KEY ROLE OF THE POTENTIALLY RECRUITABLE LUNG ON THE MECHANICAL EFFECTS OF PEEP DURING ARDS

INTRODUCTION. In acute respiratory distress syndrome (ARDS), the effect of high level of positive end-expiratory pressure (PEEP) on outcome likely depends on the balance between its beneficial and detrimental effects (reduction of intra-tidal lung opening-closing [O/C] vs. increase of alveolar strain). We hypothesized that these effects may extensively vary according to the amount of the potentially recruitable lung (PRL) of each patient. METHODS. Sixty-eight patients underwent a whole lung CT-scan at 5 and 15 cmH2O PEEP, and at airway plateau pressure of 45 cmH2O. End-inspiratory non-aerated lung tissue was estimated from the CT pressure-volume curve. Intra-tidal lung O/C was defined as the difference between the endexpiratory and end-inspiratory non-aerated lung tissue at each PEEP step, while keeping tidal-volume (VT) constant (8-10ml/kg). Alveolar strain was calculated as the ratio between the end-inspiratory lung inflation and end-expiratory lung volume. The amount of PRL was defined as the proportion of lung tissue regaining aeration between 5 and 45 cmH2O airway pressures. RESULTS. Patients were divided into those with a lower- and those with a higher-PRL, according to its median value (9 percent). At 5 cmH2O PEEP, lung O/C in patients with a higher-PRL was greater than that of patients with a lower-PRL (141±85 [SD] vs. 20±22 gr, P<0.01), which was almost negligible (2±2% of the lung weight). In contrast, alveolar strain was identical in the two groups (0.38±0.17 vs. 0.41±0.15). Increasing PEEP to 15 cmH2O reduced the lung O/C only in patients with a higher-PRL (down to 63±87 gr, P<0.01), and increased the alveolar strain at the same extent in both groups (up to 0.68±0.33 and 0.84±0.26, respectively, P<0.01 for both). Regardless the same VT (9±2 vs. 9±2 ml/kg) and PEEP (12±3 vs. 11±3 cmH2O) clinically employed, patients with a higher-PRL showed a greater clinical lung O/C (200±106 vs. 36±26 gr, P<0.01), and a higher mortality rate at ICU discharge (41 vs. 15%, P=0.02), than patients with a lower-PRL. CONCLUSION. The use of high level of PEEP appears to be physiologically advantageous only in patients with a higher-PRL, where the beneficial effects may exceed the harmful ones.