Accuracy of plateau pressure and stress index to identify injurious ventilation in patients with acute respiratory distress syndrome.

Background: Guidelines suggest plateau pressure (PPLAT)≤30 cmH2O for patients with acute respiratory distress syndrome but ventilation may still be injurious despite adhering to this guideline. The shape of the airway pressure vs. time curve (STRESS INDEX) may identify injurious ventilation. We assessed accuracy of PPLAT and STRESS INDEX to identify morphological indexes of injurious ventilation. Methods: Indexes of lung aeration (computerized-tomography) associated with injurious ventilation were used as a “reference standard”. Threshold values of PPLAT and STRESS INDEX were determined assessing the receiver-operating characteristics (“training set” N=30). Accuracy of these values was assessed in a second group of patients (“validation set” N=20). PPLAT and STRESS INDEX were partitioned between respiratory system (PPLAT,RS and STRESS INDEX,RS) and lung (PPLAT,L and STRESS INDEX,L) (esophageal pressure; “physiological set” N=50). Results: Sensitivity and specificity of PPLAT>30 cmH2O were 0.06 (95% CI: [0.002 to 0.30]) and 1.0 (95% CI: [0.87 to 1.00]). PPLAT>25cmH2O and a STRESS INDEX>1.05 best identified morphological indexes associated with injurious ventilation. Sensitivity and specificity of these values were 0.75 (95% CI:[0.35 to 0.97]) and 0.75 (95% CI:[0.43 to 0.95]) for PPLAT>25cmH2O vs. 0.88 (95% CI:[0.47 to 1.00]) and 0.50 (95% CI:[0.21 to 0.79]) for STRESS INDEX>1.05. PPLAT,RS did not correlate with PPLAT,L (R=0.0099); STRESS INDEX,RS and STRESS INDEX,L were correlated (R=0.762). Conclusions: The best threshold values for discriminating morphological indexes associated with injurious ventilation were PPLAT,RS>25 cmH2O and STRESS INDEX,RS>1.05. While a substantial discrepancy between PPLAT,RS and PPLAT,L occurs, STRESS INDEX,RS reflects STRESS