Volumetric capnography in patients with acute lung injury: effects of positive end-expiratory pressure

The aim of the study was to analyse the effects of positive end-expiratory pressure (PEEP) on volumetric capnography and respiratory system mechanics in mechanically ventilated patients. Eight normal subjects (control group), nine patients with moderate acute lung injury (AT,I group) and eight patients with acute respiratory distress syndrome (ARDS group) were studied. Respiratory system mechanics, alveolar ejection volume as a fraction of tidal volume (VAE/VT), phase IU slopes of expired CO2 beyond VAE and Bohr's dead space (VD/VT(Bohr)) at different levels of PEEP were measured. No differences in respiratory system resistances were found between the ALI and ARDS groups. VD/VT(Bohr) and expired CO2 slope beyond VAE were higher in ALI patients (0.52+/-0.01 and 13.9+/-0.7 mmHg.L-1, respectively) compared with control patients (0.46+/-0.01 and 7.7+/-0.4 mmHg.L-1, p<0.01, respectively) and in ARDS patients (0.61+/-0.02 and 24.9+/-1.6 mmHg.L-1, p<0.01, respectively) compared with ALI patients. VAE/VT differed similarly (0.6+/-0.01 in control group, 0.43+/-0.01 in ALI group and 0.31+/-0.01 in ARDS group, p<0.01). PEEP had no effect on VAE/VT, expired CO2 slope beyond VAE and VD/VT(Bohr) in any group. A significant correlation (p<0.01) was found between VAE/VT and expired CO2 slope beyond VAE and lung injury score at zero PEEP. Indices of volumetric capnography are affected by the severity of the lung injury, but are unmodified by the application of positive end-expiratory pressure.