Since its introduction in 1987, the Maze procedure has been regarded as the 'gold standard' for the surgical treatment of atrial fibrillation (AF). However, the complexity of the procedure has restricted its adoptability in the surgical community and provided an impetus for the development of less complex operations for the effective treatment of AF. The absolute number of patients who have been cured of AF with a simpler, modified Maze approach has dwarfed the number cured with the Maze III procedure. The modified procedure, which makes use of alternative forms of energy to replace the incisions made in the classic 'cut-and-sew' Maze, has opened up new avenues in the surgical treatment of AF. These new approaches include cryothermal applications, referred to by practitioners as 'Cryo Maze' procedures.
Developing a Widely Applicable and Effective Surgical Treatment for AF
In the surgical treatment of AF, lesions are produced in the atria to isolate the trigger mechanism of the arrhythmia or to create a physical barrier to the re-entrant circuits of electrical signals that induce and maintain the arrhythmia.The Maze procedure is widely regarded as the most effective surgical treatment option for AF. While it has achieved success rates approaching 99%, the complexity of the 'cut and sew' Maze procedure has limited its adoption within the surgical community. As a result, fewer patients are cured of AF with the Maze III than with modified approaches utilizing various energy modalities to create electrical isolation. For a surgical procedure to be applicable to large numbers of patients, three crucial factors must be addressed: complexity, efficacy, and adoptability.
- complexity - the more technically demanding the procedure, the higher the perceived risk, resulting in a lower adoption rate;
- efficacy - the success rate achieved by the procedure must be high enough to constitute a true benefit to patients; and
- adoptability - a gauge of surgeons' willingness to perform the procedure based on its complexity and efficacy.
In addition to these criteria, safety must remain a prime consideration when selecting an energy source to create the required lesions in a partial or mini Maze approach. Empirical experience has shown that it is not necessary to create all of the lesions of the original surgical Maze III procedure. Evidence strongly suggests that neither the septal nor the left atrial appendage lesions are crucial to the ablation of AF.
The essential left atrial lesions are:
- the pulmonary vein encircling lesion; and
- the lesion across the isthmus bridging the inferior pulmonary veins and the mitral valve annulus.
Cardiac surgeons have repeatedly noted the importance of the left atrial isthmus lesion in eliminating the re-entry responsible for AF or left atrial flutter. Also significant are the refractory period differentials in the left and right atria.As a result, treatment of AF may focus on the left atrium. To address the right atrium, a lesion may be placed in the tricuspid valve and inferior vena cava (IVC). This suggests that surgeons can successfully treat the majority of patients with AF by creating a lesion set of pulmonary vein encircling lesion, a left atrial isthmus lesion and its attendant coronary sinus lesion, and a right atrial isthmus lesion. Cryoablation has been demonstrated to be as effective as the 'cut-and-sew' Maze with shorter operative time and less bleeding. The complexity of the full Maze procedure and the availability of energy sources, such as cryothermy, for the creation of the essential lesions, have provided an initial impetus for the development of partial or mini Maze approaches. Safety considerations and efficacy rates achieved by cryothermy strongly suggest that cryosurgical applications within a full or mini Maze approach can help treat many more patients with AF than the classic 'cut-and-sew' Maze procedure.
Creating Cryolesions to Replicate the Full Maze Procedure
One practice is to employ argon-based cryothermal energy, using a Cryo Maze procedure in which the practice of making cuts in the atria are replaced by the creation of full-thickness cryo lesions. The objective of this approach is to abolish recurrent AF, improve functional capacity, eliminate or reduce the need for anti-arrhythmic medications, eliminate anticoagulants, and lessen the risk of future cerebrovascular accidents. Cryothermal energy ablates tissue as intracellular ice crystals disrupt cell membranes while leaving collagen structure intact. Lesions created by cryoenergy heal by fibrosis, leaving a dense homogeneous scar. One finding is that cryoablation has no effect on the mitral or tricuspid valve or the coronary sinus. There are no reports of collateral injuries with Cryo as have been seen with radiofrequency and microwave. The results of a series of patients treated at the Baylor University Medical Center were presented at the 4th Annual Conference on the Surgical Treatment of Atrial Fibrillation, held in January 2004 in San Antonio,Texas. All 33 patients were referred for mitral valve surgery with paroxysmal or permanent AF. In this study, ablation lines were created with the CryoCath SurgiFrost system, which uses a 60mm probe and an argon refrigerant.
The cryoprobe has a flexible cooling segment that can be shaped to anatomical structures in the heart. The system can apply temperatures at the probe/tissue interface as low as −160°C.At low temperatures, the tip of the probe adheres to the tissue (a property known as 'cryoadhesion'), which allows the surgeon to pull the atrial wall away from underlying structures without causing damage to them. At 5.8 months following treatment with the Cryo Maze approach, 32 of the 33 patients remained free of AF symptoms, 88% of the patients were in sinus rhythm, and 81% were off all antiarrhythmic medications at three months post-operation.
Looking Ahead
As mentioned in the Introduction, the possibility of establishing a widely applicable and consistent lesion set has focused on 'essential' lesions. It appears that neither the septal nor the left atrial appendage lesions are critical to the ablation of AF. Accordingly, the essential left atrial lesions would be limited to an encircling lesion of the pulmonary veins and a connecting lesion between the inferior pulmonary veins and the mitral annulus. As Dr Marc Gillinov has argued: "These new approaches are being modified for thorascopic, minimally invasive, and epicardial ablation. This minimally invasive procedure will enable rapid and effective cure of AF in large numbers of patients."
The surgical treatment of AF is a field characterized by constant progress and innovation. There are promising developments that suggest the possibility of establishing a modified Maze procedure using a safe and effective energy source - such as cryo - and consistent lesion sets applicable to all heart surgery patients.