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J Thorac Cardiovasc Surg 2006;132:212-213
© 2006 The American Association for Thoracic Surgery
Letter to the Editor |
a Cardiac Surgery Department, Polytechnic University of Valencia, Camino de Vera s/n, 46022 Valencia, Spain
b Valencia University General Hospital, Center for Research and Innovation on Bioengineering, Polytechnic University of Valencia, Camino de Vera s/n, 46022 Valencia, Spain
(Email: eberjano{at}eln.upv.es).
In the December issue of the Journal, we read with enormous interest the article entitled "Ablation of atrial fibrillation and esophageal injury: effects of energy source and ablation technique" by Aupperle and colleagues.
1
The authors performed in vivo experiments on 39 sheep to evaluate the histologic changes induced in the esophagus by using atrial fibrillation ablation with different energy types, such as cryoablation, microwave, laser, and unipolar or bipolar radiofrequency, through 2 different approaches, endocardial and epicardial. They observed esophageal alterations in numerous cases and concluded that the most significant lesions (moderate and severe damage) were principally induced by endocardial unipolar radiofrequency and cryoablation. We would like to comment on a number of issues.
The esophageal thermal lesion, which is similar to those found in myocardial thermal lesions, is mainly based on the quantity of energy absorbed by the tissue, the type of energy, and the distance between the ablation electrode and the esophagus. Aupperle and colleagues
1
have compared different types of energy applications with standard clinical protocols. However, they do not consider the distance between the electrode and the esophagus or the individual variations in myocardial thickness; that is, no allowance was made for these parameters in the groups under study. Several clinical studies have shown a short anatomic distance between the left atrium and the esophagus (around 3-5 mm) and a thickness of the myocardium of the posterior atrial wall of around 2 to 3 mm.
2,3
It is thus difficult to obtain valid conclusions with such variations in the endocardium-esophagus distance. Computer simulations using the finite-element method have suggested that the most important factor in esophageal lesions is precisely this distance.
4
In the simulation in which the total distance between the endocardium and the esophagus was fixed, neither variations in the thickness of the linking tissues nor in the thermoelectrical properties had an effect on the thermal pattern.
Regarding the temperature measurement in the esophageal lumen, variations were not found in any case. This result is fairly surprising because there were cases of severe esophageal damage. With regard to this, although temperature monitoring had been proposed as a control method for esophageal lesions during radiofrequency catheter ablation, recent studies have questioned this strategy. Meade and associates
5
showed experimentally that the esophageal temperature failed to increase when the sensors were positioned more than 1.3 cm apart. Effective measuring of the esophageal temperature depends on the position of the temperature probe relative to the heated cardiac tissue and also on good contact with the esophageal mucosa. Therefore the temperature probe, which is advanced under fluoroscopic guidance, should be placed in the optimum position at the level of the ablation lesion. Previous computer results have shown that it is possible to underestimate the maximal temperature reached in the esophagus when the temperature probe is not located exactly under the center of the electrode and on the same plane.
6
This could explain the results obtained by Aupperle and colleagues.
1
The study concludes that marked lesion were principally induced by endocardial unipolar radiofrequency. In fact, these lesions were small but reached deep into the tissue. It is possible that this conclusion was reached as a result of the higher capability of the irrigated electrodes (bipolar or unipolar) to shift the location of the hottest point toward a deeper zone compared with the case of dry electrodes (bipolar or unipolar).
Finally, it is important to point out that although the acute inflammatory reaction in the lesion caused by cryoablation can be comparable with those created by hyperthermia, it is well known that the lesion is ultrastructurally different and reaches different characteristics in the chronic phase.
7
We do not think that the degree of tissue damage in the hyperthermic lesion is similar to that of the hypothermic lesion, even though both have a similar acute inflammatory reaction from a light microscopy level. Although the esophageal inflammatory reaction described in this study implies a thermal lesion, its clinical outcome is uncertain in the cryoablation case and probably different than those created by the ablation systems based on hyperthermia.
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