Invited commentary

Abstract

To compete with percutaneous endocardial ablation techniques for the treatment of atrial fibrillation (AF), several surgical epicardial approaches have been introduced into clinical practice, and additional techniques are currently under development. These surgical epicardial procedures make use of alternate energy sources and minimally invasive access to produce atrial lesions to ablate AF. There is a plethora of new ablation devices designed specifically to facilitate these minimally invasive procedures. Gaynor and colleagues [1] note that these devices must “be able to produce transmural lesions on the arrested and beating heart safely without producing collateral damage to vital structures. This requires the development of dose response curves on living tissue.” To their credit, they have developed a rigorous model that enables generation of such dose response curves. In their porcine beating heart model, endocardial microwave energy reliably produced transmural atrial lesions on the arrested heart; however, on the beating heart, epicardial atrial lesions were transmural in only 20%. Ventricular muscle, which was 4 times thicker than atrial tissue, had a linear energy response curve, that suggested that circulating blood is an effective heat sink and limits the success of epicardial microwave ablation on the beating heart. There is of great interest in the ability to create transmural lesions with alternate energy sources. Transplant surgeons and those of us who performed the traditional cut and sew maze procedure recognize that atrial tissue has variable thickness and composition. Some tissue is so thin that it seems to be made up of only endocardium, whereas other atrial tissue is very thick and may be covered by epicardial fat or endocardial calcium. This variability of atrial tissue may influence the performance of alternate energy sources. Currently available energy sources include microwave, radiofrequency, laser, ultrasound, and cryothermy. These have been applied to perform ablations under widely varying conditions (ie, on pump vs off pump, epicardial vs endocardial, minimally invasive vs sternotomy). The aim of these procedures is to create lines of conduction block or modify conduction, or both, such that AF will be ablated. Currently we have no practical means for intraoperative assessment of the effectiveness of these procedures; we tell the patient that we will not be able to judge procedural success for several months. Gaynor and colleagues’ [1Gaynor S.L. Byrd B.D. Diodato M.D. et al.Microwave ablation for atrial fibrillation dose response curves in the cardioplegia-arrested and beating heart.Ann Thorac Surg. 2006; 81: 72-77Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar] data suggest that we need to standardize assessment of ablation modalities in 3 separate but complementary fashions by: (1) generating standard, bench-top dose response curves for each device; (2) assessing safety and quantify collateral damage, and (3) assessing lines of conduction block created at the operation. Currently such dose-response curves are generated by the industry using a variety of different models. In most cases, this data is held as proprietary information. Standardization of bench top testing would facilitate comparison of different technologies, likely aiding clinicians in their decision-making. We must also develop standard techniques to evaluate epicardial lesions placed on the beating heart, as transmural lesions may be difficult to create under such circumstances. Such techniques require basic electrophysiologic investigations, generally including assessment of entrance or exit block, or both, at the pulmonary veins. Failure to achieve acute conduction block at the time of a minimally invasive ablation should spur the operator to continue working until there is demonstration of acute procedural success. New technology and innovative, minimally invasive approaches will enable surgeons to offer therapy to patients with isolated AF; however, critical appraisal of these technologies and electrophysiologic confirmation of procedural results are necessary before we attempt wide dissemination of these new operations.