of 3 minutes is optimal for a liquid carbon dioxide cryoprobe. Cryolesions should be overlapped by at least 2.5 mm to produce continuous areas of ablation. Multiple cryolesions do not form a chronic substrate for ventricular tachycardia.     

Experimental study of large-volume microwave ablation in the liver

BACKGROUND: Microwave coagulation therapy is useful in the destruction of small, irresectable liver tumours of primary and secondary origin. Unfortunately, the small lesion size produced by currently available equipment makes it difficult and time consuming completely to ablate lesions larger than 3 cm in diameter. A microwave system capable of producing large-volume ablations in very short periods of time has been developed. Using a large-animal model the ability of the equipment to produce large-volume lesions in a safe, predictable and dose-dependent manner was tested. METHODS: Fourteen large white pigs were anaesthetized and underwent multiple microwave treatments. The animals were killed at different timepoints to investigate lesion size and evolution. RESULTS: The microwave system was able to generate large-volume ablations of up to 6.5 cm in diameter in a controlled and dose-dependent manner. CONCLUSION: This novel microwave system allows the ablation of large volumes of liver tissue in a short period of time. The ability to produce lesions reproducibly and safely highlights the potential of this system in the future treatment of irresectable liver tumours.     

Cardiac cryosurgery: effects of myocardial temperature on cryolesion size

Cryothermic ablation of myocardium and portions of the specialized cardiac conduction tissue has been employed successfully for the treatment of both supraventricular and ventricular tachyarrhythmias. As more uses have been found for cardiac cryosurgery, increasing flexibility of the cryothermia system has been required. The present study was designed to develop a method for electively increasing or decreasing the dimensions and volume of cryolesions by altering adjacent myocardial temperatures. Fourteen dogs were subjected to cardiopulmonary bypass, and standard cryothermic exposures (4 mm cryoprobe, -60 degrees C for 120 seconds) were used to create cryolesions on the left ventricular free wall. The dimensions and volumes of cryolesions created at myocardial temperatures of 37 degrees, 32 degrees, and 6 degrees to 12 degrees C (hypothermic cardioplegic arrest) were compared. Cryolesions created at 6 degrees to 12 degrees C with the dogs under cardioplegic arrest were significantly larger (P less than 0.05) than cryolesions created t 37 degrees or 32 degrees C in the perfused beating heart. This information may be useful in improving the results of myocardial cryoablation, particularly in procedures requiring the ablation of large regions of myocardium or regions deep below the epicardial or endocardial surface.