Selection of the Baseline Frame for Evaluation of Electrical Impedance Tomography of the Lungs

Abstract

Electrical impedance tomography (EIT) is a promising modality for lung ventilation monitoring. It can provide information about the distribution of regional ventilation in predefined regions of interest (ROIs), as well as estimate several ventilatory parameters including tidal volume (VT) or end-expiratory lung volume (EELV). The approaches for calculation of VT and EELV are based on the values of global tidal variation (TV) and end-expiratory lung impedance (EELI) obtained by the means of functional EIT (fEIT). For reconstruction of fEIT data, a set of reference measurements, often called as a baseline frame, needs to be determined. The aim of the study is to show how setting of this baseline frame can influence the values of ROI, global TV and EELI and thus affect the estimation of VT and EELV and the evaluation of lung recruitment as such. In order to study the effect of the baseline frame selection, an animal study (pigs, n=3) was conducted. The animals were anaesthetized and mechanically ventilated. Four incremental steps in positive end-expiratory pressure (PEEP), each having a value of 0.5 kPa were performed to reach a total PEEP level of 2.5 kPa. Continuous EIT monitoring was done during this PEEP trial. The obtained data were reconstructed using baseline frames chosen manually at five different PEEP levels. The selection of the baseline frames resulted in different values of global TV and EELI. Thus, when estimating VT and EELV by means of fEIT, it is necessary to choose one common baseline frame for data reconstruction. However, the effect on the percentage values that express the distribution of regional ventilation is negligible and below clinical significance.