Cryosurgical ablation of accessory atrioventricular connections: a method for correction of the pre-excitation syndrome

Progress in atrial fibrillation ablation during 25 years of Europace journal

The first edition of Europace journal in 1999 came right around the time of the landmark publication of the electrophysiologists from Bordeaux, establishing how elimination of ectopic activity from the pulmonary veins (PVs) resulted in a marked reduction of atrial fibrillation (AF). The past 25 years have seen an incredible surge in scientific interest to develop new catheters and energy sources to optimize durability and safety of ablation, as well as study the mechanisms for AF and devise ablation strategies. While ablation in the beginning was performed with classic 4 mm tip catheters that emitted radiofrequency (RF) energy to create tissue lesions, this evolved to using irrigation and contact force (CF) measurement while increasing power. Also, so-called single-shot devices were developed with balloons and arrays to create larger contiguous lesions, and energy sources changed from RF current to cryogenic ablation and more recently pulsed field ablation with electrical current. Although PV ablation has remained the basis for every AF ablation, it was soon recognized that this was not enough to cure all patients, especially those with non-paroxysmal AF. Standardized approaches for additional ablation targets have been used but have not been satisfactory in all patients so far. This led to highly technical mapping systems that are meant to unravel the drivers for the maintenance of AF. In the following sections, the development of energies, strategies, and tools is described with a focus on the contribution of Europace to publish the outcomes of studies that were done during the past 25 years.

Cryosurgery for Atrial Fibrillation: Physiologic Basis for Creating Optimal Cryolesions

BACKGROUND

Although cryosurgery has been used to treat cardiac arrhythmias for nearly 5 decades, the mechanism of action and the surgical technique that produces optimal cryolesions for the treatment of atrial fibrillation are still poorly understood. This has resulted in surgical outcomes that can be improved by a better understanding the mechanisms of cryothermia ablation and the proper surgical techniques that take advantage of those mechanisms.

METHODS

The cryobiology underlying cryosurgical ablation is described, as are the nuances of cryosurgical techniques that ensure the reliable creation of contiguous, uniformly transmural atrial cryolesions. The oft-misunderstood "2-minute rule" for the application of cryothermia is clarified in detail, along with its variations that depend on target myocardial temperature.

RESULTS

The creation of optimal cryolesions depends on cryoprobe temperature, the temperature of the target myocardium, the duration of cryothermia application, and the presence or absence of a "heat sink" or "cooling sink" created by intracavitary blood circulation. Cryothermia kills myocardial cells during both the freezing and thawing phases of cryoablation cycle. The critical lethal temperature for myocardium is -30°C. The slower the target tissue thaws, the higher the percentage of cell death.

CONCLUSIONS

The availability of cryosurgical techniques has revolutionized the surgical treatment of atrial fibrillation. By utilizing modern cryosurgical devices and adhering to the technical principles described, surgeons can now perform surgical procedures for atrial fibrillation that are quicker, safer, and as effective as the standard Maze-III/IV procedure.

Cryoballoon Ablation for Patients With Paroxysmal Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in clinical practice and induces cardiac dysfunction and stroke. The development of AF requires a trigger and also an electroanatomic substrate capable of both initiating and perpetuating AF. In the past decade, ectopic beats originating from the pulmonary veins (PV) have been identified as a source of paroxysmal AF. Thus, strategies that target the PV, including the PV antrum, are the cornerstone of most AF ablation procedures. Recently, alternative technologies to radiofrequency catheter ablation for paroxysmal AF such as balloon ablation modalities have been developed. The purpose of this review is to discuss cryoballoon ablation for paroxysmal AF.

Three-dimensional histologic reconstruction of remnant functional accessory atrioventricular myocardial connections in a case of Wolff-Parkinson-White syndrome

Myocardial bundles working as accessory pathways in Wolff-Parkinson-White (WPW) syndrome are generally tiny tissues, so elucidating the culprit histology of atrioventricular (AV) myocardial connections requires careful serial sectioning of the AV junction. We performed a postmortem examination of accessory AV myocardial connections in an 84-year-old man who died from pneumonia 20 years after surgical cryoablation for WPW syndrome. Three-dimensional reconstruction images of serial histologic sections revealed accessory AV connections between the atrial and ventricular myocardium in the vicinity of the cryoablation scar. The remnant myocardial bridge was 4 mm wide and made up of multiple discontinuous fibers. This case was informative in that it provided for visualization of the histologic morphology of a remnant bundle of Kent.

Cryotherapy increases features of plaque stability in atherosclerotic rabbits

AIMS

In the last 10 years, cryotherapy has been investigated as a new technology to treat vascular disease. The efficiency of cryotherapy in stabilising atherosclerotic plaques has never been described. The purpose of the present study was to evaluate the effect of catheter-based cryotherapy on atherosclerotic plaque composition in a rabbit model of atherosclerosis.

METHODS AND RESULTS

Twenty-four New Zealand white rabbits were fed a 0.3% cholesterol-supplemented diet for 24 weeks. At two predefined sites of the atherosclerotic thoracic aorta, catheter-based cryotherapy, applying either single-dose, double-dose cryotherapy or control inflation, was performed after randomisation. Rabbits were continued on a cholesterol-supplemented diet for one day (acute) or four weeks (chronic). One day after cryotherapy, apoptotic cell death of smooth muscle cells (SMCs) and endothelial cells (ECs) was observed, whereas macrophages were unaffected. Four weeks later, the amount of SMCs was restored, the EC layer was regenerated, and a subendothelial macrophage-free layer was formed, indicative of a more stable plaque. In addition, both the thickness and the type I collagen content of the fibrous cap were increased.

CONCLUSIONS

The present study demonstrated that cryotherapy is feasible and appears to stabilise atherosclerotic plaques in a rabbit model.

Preexcitation Syndromes

The classic electrocardiogram in Wolff-Parkinson-White (WPW) syndrome is characterized by a short PR interval and prolonged QRS duration in the presence of sinus rhythm with initial slurring. The clinical syndrome associated with above electrocardiogram finding and the history of paroxysmal supraventricular tachycardia is referred to as Wolff-Parkinson-White syndrome. Various eponyms describing accessory or anomalous conduction pathways in addition to the normal pathway are collectively referred to as preexcitation syndromes. The latter form and associated eponyms are frequently used in literature despite controversy and disagreements over their actual anatomical existence and electrophysiological significance. This communication highlights inherent deficiencies in the knowledge that has existed since the use of such eponyms began. With the advent of curative ablation, initially surgical, and then catheter based, the knowledge gaps have been mostly filled with better delineation of the anatomic and electrophysiological properties of anomalous atrioventricular pathways. It seems reasonable, therefore, to revisit the clinical and electrophysiologic role of preexcitation syndromes in current practice.

Surgical Ablation of Atrial Fibrillation Using Energy Sources

Surgical ablation, concomitant with other operations, is an option for treatment in patients with chronic atrial fibrillation. The aim of this study is to present a literature review on surgical ablation of atrial fibrillation in patients undergoing cardiac surgery, considering energy sources and return to sinus rhythm. A comprehensive survey was performed in the literature on surgical ablation of atrial fibrillation considering energy sources, sample size, study type, outcome (early and late), and return to sinus rhythm. Analyzing studies with immediate results (n=5), the percentage of return to sinus rhythm ranged from 73% to 96%, while those with long-term results (n=20) (from 12 months on) ranged from 62% to 97.7%. In both of them, there was subsequent clinical improvement of patients who underwent ablation, regardless of the energy source used. Surgical ablation of atrial fibrillation is essential for the treatment of this arrhythmia. With current technology, it may be minimally invasive, making it mandatory to perform a procedure in an attempt to revert to sinus rhythm in patients requiring heart surgery.

Atrial fibrillation ablation in the era of cryoballoon and force-sensing catheters: freeze or burn?

OPINION STATEMENT

Atrial fibrillation can adversely affect the quality of life for many patients. Though antiarrhythmic drug therapy remains an option for the treatment of atrial fibrillation, the drugs are associated with numerous side effects. Atrial fibrillation ablation has been shown to be as efficacious as antiarrhythmic drug therapy. The field of atrial fibrillation ablations has evolved over time from utilizing radiofrequency energy to using cryoenergy. Newer technologies are being developed with efforts to improve outcomes in patients undergoing atrial fibrillation ablations. This article will highlight two such technologies: cryoballoon ablation catheters and contact force-sensing catheters. These novel catheters appear to be further revolutionizing this young field in electrophysiology.

A minimally invasive cox-maze procedure: operative technique and results

OBJECTIVE

The Cox-Maze procedure (CMP) for the surgical treatment of atrial fibrillation (AF) traditionally has required a median sternotomy and cardiopulmonary bypass. This study describes a method using ablation technologies to create the full Cox-Maze lesion set through a 5- to 6-cm right minithoracotomy.

METHODS

Twenty-two consecutive patients underwent a CMP through a right mini-thoracotomy and cardiopulmonary bypass. All patients were followed prospectively with electrocardiogram and 24-hour Holter monitoring at 3, 6, and 12 months. The CMP lesion set was created using bipolar radiofrequency energy and cryotherapy.

RESULTS

There was no operative mortality or major complications.Two patients required a permanent pacemaker. Five patients (23%) had early atrial tachyarrhythmias. At last follow-up(mean, 18 ± 12 months), all the patients (n=22) were free from atrial dysrhythmias. At 3 months (n=19), 84% of patients were off antiarrhythmic drugs. At 6 months (n=18), 94% of patients were free from AF and off antiarrhythmic medications. At 12 months (n=16), 81% of patients were free from AF and off antiarrhythmic drugs and three patients remained on warfarin for a mechanical mitral valve.

CONCLUSIONS

A full CMP can be performed through a right mini-thoracotomy with outstanding short-term results. This less invasive procedure can be offered to patients without compromising efficacy.

Cryoballoon catheter ablation in atrial fibrillation

Pulmonary vein isolation with catheter ablation is an effective treatment in patients with symptomatic atrial fibrillation refractory or intolerant to antiarrhythmic medications. The cryoballoon catheter was recently approved for this procedure. In this paper, the basics of cryothermal energy ablation are reviewed including its ability of creating homogenous lesion formation, minimal destruction to surrounding vasculature, preserved tissue integrity, and lower risk of thrombus formation. Also summarized here are the publications describing the clinical experience with the cryoballoon catheter ablation in both paroxysmal and persistent atrial fibrillation, its safety and efficacy, and discussions on the technical aspect of the cryoballoon ablation procedure.

[Intraoperative interruption of an accessory pathway: is it always a long-term cure for arrhythmia?]

We report the case of a male patient who had undergone successful cardiac surgery about 20 years previously for a posteroseptal accessory pathway that caused almost incessant orthodromic AV reentrant tachycardia, but presented again recently with new onset cardiac arrhythmia. 3D mapping demonstrated scar-related reentrant tachycardia in the right atrium with a critical isthmus between the atriotomy scar and terminal crest, which could be successfully treated using irrigated-tip high-frequency catheter ablation.

Extensive scar formation and regression during heart regeneration after cryoinjury in zebrafish

The zebrafish heart has the capacity to regenerate after ventricular resection. Although this regeneration model has proved useful for the elucidation of certain regeneration mechanisms, it is based on the removal of heart tissue rather than its damage. Here, we characterize the cellular response and regenerative capacity of the zebrafish heart after cryoinjury, an alternative procedure that more closely models the pathophysiological process undergone by the human heart after myocardial infarction (MI). Localized damage was induced in 25% of the ventricle by cryocauterization (CC). During the first 24 hours post-injury, CC leads to cardiomyocyte death within the injured area and the near coronary vasculature. Cell death is followed by a rapid proliferative response in endocardium, epicardium and myocardium. During the first 3 weeks post-injury cell debris was cleared and the injured area replaced by a massive scar. The fibrotic tissue was subsequently degraded and replaced by cardiac tissue. Although animals survived CC, their hearts showed nonhomogeneous ventricular contraction and had a thickened ventricular wall, suggesting that regeneration is associated with processes resembling mammalian ventricular remodeling after acute MI. Our results provide the first evidence that, like mammalian hearts, teleost hearts undergo massive fibrosis after cardiac damage. Unlike mammals, however, the fish heart can progressively eliminate the scar and regenerate the lost myocardium, indicating that scar formation is compatible with myocardial regeneration and the existence of endogenous mechanisms of scar regression. This finding suggests that CC-induced damage in zebrafish could provide a valuable model for the study of the mechanisms of scar removal post-MI.

Endocardial hypothermia and pulmonary vein isolation with epicardial cryoablation in a porcine beating-heart model

OBJECTIVE

The objective of this study was to investigate whether epicardial cryoablation could achieve sufficient endocardial hypothermia to create transmural lesions leading to acute and sustained pulmonary vein isolation in a normothermic beating-heart model.

METHODS

Thirty-five- to 45-kg pigs underwent median sternotomy. Epicardial cryoablation was performed on the right ventricle after insertion of thermocouples. Endocardial temperatures from thermocouples were recorded continuously and correlated with the thickness of the myocardium. Thirteen animals underwent pulmonary vein isolation as a box lesion by using 5-minute epicardial cryoablation. Endocardial temperatures were measured in 5 of these animals. Ten animals survived for 7 or 30 days. Electrical isolation was tested at the time of surgical intervention and again at death. Hearts were removed en bloc and submitted for gross and microscopic examination.

RESULTS

Endocardial temperature varied inversely with tissue thickness, ranging from -60 degrees C in 5-mm-thick tissue to staying unchanged in tissue more than 10 mm thick. During pulmonary vein isolation, median endocardial temperatures were between -1 degrees C and -22 degrees C. Acute electrical isolation was achieved in all 13 animals. All except one of the animals maintained electrical isolation long-term. Histologic analysis revealed transmurality in 89% of sections, although none of the box lesions were completely transmural.

CONCLUSION

Epicardial cryoablation can produce long-term pulmonary vein isolation in a beating heart. Dose-response studies demonstrate consistent endocardial hypothermia in tissues up to 7 mm thick. To our knowledge, this is the first report documenting endocardial hypothermia during epicardial cryoablation. This technology holds promise for performing the complete maze procedure on a beating heart.

A randomized trial comparing effects of radiofrequency and cryoablation on the structural integrity of esophageal tissue

BACKGROUND

Esophageal injury is a rare, but catastrophic complication of radiofrequency (RF) pulmonary vein isolation. It is not known if cryoablation is less likely to injure esophageal tissue. The purpose of this study is to compare the effects of RF and cryoablation on the structural integrity of esophageal tissue.

METHODS AND RESULTS

Porcine esophageal tissue was sectioned into 396 strips measuring 3 mm in width by 30 mm in length. Samples were randomly assigned to receive no ablation (149 specimens in the control group), RF ablation (126 specimens) or cryoablation (121 specimens). A single ablation was administered in the center of the tissue sample. A force gauge was used to measure the tensile strength of the tissue sample in Newtons. Groups were compared using ANOVA and a Bonferroni post-test. The mean tensile strength in the control group was 2.19 N (SD, 2.17), 1.66 N (SD, 0.88) for RF ablated tissue and 1.96 N (SD, 1.68) for cryo. RF ablation resulted in a significant reduction in esophageal tensile strength when compared to control (t = 2.59), however cryo did not (t = 1.11). On microscopic evaluation RF ablation disrupted elastic fiber architecture whereas cryoablation did not.

CONCLUSIONS

Cryoablation has no significant adverse impact on the structural integrity of esophageal tissue. Cryoablation may be a safer alternative to RF for left atrial ablation and reduce the risk of esophageal injury and atrial-esophageal fistula formation.

Cryoinjury: a model of myocardial regeneration

INTRODUCTION

Although traditionally adult cardiomyocytes are thought to be unable to divide, recent observations provide evidence for cardiomyocyte proliferation after myocardial injury. Myocardial cryoinjury has been shown to be followed by neovascularization. We hypothesize that, in addition to neovascularization, cardiomyocyte proliferation after myocardial cryoinjury contributes to regeneration.

METHOD

Cryolesions were applied to the left ventricle of mouse hearts. Inflammatory cell infiltration (F4/80, neutrophils), neovascularization (CD31), and cardiomyocyte proliferation (5-bromo-2-deoxyuridine, Ki-67, mitotic spindle) were determined at different time points (2-70 days) after cryoinjury.

RESULTS

Between Days 7 and 14 after injury, a 150- and 280-fold increase in number of proliferating cardiomyocytes was observed, as compared to controls. At the same time, numerous proliferating capillaries were found in between the proliferating cardiomyocytes. Presence of high numbers of macrophages in the cryolesion preceded and coincided with this proliferation. The area of cryolesion decreased significantly between Days 7 (23+/-5%) and 14 (8+/-2%) after cryoinjury. Moreover, regeneration of viable, nonhypertrophied myocardium was observed. After 14 days, cardiomyocyte proliferation decreased to numbers observed in controls, and concomitantly, the number of macrophages strongly decreased.

CONCLUSION

Our data show that adult cardiomyocytes proliferate in sufficiently high numbers to effectuate myocardial regeneration after left ventricular cryoinjury in mice.

Safety and Feasibility of Cryothermal Ablation Within the Mid‐ and Distal Coronary Sinus

INTRODUCTION

The aim of this study was to assess the safety and feasibility of catheter-based cryothermal ablation lesions in the mid- and distal coronary sinus.

METHODS AND RESULTS

Cryothermal ablation lesions were delivered using a 7-French catheter at the mid- (n = 13) and distal (n = 12) coronary sinus in 14 swine under general anesthesia. Lesions were delivered for 2 or 4 minutes in a 1:2 randomized ratio such that seven 2-minute lesions and eighteen 4-minute lesions were delivered to a maximum negative temperature of -70 degrees C. Integrity of the circumflex artery was assessed by angiography before and after each lesion application. In five animals, arterial Doppler flow velocity was continuously monitored and coronary flow reserve assessed. Histologic assessment of the left AV ring was made after a 48-hour survival period and lesions graded for depth and transmurality. Eighteen of 25 lesions were >3 mm deep: five of seven 2-minute lesions and thirteen of eighteen 4-minute lesions. Lesions were transmural in 18 of 25 cases. Two transmural lesions were limited in depth due to their epicardial position. One 2-minute mid-coronary sinus lesion was not found. Adherent thrombus was seen grossly in the coronary sinus at one site and only on microscopic examination in three other lesions. Angiography demonstrated no arterial spasm or thrombosis. Continuous-flow Doppler remained unchanged throughout lesion production. Coronary flow reserve was unchanged (1.7 +/- 0.8 preablation vs 1.7 +/- 1.0 postablation, P = 0.6). The media and intima were preserved in all cases. Necrosis of the adventitia was seen in one arterial segment.

CONCLUSION

Catheter-based cryoablation can produce lesions in the musculature of the adjacent atrium and ventricle when accessed from the coronary sinus without significant injury to the coronary sinus or adjacent artery. This method has potential application as the ablation method of choice when such lesions are required.

Feasibility of Catheter Cryoablation in Normal Ventricular Myocardium and Healed Myocardial Infarction

Although novel cryoablation systems have recently been introduced into clinical practice for catheter ablation of supraventricular tachycardia, the feasibility of catheter cryoablation of VT is unknown. Thus, the present study evaluates catheter cryoablation of the ventricular myocardium (1) in healthy sheep and (2) of VT in chronic myocardial infarction (MI). In three healthy sheep, 21 ventricular lesions (12 left and 9 right ventricle) were created with a catheter cryoablation system. Different freeze/thaw characteristics were used for lesion creation. The mean nadir temperature was -84.1 degrees C +/- 0.9 degrees C, mean lesion volume was 175.8 +/- 170.3 mm3, and 5 of 21 lesions were transmural. Lesion dimensions were 7.5 +/- 3.1 mm (width) and 4.2 +/- 2.5 mm (depth). Left ventricular lesions were significantly larger than right ventricular lesions (262 +/- 166 vs 60.5 +/- 91.6 mm3, P=0.0025). There was no difference in lesion volume with respect to different freeze/thaw characteristics. Anatomically (n=3) or electrophysiologically (n=3) guided catheter cryoablation was attempted in six sheep 105 +/- 56 days after MI, three of six animals had reproducibly inducible VT with a mean cycle length of 215 +/- 34 ms prior to ablation. In these animals, five VTs were targeted for ablation. A mean of 6 +/- 3 applications for nine left ventricular lesions were applied, six of nine lesions were transmural. The mean lesion volume was 501 +/- 424 mm3. No VT was inducible in two of three animals after cryoablation using an identical stimulation protocol. Therefore, catheter cryoablation of VT in healed MI is feasible, and no acute complications were observed.

Catheter cryoablation of supraventricular arrhythmias: a painless alternative to radiofrequency energy

Cryothermy has potential advantages over RF energy for catheter ablation, including reversibility of lesion formation, catheter stability, and less procedural discomfort. Cryoablation procedures were performed in 14 patients with atrioventricular reentrant tachycardias (AVNRTs), 13 patients with accessory pathway (AP)-mediated tachycardias, and 5 patients with atrial fibrillation. The numbers of energy applications, pain scores, procedural times, and outcomes were recorded and compared with age- and sex-matched patients undergoing similar RF procedures. Cryoablation was successful in 26 of 32 patients (11/14 AVNRT, 10/13 AP, 5/5 AF) compared with 30 of 32 undergoing RF procedures, with similar numbers of energy applications and procedural times. Cryothermy was painless in all patients, and the overall procedural discomfort was significantly less than in patients treated with RF (1.3 +/- 2.2 vs 6.1 +/- 3.5). In patients with anteroseptal pathways, cryomapping successfully identified safe sites to target the delivery of energy. Cryothermy is a painless and safe alternative to RF. It may be particularly useful for catheter ablation of patients with pathways close to the atrioventricular node.

Transcatheter cryoablation of ventricular myocardium in dogs

INTRODUCTION

Surgical cryoablation, a highly effective technique used during antiarrhythmic surgery, produces voluminous, histologically uniform and discreet myocardial lesions. In contrast, radiofrequency (RF) catheter ablation, which as a result of its less invasive nature has largely supplanted antiarrhythmic surgery, produces smaller, histologically heterogeneous myocardial lesions. Since small lesion size and heterogeneity may reduce antiarrhythmic efficacy, we sought to reproduce the large, histologically homogeneous lesions created by surgical cryoablation, using a catheter cryoablation system (Cryogen, Inc., San Diego, CA) in the canine ventricle.

METHODS AND RESULTS

In seven dogs, nineteen ventricular lesions (two right and seventeen left) were created with a 10F cryoablation catheter with either a 2 or 6 mm tip. In one dog AV node ablation was also performed. For each 'freeze', catheter tip nadir temperature, lesion width, depth, and transmurality were recorded, and lesion volume calculated. Average tip nadir temperature was -79.6+/-4.9 degrees C. Cooler nadir tip temperature was associated with deeper (p=.007) and more voluminous lesions (p=.042), and a greater likelihood of lesion transmurality (p=.034). Average lesion volume was 500+/-356 mm(3). No other variables predicted lesion volume or transmurality. Histologically, the catheter cryoablation lesions were sharply demarcated and homogeneous. The single freeze performed at the AV junction produced complete AV block. One complication, catheter rupture following its repetitive use, resulted in a coronary air embolus and death.

CONCLUSION

Catheter cryoablation of canine ventricular myocardium produced voluminous, discrete, transmural lesions, which might be effective for ablation of ventricular tachycardia. Lesion volume and transmurality were dependent on catheter tip nadir temperature.

Limited posterior left atrial cryoablation in patients with chronic atrial fibrillation undergoing valvular heart sugery

OBJECTIVES

We sought to evaluate whether a limited surgical cryoablation of the posterior region of the left atrium was safe and effective in the cure of atrial fibrillation (AF) in patients with associated valvular heart disease.

BACKGROUND

Extensive surgical ablation of AF is a complex and risky procedure. The posterior region of the left atrium seems to be important in the initiation and maintenance of AF.

METHODS

In 32 patients with chronic AF who underwent heart valve surgery, linear cryolesions connecting the four pulmonary veins and the posterior mitral annulus were performed. Eighteen patients with AF who underwent valvular surgery but refused cryoablation were considered as the control group.

RESULTS

Sinus rhythm (SR) was restored in 25 (78%) of 32 patients immediately after the operation. The cryoablation procedure required 20 +/- 4 min. There were no intraoperative and perioperative complications. During the hospital period, one patient died of septicemia. Thirty-one patients reached a minimum of nine months of follow-up. Two deaths occurred but were unrelated to the procedure. Twenty (69%) of 29 patients remained in SR with cryoablation alone, and 26 (90%) of 29 patients with cryoablation, drugs and radiofrequency ablation. Three (10%) of 29 patients remained in chronic AF. Right and left atrial contractility was evident in 24 (92%) of 26 patients in SR. In control group, two deaths occurred, and SR was present in only four (25%) of 16 patients.

CONCLUSIONS

Linear cryoablation with lesions connecting the four pulmonary veins and the mitral annulus is effective in restoration and maintenance of SR in patients with heart valve disease and chronic AF. Limited left atrial cryoablation may represent a valid alternative to the maze procedure, reducing myocardial ischemic time and risk of bleeding.

Alternate energy sources for catheter ablation

Because of the limitations of conventional radiofrequency ablation in creating large or linear lesions, alternative energy sources have been used as possible methods of catheter ablation. Modified radiofrequency energy, cryoablation, and microwave, laser, and ultrasound technologies may be able to create longer, deeper, and more controlled lesions and may be particularly suited for the treatment of ventricular tachycardias and for linear atrial ablation. Future studies will establish the efficacy of these new and promising technologies.

Cryothermal ablation: mechanism of tissue injury and current experience in the treatment of tachyarrhythmias

Cryosurgery has been an integral part of the surgical management of cardiac arrhythmias since the late 1970s. With the recent development of intravenous cryocatheters, the use of cryothermy in the treatment of cardiac arrhythmias will increase in the near future. The following discussion includes a detailed consideration of the mode of tissue injury associated with cryothermy and a comprehensive review of cryosurgery in the management of a variety of cardiac arrhythmias. Cryosurgical management of supraventricular and ventricular tachycardias has proven to be both safe and effective. Cryothermal tissue injury is distinguished from hyperthermic injury by the preservation of basic underlying tissue architecture and minimal thrombus formation. Such differences will be particularly important in settings requiring extensive lesion formation, such as catheter-based maze procedures for the treatment of atrial fibrillation.

Thromboembolic complications of cardiac radiofrequency catheter ablation: a review of the reported incidence, pathogenesis and current research directions

INTRODUCTION

Radiofrequency catheter ablation (RFCA) has become established as an effective therapy for the treatment of many cardiac tachyarrhythmias. The principle limitation of conventional RFCA continues to be the risk of thromboembolism. This risk is of particular concern for the ongoing development of the catheter maze procedure for the treatment of atrial fibrillation, which currently involves the creation of extensive linear lesions in the left atrium.

METHODS AND RESULTS

A Medline search of the literature over the last ten years was performed. Focused review of the reported thromboembolic complications of RFCA indicates an overall incidence of 0.6%. However, the risk is increased when ablation is performed in the left heart (1.8% to 2%) and for ventricular tachycardia (2.8%). It is of concern that intravenous heparin and the use of temperature feedback to control radiofrequency current do not eliminate the risk of thromboembolic events.

CONCLUSION

The thromboembolic complications of RFCA are not eliminated by the treatment of intravenous herapin and mode of temperature control during ablation. Potential approaches to further reduce the risk of thromboembolism include the adjunctive administration of specific inhibitors of platelet activation and aggregation, intraprocedural intracardiac echocardiography, irrigated radiofrequency ablation, and cryoablation catheter systems.

Experimental application of microwave tissue coagulation to ventricular myocardium

BACKGROUND

It is difficult to create transmural lesions in a beating, normothermic perfused heart. The aim of this study was to evaluate the effect of microwave tissue coagulation on a beating heart.

METHOD

We used a microwave tissue coagulator that emits microwaves of 2,450 MHz. Studies were conducted on 30 mongrel dogs weighing between 9 and 13 kg, and microwave tissue coagulation was performed at the free wall of the left ventricle in a beating heart without cardiopulmonary bypass.

RESULTS

Microwave tissue coagulation created transmural degenerated lesions in the left ventricle without risk of ventricular rupture. The lesion width of microwave ablation increased from 10 to 60 seconds. Histologic examinations revealed well-demarcated areas of heat degeneration consisting of coagulation necrosis and contraction band necrosis of the myocardium. The lesion healed to hard scar tissue, which was sharply demarcated from the normal myocardium. No animals had inducible ventricular tachycardia through programmed ventricular stimulation.

CONCLUSIONS

Microwave ablation with a monopolar antenna created transmural lesions with only a few proarrhythmic events occurring during ablation.

Prediction of lesion size through monitoring the 0 degree C isothermic period following transcatheter cryoablation

A prototype steerable 8.5 F bipolar catheter fitted with a feedback thermocouple was tested in 7 anaesthetized pigs (30 kg) guided by the electrocardiogram in order to modify the AV nodal and His-Purkinje system conductive properties. Thermal energy was delivered by a pressurized N2O tank (> 650 psi) via a cardiac cryo unit (Spembly, Hampshire, UK) into the catheter wherein gas expands resulting in a tip temperature as low as -70 +/- 2 degrees C within 10 seconds. Cryoablation under fluoroscopic and electrocardiographic guidance was applied at distinct sites in both ventricles for 60 or 120 seconds. After a follow-up period of 6 weeks, the ablation lesions found were well demarcated with small margins of hypertrophy of myocardial cells. With respect to lesion volume variability (8-207 mm3) and geometry, a relationship between the 0 degree C isothermic period and cryolesion volume was found. Results of an in vitro model corroborated this relationship. Therefore, an isothermic period probably can predict the lesion size and its geometry in terms of lesion depth. This potential therapeutic mode of transcatheter cryoablation deserves further investigation.

Transvenous cold mapping and cryoablation of the AV node in dogs: observations of chronic lesions and comparison to those obtained using radiofrequency ablation

INTRODUCTION

Radiofrequency (RF) is the most commonly used energy source for the treatment of cardiac arrhythmias. Surgical experience has shown that cryoablation also is effective for ablating arrhythmias. The aims of this study were to (1) investigate the feasibility of inducing permanent complete AV block (CAVB), (2) investigate the value of cold mapping to select the cryoablation site to produce permanent CAVB, (3) study the macro- and microscopic lesion characteristics 6 weeks later, and (4) compare them to those produced with RF energy.

METHODS AND RESULTS

A new steerable 8.5-French bipolar electrode catheter having a thermocouple with a 3-mm tip using N2O as the refrigerant controlled by a cryoconsole was used. Six mongrel dogs were anesthetized, and the catheter was positioned via the femoral vein across the tricuspid valve to record a large low right atrial and a small His-bundle potential. After cold mapping (-15 degrees to -20 degrees C tip temperature) resulted in ECG modifications, cryothermia (-70 degrees C) was given twice, lasting 5 minutes each, to create permanent CAVB (Cryo group). Additionally, RF catheter ablation of the AV node was performed in two anesthetized mongrel dogs (RF group). In the Cryo group, a permanent proximal CAVB was created in four dogs (block occurred within 10 to 20 sec of cryothermia). Permanent right bundle branch block was obtained in one dog and transient CAVB in the remaining dog. In both dogs of the RF group, permanent CAVB was obtained. The cryolesions consisted of well-circumscribed, homogeneous areas of fibrotic tissue without viable cardiomyocytes. Lesions produced with RF were less circumscribed and inhomogeneous, with clear evidence of viable cardiomyocytes and cartilage formation (patchy lesions).

CONCLUSIONS

(1) Permanent CAVB can be created by using a steerable cryoablation catheter. (2) Histologically, cryoablated sites were homogeneous and showed fibrotic tissue without signs of chronic inflammation and no evidence of viable myocytes. (3) Lesions created with RF were less homogenous and still contained viable myocytes within the lesion and cartilage formation. (4) The arrhythmogenic significance of these differences requires further study. (5) The technology of using reversible cold mapping has the potential to identify the successful ablation site and warrants further clinical study.

[High frequency current catheter ablation of accessory conduction pathways]

Radiofrequency catheter ablation has established as the first line therapy for the curative treatment of patients with accessory pathway. Atrioventricular accessory pathways irrespective of the exact localisation can be successfully ablated in more than 90% of all cases. Severe complications associated with the ablation procedure are rare and occur in approximately 2% of patients treated. The recurrence rate after successful ablation is approximately 8 to 10%. Recurrence of accessory pathway conduction occurs almost exclusively within the first 3 months following successful ablation, late recurrences are rare. Patients with variants of accessory pathways such as atriofascicular pathways or retrogradely conducting accessory pathways with decremental conduction properties can also be cured with a high success rate. Because of its well balanced efficacy-risk profile radiofrequency catheter ablation should be recommended as the first line therapy to all symptomatic patients with accessory pathway.

Insights into the electrophysiology of accessory pathway-mediated arrhythmias provided by the catheter ablation experience: "learning while burning, part III"

The success of catheter ablation has greatly improved the care of patients with paroxysmal tachycardias and has caused a revolution in the practice of electrophysiology. Some investigators have expressed that concern over procedural success in an increasingly interventional specialty threatens to eclipse attempts to understand the physiology of arrhythmia syndromes. Alternatively, due to the precise and directed nature of the lesions created with radiofrequency energy, catheter ablation procedures have allowed investigation to continue at a more focused level. In this article, the insights provided by the catheter ablation experience into the physiology of arrhythmias mediated by accessory AV pathways will be reviewed. Although the learning process was sometimes delayed by the nearly immediate success of radiofrequency catheter ablation, difficult situations have continued to renew efforts for understanding at a deeper level. Conscious attempts at "learning while burning" will provide the opportunity to investigate aspects of bypass tract physiology that remain incompletely characterized, such as partial response to therapy and late recurrence.

Histologic findings of long-term cryolesions in a patient with ventricular tachycardia

This report describes cardiac morphologic alterations in a patient who died 14 months following cryoablation. Histologic examination of the long-term cryolesion revealed a homogeneous fibrous scar with surrounding sinusoid capillary proliferation. The border between the fibrosis and the normal myocardium was sharply defined. Cryothermia is an excellent technique for the surgical correction of arrhythmias because it provides sharply demarcated lesions which lack arrhythmogenic potential.

Catheter ablation of the accessory pathways of the Wolff-Parkinson-White syndrome and its variants

The basis of arrhythmias in the Wolff-Parkinson-White (WPW) syndrome and its variants is the presence of accessory atrioventricular connections. Those variants include the concealed form of the WPW syndrome, the permanent form of junctional reciprocating tachycardia, and Mahaim preexcitation. In all forms of symptomatic WPW syndrome, catheter ablation of the accessory atrioventricular connections using radiofrequency current has become the treatment of choice. This review traces the development of this therapy, outlines the basics of the technique, summarizes the results reported in the largest series, indicate remaining areas of controversy, and discusses the indications and limitations of radiofrequency ablation therapy.

Catheter induced mechanical stunning of accessory pathway conduction: useful guide to successful transcatheter ablation of accessory pathways

Established electrophysiological criteria indicating anatomical proximity to an accessory pathway include early ventricular or atrial activation during antegrade or retrograde accessory pathway conduction, recording of accessory pathway potentials, and pace map concordance. This article describes two cases of RF catheter ablation of accessory pathways, during which positioning of the mapping catheter at specific sites on the endocardial aspect of the atrioventricular annulus led to prolongation of accessory pathway refractoriness and/or slowing of conduction. RF energy application at these sites successfully abolished accessory pathway conduction. When observed on an "internal" basis during catheter mapping, catheter induced stunning of accessory pathway conduction provides evidence of satisfactory electrode-tissue contact in addition to anatomical proximity, and may give additional predictive value to successful transcatheter accessory pathway ablation.

Physics and engineering of transcatheter cardiac tissue ablation

Ablation of arrhythmogenic cardiac tissues has emerged as one of the most important advances in cardiac electrophysiology. With the introduction of transcatheter ablation, the treatment of ventricular tachycardia, Wolff-Parkinson-White syndrome and other cardiac arrhythmias has progressed from an expensive and painful surgical therapy accompanied by a long recovery period to the less expensive, less traumatic transcatheter approach. The feasibility of cardiac ablation, along with the increasing number of physicians using the technique, requires understanding of the anatomic and electrophysiologic bases of transcatheter ablation as well as the different technologies, their limitations and complications. This report provides an overview of the physical, scientific and technical aspects of cardiac ablation performed with the methods currently available and a summary of the limitations of each method and expected future technologic developments in this growing field. Emphasis is placed on radiofrequency and direct current energies, the primary methods now used. Methods such as cryoablation and laser, and microwave and chemical ablation are discussed with less detail because the method of delivering energy for these ablative procedures has not been fully developed.

Surgical treatment of Wolff-Parkinson-White syndrome: epicardial approach without the use of cardiopulmonary bypass

Epicardial dissection without the use of cardiopulmonary bypass (CPB) was performed in 88 patients (56 males and 32 females, mean age 31.9 years). With intraoperative epicardial mapping, 101 accessory pathways were detected, with multiple pathways in 11 patients. CPB was avoided in all but one patient due to frequent onset of atrial fibrillation with rapid ventricular rate. Surgical ablation was successful in 86 patients (97.6%). Three patients required multiple surgical procedures because of persistence of conduction along a component of the original pathway. All but two patients were discharged without antiarrhythmic medication; these two patients were given quinidine therapy because of atrial fibrillation, but had normal early and late electrophysiological studies. Surgical ablation of Kent bundles by the epicardial approach for the treatment of Wolff-Parkinson-White syndrome can be achieved without the use of CPB. Optimal and steady exposure of the area are mandatory for the procedure, and dissection is eased by avoidance of heparin required for CPB.

Effects of cryothermia during cold cardioplegia on epicardial and intramural coronary arteries

Cryosurgery is playing an increasingly important role in the surgical treatment of various supraventricular and ventricular tachyarrhythmias. The short-term and long-term effects of cryogenic injury on epicardial and intramural coronary arteries were studied in 22 dogs. Transmural cryolesions encompassing a posterolateral branch of the circumflex coronary artery were produced in the inferior left ventricular wall during extracorporeal circulation and cold cardioplegic arrest. The mean epicardial cryolesion area +/- standard deviation was 10.4 +/- 1.8 cm2. The mean epicardial coronary artery diameter +/- standard deviation measured 1.2 +/- 0.4 mm. At 6 hours, no important structural changes were noted in any of 6 dogs. At 48 hours, 1 of 2 epicardial coronary arteries showed recent thrombus. At 1, 3, and 6 months, the epicardial coronary arteries were occluded due to thrombosis and intimal hyperplasia in 13 of 14 dogs. A limited degree of recanalization was observed. At all follow-up intervals, the intramural coronary arteries exhibited a histologic pattern similar to that of the epicardial coronary arteries. It is concluded that the exposure of major epicardial coronary arteries to cryoinjury during cold cardioplegic arrest should be avoided where possible.

Transvenous cryoablation of the bundle of His

Cardiac dysrhythmias are a prominent cause of morbidity and mortality. Pharmacological treatment is ineffective in a large number of patients and is associated with many serious side effects. Thus, direct treatment of cardiac arrhythmias has been used with increasing frequency. Each form of direct treatment, such as surgical ablation, DC catheter ablation, radiofrequency catheter ablation, laser catheter ablation suffer serious drawbacks. Thus, we investigated the utility of transvenous catheter cryoablation of the bundle of His in five miniature swine, 40-60 lbs. in weight. Complete atrioventricular block was produced in each animal during cryothermia and persisted for 1 hour of observation in four out of five swine. In the fifth animal, 2:1 atrioventricular block within the atrioventricular node persisted for 1 hour of observation. Morphological and histologic examination revealed no dysfunction of capillaries and myofibriles in the atrioventricular node and proximal bundle of His. This potential mode of transcatheter therapy deserves further investigation.

The role of cryosurgery in the management of cardiac arrhythmias

In the late 1970s, cryoablation of the AV node, accessory pathways, and ventricular tachycardia was first demonstrated and the technique was thought likely to assume an increasing importance in the surgical management of cardiac arrhythmias. However, more than 10 years later, cryotherapy is relatively sparingly used in these situations, and is at best an adjunctive means of therapy. The principal reason for this may lie in what was thought to be its major advantage: the fact that it is a highly selective, precise means of ablating myocardial tissue. Whereas electrophysiological mapping of tachycardia is able to identify apparently localized areas of arrhythmia substrate, relatively wide surgical destruction of myocardial tissue is frequently required to ensure successful tachycardia abolition: discrete, precise means of ablation are at a disadvantage. The future role for cryosurgery would seem to lie in those areas of arrhythmia management where selective ablation of substrate is essential. The ability of cryosurgery to modify (rather than simply ablate) AV nodal physiology in patients with AV nodal re-entrant tachycardia is such that it is likely to rival the recently reported catheter techniques for modification of the AV node.