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

In: Respiratory Failure

3 Oct 2014

Electrical Impedance Tomography in the Assessment of Extravascular Lung Water in Noncardiogenic Acute Respiratory Failure: DiscussionOur data show that EIT, by using an IR that is based on ventilation-induced impedance changes in the anterior and the posterior parts of the lung, is a noninvasive technique that may be a reasonable estimate of EVLW in ARF. read more

The explanation for the increase in the IR, and thus the presence of EVLW, which reflects the ventilation-induced impedance changes of the posterior part divided by those of the anterior part of the lungs, might be twofold. First, ALI is known to result in a decreased gas/tissue ratio in the anterior and the posterior parts of the lung, as compared to the healthy state. Therefore, a substantial decrease in ventilation-induced impedance changes can be expected in the anterior part. And second, as lung weight increases with an increasing EVLW, the lung is moved to the back in the supine position due to gravitational forces. This is reflected by a bigger decrease in the ventilation-induced impedance change in the anterior part of the lung than that of the posterior part because the effect of gravity on the dense lung results in a greater than normal gradient from the top to the bottom of the lung (Fig 2, top right, B and bottom left, C). Thus, in the EIT image, the lung is moved to the back, and because the EIT image was divided into an anterior and posterior part of the lung by a standardized ROI analysis, the ventilation-induced impedance changes in the anterior part of the lung may decrease and those in the posterior part of the lung may even increase due to a shift in the amount of alveoli in the parts of the lung selected, thereby increasing the IR.
Theoretically, several factors might influence the IR. First, PEEP may increase the lung volume predominantly in the dependent parts of the lung” and thereby increase the ventilation-induced impedance changes in those parts of the lung. In order to avoid this interference, we chose the center of the image in which the highest ventilation-induced impedance changes occur as the ROI for our analysis.


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