Dynamic bedside assessment of the physiologic effects of prone position in acute respiratory distress syndrome patients by electrical impedance tomography.

BACKGROUND: Prone position (PP) improves acute respiratory distress syndrome (ARDS) survival by reducing the risk of ventilation-induced lung injury. However, inter-individual variability is a hallmark of ARDS and lung protection by PP might not be optimal in all patients. In the present study, we dynamically assessed physiologic effects of PP by electrical impedance tomography (EIT) and identified predictors of improved lung protection by PP in ARDS patients. METHODS: Prospective physiologic study on 16 intubated, sedated and paralyzed patients with ARDS undergoing PP as per clinical decision. EIT data were recorded during two consecutive steps: 1) baseline supine position before and after a recruitment maneuver (RM); 2) prone position before and after a RM. "Improved lung protection" by PP was defined in the presence of simultaneous improvement of ventilation homogeneity (Hom), alveolar overdistension and collapse (ODCL) and amount of recruitable lung volume by RM in comparison to supine. RESULTS: PP versus supine increased the tidal volume distending the dependent regions (Vtdep), resulting in improved Hom (1.1±0.9 vs. 1.7±0.9, P=0.021). PP also reduced ODCL (19±9% vs. 28±8%, P=0.005) and increased the recruitable lung volume (80 [71-157] vs. 59 [1-110] mL, P=0.025). "Improved lung protection" by PP was predicted by lower Vtdep, higher Vtndep and poorer Hom measured during baseline supine position (P<0.05). CONCLUSIONS: EIT enables dynamic bedside assessment of the physiologic effects of PP and might support early recognition of ARDS patients more likely to benefit from PP.

Bedside selection of positive end-expiratory pressure by electrical impedance tomography in hypoxemic patients: a feasibility study.

BACKGROUND: Positive end-expiratory pressure (PEEP) is a key element of mechanical ventilation. It should optimize recruitment, without causing excessive overdistension, but controversy exists on the best method to set it. The purpose of the study was to test the feasibility of setting PEEP with electrical impedance tomography in order to prevent lung de-recruitment following a recruitment maneuver. We enrolled 16 patients undergoing mechanical ventilation with PaO2/FiO2 <300 mmHg. In all patients, under constant tidal volume (6-8 ml/kg) PEEP was set based on the PEEP/FiO2 table proposed by the ARDS network (PEEPARDSnet). We performed a recruitment maneuver and monitored the end-expiratory lung impedance (EELI) over 10 min. If the EELI signal decreased during this period, the recruitment maneuver was repeated and PEEP increased by 2 cmH2O. This procedure was repeated until the EELI maintained a stability over time (PEEPEIT). RESULTS: The procedure was feasible in 87% patients. PEEPEIT was higher than PEEPARDSnet (13 ± 3 vs. 9 ± 2 cmH2O, p < 0.001). PaO2/FiO2 improved during PEEPEIT and driving pressure decreased. Recruited volume correlated with the decrease in driving pressure but not with oxygenation improvement. Finally, regional alveolar hyperdistention and collapse was reduced in dependent lung layers and increased in non-dependent lung layers. CONCLUSIONS: In hypoxemic patients, a PEEP selection strategy aimed at stabilizing alveolar recruitment guided by EIT at the bedside was feasible and safe. This strategy led, in comparison with the ARDSnet table, to higher PEEP, improved oxygenation and reduced driving pressure, allowing to estimate the relative weight of overdistension and recruitment.