Gas distribution and optimal PEEP with lowest transpulmonary driving pressure (without esophageal pressure) during a PEEP trial. A post hoc analysis in patients with acute respiratory failure.

Background: A protective ventilation strategy should be based on assessment of lung mechanics and transpulmonary pressure (PTP), as this is the pressure that directly “hits” the lung. Protective ventilation with tidal volumes of 6 ml/kg ideal body weight is generally accepted, but there is no consensus on selection of optimal PEEP. We have shown that the change in end-expiratory lung volume (ΔEELV) is determined by the size of the PEEP step and the elastic properties of the lung (only), ΔEELV/ΔPEEP. Consequently, lung elastance (EL) can be determined as ΔPEEP/ΔEELV.The aim was to determine the PEEP level with lowest (least injurious) transpulmonary driving pressure and to identify what signifies high or low PEEP response. Methods: In ten patients with acute respiratory failure, global and ventral-dorsal tidal volume and ΔEELV were determined by inspiratory tidal volume calibrated EIT during a PEEP step procedure from 0 to 16 cmH 2 O. As the lung pressure/volume (P/V) curve coincides with the line connecting the end-expiratory airway P/V points, a complete lung P/V curve can be obtained for the whole lung or by combination with EIT, for the ventral and dorsal lung separately. Results: Global lung P/V curves showed marked individual variation with overall lung elastance of 7.0 – 23.2 cmH 2 O/L. Optimal PEEP level was 12.9 cmH 2 O (10-16) with ΔPTP of 4.1 cmH2O (2.8-7.6). The ratio of ΔPTP to airway driving pressure (ΔPAW), ΔPTP/ΔPAW, varied between patients and changed with changes of PEEP, making ΔPAW a less suitable surrogate for ΔPTP in individual patients. The two patients with highest EL were non-responders to PEEP with increasing global, ventral and dorsal lung elastance at high PEEP levels, indicating non-dependent over-distension. In patients with lower lung elastance, there was positive response to PEEP with successively lower lung elastance when increasing PEEP, especially in dorsal (dependent) lung. Conclusion: Only a two PEEP step up-down procedure is required for obtaining a full best-fit lung P/V curve, from which the PEEP level with the lowest transpulmonary driving pressure could be calculated. High PEEP response is signified by low lung elastance, which increases at high PEEP levels both globally and in dorsal lung.