Background: A protective ventilation strategy should be based on lung mechanics and transpulmonary pressure, as this is the pressure that directly “hits” the lung. Esophageal pressure has been used for this purpose but has not gained widespread clinical acceptance. Instead, respiratory system mechanics and airway driving pressure have been used as surrogate measures. We have shown that the lung pressure/volume (P/V) curve coincides with the line connecting the end-expiratory airway P/V points of a PEEP trial. Consequently, transpulmonary pressure increases as much as PEEP and lung compliance (CL) can be determined as ΔEELV/ΔPEEP and transpulmonary driving pressure (ΔPTP) as tidal volume divided by ΔEELV/ΔPEEP.
Methods: In ten patients with acute respiratory failure, ΔEELV was measured during each 4 cmH2O PEEP-step from 0 to 16 cmH2O and CL for each PEEP interval calculated as ΔEELV/ΔPEEP giving a lung P/V curve for the whole PEEP trial. Similarily, a lung P/V curve was obtained also for the PEEP levels 8, 12, and 16 cmH2O only.
Results: A two-step PEEP procedure starting from a clinical PEEP level of 8 cmH2O gave almost identical lung P/V curves as the four PEEP-step procedure. The lung P/V curves showed a marked individual variation with an over-all CL (CLoa ) 50-137 ml/cmH2O. ΔPTP of a tidal volume of 6-7 ml/kg ideal body weight divided by CLoa ranged from 8.6-2.8 cmH2O, while ΔPTP of tidal volume adapted to CLoa ranged from 3.3 in the patient with lowest to 4.3 cmH 2 O in the patient with highest CLoa . The ratio of airway driving pressure to transpulmonary driving pressure (ΔPTP/ΔPAW) varied between patients and changed with PEEP, reducing the value of ΔPAW as surrogate for ΔPTP in individual patients.
Conclusion: Only a two PEEP-step procedure is required for obtaining a lung P/V curve from baseline clinical PEEP to end-inspiration at the highest PEEP level, i.e. without esophageal pressure measurements. The best-fit equation for the curve can be used to determine a tidal volume related to lung compliance instead of ideal body weight and the PEEP level where transpulmonary driving pressure is lowest and possibly least injurious for any given tidal volume.