[Significance of static pressure-volume loop for differential diagnostics and optimization of respiratory support in parenchimal respiratory failure].

PURPOSE OF THE STUDY: To determine significance of static pressure-volume loop (PV loop) for differential diagnostics of parenchymal respiratory failure, setting of positive end-expiratory pressure (PEEP) and recruit ability of the lung.

MATERIALS AND

METHODS

76 patients (52 males) with parenchymal respiratory failure were included in the study (oxygenation index (PaO2/ FiO2) < 250 torr infiltrates on chest X-ray or CT-scan of the lungs, no data on left ventricular failure). We plot static PV loop by low flow technique in range of 0 to 40 mbar, fixing lower inflection point (LIP), linear compliance (Clin), upper inflection point (UIP), expiratory inflection point (EIP), compliance of linear deflation limb (C defl), hysteresis (Hyst) and volume of PEEP-induced recruitment of the lung (V(peep)). Then we plot another static PV loop with sustained inflation of 40 mbar for 30 seconds, fixing changes in lung volume at 40 mbar. After 10 minutes of sustained inflation we measured changes of oxygenation index. For 69 patient we performed lung CT-scan and defined diffuse (acute respiratory distress syndrome) or local lung injury (pneumonia, atelectasis).

RESULTS

LIP value can differentiate diffuse and local lung injury. LIP more than 10 mbar corresponds to diffuse lung injury on CT scan (sensitivity 76%, specificity 85%, AUROC 0.81). LIP cannot predict PEEP-induced alveolar recruitment and changes of PaO2/FiO2 after sustained inflation maneuver (p > 0.05). Empirically set PEEP (by maximum PaO2/FiO2) was much higher than LIP (p < 0.0001), but LIP correlates with empirically set PEEP in diffuse lung injury (rho = 0.642, p = 0.003). Clin cannot differentiate diffuse from local lung injury (p > 0.05), but predicts PEEP-induced alveolar recruitment during static PV loop plotting (rho = 0.493, p < 0.0001). We did not find any statistically significant values of UIP and EIP for differential diagnosis, setting of PEEP or recruit ability of the lung. Hysteresis value (defined as volume difference at 20 mbar between deflation and inflation limbs) cannot predict influence of PEEP setting and sustained inflation maneuver on PaO2/FiO2 changes and recruit ability of the lung (p > 0.05). After static PV loop plotting combined with sustained inflation maneuver recruited volume of the lungs was 350 (250-450) ml. We didn't find significant differences between recruit ability of the diffuse and locally injured lungs (p > 0.05). Recruitment volume has no correlations with all points and segments of static PV loop.

CONCLUSIONS

Static PV loop has limited prognostic value for differential diagnostics of diffuse or local lung injury and brings potential harm for setting PEEP according to LIP. LIP more than 10 mbar can predict diffuse lung injury. Clin can predict volume of PEEP-induced recruitment. In diffuse lung injury LIP correlates with empirically set PEEP.