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Letter with respect to the article Tormos et al.
Medical Engineering & Physics 34 (2012) 795
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Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy
Letter to the editor Letter with respect to the article Tormos et al. With great interest we have read the article published by Tormos et al. [1]. The authors have developed a 128-channel epicardial mapping electrode which can alter tissue temperature in a circular area of 16 mm diameter by cooling or heating. The device was applied for investigating electrical activity during thermally induced heterogeneity in the Langendorff perfused rabbit ventricles during sinus rhythm and ventricular fibrillation. The electrophysiological data presented reveal an interesting insight into temperature-induced changes of cardiac electrical function and are in good agreement with other work published in this field. However, the temperatures measured across the ventricular wall are questionable. At a depth of only 1 mm tissue published results were almost at tissue temperature when cooling or heating the epicardium to a target temperature of 22 ◦ C or 42 ◦ C, respectively. Admittedly, in the discussion Tormos et al. raise some doubts about the correctness of these data, but a final conclusion about the temperature profile is missing. This letter now presents a biophysical model, explaining the measured data. The temperature field is computed by the Pennes bio-heat equation as described in [2] and the model is implemented in a customized software tool of our group as described in [3]. The thermocouple needle used by Tormos et al. is modeled by a cave steel cylinder inserted into the myocardium. The experimentally observed temperature field is qualitatively reproducible if two assumptions are made: First, a good thermal contact of the needle to the deep (near endocardial) myocardium is reached assuming that interstitial liquid or the perfusion solution of the Langendorff apparatus fills the distal end of the needle. Second, a weak thermal coupling to the epicardium is achieved assuming that the space between the needle and the mapping electrode is filled by a small air gap. This seems reasonable as here the cables of the thermocouples are guided out of the needle. Fig. 1 shows the simulated temperature field computed for the situation that the device cools the tissue. It can be seen from this that the high thermal conductivity of the metallic needle significantly reduces all temperature gradients along the needle. Because of a better coupling of the needle to the deep myocardium its temperature is close to body temperature. Further on, due to the air inclusion at the epicardium a sharp temperature drop occurs near to the mapping electrode. Thus, the temperature variation along the needle remarkably differs from the temperature profile in the tissue. In conclusion, our model predicts the significant alteration of the temperature profile within the myocardial wall by a metallic needle. Hardly controllable situations in the experiment such as air inclusions strongly impact the measured temperature profile. Therefore, different experimental methods need to be developed when assessing the temperature profile within the myocardium.
Fig. 1. Simulated temperature profile for a cylindrical needle in the tissue above the cooling device. Temperatures in observation points at 1 mm spacing within the needle and within the tissue are indicated for comparison.
Hence, precise temperature data in combination with valuable high resolution mapping data accessed by the device of Tormos et al. will further gain insight into temperature dependency of cardiac electric function. Conflict of interest statement The authors have no conflict of interest to declare. Research is supported by the Standortargentur Tirol. References [1] Tormos A, Guill A, Millet J, Roses EJ, Trapero I, Such-Miquel L, et al. New epicardial mapping electrode with warming/cooling function for experimental electrophysiology studies. Med Eng Phys 2011;33(June (5)):653–9. [2] Liu J. In: Akay M, editor. Bioheat transfer model. Wiley Encyclopedia of Biomedical Engineering; 2006. [3] Seger M, Fischer G, Handler M, Stöger M, Nowak C.-N., Hintringer F, et al. Achieving elongated lesions employing cardiac cryoablation: a preclinical evaluation study. Cryobiology, submitted for publication [Published online 8 May 2012].
DOI of original article: http://dx.doi.org/10.1016/j.cryobiol.2012.04.007. 1350-4533/$ – see front matter © 2012 IPEM. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.medengphy.2012.04.018
G. Fischer ∗ M. Handler R. Kienast C. Baumgartner Institute of Electrical and Biomedical Engineering, UMIT, The Health and Life Sciences University, Hall/Tyrol, Austria ∗ Corresponding
author. E-mail address: [email protected] (G. Fischer) 28 February 2012
Contents lists available at SciVerse ScienceDirect
Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy
Letter to the editor Letter with respect to the article Tormos et al. With great interest we have read the article published by Tormos et al. [1]. The authors have developed a 128-channel epicardial mapping electrode which can alter tissue temperature in a circular area of 16 mm diameter by cooling or heating. The device was applied for investigating electrical activity during thermally induced heterogeneity in the Langendorff perfused rabbit ventricles during sinus rhythm and ventricular fibrillation. The electrophysiological data presented reveal an interesting insight into temperature-induced changes of cardiac electrical function and are in good agreement with other work published in this field. However, the temperatures measured across the ventricular wall are questionable. At a depth of only 1 mm tissue published results were almost at tissue temperature when cooling or heating the epicardium to a target temperature of 22 ◦ C or 42 ◦ C, respectively. Admittedly, in the discussion Tormos et al. raise some doubts about the correctness of these data, but a final conclusion about the temperature profile is missing. This letter now presents a biophysical model, explaining the measured data. The temperature field is computed by the Pennes bio-heat equation as described in [2] and the model is implemented in a customized software tool of our group as described in [3]. The thermocouple needle used by Tormos et al. is modeled by a cave steel cylinder inserted into the myocardium. The experimentally observed temperature field is qualitatively reproducible if two assumptions are made: First, a good thermal contact of the needle to the deep (near endocardial) myocardium is reached assuming that interstitial liquid or the perfusion solution of the Langendorff apparatus fills the distal end of the needle. Second, a weak thermal coupling to the epicardium is achieved assuming that the space between the needle and the mapping electrode is filled by a small air gap. This seems reasonable as here the cables of the thermocouples are guided out of the needle. Fig. 1 shows the simulated temperature field computed for the situation that the device cools the tissue. It can be seen from this that the high thermal conductivity of the metallic needle significantly reduces all temperature gradients along the needle. Because of a better coupling of the needle to the deep myocardium its temperature is close to body temperature. Further on, due to the air inclusion at the epicardium a sharp temperature drop occurs near to the mapping electrode. Thus, the temperature variation along the needle remarkably differs from the temperature profile in the tissue. In conclusion, our model predicts the significant alteration of the temperature profile within the myocardial wall by a metallic needle. Hardly controllable situations in the experiment such as air inclusions strongly impact the measured temperature profile. Therefore, different experimental methods need to be developed when assessing the temperature profile within the myocardium.
Fig. 1. Simulated temperature profile for a cylindrical needle in the tissue above the cooling device. Temperatures in observation points at 1 mm spacing within the needle and within the tissue are indicated for comparison.
Hence, precise temperature data in combination with valuable high resolution mapping data accessed by the device of Tormos et al. will further gain insight into temperature dependency of cardiac electric function. Conflict of interest statement The authors have no conflict of interest to declare. Research is supported by the Standortargentur Tirol. References [1] Tormos A, Guill A, Millet J, Roses EJ, Trapero I, Such-Miquel L, et al. New epicardial mapping electrode with warming/cooling function for experimental electrophysiology studies. Med Eng Phys 2011;33(June (5)):653–9. [2] Liu J. In: Akay M, editor. Bioheat transfer model. Wiley Encyclopedia of Biomedical Engineering; 2006. [3] Seger M, Fischer G, Handler M, Stöger M, Nowak C.-N., Hintringer F, et al. Achieving elongated lesions employing cardiac cryoablation: a preclinical evaluation study. Cryobiology, submitted for publication [Published online 8 May 2012].
DOI of original article: http://dx.doi.org/10.1016/j.cryobiol.2012.04.007. 1350-4533/$ – see front matter © 2012 IPEM. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.medengphy.2012.04.018
G. Fischer ∗ M. Handler R. Kienast C. Baumgartner Institute of Electrical and Biomedical Engineering, UMIT, The Health and Life Sciences University, Hall/Tyrol, Austria ∗ Corresponding
author. E-mail address: [email protected] (G. Fischer) 28 February 2012