Electrical Impedance Tomography in the Assessment of Extravascular Lung Water in Noncardiogenic Acute Respiratory Failure: Protocol

In: Respiratory Failure

27 Sep 2014

Electrical Impedance Tomography in the Assessment of Extravascular Lung Water in Noncardiogenic Acute Respiratory Failure: ProtocolEIT: In this study, EIT measurements were performed using a portable data acquisition system (Sheffield Applied Potential Tomograph, DAS-01P, Mark I; IBEES; Sheffield, UK), which has been described before. In the present study, the 16 electrodes (Meditrace; Technomed; Beek, The Netherlands) were equidistantly spaced around the thorax. Impedance measurements were performed at the third intercostal level, with the patient in a supine position. A source that generates an alternating current (50 KHz, 5 mA peak to peak) was used to measure impedance. Reading here
Data sets, consisting of 50 images, with an interval time of 1.13 s were recorded. One image consisted of 10 averaged data collection cycles used to minimize artifacts in impedance. Each image visualizes the impedance distribution within the thorax. Because inspiration and expiration of air cause relatively large impedance changes, these changes can be imaged. In the sequence of images, the impedance changes during ventilation can be visualized and studied. By defining a region of interest (ROI), specific areas can be examined. The computer calculated impedance changes within the ROI over the whole sequence of images. The average impedance change in this ROI is plotted in a curve as a function of time in order to show the impedance changes during ventilation. Because the reconstruction algorithm of the EIT is based on normalized differences, the average impedance change has no unit and is expressed as an arbitrary unit.
Image Analysis: As expected, ventilation-induced impedance changes increased after the application of PEEP. To avoid these PEEP-associated changes, we chose the region where only high ventilation-induced impedance changes (the center of the image: 128 pixels) are seen as ROI, thereby assuming that alveolar recruitment in this area by PEEP would be minimal.

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