Determination of tidal volume by electrical impedance tomography (EIT) after indirect two-point calibration

Objective. A linear relationship between impedance change (Delta Z) measured by thoracic electrical impedance tomography (EIT) and tidal volume (VT) has been demonstrated. This study evaluated the agreement between the displayed VT calculated by the EIT software (VTEIT) and spirometry (VTSPIRO) after an indirect two-point calibration. Approach. The EIT software was programmed to execute a bedside two-point calibration from the subject-specific, linear equation defining the relationship between Delta Z and VTSPIRO and displaying VTEIT breath-by-breath in 20 neutered male, juvenile pigs. After EIT calibration VTs of 8, 12, 16 and 20 ml kg(-1) were applied to the lungs. VTEIT and VTSPIRO were recorded and analysed using Bland-Altman plot for multiple subject measurements. Volumetric capnography (VCap) and spirometry data were explored as components of variance using multiple regression. Main results. A mean relative difference (bias) of 0.7% with 95% confidence interval (CI) of -10.4% to 10.7% were found between V-TEIT and VTSPIRO for the analysed data set. The variance in VTEIT could not be explained by any of the measured VCap or spirometry variables. Significance. The narrow CI estimated in this study allows the conclusion that EIT and its software can be used to measure and accurately convert Delta Z into mililitre VT at the bedside after applying an indirect two-point calibration.