Respiratory mechanics in patients ventilated for critical lung disease

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Abstract
Respiratory mechanics, using flow interruption, was previously studied during the complete breath in healthy ventilated man, numerical techniques relieving constraints regarding flow pattern. The classical linear model of non-Newtonian behaviour was found to be valid. The present study was extended to subjects with critical lung disease. Subjects with acute lung injury (ALI; n = 2), acute respiratory distress syndrome (ARDS; n = 4), and chronic obstructive pulmonary disease (COPD; n = 3) were studied with and without positive end-expiratory pressure (PEEP). Functional residual capacity (FRC) was measured with sulphur hexafluoride (SF6) wash-out. The static pressure-volume (P-V) curve was linear at zero end-expiratory pressure (ZEEP), but nonlinear at PEEP. Its hysteresis was nonsignificant. In ALI/ARDS, PEEP increased lung volume by distension and recruitment, but only by distension in COPD. In ALI/ARDS, resistance was increased, at ZEEP. In COPD, resistance became extremely high during expiration at ZEEP. In ALI/ARDS at ZEEP, non-Newtonian behaviour, representing tissue stress relaxation and pendel-luft, complied with the classical linear model. At PEEP, the non-Newtonian compliance became volume-dependent to an extent correlated to the nonlinearity of the static P-V curve. In COPD, non-Newtonian behaviour was adequately explained only with a model with different inspiratory and expiratory behaviour. The classical model of the respiratory system is valid in ALI/ARDS at ZEEP. More advanced models are needed at PEEP and in COPD.

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