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Use of electrical impedance tomography for checking the correct placement of double-lumen tube
- Inéz Frerichs, Sven Pulletz, Gunnar Elke, and Norbert Weiler (31 August 2008)
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Inéz Frerichs Dept. of Anaesthesiology and Intensive Care Medicine, University Medical Centre S-H, Kiel, Germany, Sven Pulletz, Gunnar Elke, and Norbert Weiler
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Editor - We read with interest the article by Steinmann and colleagues1 on the use of electrical impedance tomography (EIT) during one -lung ventilation. The authors showed that EIT reliably identified all gross misplacements of the double-lumen tube in the contralateral bronchus. However, when the tube was positioned in the correct bronchus and only the endobronchial cuff was displaced, EIT was not able to determine any changes in the ventilation distribution between the right and left lungs after the repositioning of the cuff. Representative EIT images shown in this article in Figs. 2, 4 and 5 indicate that regions with significant time-dependent variation of electrical impedance were present in the non-ventilated halves of the images during one lung ventilation (although of a smaller amplitude than in the ventilated lungs). In a recent study from our research group2 performed in a very similar group of patients and setting, we were able to show that these regions arise from ventilation-related cyclic changes of the EIT signal which are synchronous with the breathing rate but out-of- phase with the fluctuations of the signal in the ventilated lung. In our study, this phenomenon was found in all 20 studied cases of one-lung ventilation of either right or left lungs. The figure illustrates this typical finding in one of our patients. We postulate that the source of these cyclic out-of-phase changes in electrical impedance may be the passive movement and lung volume changes of the non-ventilated lung elicited indirectly by inflation and deflation of the ventilated lung or the image reconstruction algorithm. Steinmann and colleagues1 state in the Methods section of their article that, for each image quadrant, “the intra-tidal impedance changes (deltaZ) between maximal expiration and maximal inspiration were averaged for three consecutive breaths”. If the phase lag of the signal over the non-ventilated lung was not taken into account this type of evaluation would lead to an underestimation of ventilation in the ventilated lung. This may have happened in the data analysis of Steinmann and colleagues1 and can be inferred from the fact that, during one lung ventilation, the calculated percentage of ventilation distributed to the ventilated lung was only about 80%. This means that in spite of the verified correct position of the double-lumen tube 20% of ventilation was attributed to the non-ventilated lung. In our study2, we have analyzed how different types of EIT data evaluation affect the calculated right-to-left lung ventilation distribution during one-lung ventilation. The worst separation between the ventilated and non-ventilated lungs was noted when a simple intra-tidal variation of the EIT signal (i.e. the standard deviation of the signal), was used as a measure of ventilation and calculated in the whole right and left halves of the scans. Using this approach for instance during the left lung ventilation, the calculated percentage of ventilation was just 80% which is comparable to the findings by Steinmann and colleagues1. If the difference between the maximum values of electrical impedance at inspiration and minimum values at expiration was determined as a measure of tidal volume, the calculated percentage of ventilation increased to 90%. The use of a region-of-interest, based on the generation of functional EIT scans3 4, further improved the assessed percentage of ventilation to 92%. Finally, a percentage of ventilation of 97% was achieved when another type of region-of-interest2 5 was applied which eliminated the regions with the out-of-phase EIT signal from further analysis. These results indicate that the EIT data evaluation tools as used by Steinmann and colleagues1 which may work well under the conditions of relatively homogeneous ventilation distribution may not be sufficient when extremely heterogeneous distribution patterns are studied. Advanced data analysis most likely is needed to obtain optimum results. Integration of such analysis tools into existing software packages and development of new evaluation methods may further improve the performance of EIT in monitoring regional lung function. I. Frerichs* S. Pulletz G. Elke N. Weiler Kiel, Germany *E-mail: frerichs@anaesthesie.uni-kiel.de 1 Steinmann D, Stahl CA, Minner J et al. Electrical impedance tomography to confirm correct placement of double-lumen tube: a feasibility study. Br J Anaesth 2008; 101: 411-418 2 Pulletz S, Elke G, Zick G et al. Performance of electrical impedance tomography in detecting regional tidal volumes during one-lung ventilation. Acta Anaesthesiol Scand 2008; 52: 1131-1139 3 Hahn G, Šipinková I, Baisch F, Hellige G. Changes in the thoracic impedance distribution under different ventilatory conditions. Physiol Meas 1995; 16: A161-73 4 Frerichs I, Hahn G, Schröder T, Hellige G. Electrical impedance tomography in monitoring experimental lung injury. Intensive Care Med 1998; 24: 829-36 5 Pulletz S, van Genderingen H, Schmitz G et al. Comparison of different methods to define regions of interest for evaluation of regional lung ventilation by EIT. Physiol Meas 2006; 27:S115-27 Figure legend Functional EIT scan obtained in a patient during one-lung ventilation of the left lung (anterior is at the top and right side of the chest is on the left of the image). The scan was generated using the same procedure as used by Steinmann and colleagues1 which was originally described in e.g. 3 4. The scan shows the distribution of regional variation of electrical impedance (i.e. the standard deviation of the EIT signal in each image pixel). The time courses of regional EIT data in two image pixels located in the non-ventilated right and ventilated left lung indicate that the EIT signal in the right lung region is out-of-phase with the left lung region. rel. deltaZ, relative impedance change. Conflict of Interest:NW has received an unrestricted research grant of 40.000,- Euro from Viasys Healthcare. IF has received 3000,- Euro from Viasys Healthcare for reimbursement of travel costs to the ESICM and ATS congresses. |
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