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Credits:Marc A. GillinovThe Department of Thoracic and Cardiovascular Surgery, The Cleveland Clinic, Cleveland, OH
Disclosure : Dr. Gillinov is a consultant to Edwards Lifesciences, LLC, St. Jude Medical Inc. and Medtronic, Inc. The Cleveland Clinic has an indirect equity interest in AtriCure, Inc. through its interest in a private fund that has an equity interest in AtriCure, Inc. This work was supported by the Atrial Fibrillation Innovation Center. The Atrial Fibrillation Innovation Center is a Wright Center of Innovation funded by a grant from the State of Ohio.
Corresponding Author : Marc A. Gillinov, M.D., Department of Thoracic and Cardiovascular Surgery, The Cleveland Clinic Foundation/F24, 9500 Euclid Avenue, Cleveland, OH 44195. Tel: 216-445-8841,Fax: 216-444-0777, Email: gillinom@ccf.org
Submitted : 2008-03-05Accepted : 2008-05-11.
Copyright: Copyright
belongs to Dr. Marc A Gillinov under Open Access License details available online at http://creativecommons.org
doi : 10.4022/jafib.v1i1.408
Atrial fibrillation (AF) is now commonly treated at the time of valvular heart surgery or coronary artery bypass grafting. Surgical ablation of AF, which is predicated upon the Maze procedure, includes creation of lines of conduction block and excision of the left atrial appendage. A full bi-atrial lesion set is associated with success in 80% to 95% of patients and virtually eliminates the risk of late stroke. A complex but safe operation, the classic cut-and-sew Maze procedure has been applied by relatively few surgeons. However, recent advances in understanding of the pathogenesis of AF and development of new ablation technologies enable surgeons to perform pulmonary vein isolation, create linear left and right atrial lesions, and remove the left atrial appendage rapidly and safely. Lesions are created under direct vision, minimizing the risk of damage to the pulmonary veins and adjacent mediastinal structures. Recently developed instrumentation now enables thoracoscopic and keyhole approaches, facilitating extension of epicardial AF ablation and excision of the left atrial appendage to patients with isolated AF and no other indication for cardiac surgery. In addition, novel devices designed specifically for minimally invasive epicardial exclusion of the left atrial appendage will broaden the range of treatment options for patients with AF, possibly eliminating the need for anticoagulation in selected patients.
Atrial fibrillation (AF) is common in patients
presenting for management of valvular heart disease or coronary artery disease.
Developed by Dr. James Cox, the classic Maze procedure is the predicate
operation for ablation of AF; long-term data suggest that the Maze procedure eliminates
AF in more than 90% of patients (1-4).
In spite of these unmatched results, the complexity and time associated with
the Maze procedure have prevented widespread application by surgeons.
Recently, however, there has been increased interest
in surgical ablation of AF, fueled by technological advances and demonstration
that the pulmonary veins and left atrium are the drivers of AF in most patients
(5). New ablation technologies enable surgeons to
perform pulmonary vein isolation, create strategically-placed, linear left and
right atrial lesions, and excise the left atrial appendage rapidly and safely (6-12). Alternate energy sources used
to create lines of conduction block and replace surgical incisions of the Maze
procedure include radiofrequency, microwave, ultrasound, cryothermy, and laser
(6-12). Applied primarily in
patients with valvular heart disease, procedures using these ablation
technologies add 15 to 20 minutes to operative time and appear to cure AF in 70%
to 85% of patients. In addition, recent adaptation of these ablation
technologies for thoracoscopic and keyhole approaches now enables minimally
invasive surgery in selected patients having stand-alone ablation.
In this report, we will 1) Review surgical technique
and results of the Maze procedure, 2) Describe new technologies and approaches
for surgical AF ablation and, 3) Discuss the development of technology for
minimally invasive epicardial ablation and ligation of the left atrial
appendage.
The
Cox-Maze III operation, or Maze procedure, is the gold standard for surgical
treatment of AF. In fact, it is the most effective curative therapy for AF yet
devised, and it sets the standard for new surgical approaches to AF (1-4,13,14).
Cox and colleagues designed the procedure based on early experimental and
clinical evidence concerning the pathophysiology of AF (1-4). To improve results and simplify the operation, they
modified the procedure twice, culminating in the Cox-Maze III.
The
Maze procedure includes a complex, bi-atrial lesion set that is applied to all
patients having the operation; therefore, it does not include an ablation
strategy that is specifically tailored to individual patients (15-19) In the Maze procedure, right
and left atrial incisions and cryolesions are constructed to interrupt the
multiple, disorganized reentrant circuits that characterize AF (Figure 1). This is termed “cut-and-sew” technique. These
lesions direct the sinus impulse from the sinoatrial node to the
atrioventricular node along a specified route. Multiple “blind alleys” off
this main conduction pathway (the Maze analogy) facilitate coordinated
electrical activation of the atrial myocardium. Key components of the Maze
procedure include isolation of the pulmonary veins and excision of the left
atrial appendage. These features are maintained in most of the newer
operations designed to ablate AF.
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Figure 1 : Left atrial lesion set of the Cox-Maze III procedure. Schematic illustration of the posterior left atrium. White ovals represent mitral valve, and sets of 4 black ovals represent pulmonary veins. Dashed lines indicate surgical incisions. The pulmonary veins are encircled by a surgical incision, and there is a connecting incision to the mitral valve annulus. The left atrial appendage is excised, and this incision is connected to the pulmonary vein encircling incision.
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Although
the Maze procedure can be completed minimally invasively through a small chest
wall incision, the operation entails cardiopulmonary bypass and cardiac
arrest. In experienced centers, the Maze procedure requires 45 to 60 minutes
of cardiopulmonary bypass and cardiac arrest (4,13,14). The operation may be
performed alone or in conjunction with other cardiac surgical procedures, such
as mitral valve repair or coronary artery bypass grafting.
Cox
and colleagues (4) have reported the largest
series of patients undergoing the Maze procedure. Among 346 patients,
operative mortality was 2%. Reported AF cure rate was 99%, and only 2%
required long-term postoperative anti-arrhythmic medication. Of note, results
were tabulated by calculating freedom from “symptomatic AF”; long-term rhythm
monitoring was not employed. Therefore, these results likely over-state the
effectiveness of the cut-and-sew Maze procedure. Successful ablation of AF was
unaffected by presence of mitral valve disease, left atrial size, and type of
AF (paroxysmal, persistent or permanent). Temporary postoperative AF, attributed
to a shortened atrial refractory period, was common and did not diminish
long-term results. Fifteen percent of patients required new pacemakers after
surgery, and this was generally necessary in patients with underlying sinus
node dysfunction. In spite of multiple right and left atrial incisions, right
atrial transport function was demonstrated in 98% and left atrial transport in
93%. However, there is some controversy concerning the extent to which atrial
mechanical function returns after surgical ablation. Perhaps most importantly,
in addition to restoring sinus rhythm the Maze procedure virtually eliminated
the risk of stroke or other thromboembolism (20,21).
Other
centers have documented excellent results with the Maze procedure, with
restoration of sinus rhythm in 75% to 95% of patients, low risk of late stroke,
and very low operative morbidity and mortality (13,14,21). In recent series, the need
for a new pacemaker has decreased to 5% to 10% (22).
These results confirm the safety of the Maze procedure, its efficacy at
restoring sinus rhythm, and its prevention of late strokes. In spite of these
findings, the Maze procedure has been relatively underutilized. Today, few
patients are referred for a surgical Maze procedure for stand-alone AF ablation,
and, even in patients requiring cardiac surgery for other reasons, surgeons are
reluctant to add a Maze procedure. The perceived surgical complexity and
magnitude of the operation account for these practices.
The
development of new surgical approaches to AF has been predicated upon 2
factors: 1) Recognition that the pulmonary veins and left atrium are critical
to the initiation and maintenance of AF and 2) Development of ablation
technologies that use alternate energy sources to facilitate rapid and safe
creation of lines of conduction block under direct vision.
While
the Maze procedure was designed to interrupt the multiple macro- reentrant
circuits that characterize AF, new approaches are more precisely anatomically
focused. There is general agreement that AF requires a substrate and a
trigger, and that these substrates and triggers are usually located in the
pulmonary veins and left atrium (23,24).
Haissaguerre and colleagues (5) demonstrated that
paroxysmal AF originates from ectopic beats in the pulmonary veins in 94% of
cases. In addition, autonomic innervation of these regions may contribute to
the pathogenesis of AF (25). Catheter ablation
of the posterior left atrium, including the antra surrounding the pulmonary
veins, has proven effective at ablating both paroxysmal and permanent AF (25,26). These data suggest that
modification of the left atrial substrate, in combination with pulmonary vein
isolation, is an effective therapy for all forms of AF.
Surgeons
have used alternate energy sources to create a variety of left atrial lesions
sets, ranging from wide pulmonary vein isolation with excision of the left
atrial appendage to a lesion set that resembles that of the Maze procedure (Figure 2). There is growing consensus that patients with
persistent or long-standing persistent AF should receive a lesion set that
incorporates more than simple pulmonary vein isolation (27,28). In such patients, the lesion from the right
pulmonary veins to the mitral annulus may be particularly important in
prevention of post-procedure left atrial flutter (29).
The addition of right atrial lesions appears to increase freedom from recurrent
AF and atrial flutter, but their importance is controversial (30). However, creation of right atrial lesions is simple
and safe, and we currently favor their incorporation at the time of surgical
ablation.
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Figure 2 : Left atrial lesion sets created with alternate energy sources. Schematic illustration of the posterior left atrium as above. These require approximately 10 to 20 minutes of operative time. Dashed lines indicate lesions created with heat-based energy sources or cryothermy. (A) Bilateral pulmonary vein isolation with excision of the left atrial appendage. (B) Maze-like lesion set created with bipolar radiofrequency and cryothermy (for lesion to the mitral annulus).
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Based upon these advances in understanding of the
pathophysiology of AF, a variety of new ablation tools have been developed to facilitate
surgical ablation of AF. These probes and catheters rely on alternate energy
sources to create long, continuous, linear lesions that block conduction.
Energy sources that have been used clinically include radiofrequency, laser,
ultrasound, microwave, and cryothermy (10,11).
Radiofrequency, laser, ultrasound and microwave are heat-based energy sources
that create lines of conduction block through thermal injury. Each of these
modalities can be employed for ablation during concomitant open heart surgery
or, alternatively, for stand-alone, minimally invasive epicardial ablation.
Because
the surgeon has the advantages of 1) direct visualization of cardiac structures
and 2) catheters that facilitate rapid creation of transmural lesions, there is
great interest in ablating AF in patients presenting for other cardiac surgical
procedures. Completion of left atrial lesion sets requires only 10 to 20
minutes. This amount of time contrasts with the 1 hour required to perform the
traditional cut-and-sew Maze procedure. In addition, because incisions are
replaced by heat- or cryo-based lesions, the risk of bleeding is virtually
eliminated when alternate energy sources are employed.
Although
approaches vary somewhat, results are similar with a variety of energy sources.
AF is ablated in 70% to 80% of patients having concomitant heart surgery (10,11). Thus far, most treated
patients have had organic heart disease and have received a mitral valve
procedure in addition to AF ablation. For them, results with alternate energy
sources fall just short of those reported for the classic cut-and-sew Maze
procedure (31,32).
After
surgical ablation, perioperative AF is common, occurring in approximately 50%
of patients. Although 30% to 40% of patients leave the hospital in AF, many
return to sinus rhythm over the ensuing 3 months. Thus, discharge in AF is not
an indication of procedure failure (33). Given
the high incidence of perioperative AF, a strategy that includes 3 months of
routine postoperative anti-arrhythmic therapy and anticoagulation in all
patients is recommended. Because heart rhythm varies in the first 3 months
after surgery, we recommend aggressive attempts to restore sinus rhythm during
this time frame when patients develop AF or atrial flutter. Heart rhythm
generally stabilizes by 3 to 6 months after surgery. Factors that influence
procedure success include larger left atrial size, longer duration of AF, and
choice of lesion set in permanent AF (16,34,35).
Open
surgical treatment of AF has a long track record of success. The surgeon has
the advantage of direct visualization of the left atrium and pulmonary veins,
either from the epicardial or endocardial surface of the heart, and this
factor, coupled with new ablation technology, enables rapid and safe ablation.
Because the surgeon can see cardiac structures, ablation lines can be placed
safely on the left atrial cuffs adjacent to the pulmonary vein orifices,
thereby avoiding the dreaded complication of pulmonary vein stenosis.
Epicardial ablation eliminates the risk of esophageal injury; however, there is
uncertainty concerning the ability of unipolar energy sources to create
continuous, transmural lesions from the epicardium of the beating heart. With
surgical approaches, the left atrial appendage is excised, and this is likely
important in decreasing the risk of late stroke. Finally, the simplicity of
these techniques makes them generally applicable; all cardiac surgeons can now
ablate AF. In current clinical practice, almost all patients with AF who
present for cardiac surgery should have both AF ablation and the intended
cardiac procedure.
Using
this experience as a spring-board, surgeons have developed epicardially-based,
minimally invasive and thoracoscopic approaches to offer stand-alone AF
ablation. While these procedures are currently in early stages of development
and application, results are promising. Pulmonary vein isolation and excision
of the left atrial appendage can be performed using a minimally invasive
“keyhole approach” or thoracoscopically; neither approach requires
cardiopulmonary bypass (36-42).
These minimally invasive procedures enable wide, circumferential pulmonary vein
isolation, either with a single “box” lesion or separate oval-shaped ablations
on the right and left. Connecting lesions across the dome of the left atrium
and to the mitral annulus can now be created using specially-designed unipolar
energy sources (42). Procedure times are
generally 2 to 4 hours, and median length of hospital stay is 3 days.
Early results, obtained primarily in patients with
paroxysmal AF, demonstrate 80% to 90% freedom from AF 6 months after ablation.
With continued experience and further advances in instrumentation, procedure
time is expected to fall to less than 2 hours, and hospital length of stay to
decline to a single day. Application of these minimally invasive,
thoracoscopic and keyhole procedures will offer the potential for cure of AF,
with improved quality of life and freedom from anticoagulation and anti-arrhythmic
medications, to large numbers of patients.
Currently
there is great interest in development and assessment of endocardial and
epicardial procedures for exclusion of the left atrial appendage (43,44). It is widely believed that
formation and embolism of left atrial appendage thrombi are responsible for the
increased risk of stroke in AF patients (43,44). In AF patients, warfarin inhibits formation of
atrial appendage thrombi and reduces cardioembolic strokes, while aspirin prevents
smaller, noncardioembolic strokes. Based upon data from the Stroke Prevention
in Atrial Fibrillation (SPAF) trials, one-third of AF patients are at high risk
for stroke and should be treated with warfarin (4,45,46). In spite of these
observations and recommendations, warfarin is under-prescribed in AF patients.
Furthermore, many patients cannot or will not take warfarin. Therefore,
interventional therapies that specifically address the left atrial appendage in
AF patients are being investigated as potential alternatives to warfarin
therapy.
Excision
or exclusion of the left atrial appendage is currently performed during
surgical ablation of AF and is recommended in ACC/AHA guidelines for patients
undergoing mitral valve surgery. However, standard surgical exclusion by
suture closure is incomplete in 30% of cases, and stapled closure or excision
has been associated with bleeding complications (47).
Thus, there is a need for new surgical approaches to the left atrial
appendage. Several devices for epicardial exclusion of the left atrial
appendage are under development (Figure 3).
Pre-clinical studies suggest that device-based epicardial exclusion of the left
atrial appendage is rapid and safe and has the advantage of avoiding placement
of a foreign body in the fibrillating atrium (50).
Early clinical results with this epicardially-based left atrial appendage clip
are promising. This and other new technologies for minimally invasive
epicardial ablation of the left atrial appendage will soon be available for widespread
clinical use. Clinical trials are being designed to test the hypothesis that
epicardial, device-based exclusion of the left atrial appendage will reduce the
risks of stroke and other thromboembolism in patients with AF. Should this
hypothesis prove correct, physicians will be able to offer a new strategy for
primary or secondary stroke prevention in AF patients.
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Figure 3 : Left atrial appendage clip. A cloth-covered, nitinol and titanium clip has been applied to the epicardial aspect of a canine left atrial appendage.
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