Histopathology and transmurality of acute microwave lesions on the beating human atrium
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《血管的通路杂志》
a Department of Surgery, Duke University Medical Center, Division of Cardiovascular and Thoracic Surgery, Durham, NC 27710, USA
b Duke University Medical Center, Department of Pathology, Durham, NC, USA
Abstract
Microwave energy allows thoracoscopic beating-heart ablation for the treatment of atrial fibrillation. However, there is a paucity of data on the histologic effects of microwave energy on the beating human heart. This study aims to histopathologically characterize microwave lesions on the beating human atrium. Microwave energy was applied prior to cardiectomy on the beating native right atrium in eight patients undergoing heart transplantation and as a circumferential left atrial ‘box’ lesion in one patient undergoing heart-lung transplantation. Lesions were applied following heparinization and cannulation, but before initiation of cardiopulmonary bypass. Following cardiectomy, specimens were resected, fixed and subjected to histologic preparation. Grossly, all atrial lesions were ‘comma-shaped’ with an area of maximum injury on the surface. Microscopically, myocyte injury manifested as acute coagulation necrosis with hypereosinophilic myocytes with both nuclear loss and pyknosis. Contraction bands were noted at the periphery of lesions. The injury was transmural in all right atrial lesions. The left atrial sample contained a circumferential lesion ranging from 0.1 to 0.8 cm in width. The cut edge demonstrated lesion depths of 0.2–0.6 cm, maximum (transmural) in the inferior margin. Microwave ablation represents an acceptable energy source to create characteristic lesions on the beating human atrium.
Key Words: Atrial fibrillation; Microwave ablation; Human atrium; Histopathology; Pulmonary veins
1. Introduction
Electrophysiologic studies have implicated abnormal atrial tissue in the propagation of electrical re-entrant wavelets constituting atrial fibrillation (AF) [1]. In particular, the pulmonary veins appear to serve as the trigger for this re-entry phenomenon in a vast majority of patients with paroxysmal AF [2]. As a result, pulmonary vein isolation remains the focus of surgical ablation for paroxysmal AF, whereas both right and left atrial lesions may be justified in attempting to treat chronic AF. The Flex 10 [Guidant Corporation, Santa Clara, CA] microwave probe enables thoracoscopic beating heart ablation. The pulmonary vein ‘box’ lesion created with this probe has been studied in dogs [3], however, there is a paucity of literature on the effects of microwave energy on beating human cardiac tissues likely to be the targets of ablation, such as the free wall of the atrium or the pulmonary veins. The aim of this work was to histopathologically characterize microwave lesions created with a Flex 10 probe on the beating human right and left atrium.
2. Materials and methods
This study was pre-approved by the institutional review board on human research and informed consent for participation was obtained from all patients. There was no ablated tissue remaining in transplant recipients at the end of the procedure. This precluded us from analyzing left atrial ablation sites on isolated heart transplant recipients since, with our current heart transplant technique, a significant portion of the recipient left atrium is preserved. However, microwave ablation performed on a patient undergoing cardiopulmonary transplant offered the unique ability to investigate ablative effects on left atrial tissue.
Using the Flex 10 microwave ablation probe and microwave surgical ablation system, microwave energy was applied to the right atrial free wall in eight patients undergoing heart transplantation just prior to cardiectomy. Ablation was performed following heparinization and cannulation, but before the institution of cardiopulmonary bypass, at 65 watts for 90 s. Visual evidence of satisfactory microwave application was verified. Following cardiectomy, ablated portions of the right atrium were widely resected and placed in 10% formalin and stored at 4 °C until sectioning. Sections of the area of maximum injury and the injury margin were taken based upon gross inspection. Standard histology with hematoxylin and eosin and Masson's trichrome stain was performed.
For ablation of left atrial tissue, a patient listed for cardiopulmonary transplant and carrying the diagnosis of severe pulmonary sarcoidosis and resultant pulmonary hypertension was enrolled in this investigation. A circumferential left atrial ‘box’ lesion was created with the Flex 10 probe in this patient while undergoing heart-lung transplantation. Circumferential lesions were each applied individually and in sequence at 65 watts for 90 s. These settings were selected based upon those commonly used in the clinical setting [4]. The lesions were performed following heparinization and cannulation, but before the institution of cardiopulmonary bypass. Following cardiopulmonectomy, the circumference of the left atrium containing the lesions and the proximal pulmonary veins was resected, fixed in 10% formalin and stored at 4 °C until sectioning. Standard histology with hematoxylin and eosin and Masson's trichrome stain was performed at the margin of the lesions. Multiple sections were submitted demonstrating circumferential, tangential sections for depth and perpendicular sections for breadth of the lesion.
3. Results
Eight patients undergoing heart transplantation were subjected to right atrial microwave ablation as previously described (Table 1). On gross inspection, satisfactory ablation bands were present on the epicardial surface. The microwave lesions were ‘comma-shaped’ with an area of maximum injury on the surface and a tail of diminishing injury. Lesions were generally over epicardium free of adipose tissue but occasionally there was adipose tissue present at the injury site.
Microscopically, injury in all cases was localized to within a few myocytes of the superficial margins. Myocyte injury was manifested as acute coagulation necrosis with anucleate, hypereosinophilic myocytes with both nuclear loss and pyknosis (Fig. 1). Such hypereosinophilia is likely a result of enhanced eosin binding to denatured proteins and the loss of cytoplasmic RNA with its corresponding basophilia. Contraction bands were noted at the periphery of lesions in most instances (Fig. 2). These hypereosinophilic bands run at right angles to the long axis of the cardiac myocyte and represent, in this case, thermal necrosis resulting in condensed contractile proteins. The maximum injury was transmural in all lesions (Fig. 3) with right atrial thickness measuring up to 0.6 cm. Penetrating vessels demonstrated injury with mural and endothelial coagulative necrosis. Additionally, although it would be less likely to detect thrombus formation immediately after ablation performed under effective heparinization, no thrombosis was seen in these studies. Separate ablations performed with overlying adipose tissue at the site of ablation revealed a diminished penetrance of the microwave energy (Fig. 4).
A 46-year-old black female with cardiopulmonary sarcoidosis undergoing heart-lung transplantation was subjected to sequential pulmonary vein ablation as previously described. The maximum achieved probe temperature was measured at 48.0 °C. The epicardial lesion was clearly observed circumferentially around the left atrium, including the thoracoscopically ‘blind’ areas in between the two superior and inferior pulmonary veins. The formalin-preserved specimen contained a ring of left atrial tissue including the proximal portions of the right superior and inferior pulmonary veins as well as the ostia of the left pulmonary veins. Along the resection margin, there was a pale tan, circumferential ablation lesion that ranged in width from 0.1 to 0.8 cm. Examination of the cut edge demonstrated lesion depths of 0.2–0.6 cm, maximum (transmural) in the inferior margin. No acute thrombus was seen overlying the endocardial surface of the submitted ring/ablation lesion, though focal small areas of mural hemorrhage were noted. Hematoxylin and eosin staining of paraffin embedded sections demonstrated myocyte injury manifested as acute coagulation necrosis with hypereosinophilic myocytes with both nuclear loss and pyknosis. The necrosis was only focally transmural (Fig. 5). Multiple small foci of contraction bands were noted at the periphery of the lesions. Small penetrating vessels within the ablation lesions exhibited necrosis of the endothelial cells with pyknotic nuclei and cell swelling, features that were also seen within the medial smooth muscle in some vessels (Fig. 6). Masson trichrome staining highlighted the necrotic cells as well as the foci of contraction band necrosis (Fig. 7). Also present within the sections was mild to moderate epicardial inflammation with numerous neutrophils and scattered mononuclear plasma cells and lymphocytes.
4. Discussion
Atrial fibrillation (AF) afflicts more than one percent of the population, with a prevalence increasing with advanced age [5]. Symptomatic AF is quite disabling and loss of atrial contraction may decrease cardiac function with or without increased ventricular rate. Stroke is the most serious consequence of AF and, without prophylaxis, has an estimated annual rate of 4.5% [6]. Even more concerning, strokes associated with AF are more devastating than those due to other causes [7]. These devastating consequences, combined with the frequent failure and complications with pharmacologic management, have stimulated investigation into surgical treatment for AF.
Experimental and human surgical mapping studies have shown AF to be perpetuated by re-entrant wavelets propagating in an abnormal atrial tissue substrate [1]. Evidence suggests that the pulmonary veins harbor triggers for these wavelets in >90% of patients with paroxysmal AF [2]. The elegant and pioneering work of Cox et al. exploits this phenomenon to cure AF by surgically creating lesions that block re-entrant wavelets [8]. This novel technique has enjoyed widespread success, electrically curing AF in 86% of patients in just one month postoperatively [9]. Much of this success can be attributed to the transmural nature of the operation. Nevertheless, the Cox-maze ‘cut-and-sew’ technique remains a major cardiac procedure with a low, but ever-present mortality and physiologic perturbance despite its simplified version. Atrial transport, thus reduction in stroke risk, may be lacking postoperatively. This principle of the operation has served as the impetus for catheter-based ablation attempts to cure AF. These attempts, however, have so far been protracted, frequently unsuccessful (in up to 15% of cases) and not without complications. Even in the most experienced hands, thromboembolic complications and pulmonary vein stenosis occurs in up to 2% of cases [10].
This milieu has stimulated alternative surgical approaches of creating maze-like lesions on the heart in a less invasive fashion. Energy sources such as radiofrequency, cryotherapy and microwave have been attempted to simulate lesions of the Cox-maze procedure. These techniques, when applied endocardially, may shorten the operative time that would be required to ‘cut-and-sew’. More excitingly, energy sources that could be applied epicardially may allow ablation while the heart is beating. One such source that appears to be versatile, efficient and safe is that of microwave energy [11].
Microwave (electromagnetic) energy, such as that created by the Flex 10 probe, heats tissue by inducing dielectric losses in polar molecules such as water [12]. The versatility of the commercially available microwave probes has resulted in increasing usage of this technology [11]. The energy can be applied safely to the epicardium and could be offered thoracoscopically for lone atrial fibrillation, avoiding sternotomy. Despite this enthusiasm, there has been a paucity of basic work on the effects of microwave energy on the human heart. It remains to be proven whether microwave energy creates transmural cardiac lesions, a feature that would be a prerequisite to duplicate the procedural success of the Cox-maze procedure. To this end, a limited number of investigations have addressed this issue. Williams [13] was able to achieve 4 mm lesions with microwave ablation at 40 watts for 24 s. More recently, transmural myocardial injury was demonstrated histopathologically after the application of microwave energy to the right atrium at 65 watts for 90 s [14]. In regard to observed AF cure rates with this technology, a recent study by Knaut establishes similar outcomes to the Cox-maze procedure [15]. Using a previous generation Flex probe and a modified ablation line concept, 88% of patients with mitral valve disease, 78% with coronary artery disease and 85% with aortic valve disease were in sinus rhythm at six-month follow-up. These results were accomplished with a 97.3% survival rate. The power setting and duration of the ablation technique used in our study have been the most common ones in clinical practice, accounting for 40% power lost between the microwave generator and the probe.
The studies presented herein demonstrate that the epicardial beating-heart application of the Flex 10 microwave probe creates typical, acutely transmural lesions on the free wall of the right atrium. Importantly, transmurality was achieved on beating hearts despite a cooling effect on endocardial tissue created by continuous pulmonary vein blood flow. Additionally, a circumferentially continuous ‘box’ lesion around the left atrium can induce myocyte injury that is focally transmural and involves myocyte necrosis acutely. Interestingly, prior work by van Brakel in dogs using the Flex 10 probe demonstrates that histologic evaluation of transmurality trails electrophysiologic assessment by a matter of weeks [3]. This suggests that histopathology underestimates true lesion depth in the acute stage and could account for incomplete transmurality along the ablation interface in this patient. Thus, the clinical success of this treatment may be superior to that predicted by histologic transmurality alone. Furthermore, functional results could be achieved with less than transmural ablation through microwave denervation of epicardial ganglia. Conversely, diminished left atrial penetrance by microwave energy in this study, as well as in other recent studies, could be explained by poor transmission through the adipose-rich connection between the pulmonary vein and left atrium.
While this study was performed on cardiac transplant patients that are not necessarily representative of normal atrial fibrillation patients, the data still provide valuable insight into atrial tissue penetrance and histologic characteristics of human beating heart microwave ablation. In the application of microwave energy for pulmonary vein isolation in atrial fibrillation, penetrance through thickened and fibrotic atrial tissue must be considered. Nevertheless, this technology carries the potential to match the procedural success of the Cox-maze procedure in curing AF while allowing the flexibility to be performed on the beating heart and in a thoracoscopic fashion.
Acknowledgements
This work has been supported by funding from the Guidant Corporation, Santa Clara, CA.
References
Allessie MA, Rensma PL, Brugada J, Smeets J, Penn O, Kirchhof C. Zipes DP, Jalife J. Pathophysiology of atrial fibrillation. eds Cardiac Electrophysiology: From Cell to Bedside Philadelphia: WB Saunders, 1990:548–559. In:.
Haissaguerre M, Jais P, Shah DC, Takahashi A, Hocini M, Quiniou G, Garrigue S, Le Mouroux A, Le Metayer P, Clementy J. Spontaneous initiation of atrial fibrillation by ectopic beats originating in the pulmonary veins. N Engl J Med 1998; 339:659–666.
van Brakel TJ, Bolotin G, Salleng KJ, Nifong LW, Allessie MA, Chitwood Jr WR, Maessen JG. Evaluation of epicardial microwave ablation lesions: histology vs. electrophysiology. Ann Thorac Surg 2004; 78:1397–1402.
Manasse E, Colombo PG, Barbone A, Braidotti P, Bulfamante G, Roincalli M, Gallotti R. Clinical histopathology and ultrastructural analysis of myocardium following microwave energy ablation. Eur J Cardiothorac Surg 2003; 23:573–577.
Phillips SJ, Whisnant JP, O'Fallon WM, Frye RL. Prevalence of cardiovascular disease and diabetes mellitus in residents of Rochester, Minnesota. Mayo Clin Proc 1990; 65:344–359.
American Heart Association 1998. Heart and stroke statistical update 1998;Dallas: American Heart Association.
Leckey R, Aguilar EG, Phillips SJ. Atrial fibrillation and the use of warfarin in patients admitted to an acute stroke unit. Can J Cardiol 2000; 16:481–485.
Cox JL, Schuessler RB, Cain ME, Corr PB, Stone CM, D'Agostino Jr HJ, Harada A, Chang BC, Smith PK, Boineau JP. Surgery for atrial fibrillation. Semin Thorac Cardiovasc Surg 1989; 1:67–73.
Cox JL, Boineau JP, Schuessler RB, Kater KM, Lappas DG. Five-year experience with the maze procedure for atrial fibrillation. Ann Thorac Surg 1993; 56:814–823.
Shah D. Catheter ablation for atrial fibrillation: mechanism-based curative treatment. Expert Rev Cardiovasc Ther 2004; 2:925–933.
Williams MR, Argenziano M, Oz MC. Microwave ablation for surgical treatment of atrial fibrillation. Semin Thorac Cardiovasc Surg 2002; 14:232–237.
Johnson CC, Guy AW. Nonionizing electromagnetic wave effects in biological materials and systems. Proc IEEE 1972; 60:692–709.
Williams MR, Knaut M, Berube D, Oz MC. Application of microwave energy in cardiac tissue ablation: from in vitro analyses to clinical use. Ann Thorac Surg 2002; 74:1500–1505.
Manasse E, Colombo PG, Barbone A, Braidotti P, Bulfamante G, Roincalli M, Gallotti R. Clinical histopathology and ultrastructural analysis of myocardium following microwave energy ablation. Eur J Cardiothorac Surg 2003; 23:573–577.
Knaut M, Tugtekin SM, Jung F, Matschke K. Microwave ablation for the surgical treatment of permanent atrial fibrillation – a single center experience. Eur J Cardiothorac Surg 2004; 26:742–746.(Joseph W. Turek, Louis R.)
b Duke University Medical Center, Department of Pathology, Durham, NC, USA
Abstract
Microwave energy allows thoracoscopic beating-heart ablation for the treatment of atrial fibrillation. However, there is a paucity of data on the histologic effects of microwave energy on the beating human heart. This study aims to histopathologically characterize microwave lesions on the beating human atrium. Microwave energy was applied prior to cardiectomy on the beating native right atrium in eight patients undergoing heart transplantation and as a circumferential left atrial ‘box’ lesion in one patient undergoing heart-lung transplantation. Lesions were applied following heparinization and cannulation, but before initiation of cardiopulmonary bypass. Following cardiectomy, specimens were resected, fixed and subjected to histologic preparation. Grossly, all atrial lesions were ‘comma-shaped’ with an area of maximum injury on the surface. Microscopically, myocyte injury manifested as acute coagulation necrosis with hypereosinophilic myocytes with both nuclear loss and pyknosis. Contraction bands were noted at the periphery of lesions. The injury was transmural in all right atrial lesions. The left atrial sample contained a circumferential lesion ranging from 0.1 to 0.8 cm in width. The cut edge demonstrated lesion depths of 0.2–0.6 cm, maximum (transmural) in the inferior margin. Microwave ablation represents an acceptable energy source to create characteristic lesions on the beating human atrium.
Key Words: Atrial fibrillation; Microwave ablation; Human atrium; Histopathology; Pulmonary veins
1. Introduction
Electrophysiologic studies have implicated abnormal atrial tissue in the propagation of electrical re-entrant wavelets constituting atrial fibrillation (AF) [1]. In particular, the pulmonary veins appear to serve as the trigger for this re-entry phenomenon in a vast majority of patients with paroxysmal AF [2]. As a result, pulmonary vein isolation remains the focus of surgical ablation for paroxysmal AF, whereas both right and left atrial lesions may be justified in attempting to treat chronic AF. The Flex 10 [Guidant Corporation, Santa Clara, CA] microwave probe enables thoracoscopic beating heart ablation. The pulmonary vein ‘box’ lesion created with this probe has been studied in dogs [3], however, there is a paucity of literature on the effects of microwave energy on beating human cardiac tissues likely to be the targets of ablation, such as the free wall of the atrium or the pulmonary veins. The aim of this work was to histopathologically characterize microwave lesions created with a Flex 10 probe on the beating human right and left atrium.
2. Materials and methods
This study was pre-approved by the institutional review board on human research and informed consent for participation was obtained from all patients. There was no ablated tissue remaining in transplant recipients at the end of the procedure. This precluded us from analyzing left atrial ablation sites on isolated heart transplant recipients since, with our current heart transplant technique, a significant portion of the recipient left atrium is preserved. However, microwave ablation performed on a patient undergoing cardiopulmonary transplant offered the unique ability to investigate ablative effects on left atrial tissue.
Using the Flex 10 microwave ablation probe and microwave surgical ablation system, microwave energy was applied to the right atrial free wall in eight patients undergoing heart transplantation just prior to cardiectomy. Ablation was performed following heparinization and cannulation, but before the institution of cardiopulmonary bypass, at 65 watts for 90 s. Visual evidence of satisfactory microwave application was verified. Following cardiectomy, ablated portions of the right atrium were widely resected and placed in 10% formalin and stored at 4 °C until sectioning. Sections of the area of maximum injury and the injury margin were taken based upon gross inspection. Standard histology with hematoxylin and eosin and Masson's trichrome stain was performed.
For ablation of left atrial tissue, a patient listed for cardiopulmonary transplant and carrying the diagnosis of severe pulmonary sarcoidosis and resultant pulmonary hypertension was enrolled in this investigation. A circumferential left atrial ‘box’ lesion was created with the Flex 10 probe in this patient while undergoing heart-lung transplantation. Circumferential lesions were each applied individually and in sequence at 65 watts for 90 s. These settings were selected based upon those commonly used in the clinical setting [4]. The lesions were performed following heparinization and cannulation, but before the institution of cardiopulmonary bypass. Following cardiopulmonectomy, the circumference of the left atrium containing the lesions and the proximal pulmonary veins was resected, fixed in 10% formalin and stored at 4 °C until sectioning. Standard histology with hematoxylin and eosin and Masson's trichrome stain was performed at the margin of the lesions. Multiple sections were submitted demonstrating circumferential, tangential sections for depth and perpendicular sections for breadth of the lesion.
3. Results
Eight patients undergoing heart transplantation were subjected to right atrial microwave ablation as previously described (Table 1). On gross inspection, satisfactory ablation bands were present on the epicardial surface. The microwave lesions were ‘comma-shaped’ with an area of maximum injury on the surface and a tail of diminishing injury. Lesions were generally over epicardium free of adipose tissue but occasionally there was adipose tissue present at the injury site.
Microscopically, injury in all cases was localized to within a few myocytes of the superficial margins. Myocyte injury was manifested as acute coagulation necrosis with anucleate, hypereosinophilic myocytes with both nuclear loss and pyknosis (Fig. 1). Such hypereosinophilia is likely a result of enhanced eosin binding to denatured proteins and the loss of cytoplasmic RNA with its corresponding basophilia. Contraction bands were noted at the periphery of lesions in most instances (Fig. 2). These hypereosinophilic bands run at right angles to the long axis of the cardiac myocyte and represent, in this case, thermal necrosis resulting in condensed contractile proteins. The maximum injury was transmural in all lesions (Fig. 3) with right atrial thickness measuring up to 0.6 cm. Penetrating vessels demonstrated injury with mural and endothelial coagulative necrosis. Additionally, although it would be less likely to detect thrombus formation immediately after ablation performed under effective heparinization, no thrombosis was seen in these studies. Separate ablations performed with overlying adipose tissue at the site of ablation revealed a diminished penetrance of the microwave energy (Fig. 4).
A 46-year-old black female with cardiopulmonary sarcoidosis undergoing heart-lung transplantation was subjected to sequential pulmonary vein ablation as previously described. The maximum achieved probe temperature was measured at 48.0 °C. The epicardial lesion was clearly observed circumferentially around the left atrium, including the thoracoscopically ‘blind’ areas in between the two superior and inferior pulmonary veins. The formalin-preserved specimen contained a ring of left atrial tissue including the proximal portions of the right superior and inferior pulmonary veins as well as the ostia of the left pulmonary veins. Along the resection margin, there was a pale tan, circumferential ablation lesion that ranged in width from 0.1 to 0.8 cm. Examination of the cut edge demonstrated lesion depths of 0.2–0.6 cm, maximum (transmural) in the inferior margin. No acute thrombus was seen overlying the endocardial surface of the submitted ring/ablation lesion, though focal small areas of mural hemorrhage were noted. Hematoxylin and eosin staining of paraffin embedded sections demonstrated myocyte injury manifested as acute coagulation necrosis with hypereosinophilic myocytes with both nuclear loss and pyknosis. The necrosis was only focally transmural (Fig. 5). Multiple small foci of contraction bands were noted at the periphery of the lesions. Small penetrating vessels within the ablation lesions exhibited necrosis of the endothelial cells with pyknotic nuclei and cell swelling, features that were also seen within the medial smooth muscle in some vessels (Fig. 6). Masson trichrome staining highlighted the necrotic cells as well as the foci of contraction band necrosis (Fig. 7). Also present within the sections was mild to moderate epicardial inflammation with numerous neutrophils and scattered mononuclear plasma cells and lymphocytes.
4. Discussion
Atrial fibrillation (AF) afflicts more than one percent of the population, with a prevalence increasing with advanced age [5]. Symptomatic AF is quite disabling and loss of atrial contraction may decrease cardiac function with or without increased ventricular rate. Stroke is the most serious consequence of AF and, without prophylaxis, has an estimated annual rate of 4.5% [6]. Even more concerning, strokes associated with AF are more devastating than those due to other causes [7]. These devastating consequences, combined with the frequent failure and complications with pharmacologic management, have stimulated investigation into surgical treatment for AF.
Experimental and human surgical mapping studies have shown AF to be perpetuated by re-entrant wavelets propagating in an abnormal atrial tissue substrate [1]. Evidence suggests that the pulmonary veins harbor triggers for these wavelets in >90% of patients with paroxysmal AF [2]. The elegant and pioneering work of Cox et al. exploits this phenomenon to cure AF by surgically creating lesions that block re-entrant wavelets [8]. This novel technique has enjoyed widespread success, electrically curing AF in 86% of patients in just one month postoperatively [9]. Much of this success can be attributed to the transmural nature of the operation. Nevertheless, the Cox-maze ‘cut-and-sew’ technique remains a major cardiac procedure with a low, but ever-present mortality and physiologic perturbance despite its simplified version. Atrial transport, thus reduction in stroke risk, may be lacking postoperatively. This principle of the operation has served as the impetus for catheter-based ablation attempts to cure AF. These attempts, however, have so far been protracted, frequently unsuccessful (in up to 15% of cases) and not without complications. Even in the most experienced hands, thromboembolic complications and pulmonary vein stenosis occurs in up to 2% of cases [10].
This milieu has stimulated alternative surgical approaches of creating maze-like lesions on the heart in a less invasive fashion. Energy sources such as radiofrequency, cryotherapy and microwave have been attempted to simulate lesions of the Cox-maze procedure. These techniques, when applied endocardially, may shorten the operative time that would be required to ‘cut-and-sew’. More excitingly, energy sources that could be applied epicardially may allow ablation while the heart is beating. One such source that appears to be versatile, efficient and safe is that of microwave energy [11].
Microwave (electromagnetic) energy, such as that created by the Flex 10 probe, heats tissue by inducing dielectric losses in polar molecules such as water [12]. The versatility of the commercially available microwave probes has resulted in increasing usage of this technology [11]. The energy can be applied safely to the epicardium and could be offered thoracoscopically for lone atrial fibrillation, avoiding sternotomy. Despite this enthusiasm, there has been a paucity of basic work on the effects of microwave energy on the human heart. It remains to be proven whether microwave energy creates transmural cardiac lesions, a feature that would be a prerequisite to duplicate the procedural success of the Cox-maze procedure. To this end, a limited number of investigations have addressed this issue. Williams [13] was able to achieve 4 mm lesions with microwave ablation at 40 watts for 24 s. More recently, transmural myocardial injury was demonstrated histopathologically after the application of microwave energy to the right atrium at 65 watts for 90 s [14]. In regard to observed AF cure rates with this technology, a recent study by Knaut establishes similar outcomes to the Cox-maze procedure [15]. Using a previous generation Flex probe and a modified ablation line concept, 88% of patients with mitral valve disease, 78% with coronary artery disease and 85% with aortic valve disease were in sinus rhythm at six-month follow-up. These results were accomplished with a 97.3% survival rate. The power setting and duration of the ablation technique used in our study have been the most common ones in clinical practice, accounting for 40% power lost between the microwave generator and the probe.
The studies presented herein demonstrate that the epicardial beating-heart application of the Flex 10 microwave probe creates typical, acutely transmural lesions on the free wall of the right atrium. Importantly, transmurality was achieved on beating hearts despite a cooling effect on endocardial tissue created by continuous pulmonary vein blood flow. Additionally, a circumferentially continuous ‘box’ lesion around the left atrium can induce myocyte injury that is focally transmural and involves myocyte necrosis acutely. Interestingly, prior work by van Brakel in dogs using the Flex 10 probe demonstrates that histologic evaluation of transmurality trails electrophysiologic assessment by a matter of weeks [3]. This suggests that histopathology underestimates true lesion depth in the acute stage and could account for incomplete transmurality along the ablation interface in this patient. Thus, the clinical success of this treatment may be superior to that predicted by histologic transmurality alone. Furthermore, functional results could be achieved with less than transmural ablation through microwave denervation of epicardial ganglia. Conversely, diminished left atrial penetrance by microwave energy in this study, as well as in other recent studies, could be explained by poor transmission through the adipose-rich connection between the pulmonary vein and left atrium.
While this study was performed on cardiac transplant patients that are not necessarily representative of normal atrial fibrillation patients, the data still provide valuable insight into atrial tissue penetrance and histologic characteristics of human beating heart microwave ablation. In the application of microwave energy for pulmonary vein isolation in atrial fibrillation, penetrance through thickened and fibrotic atrial tissue must be considered. Nevertheless, this technology carries the potential to match the procedural success of the Cox-maze procedure in curing AF while allowing the flexibility to be performed on the beating heart and in a thoracoscopic fashion.
Acknowledgements
This work has been supported by funding from the Guidant Corporation, Santa Clara, CA.
References
Allessie MA, Rensma PL, Brugada J, Smeets J, Penn O, Kirchhof C. Zipes DP, Jalife J. Pathophysiology of atrial fibrillation. eds Cardiac Electrophysiology: From Cell to Bedside Philadelphia: WB Saunders, 1990:548–559. In:.
Haissaguerre M, Jais P, Shah DC, Takahashi A, Hocini M, Quiniou G, Garrigue S, Le Mouroux A, Le Metayer P, Clementy J. Spontaneous initiation of atrial fibrillation by ectopic beats originating in the pulmonary veins. N Engl J Med 1998; 339:659–666.
van Brakel TJ, Bolotin G, Salleng KJ, Nifong LW, Allessie MA, Chitwood Jr WR, Maessen JG. Evaluation of epicardial microwave ablation lesions: histology vs. electrophysiology. Ann Thorac Surg 2004; 78:1397–1402.
Manasse E, Colombo PG, Barbone A, Braidotti P, Bulfamante G, Roincalli M, Gallotti R. Clinical histopathology and ultrastructural analysis of myocardium following microwave energy ablation. Eur J Cardiothorac Surg 2003; 23:573–577.
Phillips SJ, Whisnant JP, O'Fallon WM, Frye RL. Prevalence of cardiovascular disease and diabetes mellitus in residents of Rochester, Minnesota. Mayo Clin Proc 1990; 65:344–359.
American Heart Association 1998. Heart and stroke statistical update 1998;Dallas: American Heart Association.
Leckey R, Aguilar EG, Phillips SJ. Atrial fibrillation and the use of warfarin in patients admitted to an acute stroke unit. Can J Cardiol 2000; 16:481–485.
Cox JL, Schuessler RB, Cain ME, Corr PB, Stone CM, D'Agostino Jr HJ, Harada A, Chang BC, Smith PK, Boineau JP. Surgery for atrial fibrillation. Semin Thorac Cardiovasc Surg 1989; 1:67–73.
Cox JL, Boineau JP, Schuessler RB, Kater KM, Lappas DG. Five-year experience with the maze procedure for atrial fibrillation. Ann Thorac Surg 1993; 56:814–823.
Shah D. Catheter ablation for atrial fibrillation: mechanism-based curative treatment. Expert Rev Cardiovasc Ther 2004; 2:925–933.
Williams MR, Argenziano M, Oz MC. Microwave ablation for surgical treatment of atrial fibrillation. Semin Thorac Cardiovasc Surg 2002; 14:232–237.
Johnson CC, Guy AW. Nonionizing electromagnetic wave effects in biological materials and systems. Proc IEEE 1972; 60:692–709.
Williams MR, Knaut M, Berube D, Oz MC. Application of microwave energy in cardiac tissue ablation: from in vitro analyses to clinical use. Ann Thorac Surg 2002; 74:1500–1505.
Manasse E, Colombo PG, Barbone A, Braidotti P, Bulfamante G, Roincalli M, Gallotti R. Clinical histopathology and ultrastructural analysis of myocardium following microwave energy ablation. Eur J Cardiothorac Surg 2003; 23:573–577.
Knaut M, Tugtekin SM, Jung F, Matschke K. Microwave ablation for the surgical treatment of permanent atrial fibrillation – a single center experience. Eur J Cardiothorac Surg 2004; 26:742–746.(Joseph W. Turek, Louis R.)