1. Effect of Positive End Expiratory Pressure and level of Pressure Support Ventilation on ventral-to-dorsal tidal ventilation redistribution and heterogeneity in lung ventilation evaluated with Electrical Impedance Tomography Turella M 1, Mauri T 2, Confalonieri A 2, Pradella A 1, Sala V 1, Coppadoro A 2, Bellani G 2, Patroniti N 2, Pesenti A 1,2 1 Postgraduate school of Anesthesia and Intensive Care, University of Milan-Bicocca, Monza, Italy; 2 Department of Perioperative Medicine and Intensive Care, San Gerardo Hospital, University of Milan-Bicocca, Monza, Italy AIM OF THE STUDY Acute lung injury (ALI) is a clinical syndrome characterized by bilateral inflammatory lung edema, mostly involving dependent regions. Mechanical ventilation (MV) is usually implemented in ALI patients to correct gas exchange impairment. Application of assisted MV modes and positive end expiratory pressure (PEEP) promote redistribution of tidal ventilation towards dependent collapsed lung regions. Electrical impedance tomography (EIT) is a non-invasive bedside lung imaging technique. EIT might be useful to investigate the effects of different assisted MV and PEEP settings on tidal ventilation distribution in ALI patients. METHODS We enrolled 10 intubated semi-recumbent ALI patients undergoing Pressure Support Ventilation (PSV). We applied on each patient’s thorax a 16-electrode belt connected with a bedside EIT continuous recording monitor (PulmoVista 500®, Dräger Medical GmbH, Lübeck, Germany); (Figure 1,2). Figure 1: : Electrode belt with patient cable connected. Low PS (n=10) High PS (n=10) P-value Low PEEP (n=10) High PEEP (n=10) P-value PS (cmH 2 0) 3±212±3<0,0018±4,75 p 0,1 3,3±1,21,0±0,6<0,0012,03±1,731,81±1,670,61 PEEP (cmH 2 0) 7±2 12,3±2,66<0,001 Crs (mL/cmH 2 0) 62,6±31,360,3±25,90,6859,5±28,4264,9±26,00,26 Paw (cmH 2 0) 8,7±2,510,1±2,70,01289,9±2,8414,5±3,24<0,01 PaO c /FiO 2 (mmHg) 262,6±31,7250,0±53,90,38248,8±51,68268,3±58,10,00103 PaCO 2 (mmHg) 41,6±5,8139,12±4,190,07739,9±4,6240,81±4,860,37 pH 7,40±0,0667,42±0,0530,0467,41±0,0497,44±0,0540,42 Table 1: Characteristics of the study population. We randomly performed 2 steps of PSV for 15 minutes: one at lower levels of PSV (PS low ) and one at higher levels (PS high ). In the same population, we also performed 2 steps at different PEEP levels for 15 minutes: the level used for clinical purpose (PEEP low ) and a higher level (PEEP high : PEEP low +5 cmH 2 O). Figure 2: Current application and voltage measurements around the thorax. EIT imaging field was divided into 2 contiguous equal-size regions of interest (ROI): ventral and dorsal. From EIT data offline analysis we calculated, for each phase: percentage of tidal ventilation distribution in ventral and dorsal ROIs; heterogeneity, calculated as the standard deviation of the voxel-level impedance differences between end- inspiration and end-expiration/global impedance changes. Together, we collected ventilation parameters and arterial blood gas analysis. RESULTS  PS low was set at 3±2 cmH 2 O and PS high at 12±3 cmH 2 O; Dorsal ventilation increased during PS low, as compared to PS high (42±10% vs. 32±11%, p=0.004), while ventral ventilation decreased (58±9% vs. 68±10%, p=0.005);(Figure 3). Comparing PS high with PS low, heterogeneity increased in dorsal ROI while it decreased in ventral ROI (p=0.006 and p=0.008, respectively). * * Figure 3: Ventilation in ventral and Dorsal ROI at different PS levels.* p<0.05  PEEP low was 7±2 cmH 2 O and PEEP high 12±3 cmH 2 O. PEEP high was associated with increased dorsal (41±16% vs. 36±15%, p =0.006) and decreased ventral ventilation (59±16% vs. 65±14%, p =0.006), as compared to PEEP low ;(Figure 4). During PEEP high, ventral heterogeneity decreased, as compared to PEEP low (p =0.008), while dorsal heterogeneity did not differ (p = 0.26). * * Figure 4: Ventilation in Ventral an Dorsal ROI at different PEEP levels. * p<0.05 CONCLUSIONS  EIT was able to detect ventral-to-dorsal tidal ventilation redistribution associated with higher PEEP and lower PSV levels.  Increased dorsal ventilation was associated with better oxygenation probably because of improved ventilation-perfusion matching.  Comparable heterogeneity levels in presence of increased dorsal ventilation may be related to alveolar recruitment induced by PEEP.  PaO 2 /FiO 2 ratio correlated with dorsal tidal ventilation during both PEEP and PS phases (R 2 =0.390 and 0.251, respectively p<0.05;(Figure 5,6). Figure 5: PEEP study: changes in PaO2/FiO2 and percentage of Tidal Volume (Vt) to Dorsal ROI. Figure 5: PS study: changes in PaO2/FiO2 and percentage of Tidal Volume (Vt) to Dorsal ROI. R 2 =0,251 p <0.05 R 2 =0,39 p <0.05