Electrical Impedance Tomography
In ARDS patients, the amount of potentially recruitable lung volume and the difference between the open, fully recruited, and the not recruited lung volume at 40 cmH2O was computed as 26% ± 11% (range 11-47%) using EIT [36]. Thanks to its dynamic analysis, EIT has also allowed to identify alveolar opening of collapsed regions, the overdistension of opened areas, the collapse of previously opened regions, and the recovery of previously overdistended alveoli, during stepwise changes of mean airway pressure [37]. In particular, it was used to further study the regional intratidal gas distribution during the lung recruitment at different PEEP levels. For these purposes, EIT image can be divided in four anteroposterior quadrants (dorsal and ventral regions for the left and right lungs), and a regional analysis can be performed for each quadrant.
The study of the dynamic behaviors of the lung during slow recruitment maneuvers, in particular the delay time needed for the regional impedance in each quadrant to reach a certain threshold value of the maximal local impedance change, has allowed to construct maps of regional delays in ventilation that correlate with the amount of recruited lung and lung inhomogeneity and showed that distribution of gas started in the nondependent lung regions and ended in the dependent lung regions [38, 39].
The advantage of EIT is that the image might change following recruitment maneuvers, physiotherapy, or prone positioning, indicating a response to such treatments. In this context, EIT can open new scenarios to guide the PEEP titration strategy aimed at a decrease of the regional ventilation inhomogeneity, minimizing tidal collapse and hyperinflation.
