Microwave catheter ablation of myocardium in vitro. Assessment of the characteristics of tissue heating and injury

Advancements in microwave ablation for tumor treatment and future directions

Microwave ablation (MWA) is a minimally invasive treatment that uses thermal energy to target and destroy tumors. Compared to other ablation methods, such as radiofrequency ablation (RFA), MWA operates at higher frequencies, allowing for faster ablation and larger treatment areas. In addition to its direct tumor-destroying effects, MWA has been shown to activate immune responses, contributing to long-term antitumor effects. MWA can also be combined with surgery, chemotherapy, and immunotherapy to enhance treatment outcomes. This review examines the current research on MWA's technical innovations, clinical applications, and its potential in improving cancer treatment efficacy.

Temperature Monitoring in Hyperthermia Treatments of Bone Tumors: State-of-the-Art and Future Challenges

Bone metastases and osteoid osteoma (OO) have a high incidence in patients facing primary lesions in many organs. Radiotherapy has long been the standard choice for these patients, performed as stand-alone or in conjunction with surgery. However, the needs of these patients have never been fully met, especially in the ones with low life expectancy, where treatments devoted to pain reduction are pivotal. New techniques as hyperthermia treatments (HTs) are emerging to reduce the associated pain of bone metastases and OO. Temperature monitoring during HTs may significantly improve the clinical outcomes since the amount of thermal injury depends on the tissue temperature and the exposure time. This is particularly relevant in bone tumors due to the adjacent vulnerable structures (e.g., spinal cord and nerve roots). In this Review, we focus on the potential of temperature monitoring on HT of bone cancer. Preclinical and clinical studies have been proposed and are underway to investigate the use of different thermometric techniques in this scenario. We review these studies, the principle of work of the thermometric techniques used in HTs, their strengths, weaknesses, and pitfalls, as well as the strategies and the potential of improving the HTs outcomes.

Study on the effect of different blood flow velocities of pulmonary vein on endocardial microwave ablation of atrial fibrillation

OBJECTIVE

To cure atrial fibrillation, the maximum ablation depth (⩾ 50∘C) should exceed the myocardial thickness to achieve the effect of transmural ablation. The blood flow of pulmonary vein in the endocardium can cause the change in the myocardial temperature distribution. Therefore, the study investigated the effect of different pulmonary vein blood flow velocities on the endocardial microwave ablation.

METHODS

The finite element model of the endocardial microwave ablation of pulmonary vein was simulated by electromagnetic thermal flow coupling. The ablation power was 30 W and the ablation time was within 30 s. The blood flow in the coupling of fluid mechanics equation and heat transfer equation results in the heat damage. Furthermore, the cause of the different lesion dimensions is the blood flow velocity. The flow velocities were set as 0, 0.02, 0.05, 0.07, 0.12, 0.16, 0.20, 0.25 and 0.30 m/s.

RESULTS

When the flow velocities were 0, 0.02, 0.05, 0.07, 0.12, 0.16, 0.20, 0.25 and 0.30 m/s, the maximum ablation depth were 6.0, 5.56, 5.16, 5.12, 5.04, 5.01, 4.98, 4.96 and 4.94 mm, respectively; the maximum ablation width were 12.52, 9.63, 9.23, 9.16, 9.07, 9.05, 8.94, 8.91, 8.90 mm, respectively; the maximum ablation length were 12.00, 11.61, 8.98, 8.59, 8.37, 8.23, 8.16, 8.06 and 8.04 mm respectively. To achieve transmural ablation, the time was 3, 3, 3, 3, 3, 4, 4, 4, 4 s, respectively when the myocardial thickness was 2 mm; the time was 7, 8, 8, 8, 9, 9, 9, 9, 9 s, respectively when 3 mm; the time was 15, 16, 18, 19, 19, 20, 20, 20, 20 s, respectively when 4 mm.

CONCLUSIONS

When the velocity increases from 0 m/s to 3 m/s, the microwave lesion depth decreases by 1.06 mm. To achieve transmural ablation, when the myocardial thickness is 2 mm, 3 and 4 s should be taken when the velocity is 0-0.12 and 0.120.30 m/s, respectively; when the myocardial thickness is 3 mm, 7, 8 and 9 s should be taken when 0, 0-0.07 and 0.07-0.30 m/s respectively; when the myocardial thickness is 4 mm, 15, 16, 18, 19, 20 s should be taken when 0, 0-0.02, 0.02-0.05, 0.05-0.12, 0.12 m/s-0.30 m/s.

Techniques for Temperature Monitoring of Myocardial Tissue Undergoing Radiofrequency Ablation Treatments: An Overview

Cardiac radiofrequency ablation (RFA) has received substantial attention for the treatment of multiple arrhythmias. In this scenario, there is an ever-growing demand for monitoring the temperature trend inside the tissue as it may allow an accurate control of the treatment effects, with a consequent improvement of the clinical outcomes. There are many methods for monitoring temperature in tissues undergoing RFA, which can be divided into invasive and non-invasive. This paper aims to provide an overview of the currently available techniques for temperature detection in this clinical scenario. Firstly, we describe the heat generation during RFA, then we report the principle of work of the most popular thermometric techniques and their features. Finally, we introduce their main applications in the field of cardiac RFA to explore the applicability in clinical settings of each method.

Epicardial Ablation Biophysics and Novel Radiofrequency Energy Delivery Techniques

Important physiologic and anatomic differences exist between the epicardium and endocardium, particularly of the ventricles, and these differences affect ablation biophysics. Absence of passive convective effects conferred by circulating blood as well as the presence of epicardial fat and vessels and absence of intracavitary ridges and structures affect ablation lesion size when performing epicardial catheter-based ablation, whether using radiofrequency or cryothermal energy. Understanding differential effects in each environment is important in informing strategies to increase ablation lesion depth. When using actively cooled radiofrequency ablation, local impedance can be altered to selectively augment energy delivery.

Cardiac radiofrequency ablation tracking using electrical impedance tomography

There is a need for accessible high speed imaging of Radiofrequency (RF) cardiac electrosurgery to improve safety and efficacy of the ablation time course, where lesion information is critical to safety and efficacy but currently lacking in real time. In this paper, Electrical Impedance Tomography (EIT) using existing cardiac EP electrodes was optimised to confirm (1) that removal of measurements with low signal sensitivity leads to improved images and (2) that multiple signal thresholds are needed to track the lesion accurately over time. A novel ventricle-shaped gel phantom with realistic fluid flow to mimic blood flow, lung ventilation and myocardium conductivity was developed to study the capability and motivate transition to in-vivo measurements. When using 8 external (ECG) electrodes, 4 internal coronary sinus electrodes and 4 RF catheter-based electrodes, the optimal setup for sensitivity and dynamic tracking was 77 measurements within an error of 20%. Higher thresholds were more suitable for the earlier phase of the ablation when lesions are small while lower thresholds suited later phases. Patient-specific thresholds could be optimised in pre-surgical planning where detailed anatomical images are available. While the error reported in this initial study appears large, it is a major advance over the current situation for the cardiologist where no real-time lesion visualization is accessible in a regular EP suite/cath lab.

FEM-based elasticity reconstruction using ultrasound for imaging tissue ablation

PURPOSE

Success of ablation treatment depends on the accurate placement of the target ablation focus and the complete destruction of the pathological tissue. Thus, monitoring the formation, location, and size of the ablated lesion is essential. As ablated tissue gets stiffer, an option for ablation monitoring is ultrasound elastography, for imaging the tissue mechanical properties. Reconstruction of elasticity distribution can be achieved by solving an inverse problem from observed displacements, based on a deformable tissue model, commonly discretized by the finite element method (FEM). However, available reconstruction techniques are prone to noise and may achieve suboptimal accuracy.

METHODS

We propose a novel inverse problem formulation and elasticity reconstruction method, in which both the elasticity parameters and the model displacements are estimated as independent parameters of an unconstrained optimization problem. Total variation regularization of spatial elasticity distribution is introduced in this formulation, providing robustness to noise.

RESULTS

Our approach was compared to state of the art direct and iterative harmonic elastography techniques. We employed numerical simulation studies using various noise and inclusion contrasts, given multiple excitation frequencies. Compared to alternatives, our method leads to a decrease in RMSE of up to 50% and an increase in CNR of up to 11 dB in numerical simulations. The methods were also compared on an ex vivo bovine liver sample that was locally subjected to ablation, for which improved lesion delineation was obtained with our proposed method. Our method takes [Formula: see text] for [Formula: see text] reconstruction grid.

CONCLUSIONS

We present a novel FEM problem formulation that improves reconstruction accuracy and inclusion delineation compared to currently available techniques.

Cryoablation Versus Radiofrequency Ablation in AVNRT: Same Goal, Different Strategy

Catheter ablation is nowadays the first therapeutic option for AVNRT, the most common benign supraventricular tachycardia. Both cryotherapy and radiofrequency energy may be used to ablate the slow pathway. This paper compares both techniques, evaluates results published in literature and gives feedback on some typical aspects of cryo- and RF ablation. Although both techniques have satisfying success rates in AVNRT ablation, with a higher safety profile of cryoablation towards creation of inadvertent atrioventricular block, it remains paramount that the operator respects the distinctive traits of each technique in order to obtain an optimal result in every patient.

Electrophysiological changes correlated with temperature increases induced by high-intensity focused ultrasound ablation

To gain better understanding of the detailed mechanisms of high-intensity focused ultrasound (HIFU) ablation for cardiac arrhythmias, we investigated how the cellular electrophysiological (EP) changes were correlated with temperature increases and thermal dose (cumulative equivalent minutes [CEM43]) during HIFU application using Langendorff-perfused rabbit hearts. Employing voltage-sensitive dye di-4-ANEPPS, we measured the EP and temperature during HIFU using simultaneous optical mapping and infrared imaging. Both action potential amplitude (APA) and action potential duration at 50% repolarization (APD50) decreased with temperature increases, and APD50 was more thermally sensitive than APA. EP and tissue changes were irreversible when HIFU-induced temperature increased above 52.3 ± 1.4°C and log10(CEM43) above 2.16 ± 0.51 (n = 5), but were reversible when temperature was below 50.1 ± 0.8°C and log10(CEM43) below -0.9 ± 0.3 (n = 9). EP and temperature/thermal dose changes were spatially correlated with HIFU-induced tissue necrosis surrounded by a transition zone.

Electrical homogenization of ventricular scar by application of collagenase: a novel strategy for arrhythmia therapy

BACKGROUND

Radiofrequency ablation for ventricular tachycardia is an established therapy. Use of chemical agents for scar homogenization represents an alternative approach. The purpose of this study was to characterize the efficacy of collagenase (CLG) application on epicardial ventricular scar homogenization.

METHODS AND RESULTS

Myocardial infarcts were created in Yorkshire pigs (n=6) by intracoronary microsphere injection. After 46.6±4.3 days, CLG type 2, type 4, and purified CLG were applied in vitro (n=1) to myocardial tissue blocks containing normal myocardium, border zone, and dense scar. Histopathologic studies were performed to identify the optimal CLG subtype. In vivo high-density electroanatomic mapping of the epicardium was also performed, and border zone and dense scar surface area and late potentials were quantified before and after CLG-4 application (n=5). Of the CLG subtypes tested in vitro, CLG-4 provided the best scar modification and least damage to normal myocardium. During in vivo testing, CLG-4 application decreased border zone area (21.3±14.3 to 17.1±11.1 mm(2), P=0.043) and increased dense scar area (9.1±10.3 to 22.0±20.6 mm(2), P=0.043). The total scar area before and after CLG application was 30.4±23.4 and 39.2±29.5 mm(2), respectively (P=0.08). Late potentials were reduced by CLG-4 application (28.8±21.8 to 13.8±13.1, P=0.043). During CLG-4 application (50.0±15.5 minutes), systolic blood pressure and heart rate were not significantly changed (68.0±7.7 versus 61.8±5.3 mmHg, P=0.08; 77.4±7.3 versus 78.8±6.0 beats per minute, P=0.50, respectively).

CONCLUSIONS

Ventricular epicardial scar homogenization by CLG-4 application is feasible and effective. This represents the first report on bioenzymatic ablation of arrhythmogenic tissue as an alternative strategy for lesion formation.

Achieving renal denervation: catheter-based and surgical management for neural ablation in the management of hypertension

Hypertension refractory to conventional management with medication remains a significant cause of cardiovascular morbidity and mortality. Alternative strategies are warranted in this subgroup of patients. The target of these strategies centers around sympathetic neural activity, which is thought to play a key role in hypertension. We will review the historic and current approaches toward altering sympathetic neural activity, specifically discussing surgical sympathectomy, catheter-based renal denervation strategies, and baroreflex activation therapy.

Radiofrequency ablation as a possible method for preparing pathologically altered myocardium for intramyocardial cell transplantation

We studied the effects of radiofrequency ablation on the results of intramyocardial transplantation of bone marrow NSC into the myocardium of rats with postinfarction cardiosclerosis. It was shown that exposure of the pathologically changed myocardium to radiofrequency radiation led to destruction of formed connective tissue. Transplantation of MSC into sites exposed to radiofrequency radiation promoted the development of regenerative processes (abundant infiltration with mononuclear cells, presence of granulation tissue, and numerous newly formed blood vessels). We concluded that preliminary radiofrequency irradiation of the myocardial areas promotes realization of the regenerative potential of cell transplantation.

Biophysics and clinical utility of irrigated-tip radiofrequency catheter ablation

Catheter ablation by radiofrequency (RF) energy has successfully eliminated cardiac tachyarrhythmias. RF ablation lesions are created by thermal energy. Electrode catheters with 4-mm-tips have been adequate to ablate arrhythmias located near the endocardium; however, the 4-mm-tip electrode does not readily ablate deeper tachyarrhythmia substrate. With 8- and 10-mm-tip RF electrodes, ablation lesions were larger; yet, these catheters are associated with increased risk for coagulum, char and thrombus formation, as well as myocardial steam rupture. Cooled-tip catheter technology was designed to cool the electrode tip, prevent excessive temperatures at the electrode tip-tissue interface, and thus allow continued delivery of RF current into the surrounding tissue. This ablation system creates larger and deeper ablation lesions and minimizes steam pops and thrombus formation. The purpose of this article is to review cooled-tip RF ablation biophysics and outcomes of clinical studies as well as to discuss future technological improvements.

Alternative energy sources for surgical treatment of atrial fibrillation in patients undergoing mitral valve surgery: microwave ablation vs cryoablation

The study aim was to compare maze outcomes using microwave ablation or cryoablation in patients with mitral disease and atrial fibrillation (AF). Between 1999 and 2005, 340 patients underwent mitral valve surgery and concomitant maze procedure involving either microwave ablation (n=96, MW group) or cryoablation (n=244, Cryo group). Mean age at operation was 50.0±12.5 yr. Follow-up period was 46.1±28.2 months. The Cryo group showed a longer aortic clamping time than the MW group (P=0.005). There were no differences in operative mortality and morbidity rates. The unadjusted 5-yr AF free rate was 61.3±1.2% in the MW group and 79.9±3.2% in the Cryo group (P=0.089). After adjustment, the MW group only showed a tendency toward more frequent AF recurrence than the Cryo group (Hazard ration 1.66, 95% confidence interval 0.89 to 3.07). Multivariate analysis revealed that older patient age (P<0.001) and greater left atrial size (P<0.001) were independent risk factors for AF recurrence. Although the use of microwave ablation results in shorter aortic clamping time, it has a tendency toward more frequent late AF recurrence than with cryoablation.

An Update on the Energy Sources and Catheter Technology for the Ablation of Atrial Fibrillation

The ablation of atrial fibrillation (AF) is an area of intense research in cardiac electrophysiology. In this review, we discuss the development of catheter-based interventions for AF ablation. We outline the pathophysiologic and anatomic bases for ablative lesion sets and the evolution of various catheter designs for the delivery of radiofrequency (RF), cryothermal, and other ablative energy sources. The strengths and weaknesses of various specialized RF catheters and alternative energy systems are delineated, with respect to efficacy and patient safety.

Energy Sources for Ablation in the Pericardial Space

Catheter ablation has been widely used for the management of cardiac arrhythmias. Transvenous endocardial catheter ablation successfully eliminates or modifies the critical substrate for most arrhythmias. Most arrhythmias can be eliminated with conventional endocardial mapping and radiofrequency energy delivery, but some critical arrhythmic substrates are not accessible via endocardial access and this has led to epicardial mapping and ablation in addition to traditional endocardial mapping techniques. This article reviews current approaches to epicardial ablation and discusses the specialized tools that increase ablation efficacy and safety.

Laser Ablation Of Atrial Fibrillation: Mid-term Clinical Experience

Background: Atrial Fibrillation is known to account for one third of all the strokes caused in the US in the population above the age of 70. Patients treated with the surgical Cox MAZE operation have been shown to have a 150 fold decrease in the incidence of stroke over an 18 year period. However, the original Cox MAZE although extremely successful in treating atrial fibrillation and decreasing the incidence of strokes was not performed widely because of complexity and invasiveness of the procedure. A variety of alternative energy based curative ablation strategies are now available for more minimally invasive therapeutic management of atrial fibrillation (AF). In this communication, we report our clinical experience in AF therapy utilizing laser energy ablation technology. Methods: Fifty two consecutive AF patients underwent concomitant or isolated ablation prior to any coexisting cardiac procedures that included CABG (coronary artery bypass surgery, MV (mitral valve) or AV (aortic valve) repairs. All patients had an epicardially based ablation pattern with basic lesions being en bloc box type pulmonary vein isolation which included the antral surface of the left atrium, directed ganglionectomies of the the right anterior and inferior ganglions, posteriomedial ablation of the IVC ( inferior vena cava), and a right isthmus ablation. Twenty seven patients had ligation of their left atrial appendage, 14 patients had resection of the ligament of Marshall, and three patients had endocardial placed lesions of a mitral annular connecting type lesion. In order to maintain the patients in normal sinus rhythm (NSR), electrical cardioversion and anti-arrhythmic drugs were employed as required. Results: At a median follow-up of 250 days, 44 of the total 52 patients (84.6%) exhibited NSR.. No complications or mortality were reported due to the laser procedure. Conclusion: Laser ablation was successfully and safely used for endocardial and epicardial AF ablation concomitant to other cardiovascular procedures and in the lone atrial fibrillation treatment utilizing a two port thoracoscopic approach.

Determinants of lesion dimensions during transcatheter microwave ablation

BACKGROUND

Transcatheter microwave ablation is a novel technique for treating cardiac arrhythmias.

METHODS

We investigated the effects of catheter temperature, application duration, and antenna length on lesion dimensions during catheter-based microwave ablation. In a swine thigh muscle preparation, microwave was delivered at targeted temperatures of 60 degrees C (n = 18), 70 degrees C (n = 27), 80 degrees C (n = 43), or 90 degrees C (n = 18) for 120 seconds with 10-mm antenna; and at targeted temperatures of 80 degrees C for 120 seconds (n = 22), 150 seconds (n = 18), 180 seconds (n = 18), 210 seconds (n = 18), and 240 seconds (n = 17) with 20-mm antenna using 10 F catheter (MedWaves, San Diego, CA, USA) during parallel orientation. Conventional radiofrequency ablation (RF) using a 4-mm tip electrode was performed as control.

RESULTS

With 120-second energy applications, lesion length and depth were significantly larger with targeted temperatures of 80 degrees C and 90 degrees C than 60 degrees C (P< 0.05). Furthermore, lesion depth and width, but not length, were significantly increased by prolonging energy application duration from 120 to 240 seconds at targeted temperature of 80 degrees C (P< 0.05). Compared to RF, microwave lesions were significantly longer but had comparable depth and width. A 20-mm microwave antenna produced longer lesions than either a 10-mm antenna or RF ablation catheter. Multivariate analysis demonstrated that targeted temperature >or=80 degrees C, application duration >or=150 seconds, and use of 20-mm antenna were independent predictors for lesion depth and width (P< 0.05). Surface dessication was observed in 4/18 (22%) lesions at 90 degrees C, as compared with 1/136 (0.7%) at 80 degrees C targeted tip temperature (P < 0.05).

CONCLUSIONS

This study demonstrated that lesions size with transcatheter microwave ablation can be controlled by adjusting targeted temperature, energy application duration, and antenna length. A targeted temperature of 80 degrees C for more than 150 seconds should provide optimal lesion dimensions and lower risk of surface dessication or charring.

Successful radiofrequency ablation of the cerebral cortex in pigs using the venous system: possible implications for treating CNS disorders

BACKGROUND

When pharmacotherapy for epilepsy fails, surgical options, although efficacious, are highly invasive. We explored whether ablation of the cerebral cortex can be performed utilizing the cerebral venous system.

METHODS

Mapping and radiofrequency ablation was performed via the venous system in two pigs.

RESULTS

Eight targeted sites were successfully accessed and four targeted sites successfully ablated via the central cerebral venous network.

CONCLUSION

Electrophysiological mapping and radiofrequency ablation of the cerebral cortex can be performed via the cerebral veins.

An overview of energy sources in clinical use for the ablation of atrial fibrillation

Recent years have seen many developments in the field of alternative energy sources for arrhythmia surgery. The impetus behind these advances is to replace the traditional, "cut-and-sew" Cox maze III procedure with lesion sets that are simpler, shorter, and safer but just as effective. There is demand for technology to make continuous, linear, transmural ablations reliably with a versatile energy source via an epicardial approach. This would make minimally invasive endoscopic surgical ablation of atrial fibrillation (AF) without cardiopulmonary bypass and with a closed chest feasible. These advances would shorten cardio-pulmonary bypass and improve outcomes in patients having surgical ablation and concomitant cardiac surgery. This review summarizes the technology behind alternative energy sources used to treat AF. Alternative energy sources include hypothermic sources (cryoablation) and hyperthermic sources (radiofrequency, microwave, laser, ultrasound). For each source, the biophysical background, mode of tissue injury, factors affecting lesion size, and advantages and complications are discussed.

Preliminary Results with Percutaneous Transcatheter Microwave Ablation of Typical Atrial Flutter

BACKGROUND

Linear microwave ablation has been shown to be effective for treatment of atrial fibrillation during open-heart surgery by producing transmural lesions in the atrium to isolate the pulmonary veins. However, the safety and efficacy of percutaneous, transcatheter, linear microwave ablation for atrial arrhythmias, while demonstrated in animal models, is unknown in humans. Therefore, we studied the safety and efficacy of linear microwave ablation of the cavotricuspid isthmus (CTI) in humans with typical atrial flutter, utilizing a 2-cm long microwave antenna mounted on a steerable 9-French catheter.

METHODS AND RESULTS

In seven consecutive patients, multielectrode catheters were positioned at the His bundle (quadripolar) and around the TV annulus (duo-decapolar) for pacing and recording atrial activation sequence before and after ablation. The microwave antenna was withdrawn gradually from tricuspid annulus towards inferior vena cava to ablate the CTI. Intracardiac ultrasound was used to ensure adequate endocardial contact of the microwave ablation catheter with the CTI. Microwave energy was applied at a power of 18 to 21 W at each ablation point for 120 seconds. Ablation was repeated until bidirectional CTI block was confirmed by demonstrating a descending activation wavefront in the contralateral atrial wall during pacing from the coronary sinus ostium or low lateral right atrium, respectively. Bidirectional isthmus block was achieved in all patients, after a mean number of 27.4 +/- 14.7 energy applications per patients. There were no acute procedural complications.

CONCLUSIONS

Percutaneous, transcatheter microwave ablation of CTI dependent atrial flutter was demonstrated to be safe and effective in this preliminary feasibility study.

Diagnostic accuracy of cardiac markers for myocardial damage after radiofrequency catheter ablation

AIMS

This study compares serum markers of myocardial damage incurred during radiofrequency catheter ablation (RFCA).

METHODS AND RESULTS

Blood was sampled from 34 patients with atrial flutter (n = 16), atrioventricular nodal reentrant tachycardia (AVNRT; n = 13), and Wolff-Parkinson-White syndrome (WPW; n = 5) to measure creatine kinase MB subfraction (CK-MB), human heart-type fatty acid protein (h-FABP), and cardiac troponin T (cTnT) values at baseline and after RFCA. The controls comprised 12 patients without significant elevation of all myocardial markers during electrophysiological study (EPS) without RFCA. h-FABP values did not elevate significantly, whereas CK-MB and cTnT demonstrated significant change after RFCA (P < 0.001). Neither peak h-FABP nor CK-MB correlated with following RFCA parameters: delivery duration, number of RFCA discharges, and cumulative RFCA energy. In contrast, correlations were significant between mean peak values of cTnT and these RFCA parameters (all P < 0.05). The sensitivity (71.6%) and specificity (35.6%) of h-FABP were inferior to those of cTnT (93.3% and 89.8%, respectively).

CONCLUSION

h-FABP is an insensitive and less specific marker of myocardial damage in RFCA much along the lines of CK-MB and when compared with the proven accuracy of cTnT.

Microwave ablation for atrial fibrillation: dose-response curves in the cardioplegia-arrested and beating heart

BACKGROUND

Microwave ablation has been used to replace the traditional incisions used in the surgical treatment of atrial fibrillation. However, dose-response curves have not been established in surgically relevant models. The purpose of this study was to develop dose-response curves for the Flex 10 (Guidant, Inc) microwave device in both the acute cardioplegia-arrested heart and on the beating heart.

METHODS

Twelve domestic pigs (40 to 45 kg) were subjected to microwave ablation in either the arrested (n = 6) or beating heart (n = 6). The cardioplegia-arrested heart was maintained at 10 degrees to 15 degrees C while six atrial endocardial and seven right ventricular epicardial lesions were created in each animal. On the beating heart, six right atrial and seven ventricular epicardial lesions were created. Ablations were performed for 15, 30, 45, 60, 90, 120, and 150 seconds (65 W, 2.45 GHz). The tissue was stained with 2,3,5-triphenyl-tetrazolium chloride, and sectioned at 5-mm intervals. Lesion depth and width were determined from digital micrographs.

RESULTS

Mean atrial wall thickness was 2.8 mm (range, 1 to 8 mm). In the arrested heart, 94% of atrial lesions were transmural at 45 seconds and 100% were transmural at 90 seconds. In the beating heart, only 20% of atrial lesions were transmural despite prolonged ablation times (90 seconds). Ventricular lesion width and depth increased with duration of application, and were similar on the arrested and beating hearts.

CONCLUSIONS

Microwave ablation produces linear dose-response curves. Transmural lesions can be reliably produced on the arrested heart, but not consistently on the beating heart.

Imaging in percutaneous ablation for atrial fibrillation

Percutaneous ablation for electrical disconnection of the arrhythmogenic foci using various forms of energy has become a well-established technique for treating atrial fibrillation (AF). Success rate in preventing recurrence of AF episodes is high although associated with a significant incidence of pulmonary vein (PV) stenosis and other rare complications. Clinical workup of AF patients includes imaging before and after ablative treatment using different noninvasive and invasive techniques such as conventional angiography, transoesophageal and intracardiac echocardiography, computed tomography (CT) and magnetic resonance imaging (MRI), which offer different information with variable diagnostic accuracy. Evaluation before percutaneous ablation involves assessment of PVs (PV pattern, branching pattern, orientation and ostial size) to facilitate position and size of catheters and reduce procedure time as well as examining the left atrium (presence of thrombi, dimensions and volumes). Imaging after the percutaneous ablation is important for assessment of overall success of the procedure and revealing potential complications. Therefore, imaging methods enable depiction of PVs and the anatomy of surrounding structures essential for preprocedural management and early detection of PV stenosis and other ablation-related procedures, as well as long-term follow-up of these patients.

Early morphologic changes following microwave endocardial ablation for treatment of chronic atrial fibrillation during mitral valve surgery

INTRODUCTION

The aim of this study was to investigate the early qualitative and quantitative structural changes in the left atrial wall after endocardial microwave ablation in patients with chronic atrial fibrillation (AF) undergoing mitral surgery.

METHODS AND RESULTS

Seven patients with chronic AF of for at least 6 months underwent surgical microwave energy ablation. Linear isolation of pulmonary veins was performed in all patients by microwave energy applications to the endocardial surface delivered by catheter at 65-W constant power for 45 seconds. Biopsies were obtained from a selected site (below the right lower pulmonary vein) of the left atrial posterior wall before and after the ablation procedure in all patients. Control tissues from the same sites were obtained at autopsy from patients with noncardiac causes of death. Light and electron microscopy was used to examine qualitative and quantitative changes in tissue morphology. Tissues after endocardial ablation procedure showed significantly increased loss of contractile material. Electron microscopy of atrial tissue demonstrated loss of profile of perinuclear and plasma membranes of myocytes, disruption of the endothelial cells of capillary vessels, and presence of macrophages.

CONCLUSION

Lesions created by endocardial microwave energy ablation revealed a transmural effect on the left atrial wall without a significant reduction in thickness but a significant increase in the myolytic areas involving the entire cytosol and occlusion of the small intramyocardial vessels within the ablative lesion.

Catheter Ablation of Ventricular Tachycardia by Intramyocardial Injection of Ethanol in an Animal Model of Chronic Myocardial Infarction

INTRODUCTION

Direct injection of ethanol into myocardium has been shown to create large, well-demarcated lesions with transmural necrosis in normal ventricular myocardium and in regions of healed myocardial infarction. The aim of this study was to investigate the effects of direct ethanol injection on the inducibility of ventricular tachycardia (VT) in an animal model of chronic myocardial infarction.

METHODS AND RESULTS

Eight sheep with reproducibly inducible VT underwent an electrophysiologic study 139 +/- 65 days after myocardial infarction. Noncontact mapping was used to analyze induced VT. Fifteen different VTs were targeted for catheter ablation. Ablation was achieved by catheter-based intramyocardial injection of a mixture of 96% ethanol, glycerine, and iopromide (ratio 3:1:1). Direct intramyocardial ethanol injection resulted in noninducibility of any VT 20 minutes after ablation in 7 of 8 animals. Four of 5 animals with initially successful ablation remained noninducible for any VT at follow-up study at least 2 days after the ablation procedure. Microscopic examination revealed homogeneous lesions with interstitial edema, intramural hemorrhage, and myofibrillar degeneration at the lesion border. The lesions were well demarcated from the surrounding tissue by a border zone of neutrophilic infiltration.

CONCLUSION

Catheter ablation of VT by direct intramyocardial injection of ethanol during the chronic phase of myocardial infarction is feasible. It may be a useful tool for catheter ablation when the area of interest is located deep intramyocardially or subepicardially or when a more regional approach requires ablation of larger amounts of tissue.

Analysis of a novel expanded tip wire (ETW) antenna for microwave ablation of cardiac arrhythmias

A novel expanded tip wire (ETW) catheter antenna is proposed for microwave ablation for the treatment of atrial fibrillation (AF). The antenna is designed as an integral part of coaxial cable so that it can be inserted via a 6F catheter. A numerical model based on the rotationally symmetric finite-difference time-domain technique incorporating the generalized perfectly matched layer as the absorbing boundary condition has been utilized to accurately model the interaction between the antenna and the myocardium. Numerical and in-vitro experimental results are presented for specific absorption rate, return loss and heating pattern produced by the antenna. Both numerical modeling and in-vitro experimentation show that the proposed ETW antenna produces a well-defined electric field distribution that provides continuous long and linear lesions for the treatment of AF.

Increasing power versus duration for radiofrequency ablation with a high superfusate flow: implications for pulmonary vein ablation?

Radiofrequency (RF) ablation of pulmonary veins (PVs) is a new treatment for atrial fibrillation. Low energy ablation is usually used for this procedure. The effect of superfusate flow on lesion formation in this setting has not been studied previously. We examined lesion dimensions and intramural temperatures with varying powers and duration of RF application in this high flow environment. Ablation of fresh bovine hearts was performed with a 4-mm tip RF catheter in temperature control mode, target temperature 50 degrees C. At power levels of 20 W, 30 W, 40 W, and 50 W, effects of PV flow (no flow or 1 L/min) and 60- and 120-second durations were tested. Tissue temperatures were recorded at depths of 1, 4, 7, and 10 mm. Without flow, no lesions were created. The lowest power setting for lesion creation was 30 W at 60 seconds and 20 W at 120 seconds. Increasing power from 30 W to 50 W for 60 seconds increased lesion depth 0.7 mm (SE 0.3), P = 0.03 and 2.5 mm (SE 0.6), P = 0.003, at 120 seconds. Increasing RF application duration from 60 to 120 seconds increased depth for 30 W by 0.9 mm (SE 0.5), P = NS, 40 W 1.7 mm (SE 0.4), P = 0.002, and 50 W 2.6 mm (SE 0.5), P < 0.001. Power of 50 W for 60 seconds and >30 W for 120 seconds created lesions deeper than the wall thickness of a PV. Flow is necessary for creation of lesions with low power, low tip temperature RF ablation. When a resistant site to ablation is encountered, increasing duration of ablation is best for increasing lesion depth. Higher power has the potential to create lesions deeper than the PV wall and may increase the risk of complications.

Precision test apparatus for evaluating the heating pattern of radiofrequency ablation devices

Radiofrequency has established itself as a useful technique for managing cardiac arrhythmias and treating soft tissue tumors. However, despite its pervasive use, many of the biophysical principals needed to fully understand and optimize the radiofrequency ablation technique have not been explored. We have designed a test rig that is useful for studying the heat transfer mechanisms that affect the outcome of radiofrequency ablation devices. Using both solid and liquid phantom materials, which simulate body tissues and blood, the test rig is designed for systematic testing of the effects of predictable flow patterns on the temperature profiles generated within the solid phantom. The test rig consists of a custom built thermistor array, a linear test chamber, and a radiofrequency generator. We calibrate the flow of a liquid phantom material to demonstrate that predictable laminar flow profiles are generated. To demonstrate the performance of the ablation system, we present preliminary data attained using a commercially available cardiac ablation catheter. The advantages of this test system are its flexibility, its reproducibility, its precision, and its low cost. Thus, it is ideally suited for studying a variety of complex ablation problems involving multiple tissues types and complex blood flow geometries.

The technology in use for the surgical ablation of atrial fibrillation

The purpose of this article is to present and evaluate the various technologies recently developed for the surgical treatment of atrial fibrillation as alternatives or adjuncts to the traditional Maze III procedure and other "cut and sew" techniques. The discussion contains a detailed consideration of the biophysical background of the most common ablation techniques, their mode of tissue injury, the methods of use, and the related complications through a review of the existing literature and analysis of experimental results. All of the current technologies presented are still being tested to augment the success rates and reduce the incidence of complications, although all are not available for clinical use. Radiofrequency and cryoablation have been used clinically on large numbers of patients with varying results. Microwave technology has been used in small groups of patients, and the results are to be evaluated. Laser technology is still in an experimental phase, and the clinical results are forthcoming. True transmurality, reduction of operative time, friendly use of ablation devices, and substantial reduction of complications appear to be the key factors for broad adoption of alternative energy sources for surgical ablation.

Catheter ablation for cardiac arrhythmias

The safety and efficacy of catheter ablation for treatment of most types of cardiac arrhythmias are well established. These arrhythmias and arrhythmia substrates include AVNRT, accessory pathways, focal atrial tachycardia, atrial flutter, idiopathic ventricular tachycardia, and bundle-branch re-entry. Catheter ablation is considered as an alternative to pharmacologic therapy in the treatment of these cardiac arrhythmias.

Linear lesions in myocardium created by Nd:YAG laser using diffusing optical fibers: in vitro and in vivo results

BACKGROUND AND OBJECTIVE

Linear lesions may be necessary for successful catheter ablation of cardiac arrhythmias such as atrial fibrillation. This study uses laser energy delivered through diffusing optical fibers as an alternative to radiofrequency energy for the creation of linear lesions in cardiac tissue in a single application.

STUDY DESIGN/MATERIALS AND METHODS

Samples of canine myocardium were placed in a heated, circulating saline bath and irradiated with a 1.06-microm, continuous-wave Nd:YAG laser during in vitro studies. Laser ablation was then performed in vivo on the epicardial surface of the right ventricle during an open-chest procedure by using similar ablation parameters. Laser energy was delivered to the tissue by being diffused radially through flexible optical fiber tips oriented parallel to the tissue surface. Histology and temperature measurements verified transmurality, continuity, and linearity of the lesions.

RESULTS

Peak tissue temperatures measured in vitro remained low (51 +/- 1 degrees C at the endocardial surface, 61 +/- 6 degrees C in the mid-myocardium, and 55 +/- 6 degrees C at the epicardial surface) with no evidence of tissue charring or vaporization. Lesion dimensions produced in vitro and in vivo were similar (depth, 6 mm; width, 8-10 mm; length, 16-22 mm), demonstrating that tissue perfusion in vivo did not significantly alter the heating.

CONCLUSION

Long linear lesions, necessary for duplication of the surgical maze procedure during catheter ablation of atrial fibrillation, may be created by using laser radiation delivered through flexible diffusing optical fiber tips. Further development of steerable catheters for endocardial atrial ablation and studies correlating thermal damage zones with electrophysiologic indicators of irreversible conduction block are warranted.

The effect of ablation sequence and duration on lesion shape using rapidly pulsed radiofrequency energy through multiple electrodes

Sequences of energy application to multiple electrodes and a study of ablation duration with distal tip and multi-electrode ablations were explored with a radiofrequency controller that distributes energy from a generator to up to 4 electrodes with various duty cycles. In vitro ablations were performed on bovine left ventricle in circulating blood and lesions in goats were performed to verify the in vitro results. All of the ablation sequences with simultaneous electrode activation of contiguous electrodes resulted in deeper lesions than those created in sequence. There was also no scalloping of the lesion if contiguous electrodes were activated simultaneously. During all distal tip ablations, lesion volume and depth was greater after 3 minutes of energy delivery than after 1 minute, but did not increase from 3 minutes to 5 minutes. There was a significant increase in multi-electrode ablation lesion depth with each additional minute in the ablation cycle. The in vivo ablations verified these results at 120 and 300 second ablations. Pulsed energy distal tip ablations resulted in deeper lesions than continuous only if power amplitudes over 50 W were employed. In conclusion, contiguous electrodes in simultaneous use create lesions that resemble one large lesion rather than two lesions positioned next to each other. Multi-electrode ablation lesions continue to grow at ablation durations of up to 5 minutes compared to distal tip lesions which reach steady-state between 1 and 3 minutes. Pulsed energy delivery to distal tips may result in deeper lesions than conventional if high powers are employed.

Safety and efficacy of a steerable temperature monitoring microwave catheter system for ventricular myocardial ablation

INTRODUCTION

Radiofrequency current delivered during cardiac ablation is limited by a rise in impedance secondary to coagulum formation on the ablation electrode. Microwave antennas continue to deliver energy despite the presence of coagulum; thus, temperature control of the ablation electrode may be even more important for microwave than for radiofrequency ablations to avoid thromboembolic risks. The purpose of this study was to test the safety and efficacy of an ablation system utilizing a feedback control system to maintain a fixed target temperature for creating lesions with multiple applications of microwave energy.

METHODS AND RESULTS

Microwave ablation was assessed using an 8.5-French catheter at 2 to 4 sites in 11 dogs. Microwave energy delivery was performed for 60 seconds three times at the same site. Power was regulated using a feedback control mechanism to maintain a target temperature of 75 degrees C. Ambulatory ECG monitoring was performed before and after ablation to assess arrhythmia occurrence. After follow-up, the dogs were euthanized, and lesion dimensions measured after fixation. The mean power applied to achieve the target temperature of 75 degrees C was 9.3+/-44 W. The mean depth of the lesions was 8.8+/-4.2 mm. The mean volume of the lesions was 304+/-240 mm3. Forty-four percent of the lesions were transmural. No endocardial thrombus was found. Ventricular tachycardia was observed acutely but resolved after 1 week.

CONCLUSION

Temperature feedback control systems for microwave ablation using a temperature-controlled system is feasible for myocardial ablation and creates uniform and large lesions; however, such large lesions can be acutely proarrhythmic.

Microwave ablation using a spiral antenna design in a porcine thigh muscle preparation: in vivo assessment of temperature profile and lesion geometry

INTRODUCTION

Theoretical studies have suggested that microwave energy can increase the depth of heating compared with radiofrequency energy. A spiral microwave antenna design may have advantages over previous designs using smaller designs because the resulting power deposition pattern is considerably larger than the catheter diameter. We tested the efficacy of a spiral antenna using microwave energy in a porcine thigh muscle preparation.

METHODS AND RESULTS

In five anesthetized pigs, the thigh muscle was exposed and bathed in heparinized bovine blood (36 degrees to 37 degrees C). A helical microwave catheter with a fiberoptic thermometer attached to the distal end was positioned perpendicular to the thigh muscle. The antenna-tissue interface and tissue temperatures at depths of 3.0 and 6.0 mm were measured. A 915-MHz microwave generator delivered energy at one of three power outputs (50, 100, or 150 W) for 60 seconds. Seventy lesions were created: 50 W (n = 23), 100 W (n = 24), and 150 W (n = 23). The mean depths at 50, 100, and 150 W were 4.3 +/- 1.8 mm, 7.2 +/- 1.7 mm, and 9.4 +/- 0.9 mm, respectively. Lesion depth (R = 0.96, P = 0.05), maximum surface dimension (R = 0.99, P = 0.06), and volume (R = 0.99, P = 0.04) were closely correlated to the power applied.

CONCLUSION

Power is an important determinant of lesion size using a spiral microwave antenna. A novel, spiral microwave antenna design can create lesions of significant depth that may be applicable for the ablative therapy of ventricular tachycardia.