Ann Thorac Surg 2005;80:887
© 2005 The Society of Thoracic Surgeons
Original article: Cardiovascular
Invited commentary
Stuart P. Thomas, BMed, PhD
Department of Cardiology, Westmead Hospital, University of Sydney, Westmead, NSW 2145 Australia
(Email: stuartpt{at}yahoo.com).
Most currently used devices for forming linear thermal lesions in the myocardium employ radiofrequency energy. The main mechanism for heating is ohmic (resistive). The direct heating effect reduces at a rate approximately proportional to the inverse of the fourth power of the radial distance from the electrode during unipolar ablation. Thus, during epicardial ablation with unipolar electrodes, effective direct resistive heating occurs only in a very thin rim of tissue very close to the epicardial surface. Lesion extension beyond this thin rim is provided by thermal conduction. Overheating is prevented by a rise in impedance when the temperature exceeds approximately 100°C. The addition of surface cooling allows the delivery of greater current density without overheating the surface. However, histological studies consistently demonstrate difficulty in achieving continuous transmural lesions with unipolar epicardial radiofrequency ablation.
Heating during microwave ablation is due to oscillation of charged dipoles (dielectric heating). A larger volume of tissue is directly heated during microwave ablation. This may mean that where similar power is delivered to the tissue, heating is slower than radiofrequency ablation. However, the increased depth of direct heating may allow formation of deeper lesions. This is attractive for those who accept that continuous transmural lesions are desirable.
Accord and colleagues [1] have made an important contribution by examining the histological results of microwave ablation from the epicardial surface in humans for the first time. Importantly, they show that the lesions are rarely transmural. It is likely that transmural lesions could be produced by microwave ablation if the power and duration of delivery were increased. However it is not clear whether such changes to the method of microwave delivery would result in increased morbidity due to unintentional damage to adjacent structures or overheating of the myocardium.
It is still unclear whether transmural lesions are required for the cure of atrial fibrillation. Accord and colleagues argue correctly that substrate modification may play an important role in procedural success. However, there are good theoretical reasons to believe that electrical isolation of the pulmonary veins is desirable. Furthermore, it is clear that incomplete lines of ablation may be pro-arrhythmic. Linear ablation procedures for treatment of atrial fibrillation are still associated with moderate failure rates, especially when ablation is performed epicardially. Therefore it is reasonable to continue efforts to find effective ways to produce continuous, transmural thermal lesions in the myocardium.
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- Accord RE, van Suylen R-J, van Brakel TJ, Maessen JG. Post-mortem histologic evaluation of microwave lesions after epicardial pulmonary vein isolation for atrial fibrillation Ann Thorac Surg 2005;80:881-887.[Abstract/Free Full Text]
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