ABSTRACT: BACKGROUND: The accuracy of end-expiratory lung volume measurement by modified nitrogen washout/washin (EELV-nitrogen washout/washin) depends on the precise determination of carbon dioxide elimination (VCO₂), which is affected by alveolar dead space (VDalv). The purpose of this study was to investigate the influence of VDalv on EELV-nitrogen washout/washin. METHODS: Six piglets with lavage-induced acute lung injury were mechanically ventilated in a decremental positive end-expiratory pressure (PEEP) trial that was reduced from 20 to 4 cmH₂O in steps of 4 cmH₂O every 10 minutes. EELV was measured by modified nitrogen washout/washin and computed tomography scan (EELV-CT), and 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-nitrogen washout/washin exhibited a high correlation (r2 = 0.86, p < .001) with the EELV-CT, with a bias of -48.6 ± 150.7 ml (1% ± 18%). In the higher PEEP group, EELV-nitrogen washout/washin was not correlated with the EELVCT, with a bias of -168.1 ± 171.5 mL (-14% ± 14%). However, in the lower PEEP group, EELVnitrogen washout/washin exhibited a high correlation (r2 = 0.86, p< .001) with the EELV-CT, with a bias of 11.2 ± 97.2mL (6% ± 17%). The measurement bias was negatively correlated with VDalv (r2 = 0.44, p = .04) and VCO₂ (r2 = 0.47, p = .03) in the higher PEEP group. CONCLUSIONS: In this surfactant-depleted model, EELV measurement by modified nitrogen washout/washin reveals a systematic underestimation at high PEEP levels that is partly due to an increase in VDalv.
Respiratory care 05/2012; · 2.01 Impact Factor