Endocardial Microwave Ablation: A New Surgical Approach for Atrial Fibrillation

Mark Levinson, MD
Chief, Cardiothoracic Surgery
Hutchinson Hospital
Hutchinson, Kansas USA

Atrial fibrillation (AF) is a major medical problem in the adult population. It is estimated that 1% of adult Americans suffer from atrial fibrillation. AF is typified by its rate of occurrence and persistence. Paroxysmal AF occurs intermittently, with episodes lasting a few minutes or days. Paroxysmal AF initiates and terminates spontaneously. Persistent AF, which does not convert back to normal sinus rhythm by itself, can be cardioverted using pharmaceutical therapy and/or electrical shock. Permanent AF is present continually and by definition cannot be cardioverted.

Although the rhythm is benign when compared to malignant ventricular irritability, the consequences of atrial fibrillation definitely affect patients in an adverse way. Multiple side effects occur once the patient develops atrial fibrillation, including tachycardia, palpitations, thromboemboli, and a 15% to 20% decrease in cardiac output from the loss of atrial priming of ventricular filling (the "atrial kick"). Medications for control of atrial fibrillation are effective, but cost money and are inconvenient for the patients to continue for many years. Also, it is no small problem to maintain safe anticoagulation for decades.

A direct cure of any form of this arrhythmia would be a major advance. Several approaches have already been advanced, including ablation of the AV node with permanent pacing, surgical incisions in the atria to block re-entrant wavefronts, and transcatheter abalation. AV nodal ablation is not a cure by any means. It is quick, but is only helpful to prevent the tachycardias and palpitations. At the same time, it commits the patient to complete heart block and pacemaker dependency for life. There is no restoration of atrial contraction and thus minimal improvement in cardiac output. Current dual chamber pacemakers are very reliable, but are not without problems.

Besides medical therapy. the only proven cure of all forms of atrial fibrillation is the Cox Maze procedure. This operation is the result of decades of continuous basic science research by James Cox, MD and his laboratory staff. Dr. Cox began his distinguished career at Duke University under the famed and pioneering arrythmia surgeon Will Sealy, inventor of the surgical procedure for abalation of accessory AV pathways (i.e. Wolff-Parkinson-White syndrome). After his tutelage in the Duke program, Dr. Cox continued his original research into the mechanism of atrial arrythmias at Washington University, St. Louis. During his tenure there, Dr. Cox's laboratory researchers used multiple atrial sensing probes to discover the timing and conduction pathways of the atrial fibrillation mechanism [Cox 1991a]. Through steady research, their team formed a surgical procedure to interfere with the onset of atrial fibrillation while preserving the pathway for sinus activity to reach the AV node [Cox 1991b, Cox 1995]. Their goal was to create an operation that would safely prevent atrial fibrillation, restore the atrial kick and its contribution to cardiac output, and to reduce the risk of thromboembolism.

The procedure developed by Cox did succeed in its mission, but at a price. First, the procedure is difficult to learn. Publications show a complex geometry of incisions that are hard to communicate to surgeons that have not actually seen the procedure before. The length of time it takes to close these incisions is significant, lengthening pump times and cross clamp times. There are significant bleeding complications from these long suture lines and pump times.

Cox has reported a 99% success rate in conversion to sinus rhythm long term [Cox 2000]. However, his excellent results have not been easy for others to duplicate. His experience with research, mapping, development, and patient selection over decades has allowed him to achieve dramatic results, but others have not been so fortunate. Thus, the Cox Maze has not been widely adopted. When combined with mitral valve surgery, the operations are long and tedious. Also, the Cox Maze is not ideally suited for atrial fibrillation associated with either coronary artery disease or aortic valve disease. In these situations, the Maze can prolong and complicate an otherwise simple straightforward procedure.

A recent breakthrough in the understanding of the electrophysiologic mechanisms of paroxysmal atrial fibrillation were published in 1998 by Haissaguerre and colleaugues from Bordeaux-Pessac, France [Haissaguerre 1998]. Their monumental paper in the New England Journal of Medicine reported the results of detailed mapping inside the left atrium in patients with paroxysmal atrial fibrillation. Haissaguerre attempted to find the earliest atrial extrasystole that initiated an episode of atrial fibrillation. In their carefully selected group of 45 patients with trans-septal catheters, a single point of origin was identified in 29 (65%) patients. Radio frequency ablation abolished these foci and cured the atrial fibrillation in 62% of patients [Haissaguerre 1998].

This study created a significant new avenue for surgical treatment of atrial fibrillation. New attempts for blocking the origin of AF quickly evolved. Taking root from the original Maze operation, a series of new lesion patterns were quickly reported that attempted to isolate these premature foci in the pulmonary vein from the body of the left atrium, and to perform connecting ablation lines to avoid macro-reentry circuits around the isolating lesions.

Melo et al reported a technique for radio frequency pulmonary vein isolation in association with mitral valve surgery. [Melo 2000]. He used a multipolar catheter adapted from equipment already in use in the catheterization laboratory. Several reports of surgical incisions or electrocautery around the pulmonary veins also confirmed the efficacy of isolating these foci [Simha 2001, Sueda 2001, Kalil 2002].

Until now, these procedures were not suited for use on the beating heart. However, there are a significant number of patients where opening the left atrium is time consuming under cross clamping and cardioplegic arrest. Alternatives that can be used in both open and closed chambers procedures are now being offered. Microwave abalation is one of those options.

AFx, Inc. has introduced a series of microwave ablation probes that are FDA approved for ablation of cardiac tissue. Microwave energy causes tissue heating by inducing dielectric losses in polar molecules such as water [Williams 2002]. The advantages of microwave over radiofrequency is a deeper and more even penetration with less surface heating. The AFx probes create a single linear line of tissue coagulation that is clean, without boiling, charring, smoking, or perforation. Energy delivery times are short, on the order of 25 to 60 seconds. The probes can heat the tissue sufficiently in this short exposure time to create an even, and symmetrical lesion of predictable and controllable depth (4 to 5 mm) and 5 mm in width. In addition, flowing blood underneath the probe is moving too fast to be injured and thus the energy source is safe to use on the beating heart. Furthermore, the unidirectionality of the microwave ablation probes insures the protection of surrounding tissues during epicardial application, which is a significant requirement for off-pump ablation procedures on the beating-heart.

The AFx microwave probes are ideally suited for a full spectrum of surgical ablation procedure from a simple pulmonary vein isolation in paroxysmal AF to the full Maze for permanent AF. The antenna in these probes is malleable, allowing the surgeon to shape the device to the contour of the tissue. Lesion creation times are short enough (60 seconds) that it is practical to perform ablation without prolonging the cross-clamp time significantly. An entire ablation procedure usually takes 10 to 15 minutes to be fully completed.

Movie 1 (High Bandwidth | Low Bandwidth) illustrates a recent case of endocardial ablation for permanent atrial fibrillation. The patient is an 80 year old male presenting with a long standing history of palpitations and documented atrial fibrillation for one and a half years prior to admission. Attempted cardioversion was unsuccessful. Echocardiogram demonstrated severe mitral regurgitation. Trans-esophageal echocardiogram revealed severe central mitral regurgitation with reduced ejection fraction at 35%. Coronary angiography revealed an 80% stenosis of the mid right coronary.

At surgery, a venous graft was placed to the posterior descending. Then, the left atrium was opened through a lateral approach. Using retraction sutures to deliver and expose the entire left atrial body, a series of microwave ablation lesions were placed as described by Kress et al [Kress 2002]. The atrial wall thickness was between 3 and 4 mm. Upon completion of the ablation procedure, ring annuloplasty was performed with correction of the mitral regurgitation. The left atrial appendage was oversewn from the inside using running polypropylene. Finally, the left atrium was closed, the proximal graft anastomosis created, and the cross clamp released. The patient was weaned from the pump in stable sinus rhythm which was overdriven for rate control using temporary atrial pacing.

Prior to release of the cross-clamp, 150 mg of amiodarone were added to the bypass circuit and a drip of 1 mg/min was started. The infusion was continued until the patient was taking oral fluids, and then he was converted to oral amidarone 200 mg twice daily. By the fourth postoperative day, he was still requiring an atrial pacer for a sinus rate of 50 beats/min so an elective decision was made to place a permanent dual chamber pacemaker. He was discharged home in sinus rhythm on postoperative day 7 and continues to enjoy stable sinus rhythm. Precautionary anticoagulation for mitral annuloplasty was continued for 3 months.

This case illustrates the importance of a thorough preoperative workup to establish the presence and duration of atrial fibrillation before valve surgery. Previously, atrial fibrillation was ignored at surgery unless one was bold enough, and trained enough to perform a Cox Maze procedure. With the evolution of ablative technology and simpler ablation patterns, it is now possible to restore sinus rhythm in the majority of such patients.

Experience with the microwave probe in multiple centers indicates that the conversion rate is around 80% at 6 months for patients with permanent AF [Maessen 2002]. The rate of conversion actually increases with time as the fibrosis matures. At present, there have been no cases of perforation, suture line dehiscence, pulmonary vein stenosis, or esophageal injury secondary to the microwave device. These encouraging results makes it possible to recommend this procedure to a larger cohort of patients undergoing elective mitral valve surgery. Maessen and Knaut also reported that patients with other cardiac lesions are adequate candidates for ablation of their permanent atrial fibrillation, such as patients with aortic stenosis or coronary artery disease. Epicardial ablation using the AFx microwave probe has already been performed with identical conversion rates [Maessen 2002]. This can be done either on-pump or off pump with external application of the probe to the outside of the left atrium.

Several different lesion patterns have now been described to accomplish the same thing, i.e. isolation of the pulmonary veins with connecting lesions. At the present time, it is not possible to recommend one particular pattern of lesions. Interestingly, the long term results of all these different lesion patterns appears to be close to 80% conversion. With increasing experience and refinement, it is conceivable that conversion rates will improve to close to 90%.

The discovery by Haissaguerre that most episodes of paroxysmal atrial fibrillation begin with premature beats coming from ectopic sites in the pulmonary veins has revolutionized the treatment of these patients. A door has been opened to correct this condition using a new surgical procedure that borrows from the discoveries of Cox, the surgical experience of the Maze procedure, and the advent of technology which can accomplish the goals quickly and safely. It is expected that surgical treatment of atrial fibrillation will become a routine procedure soon, with benefits to many thousands of patients in the future.

Key Articles:

Cox JL, Canavan TE, Scheussler RB, et al. The surgical treatment of atrial fibrillation. II. Intraoperative electrophysiologic mapping and description of the electrophysiologic basis of atrial flutter and atrial fibrillation. J Thorac Cardiovasc Surg 101:406-26, 1991.

Cox JL, Scheussler RB, D'Agostino HJ, et al. The surgical treatment of atrial fibrillation. III. Development of a definitive surgical procedure. J Thorac Cardiovasc Surg 101:569-83, 1991.

Cox JL, Jaquiss RD, Scheussler RB, Boineau JP. Modification of the Maze procedure for atrial flutter and atrial fibrillation. II: Surgical technique of the maze III procedure. J Thorac Cardiovasc Surg 110:485-95, 1995.

Cox JL, Ad N, Palazzo T, Fitzpatrick S, Suyderhoud JP, DeGroot KW, Pirovic EA, Lou HC, Duvall WZ, Kim YD. Current status of theMaze procedure for the treatment of atrial fibrillation. Semin Thorac Cardiovasc Surg 2000 Jan;12(1):15-9

Gillinov AM, Smedira NG, Cosgrove III, DM. Microwave ablation of atrial fibrillation during mitral valve operations. Ann Thorac Surg 74:1259-61, 2002.

HaissaguerreM, Pierre J, Shah, DC, Takahashi A, et al. Spontaneous initiation of atrial fibrillation by ectopic beats originating in the pulmonary veins. NEJM 339(10):649-666, Sept 3, 1998.

Kalil RAK, Lima GG, Leiria TLL, Abrahao R, Pires LM, Prates PR, Nesralla IA. Simple surgical isolation of pulmonary veins for treating secondary atrial fibrillation in mitral valve disease. Ann Thorac Surg 73:1169-73, 2002.

Kress DC, Krum D, Chekanov V, Hare J, Michaud N, Akhtar M, Sra J. Validation of a left atrial lesion pattern for intraoperative ablation of atrial fibrillation. Ann Thorac Surg 73:1160-8, 2002.

Maessen JG, Jijs JFMA, Smeets JLRM, Vainer J, Mochtar B. Beating heart surgical treatment of atrial fibrillation with microwave ablation. Ann Thorac Surg 74:S1307-11, 2002.

Melo J, Adragao P, Neves J, Ferreira M, Timoteo A. Endocardial and epicardial readiofrequency ablation in the treatment of atrial fibrillation with a new intra-operative device. Eur J Cardiothorac Surg 18:182-7, 2000.

Simha P, Seetharama Bhat, PS, Prabhudeva N. The electrocautery maze _ how I do it. Heart Surg Forum #2001-98765; 4(4):340-346, 2001.

SuedaT, Imai K, Isii O, Orihashi K, Watari M, Okada K. Efficacy of pulmonary vein isolation for the eliminat of chronic atrial fibrillation in cardiac valvular surgery. Ann Thorac Surg 71:1189-93, 2001.

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 74:1500-5, 2002.