In ARDS, lung density increases from the ventral to the dorsal lung regions in the supine position and an increased amount of EVLW causes compression atelectasis in the posterior part. Because EIT can visualize regional ventilation, we hypothesized that differences in the ventilation-induced impedance changes occurring between the anterior part and posterior part of the lungs may provide information about the EVLW content.
Therefore, the ROI was divided in an anterior half and a posterior half (Fig 1). We analyzed the curves of the whole ROI and those of the anterior half and the posterior half separately. From the obtained curves, the maximal difference in impedance between inspiration and expiration was calculated, and it was called the ventilation-induced impedance change.
To obtain information about the difference in ventilation between the anterior and posterior parts of the lung, the ventilation-induced impedance change of the whole area of interest was divided by the ventilation-induced impedance change of the anterior half. The measured ventilation-induced impedance change of the posterior part of the lung may be difficult to assess and might be miscalculated due to the compression atelectasis occurring in the posterior part of the lung when edema increases. By taking the whole area of interest, information about the posterior part is included; therefore, theoretically, information about the difference in ventilation in the posterior and anterior parts of the lung following edema accumulation should be obtained. Thus, IR = ICw/ICa = (0.5[ICa + ICp])/ICa, where IR is the impedance ratio; ICw is the ventilation-induced impedance change between inspiration and expiration measured at the whole ROI; ICa is the ventilation-induced impedance change measured at the selected anterior part of the lung; and ICp is the ventilation-induced impedance change measured at the selected posterior part of the lung. canadian-familypharmacy.com
Figure 1. An image obtained by EIT at maximal inspiration in a patient with ARDS. The letters R, L, A, and P represent the right side, left side, anterior side, and posterior side of the image, respectively. The area of interest (indicated with the white lines) is divided into a posterior (post) part and an anterior (ant) part of the lungs. Next to the image, the curve, which shows the impedance changes occurring in the area of interest during inspiration and expiration, is depicted.
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