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

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Abstract:

BACKGROUND: 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 (V. CO2 ), 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 V. CO2 (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.

Keywords: PEEP; alveolar dead space; end-expiratory lung volume; mechanical ventilation; nitrogen wash-out

Document Type: Research Article

DOI: http://dx.doi.org/10.4187/respcare.01800

Affiliations: Department of Critical Care Medicine, Zhong-Da Hospital, Southeast University School of Medicine, Nanjing, China

Publication date: December 1, 2012

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