Role of absolute lung volume to assess alveolar recruitment in acute respiratory distress syndrome patients

Objective: It is commonly assumed that lung volume at zero end-expiratory pressure (functional residual capacity) is not affected by positive end-expiratory pressure (PEEP) even in presence of alveolar recruitment, and it is often ignored when measuring lung recruitment by pressure-volume curves. Aim of the study was to investigate the effect of PEEP on functional residual capacity, and quantify the error of considering equal functional residual capacity in measuring alveolar recruitment. Design: Interventional human study. Setting: A 10-bed general intensive care unit in a university hospital. PATIENTS: Ten sedated, curarized, mechanically ventilated acute respiratory distress syndrome patients. Interventions: Three levels of PEEP (5, 10, and 15 cm H2O), were randomly applied, for 1 hr each. Measurements and Main results: At each PEEP we obtained a pressure-volume curve, the volume expired from PEEP to zero pressure (PEEP related lung volume) and functional residual capacity by helium dilution method. Functional residual capacity increased at increasing PEEP levels. Functional residual capacity was 507 ± 292, 607 ± 311, and 681 ± 312 ml (p <.05), respectively, at PEEP 5, 10, and 15 cm H2O. Pressure-volume curves were aligned starting from PEEP related lung volume (relative volume method) or from end-expiratory lung volume at PEEP (absolute volume method). Recruitment was measured as vertical distance of pressure-volume curves at 20 cm H2O airway pressure. The relative volume method led to underestimation of recruitment (43 ± 28% and 35 ± 18 %, respectively, at PEEP 10 and 15). Conclusions: Functional residual capacity is affected by PEEP. Ignoring this effect leads to relevant underestimation of alveolar recruitment as measure by pressure-volume curve displacement.