Physiological Correlation of Airway Pressure and Transpulmonary Pressure Stress Index on Respiratory Mechanics in Acute Respiratory Failure

Background： Stress index at post-recruitment maneuvers could be a method of positive end-expiratory pressure （PEEP） titration in acute respiratory distress syndrome （ARDS） patients. However, airway pressure （P_aw） stress index may not reflect lung mechanics in the patients with high chest wall elastance. This study was to evaluate the P stress index on lung mechanics and the correlation between P stress index and transpulmonary pressure （P_L） stress index in acute respiratory failure （ARF） patients. Methods： Twenty-four ARF patients with mechanical ventilation （MV） were consecutively recruited from July 2011 to April 2013 in Zhongda Hospital, Nanjing, China and Ospedale S. Giovanni Battista-Molinette Hospital, Turin, Italy. All patients underwent MV with volume control （tidal volume 6 ml/kg） for 20 rain. PEEP was set according to the ARDSnet study protocol. The patients were divided into two groups according to the chest wall elastance/respiratory system elastance ratio. The high elastance group （H group, n = 14） had a ratio _〉30%, and the low elastance group （L group, n = 10） had a ratio 〈30%. Respiratory elastance, gas-exchange, P stress index, and Pt stress index were measured. Student＇s t-test, regression analysis, and Bland-Altman analysis were used for statistical analysis. Results： Pneumonia was the major cause of respiratory failure （71.0%）. Compared with the L group, PEEP was lower in the H group （5.7 ± 1.7 cmH,O vs. 9.0 ± 2.3 cmH2O, P 〈 0.01 ）. Compared with the H group, lung elastance was higher （20.0 ± 7.8 cmH20/L vs. 11.6 ± 3.6 cmH,O/L, P 〈 0.01）, and stress was higher in the L group （7.0 ±1.9 vs. 4.9 ± 1.9, P = 0.02）. A linear relationship was observed between the P stress index and the P stress index in H group （x2 = 0.56, P 〈 0.01 ） and L group （x2 = 0.85, P 〈 0.01）. Conclusion： In the ARF patients with MV, P,w stress index can substitute for PL to guide ventilator settings.