Introduction: Simple methods to predict the effect of lung recruitment maneuvers (LRMs) in acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are lacking. It has previously been found that a static pressure-volume (PV) loop could indicate the increase in lung volume induced by positive end-expiratory pressure (PEEP) in ARDS. The purpose of this study was to test the hypothesis that in ALI (1) the difference in lung volume (DeltaV) at a specific airway pressure (10 cmH2O was chosen in this test) obtained from the limbs of a PV loop agree with the increase in end-expiratory lung volume (DeltaEELV) by an LRM at a specific PEEP (10 cmH2O), and (2) the maximal relative vertical (volume) difference between the limbs (maximal hysteresis/total lung capacity (MH/TLC)) could predict the changes in respiratory compliance (Crs), EELV and partial pressures of arterial O2 and CO2 (PaO2 and PaCO2, respectively) by an LRM.
Methods: In eight ventilated pigs PV loops were obtained (1) before lung injury, (2) after lung injury induced by lung lavage, and (3) after additional injurious ventilation. DeltaV and MH/TLC were determined from the PV loops. At all stages Crs, EELV, PaCO2 and PaO2 were registered at 0 cmH2O and at 10 cmH2O before and after LRM, and DeltaEELV was calculated.
Statistics: Wilcoxon's signed rank, Pearson's product moment correlation, Bland-Altman plot, and receiver operating characteristics curve. Medians and 25th and 75th centiles are reported.
Results: DeltaV was 270 (220, 320) ml and DeltaEELV was 227 (177, 306) ml (P < 0.047). The bias was 39 ml and the limits of agreement were - 49 ml to +127 ml. The R2 for relative changes in EELV, Crs, PaCO2 and PaO2 against MH/TLC were 0.55, 0.57, 0.36 and 0.05, respectively. The sensitivity and specificity for MH/TLC of 0.3 to predict improvement (>75th centile of what was found in uninjured lungs) were for EELV 1.0 and 0.85, Crs 0.88 and 1.0, PaCO2 0.78 and 0.60, and PaO2 1.0 and 0.69.
Conclusion: A PV-loop-derived parameter, MH/TLC of 0.3, predicted changes in lung mechanics better than changes in gas exchange in this lung injury model.