Background and Goal of Study: Spontaneous breathing (SB) activity in acute respiratory distress syndrome can further aggravate the lung injury. Positive end-expiratory pressure (PEEP) has the potential to stabilize the lungs and prevent damage. We hypothesized that in experimental moderate ARDS SB activity combined with inadequate low PEEP increases inflammation in dependent lung areas.
Materials and Methods: In total, twenty-four pigs were anesthetized, and lung injury was induced with multiple saline lung lavage followed by injurious ventilation (double-hit). For assessment of transpulmonary pressure intrapleural pressure sensors were placed in ventral, dorsal and caudal regions of the left thorax. Animals were ventilated with low tidal volumes (Vt=6ml/kg) in airway pressure release ventilation (APRV) mode. Best PEEP (PEEPb) was defined as the level needed to reach a positive end-expiratory transpulmonary pressure in all lung regions. Animals were randomized to one of four groups (n=6/group): 1) no SB activity and PEEP=PEEPb-4 cmH2O(APRV/Clow); 2) no SB activity and PEEP= PEEPb+4 cmH2O (APRV/Chigh); 3) SB activity and PEEP= PEEPb-4 cmH2O (APRV/SBlow); 4) SB activity and PEEP=PEEPb+4 cmH2O (APRV/SBhigh). Regional lung [18F]fluorodeoxyglucose ([18F]FDG) uptake as well as perfusion using 68Ga-labeled microspheres were assessed by positron emission tomography after 12 hours. The specific [18F]FDG uptake rate (Kis) was calculated and lung aeration determined by static computed tomography.
Results and Discussion: At low PEEP, global Kis [median (25% percentile – 75% percentile)], increased both with [0.032(0.028-0.043) vs. 0.057(0.042-0.075) min-1, P=0.016] and without SB activity [0.029(0.027-0.030) vs. 0.044(0.041-0.065) min-1, P=0.004] compared with high PEEP. SB activity did not increase global lung inflammation, regardless of PEEP level. Nevertheless, transpulmonary driving pressure resulting from SB activity correlated with regional Kis at low (r=0.850, P=0.032) but not high PEEP (r=0.018, P=0.972). In addition, the distribution of regional transpulmonary pressures at end-expiration across the gravity gradient was more homogeneous with high PEEP, and the centers of aeration and perfusion were shifted towards dependent lung regions in APRV/SBhigh compared with APRV/SBlow.
Conclusion: SB activity at inadequate low levels of PEEP correlates with dorsal lung inflammation in moderate ARDS.

03 / Effects of spontaneous breathing activity on regional and global lung inflammation at different PEEP levels in experimental moderate ARDS