Role of PEEP and Regional Transpulmonary Pressure in Asymmetrical Lung Injury.

RATIONALE Asymmetrical lung injury is a frequent clinical presentation. Regional distribution of tidal volume (VT) and PEEP could result in hyperinflation of the less-injured lung. The validity of esophageal pressure (Pes) is unknown. OBJECTIVES To compare, in asymmetrical lung injury, Pes with directly measured pleural pressures (Ppl) of both sides and investigate how PEEP impacts ventilation distribution and the regional driving transpulmonary pressure (Inspiratory - Expiratory). METHODS 14 mechanically ventilated pigs with lung injury were studied. One lung was blocked while the contralateral one underwent surfactant lavage and injurious ventilation. Airway pressure and Pes were measured, as well as Ppl in the dorsal and ventral pleural space adjacent to each lung. Distribution of ventilation was assessed by Electrical Impedance Tomography. PEEP was studied through decremental steps. RESULTS Ventral and dorsal Ppl were similar between the injured and the non-injured lung across all PEEP levels. Dorsal Ppl and Pes were similar. The driving transpulmonary pressure was similar in the two lungs. VT distribution between lungs was different at ZEEP (≈70% of VT going in non-injured lung) due to different Crs (8.3 ml/cmH2O non-injured lung vs 3.7 ml/cmH2O injured lung). PEEP values up to 10 cmH2O homogenized VT distribution, maintaining both lungs open. High PEEP (≥ 16 cmH2O) resulted in an even distribution of Vt but with overdistension for both lungs. CONCLUSIONS Despite asymmetrical lung injury, Ppl between injured and non-injured lungs is equalized and esophageal pressure is a reliable estimate of dorsal Ppl. Driving transpulmonary pressure is similar for both lungs. VT distribution results from regional Crs. Moderate PEEP homogenizes VT distribution between lungs without generating hyperinflation.