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Intraoperative Endocardial Radiofrequency
Ablation for Atrial Fibrillation
Stuart P Thomas, BMed, PhD
Department of Cardiology, Westmead Hospital, Westmead, Australia
Atrial fibrillation is the most common clinically important cardiac arrhythmia (1). It is associated with significant morbidity and is a leading cause of stroke in the elderly. The prevalence of atrial fibrillation increases with age from 1.7% in those aged 60-64 years to 11.6 % in those over the age of 75 years (2). Total mortality and cardiovascular mortality are significantly increased in patients with atrial fibrillation compared with controls. The risk of stroke in non-rheumatic atrial fibrillation is approximately 5%/year in the absence of warfarin or aspirin (3, 4). Treatment with warfarin may reduce this figure to 1.4% but there is an additional risk of serious haemorrhage of 1.5%/year.
Pharmacological therapy to restore and maintain sinus rhythm in patients with atrial fibrillation is often unsuccessful and may be dangerous (5, 6). The most effective drugs for maintenance of sinus rhythm are those that prolong the action potential duration. The use of these drugs is often limited by the risk of proarrhythmia. The least proarrhythmic of these agents, amiodarone, has a diverse side effect profile including pulmonary fibrosis, thyroid dysfunction and skin discolouration.
During the last two decades surgical procedures were developed which compartmentalised the atria to cure atrial fibrillation. This work was based on the theoretical and experimental studies of Moe (7) and Allessie (8). These studies suggested atrial fibrillation was due to multiple simultaneous non-stationary macro-reentrant circuits. The surgical procedures involved septation of the atria with a pattern of incisions that permitted electrical activation during sinus rhythm but left insufficient contiguous tissue mass to support reentry. Left atrial isolation. (9) (10), the corridor operation (11) and the compartment procedure (12) were all successful to varying degrees. However, the most thoroughly documented of the incisional surgical procedures was the Cox maze procedure (13, 14).
The Maze procedures abolished atrial fibrillation in patients with permanent or paroxysmal atrial fibrillation. (15, 16). Three major variations in the pattern of incisions were described (I, II and III). The mortality (» 2%) was low even when combined with other procedures. Atrial contraction was often impaired post-operatively (17) but the subsequent risk of stroke was small (18). However, the Maze Procedure is associated with significant morbidity. In particular there are problems with post-operative arrhythmias, atrial contractile function, removal of the atrial appendages and the effect of the procedure on sinus node function. Pacemakers were required by 56 % of Maze I patients and 25 % of Maze III patients for treatment of sinus node dysfunction. Inappropriate resting tachycardia (>120bpm) was a problem after the Maze III procedure
INTRAOPERATIVE RADIOFREQUENCY MAZE PROCEDURES
Radiofrequency ablation is routinely used to treat several important cardiac arrhythmias. Typically radiofrequency electrical current is passed between a small, percutaneously delivered, endocardial electrode and a large cutaneous electrode. The mechanism of tissue injury is thermal with irreversible myocardial damage produced at approximately 50°C. Heating is proportional to the square of the current density which is greatest close to the ablation electrode. Procedural success rates in patients suffering from atrioventricular junctional reentrant tachycardias, typical atrial flutter or tachycardias involving accessory atrioventricular connections are above 90%.
Several properties make radiofrequency ablation
attractive for intraoperatively use.
1. Radiofrequency energy produces irreversible cell damage but leaves the macroscopic tissue structure intact. This should reduce the risk of postoperative bleeding and may preserve some vascular and neurological structures destroyed by the incisional approach.
2. Delivery of radiofrequency current to several electrodes simultaneously allows linear lesions to be created rapidly.
3. It is technically possible to place radiofrequency lines of ablation in any part of the atria.
AIMS OF SURGERY FOR ATRIAL FIBRILLATION
1. Restoration of sinus rhythm is a key aim of curative procedures for atrial fibrillation. Restoration of sinus rhythm is necessary to prevent the irregularity and inappropriate rate response of atrial fibrillation.
2. Where possible sinus rhythm should be restored without damage to sinus node function. Although a proportion of patients with atrial fibrillation will have coincident sinus node dysfunction it is also clear that some incisions in the early versions of the Maze procedure were associated with abnormal sinus node function (15, 16).
3. Atrial contractility contributes 20% to cardiac output. Loss of atrial contractility is likely to reduce cardiac reserve. Furthermore abnormal atrial contractility may increase the risk of cerebrovascular accidents (19). Patients cardioverted from atrial fibrillation have a persistent risk of stroke which is associated with a transient reduction in atrial function. Therefore another important aim of atrial fibrillation surgery is the restoration of atrial function.
Surgery for atrial fibrillation should address all of the major problems associated with atrial fibrillation. Surgical procedures should be concerned with restoration of atrial contractile function and sinus node function in addition to restoration of sinus rhythm.
INSTRUMENTS FOR RADIOFREQUENCY ABLATION
In our early studies radiofrequency lesions were produced using one of two handheld devices. The first consisted of a straight or J tipped electrode at the end of a 20cm handle. The electrode was flat with a length of 12mm and a width of 2.5mm. The long handle and shape of the probe allowed the electrode to be positioned firmly against the endocardial surface through an atriotomy under direct vision. Radiofrequency current was delivered between the probe electrode and a large diathermy electrode positioned on the patient's skin. A thermistor in the probe electrode and a closed loop feedback system were used for temperature control. The temperature was set at 80-90°C with a settling time (time taken to achieve the set temperature) of approximately 30 seconds. Each radiofrequency application was 60 seconds in duration. The long settling time and accurate temperature control minimized overheating and endocardial charring. The large electrode size and high temperature maximized lesion depth. This electrode design was tested in dogs prior to commencement of human studies. In canine ventricular muscle the mean lesion depth was 5.3 ± 0.9mm. The mean lesion width was 7.7 ± 0.4mm. Radiofrequency lesions were placed end to end with overlap to make continuous long linear lesions. The disadvantage of these probes was the slow rate of linear lesion formation. (20)
A second ablation device had four 6mm by 2mm electrodes with a 3mm interelectrode distance mounted in sequence on a 33mm long malleable tip (). This enabled the surgeon to shape the ablation device to conform to the shape of the atrial endocardial surface. Simultaneous, in-phase, unipolar ablation was performed between all four electrodes and the large surface electrode. With this instrument a 35mm lesion could be produced using electrode temperatures of 80 - 90°C over a single period of 60 seconds. This device was also tested in canine atria during cardiopulmonary bypass and consistently produced continuous transmural linear lesions.
Figure 1: Multielectrode ablation device
Recently commercial devices for intraoperative radiofrequency ablation have become available. These consist of either a non-irrigated multielectrode probe or a probe with a single irrigated electrode. Deeper lesion formation should be possible with the irrigated electrode (21) but irrigation prevents temperature monitoring and the single electrode configuration makes formation of long lesions more difficult. (20)
THE STAR PROCEDURE
In our studies we used a different pattern of lesions to that described by Cox (15, 20) . Our aims were to avoid electrical isolation of areas of atrial myocardium and minimize disruption to the normal pattern of atrial activation. The incisions of the Cox Maze procedures encircled the pulmonary veins, electrically isolating that section of the posterior left atrium. We replaced the incision encircling the pulmonary veins with lesions that allow the posterior left atrium to be electrically activated. This area represents 36% of the surface area of the left atrial muscle (22). Preserved contraction of this area may improve the haemodynamic performance of the left atrium and reduce left atrial stasis and the tendency to thromboembolism (19). Subsequently it has become clear that pulmonary vein isolation is desirable in many cases. Our current operation includes pulmonary vein isolation with minimal isolation of atrial contractile tissue.
In order to minimize disruption to the normal pattern of atrial activation the lines of radiofrequency ablation were placed, where possible, parallel to the normal direction of wavefront propagation. We therefore created seven adjacent corridors radiating from the sinus node to the atrioventricular junction. Because of the stellate pattern of lesions we referred to the new operation as the Star procedure ().
Our group recently reported the results from the first 25 star procedures (1995-8). (20) The indications for surgery were symptomatic documented sustained atrial fibrillation not responding to drug therapy (n=13) or atrial fibrillation in patients requiring cardiac surgery for other lesions (n=12). The radial pattern of radiofrequency linear lesions was associated with normal or near normal electrical activation of the atrial myocardium but there was some loss of atrial contractility. Our experience demonstrated that in most cases (91%) atrial fibrillation was abolished, at least in the medium term, by the Star pattern of endocardial radiofrequency ablation. However, atrial flutter was a common problem postoperatively and often resulted in significant symptoms. Only one patient had no post-operative atrial flutter. Post-operative arrhythmias were studied at electrophysiological studies. Thirty-seven flutter morphologies were identified in 15 patients. Seven mechanisms (lesion discontinuity, n=6, focal mechanism, n=1) of AFlut were characterized in six patients. In these cases flutter was abolished using further catheter radiofrequency ablation. In the remaining cases flutter was usually localized to an area involving the interatrial septum but no critical isthmus was identified for ablation. After 16±10 months, 15 patients (65%) were asymptomatic with (n=3) or without (n=12) antiarrhythmic medications. Eight (35%) had persistent arrhythmias. At least two major types of mechanism were responsible for post-operative atrial flutter. Most commonly a residual discontinuity in a line of ablation facilitated reentry around one of the linear radiofrequency lesions. In the remainder, reentry within a corridor, multiple discontinuities in a line of ablation or focal mechanisms were the major possibilities. Additional catheter radiofrequency ablation can be used successfully to ablate residual AFlut after initial attempts at endocardial radiofrequency ablation for AFib.
OTHER STUDIES OF INTRAOPERATIVE RADIOFREQUENCY ABLATION
FOR TREATMENT OF ATRIAL FIBRILLATION
Patwardhan et al (23) reproduced the Cox Maze III Procedure using linear radiofrequency ablation in 18 patients undergoing surgery for rheumatic valve disease. Modified forceps were used to deliver bipolar radiofrequency energy between the endocardium and opposing epicardium. Of the 15 survivors 80% were in sinus rhythm after a mean follow up of 150 days. Only nine patients had demonstrable atrial contraction as assessed by Doppler echocardiography post-operatively.
Sie et al (24) reported the results of a radiofrequency version of the maze procedure in seventy-two patients with atrial fibrillation undergoing valve surgery. The pattern of lesions was based on the Cox Maze III operation. The in-hospital mortality was low (2.7%) and at a mean of 20 months follow up 76% of patients were in sinus rhythm. Atrial function was documented in the majority of patients.
Melo et al (25) used radiofrequency current to isolate the pulmonary veins in 65 patients with paroxysmal or chronic atrial fibrillation. Most patients had concomitant mitral valve disease. After one month 34% of patients were in sinus rhythm with biatrial contraction. After six months sinus rhythm was restored in the majority of patients but again post-operative arrhythmias and reduced atrial contractile function were common problems. This group is also investigating epicardial ablation.
Benussi et al (26) performed a relatively simple pulmonary vein isolation procedure in 40 patients. There was a single death and sinus rhythm was restored in 77% of patients at approximately one year. Atrial contractility was observed in all patients.
Other series have been reported by Kottkamp et al (27), Sueda et al (28) and Du et al (29). Comparison of techniques is confounded by differences in patient selection and methods of evaluation. However the cross-sectional reporting of the preliminary experience with linear radiofrequency ablation during cardiac surgery for treatment of atrial fibrillation suggests success rates of approximately 70% in patients with or without antiarrhythmic medications. Post-operative arrhythmias remain a major problem but some of these may be treated by percutaneous radiofrequency catheter techniques.
EPICARDIAL OR ENDOCARDIAL ABLATION?
Recent studies have suggested epicardial ablation may be used to produce linear lesions. The advantage of this approach is that lesions may be placed epicardially on the beating heart. Such an approach reduces or avoids the need for cardiopulmonary bypass and is therefore likely to reduce procedural morbidity. Cooling by circulating blood reduces the ability to achieve satisfactory temperatures at the endocardial surface and represents a theoretical limitation of this approach. This effect is in contrast to that of cooling the ablation electrode; a technique used to increase lesion size. Endocardial cooling during epicardial ablation is likely to result in surviving bridges of endocardial musculature, preventing transmurality of the lines of ablation. This effect is exacerbated by the high surface area of the endocardium created by the pectinate musculature. () Studies to date have demonstrated that it is very difficult to consistently produce continuous transmural linear lesions. This goal may be even more difficult with ablation from the epicardial surface.
Figure 3: Large surface area and blood flow facilitate endocardial cooling
HOW MANY LINEAR LESIONS AND WHERE?
The optimal set of linear ablation of atrial fibrillation is unknown. Our lesion set was selected to leave corridors similar in size to those of the Cox Maze procedure. Initially we used seven radial linear lesions. The pattern of lesions should be modified on the basis of new evidence. Such evidence is likely to arise from three sources.
1. Mapping studies in animals, and especially in humans
may identify key areas responsible for atrial fibrillation. There is a growing
body of evidence suggesting the left atrium is more important than the right
in sustaining atrial fibrillation. Such evidence may allow us to eliminate or
reduce linear lesions in the right atrium.
2. As the experience with a variety of lesion sets increases it may become clear which of these is most successful.
3. Experience with different lesion sets can be maximized by intensive study of post-procedural arrhythmias. Study of post-procedural arrhythmias reveals defects in existing lesions and may indicate where new lesions are required. We are beginning to gain insights into which lesions are most important for success. Our experience and that of others has indicated that isolation of the pulmonary veins is likely to be necessary in many cases. It is also becoming clear that several of the right sided lesions may not be necessary. Recognition of the key lesions will occur after detailed analysis of the results obtained from the various lesion sets currently under investigation.
Atrial fibrillation is a heterogeneous condition and a single standard pattern of lesions may not be appropriate for all patients with this arrhythmia. It may be necessary to tailor the procedure for each patient or for several pathological subtypes.
DEFECTS IN LINEAR LESIONS
Our study showed defects in linear lesions are a major cause of post-operative atrial flutter (20). Depolarising wavefronts can propagate through even very small defects in linear thermal lesions. (30) Such defects can be overcome by positioning lines of ablation in parts of the atria where the myocardium is thin or improving the ablation technology to allow the formation of deeper continuous linear lesions. Strategies to produce deeper lesions include irrigation (21), laser (31) or microwave ablation. (32)
Clinical experience with radiofrequency ablation surgery for atrial fibrillation is small. Clear short term benefit has been demonstrated in several studies but long term efficacy and large controlled trials have not yet been performed. Therefore, these procedures should only be offered on an investigational basis and should be thoroughly evaluated as part of clinical trials. In our center the Star procedure is offered to most patients with atrial fibrillation undergoing cardiac surgery. It is also offered to highly symptomatic patients failing medical therapy who do not meet the criteria for percutaneous pulmonary vein isolation. In these patients it is an alternative to His bundle ablation.
EVALUATION OF INTRAOPERATIVE ABLATION PROCEDURES
Evaluation of intraoperative ablation procedures should directly address the aims of such surgery.
1. Our work and that of others have indicated that post-operative
atrial arrhythmias are usually intermittent. Therefore cross sectional reporting
of results is of limited value. It is more useful to attempt documentation of
the frequency and duration of arrhythmias longitudinally in each patient.
2. Electrophysiological studies in patients with post-procedural arrhythmias assist with the development of the procedure and often provide the opportunity to treat post-operative arrhythmias.
3. Published studies to date describe short term follow up. The long term consequences of the new surgical procedures need to be documented.
4. The benefits of normal atrial function have not been proven in patients after linear radiofrequency ablation for atrial fibrillation but it seems reasonable to strive toward procedures that maximally restore atrial function.
5. Sinus node function should be documented.
Detailed documentation of procedural results will allow useful comparison between different techniques. This approach will allow rapid development of the procedure and minimize unnecessary duplication at different centers.
Intraoperative endocardial radiofrequency ablation for atrial fibrillation is a promising new treatment option for patients with atrial fibrillation undergoing cardiac surgery or patients with highly symptomatic atrial fibrillation not responding to other therapies. Atrial fibrillation is abolished in the majority of cases but post-operative arrhythmias remain a major problem. New techniques are required to ensure lesions are continuous and transmural. Further studies are also required to determine the optimal radiofrequency lesion set. Issues of atrial transport and long term safety and efficacy of the procedures also need to be addressed.
I would like to acknowledge Professor David L Ross and Drs Richard Chard, Duncan Guy, Ian Nicholson and Graham Nunn, all of whom made substantial contributions to the development of the Star Procedure.
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