The Open Lung Approach Improves Pulmonary Vascular Mechanics in an Experimental Model of Acute Respiratory Distress Syndrome

OBJECTIVE:: To test whether positive end-expiratory pressure consistent with an open lung approach improves pulmonary vascular mechanics compared with higher or lower positive end-expiratory pressures in experimental acute respiratory distress syndrome. DESIGN:: Experimental study. SETTING:: Animal research laboratory. SUBJECTS:: Ten pigs, 35 ± 5.2 kg. INTERVENTIONS:: Acute respiratory distress syndrome was induced combining saline lung lavages with injurious mechanical ventilation. The positive end-expiratory pressure level resulting in highest compliance during a decremental positive end-expiratory pressure trial after lung recruitment was determined. Thereafter, three positive end-expiratory pressure levels were applied in a random order: hyperinflation, 6 cm H2O above; open lung approach, 2 cm H2O above; and collapse, 6 cm H2O below the highest compliance level. High fidelity pressure and flow sensors were placed at the main pulmonary artery for measuring pulmonary artery resistance (Z0), effective arterial elastance, compliance, and reflected pressure waves. MEASUREMENTS AND MAIN RESULTS:: After inducing acute respiratory distress syndrome, Z0 and effective arterial elastance increased (from 218 ± 94 to 444 ± 115 dyn.s.cm and from 0.27 ± 0.14 to 0.62 ± 0.22 mm Hg/mL, respectively; p < 0.001), vascular compliance decreased (from 2.76 ± 0.86 to 1.48 ± 0.32 mL/mm Hg; p = 0.003), and reflected waves arrived earlier (0.23 ± 0.07 vs 0.14 ± 0.05, arbitrary unit; p = 0.002) compared with baseline. Comparing the three positive end-expiratory pressure levels, open lung approach resulted in the lowest: 1) Z0 (297 ± 83 vs 378 ± 79 dyn.s.cm, p = 0.033, and vs 450 ± 119 dyn.s.cm, p = 0.002); 2) effective arterial elastance (0.37 ± 0.08 vs 0.50 ± 0.15 mm Hg/mL, p = 0.04, and vs 0.61 ± 0.12 mm Hg/mL, p < 0.001), and 3) reflection coefficient (0.35 ± 0.17 vs 0.48 ± 0.10, p = 0.024, and vs 0.53 ± 0.19, p = 0.005), comparisons with hyperinflation and collapse, respectively. CONCLUSIONS:: In this experimental setting, positive end-expiratory pressure consistent with the open lung approach resulted in the best pulmonary vascular mechanics compared with higher or lower positive end-expiratory pressure settings.