The accuracy of end-expiratory lung volume measurement by the modified nitrogen wash-out/wash-in method (EELV-N2) depends on the precise determination of carbon dioxide elimination (V̇CO2), which is affected by alveolar dead space (VD-alv). The purpose of this study was to investigate the influence of VD-alv on EELV-N2.
METHODS:
Six piglets with lavage-induced acute lung injury were mechanically ventilated in a decremental PEEP trial that was reduced from 20 to 4 cm H2O in steps of 4 cm H2O every 10 min. EELV was measured by the modified EELV-N2 method and computed tomography scan (EELV-CT), volumetric capnography, blood gas measurements, and hemodynamic data were recorded at each PEEP level. The data were divided into higher and lower PEEP groups.
RESULTS:
During the decremental PEEP trial, EELV-N2 exhibited a high correlation (r2 = 0.86, P < .001) with EELV-CT, with a bias of −48.6 ± 150.7 mL (1 ± 18%). In the higher PEEP group, EELV-N2 was not correlated with EELV-CT, with a bias of –168.1 ± 171.5 mL (−14 ± 14%). However, in the lower PEEP group, EELV-N2 exhibited a high correlation (r2 = 0.86, P < .001) with EELV-CT, with a bias of 11.2 ± 97.2 mL (6 ± 17%). The measurement bias was negatively correlated with VD-alv (r2 = 0.44, P = .04) and V̇CO2 (r2 = 0.47, P = .03) in the higher PEEP group.
CONCLUSIONS:
In this surfactant-depleted model, EELV measurement by the modified EELV-N2 method reveals a systematic underestimation at high PEEP levels that is partly due to an increase in VD-alv.
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