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(aw) stress index on lung mechanics and the correlation between P(aw) 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 min. 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(aw) stress index, and P(L) 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(2)O vs. 9.0 ± 2.3 cmH(2)O, P < 0.01). Compared with the H group, lung elastance was higher (20.0 ± 7.8 cmH(2)O/L vs. 11.6 ± 3.6 cmH(2)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(aw) stress index and the P(L) stress index in H group (R(2)= 0.56, P < 0.01) and L group (R(2)= 0.85, P < 0.01). CONCLUSION: In the ARF patients with MV, P(aw) stress index can substitute for P(L) to guide ventilator settings. TRIAL REGISTRATION: ClinicalTrials.gov NCT02196870 (https://clinicaltrials.gov/ct2/show/NCT02196870).