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

In: Respiratory Failure

25 Sep 2014

Electrical Impedance Tomography in the Assessment of Extravascular Lung Water in Noncardiogenic Acute Respiratory FailureIn many cases of acute respiratory failure (ARF), pulmonary edema contributes to mechanical and gas exchange abnormalities. However, radiographic and gas exchange abnormalities may not be sensitive and specific and may not accurately reflect the extent of pulmonary edema. Assessing the amount of extravascular lung water (EVLW) at the bedside is useful because the amount of directly measured pulmonary edema may have prognostic significance in ARF, and amelioration of the edema may decrease morbidity and mortality. On the other hand, assessing the amount of EVLW at the bedside is not simple. For this purpose, the thermal dye double indicator dilution technique (TDD) is used, but the invasiveness of the technique, which utilizes two catheters, has hampered widespread clinical application. Despite limitations inherent to the technique, including the potential underestimation of EVLW in poorly perfused lung regions, the technique is considered as the “gold standard” for measuring EVLW. In fact, EVLW measured by TDD may largely reflect gravimetric EVLW measurements at autopsy of ARF patients. further

Noninvasive techniques used to assess the amount of EVLW include the transthoracic electrical impedance technique. However, although simple and noninvasive, results from studies that assess the amount of EVLW with this technique are vari-able. The influence of blood volume and ventilation on the measured impedance change are major confounders in the assessment of the amount of EVLW by transthoracic electrical impedance, Furthermore, transthoracic electrical impedance does not allow one to differentiate between EVLW and pleural fluid. Therefore, it can be considered insufficiently sensitive and accurate for quantification of the amount of EVLW. Recently, a new noninvasive electrical impedance imaging technique, electrical impedance tomography (EIT), has been developed, which has the potential to overcome most of the limitations of transthoracic electrical impedance measurements. In contrast to transthoracic electrical impedance, EIT is an imaging technique that can visualize the electrical impedance changes caused by the ventilation in a two-dimensional transverse plane. The measured impedance changes due to ventilation have a linear relationship with tidal volume (Vt), and different regions can be analyzed quantitatively by selecting specific areas of interest in the measured plane.’ Since it is known that in ARDS, lung density increases from the ventral to the dorsal lung regions in the supine position, we hypothesized that EIT might be able to detect EVLW in ARF when ventilation-induced impedance changes of the anterior and the posterior parts of the lung are compared. Therefore, EIT was compared with TDD in patients with noncardiogenic ARF.

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