The Effect of Alveolar Dead Space on the Measurement of End-Expiratory Lung Volume by Modified Nitrogen Wash-Out/Wash-In in Lavage-Induced Lung Injury

BACKGROUND: The accuracy of end-expiratory lung volume measurement by the modified nitrogen wash-out/wash-in method (EELV-N 2 ) depends on the precise determination of carbon dioxide elimination (V CO 2 ), which is affected by alveolar dead space (V D-alv ). The purpose of this study was to investigate the influence of V D-alv on EELV-N 2 . 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 H 2 O in steps of 4 cm H 2 O every 10 min. EELV was measured by the modified EELV-N 2 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-N 2 exhibited a high correlation (r 2 = 0.86, P 2 was not correlated with EELV-CT, with a bias of –168.1 ± 171.5 mL (−14 ± 14%). However, in the lower PEEP group, EELV-N 2 exhibited a high correlation (r 2 = 0.86, P D-alv (r 2 = 0.44, P = .04) and V CO 2 (r 2 = 0.47, P = .03) in the higher PEEP group. CONCLUSIONS: In this surfactant-depleted model, EELV measurement by the modified EELV-N 2 method reveals a systematic underestimation at high PEEP levels that is partly due to an increase in V D-alv .