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 vs. 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]ml 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.