As comparison, the method, called compliance-volume curve method in the following, was also included in the present study.Statistical analysisStatistical reference analysis was performed with the MATLAB software package (MATLAB 7.2 statistic toolbox, The MathWorks Inc., Natick, MA, USA). The Lilliefors test was used to evaluate the distribution of all data. For normally distributed data, results are presented as mean �� SD. Paired-sample t-test was applied in this case to assess the significance of differences in choosing PEEP levels for individuals (GI index vs. dynamic compliance; GI index vs. compliance-volume curve). A P value less than 0.05 was considered statistically significant. Due to the small amount of subjects in the study, significance levels were adjusted to maintain a statistical power above 80% in order to reduce the type II error.

Furthermore, significance levels were corrected for multiple comparisons using Holm’s sequential Bonferroni method. For not normally distributed data, results are expressed as median (interquartile range). Results were compared using the Bland-Altman analysis [22].ResultsTidal volume distribution in EIT images (i.e. tidal images) at PEEP levels 6, 14 and 22 mbar are compared in Figure Figure1.1. With increased PEEP, the lung was further dilated.Figure 1Tidal ventilation distribution in EIT images at different PEEP levels. (a) 6 mbar. (b) 14 mbar. (c) 22 mbar. The tidal images were the differences of relative impedance between end-inspiration and end-expiration in electrical impedance tomography (EIT) …

In Figure Figure2,2, a typical relation between the GI value and PEEP is depicted. Starting at ZEEP, the GI index first decreased with the increase of PEEP indicating that ventilation was more homogenously distributed. A single minimum value of the GI index was found at a middle range of PEEP levels. With further increase in PEEP the GI index rose steadily (Figure (Figure2).2). Such a curve with only single minimum value of the GI index was observed in every patient. At PEEP levels corresponding with the minimum GI index values (12.2 �� 4.6 mbar) the air is most homogenously distributed in the lungs.Figure 2A typical curve of (right) GI index of one patient (left) during a standardized PEEP trial. The x axis displays the number of breathing cycles, counted once the maneuver started.

A minimum value of the global inhomogeneity (GI) index indicated the optimal …For comparison in Figure Figure3,3, the PEEP level is depicted for the same individual as in Figure Figure22 when the global dynamic compliance reached its maximum. A quasi-plateau phase in the compliance-pressure curve was found in every patient. In a range of 8 mbar (4 PEEP steps), the maximum relative change of compliance was only 2% (1%; in relation to maximum compliance).Figure 3Dynamic compliance calculated using the least-square-fit Carfilzomib method for the same patient as in Figure 1.