|
|
NewsletterJune 2002 Newsletter: The Current Status of Intraoperative Ablation Techniques for Treating Atrial Dysrhythmias
Srikantha Lakshminarayana, MD
Stuart Weiss MD, PhD Director of Intraoperative Echocardiography
The occurrence of atrial dysrhythmias, notably atrial fibrillation (AF), is now recognized to have a significant impact on patient morbidity and the economics of health care. The reduction in diastolic ventricular filling associated with AF can increase the occurrence of heart failure and worsen its symptoms. In addition, patients in AF are subject to wide swings in heart rate, the risk of thromboembolism, and the complications associated with chronic anticoagulation. Termination of AF and restoration to a sinus rhythm have a direct clinical impact on decreasing patient morbidity, improving functional activity, and decreasing the cost of health care. Historically, pharmacologic therapy has been the mainstay of treatment in patients with chronic or paroxysmal AF. However, its success is limited and requires the chronic administration of medications that have significant side effects. Several non-pharmacological strategies have been used intraoperatively to restore sinus rhythm and thereby mechanical atrial function. Selective tissue disruption of the atria can be performed to either eliminate the source of these arrhythmias or to create a barrier to aberrant reentrant circuits. The creation of focal tissue lesions can be accomplished by the use of high-energy microwaves, radio frequency (RF), laser, or cryogenic methods to produce selective transmural lesions. 1 These techniques are based in part on an earlier surgical method developed by Maze, in which surgical incisions were used to disrupt reentrant circuits in the atria. Although initial postoperative conversion to sinus rhythm using these newer methods is only modest, ranging from 25 to 54%, 1,2 the success at six-month follow-up ranges from 50 to 90%. 3,4. As the desired effect is often not seen in the immediate postoperative period, many centers restart antiarrhythmic agents, such as amiodarone postoperatively. In addition, some patients require placement of a cardiac pacemaker to treat heart block, whereas others require continued use of antiarrhythmic agents, albeit at a lower dose. The termination of AF should not be equated to restoration of normal atrial mechanical function. Echocardiography has been used to assess the return of atrial contribution to ventricular filling. Welch noted a progressive improvement in peak transmitral velocities, i.e., atrial function, over a period of 6 weeks after the ablation procedure.5 At present, there is no consensus among practitioners as to the number and location of the lesions or the extent of tissue disruption that is required to produce the desired effect. Recent clinical and laboratory investigations suggest that atrial ectopy predominantly originates within or near the pulmonary veins. In a series of 225 consecutive patients with multi-drug resistant AF, 96% of triggering foci originated from one or several pulmonary veins. 6. Connecting lesions and RF ablation above the mitral annulus may also be performed to isolate the ectopic foci and interrupt reentrant circuits. 7. Although the lesions produced vary, they include: 1) isolation of the pulmonary veins by a circumferential lesion around the left and right pulmonary veins, 2) isolation/exclusion of the left atrial appendage, 3) an interconnecting lesion between the pulmonary veins, and 4) a lesion on the posterior aspect of the left atrium above the mitral valve annulus. Some investigators advocate performing tissue ablation in the right atrium, as prophylaxis against developing atrial flutter, but this procedure would significantly increase the duration of cardiopulmonary bypass. A study of 48 cardiac surgical patients by Williams failed to demonstrate a benefit of performing the additional RF right atrial ablation. 8 A conservative approach would be to perform an intraoperative left-sided ablation, which can be followed by a limited right atrial catheter ablation, the postulated source of atrial flutter, in patients who develop delayed onset atrial flutter/fibrillation. The tissue ablation procedure is commonly performed with the patient on cardiopulmonary bypass and requires about 15 to 20 minutes. It is performed through the atriotomy incision during mitral valve surgery. The most commonly used method utilizes a flexible hand-held RF ablation probe to heat the tissue to 70-80ºC for a period of one minute to produce a transmural lesion. The probes can be shaped to produce curvilinear lesions that circumscribe structures such as the pulmonary veins. RF ablation is usually performed on the endocardial surface; it has also been successfully applied via the epicardial approach.9 Such technologic advances have allowed limited ablation procedures to be performed during off-pump CABG or thoracoscopic surgeries. Echocardiography is commonly used during surgical procedures in which intraoperative ablation would be performed. Transesophageal echocardiography (TEE) provides an excellent vantage point to examine the atria and diagnose the presence of thrombus. The detection of left atrial thrombus would likely alter surgical management and reveal a potential contraindication of the cardiac ablation. Assessment of atrial size may have prognostic value in predicting long-term success.1,10 Melo found that the success of terminating AF and restoring atrial contractile function was dependent on the size of the left atrium. 1 Patients with a left atrium smaller than 200cm3 were more likely to have return of atrial function and sinus rhythm than those with larger left atria. Alternatively, several other investigators have failed to find a significant relationship between atrial size and the success of terminating AF.4 Further studies are required to elucidate the factors that would predict a successful outcome after RF ablation. The potential risk of trauma to adjacent structures during RF ablation is of great concern, as the depth of thermal injury is 3 to 6 mm. To date, there have not been any reports of damage to other cardiac structures such as the circumflex artery or the coronary sinus. However there have been several reports of esophageal perforation. In one case, it was suggested that the injury was related to the positioning of the TEE probe behind the heart.11 In another case, the TEE probe had been withdrawn, but the patient was extremely cachectic and likely had a thin atrium that might have predisposed to esophageal injury. 12 If TEE is performed during a cardiac surgical procedure, the probe should be withdrawn during RF ablation to prevent esophageal burns caused by the TEE probe acting as a return electrode. To summarize, the technique of producing focal tissue disruption to treat atrial dysrhythmias shows promise. Potential candidates for this procedure include cardiac surgical patients, especially those undergoing mitral valve surgery with chronic or paroxysmal AF. In addition to RF, cryogenic, laser, and microwave ablation are being studied as methods to produce the focal lesions. At this time, more research is needed to define the best method to produce the lesions and to characterize the clinical advantages of restoring atrial mechanical function. Acceptance of this treatment by the medical community and the health care reimbursement organizations will depend on prospective randomized trials that demonstrate its efficacy and better define the subset of patients most suited for this strategy.
References:
Table of Contents
Archived Newsletter Articles
|