Increasing positive end-expiratory pressure (re-)improves intraoperative respiratory mechanics and lung ventilation after prone positioning  J Spaeth, K Daume, U Goebel, S Wirth, S Schumann  British Journal of Anaesthesia  Volume 116, Issue 6, Pages 838-846 (June 2016) DOI: 10.1093/bja/aew115 Copyright © 2016 The Author(s) Terms and Conditions

Fig 1 Study protocol. After induction of anaesthesia (ITN) all patients received measurements in the supine position at a positive end-expiratory pressure (PEEP) of 6 cm H2O. After prone positioning, three different PEEP levels were applied in random order. Each PEEP interval was preceded by a lung recruitment manoeuvre (RM) and maintained for at least 15 min to allow for equilibration of the respiratory system. EIT and respiratory measurements were performed in the last five min of each PEEP interval (pink-shaded areas). VCV: volume controlled ventilation (tidal volume+6–8 ml kg−1 normal body weight). Please note that interventions started directly after induction of anaesthesia and were carried out in immediate succession. British Journal of Anaesthesia 2016 116, 838-846DOI: (10.1093/bja/aew115) Copyright © 2016 The Author(s) Terms and Conditions

Fig 2 Methodological scheme regarding the calculation of the intratidal compliance-volume profiles based on measured airway pressure and flow rate: (A) The 10–90% volume range of the tidal pressure-volume-curve is subdivided into 31 equidistant volume portions (slices); (B) For each slice, the compliance is determined based on multiple linear regression analysis and referred to the respective tidal volume; (C) The median course of the compliance-volume curves from a minimum of 12 breaths is assigned to one of six compliance profiles (CP), each characterizing a certain section of the hypothetical compliance curve, that includes vital capacity (dotted line) within the tidal volume: H: horizontal CP; I: merely increasing CP; IH: increasing turning into horizontal CP; D: merely decreasing CP; HD: horizontal turning into decreasing CP; IHD: increasing turning into horizontal and further turning into decreasing CP. Solid bars indicating main profiles; contoured bars indicating overlapping profiles. British Journal of Anaesthesia 2016 116, 838-846DOI: (10.1093/bja/aew115) Copyright © 2016 The Author(s) Terms and Conditions

Fig 3 Distribution of compliance profiles (CP) calculated in 45 patients at a certain posture and PEEP. Each bar represents the absolute number of patients in the respective CP. H: horizontal CP (orange bars); IH+increasing turning into horizontal CP (green bars); I: merely increasing CP (pink bars). The horizontal CP bars are superimposed by a pattern indicating the number of patients recruited from the increasing turning into horizontal CP (crossed pattern) and from the merely increasing CP (shaded pattern) when increasing PEEP to 9 and 12 cm H2O. Other CPs’ were not observed during the study. *P<0.05 compared with supine position; §P<0.05 compared with PEEP 6 cm H2O in the prone position; no significant differences were found between the distribution of the CPs when increasing PEEP from 9 to 12 cm H2O in the prone position. British Journal of Anaesthesia 2016 116, 838-846DOI: (10.1093/bja/aew115) Copyright © 2016 The Author(s) Terms and Conditions

Fig 4 Upper panel: Electrical impedance tomography (EIT) images recorded in 45 patients averaged for the course of inspiration. Each pixel represents the mean difference between end-inspiratory and end-expiratory impedance, standardized for the end-expiratory value. The colour of pixels indicates the level of impedance from low impedance change (dark blue) to high impedance change (dark red) following the colour scale of the visible spectrum. White stars indicate significant increase of impedance compared with the next lower PEEP level [P<0.05; including Holm-Bonferroni sequential correction for 1024 (32 · 32 pixel) comparisons]. Lower panel: Fractional regional gas distribution depending on regions of interest (ROI), postures and PEEP levels during inspiration (calculated for eight iso-volume parts of impedance tidal variation (ITV). Left EIT image in the upper panel exemplarily gives the regional separation according to ventral (V) and dorsal (D) ROI. Please note the predominantly ventral located increase in impedance at higher PEEP levels in the prone position. British Journal of Anaesthesia 2016 116, 838-846DOI: (10.1093/bja/aew115) Copyright © 2016 The Author(s) Terms and Conditions

Fig 5 Percentages of end-expiratory distribution of ventilation per region of interest. The stacked bars illustrate the fractional distribution of end-expiratory ventilation according to regions of interest (ROI; ventral (V), mid-ventral (MV), mid-dorsal (MD), dorsal (D)). Ventilation was most uniformly distributed in the supine position. Turning the patient prone led to predominant ventilation in the mid-dorsal ROI. Increasing PEEP in the prone position partially redistributed ventilation towards ventral ROIs. Please note that the stacked bars are standardized to 100%; total end-expiratory impedance would have changed related to posture and PEEP. British Journal of Anaesthesia 2016 116, 838-846DOI: (10.1093/bja/aew115) Copyright © 2016 The Author(s) Terms and Conditions