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Background  Multiple-breath washout techniques are increasingly used to assess lung function. The principal statistic obtained is the lung clearance index (LCI), but values obtained for LCI using the nitrogen (N 2 )-washout technique are higher than those obtained using an exogenous tracer gas such as sulfur hexafluoride. This study explored whether the pure oxygen (O 2 ) used for the N 2  washout could underlie these higher values.    Methods  A model of a homogenous, reciprocally ventilated acinus was constructed. Perfusion was kept constant, and ventilation adjusted by varying the swept volume during the breathing cycle. The blood supplying the acinus had a standard mixed-venous composition. Carbon dioxide and O 2  exchange between the blood and acinar gas proceeded to equilibrium. The model was initialised with either air or air plus tracer gas as the inspirate. Washouts were conducted with pure O 2  for the N 2  washout or with air for the tracer gas washout.    Results  At normal ventilation/perfusion ( V ′/ Q ′) ratios, the rate of washout of N 2  and exogenous tracer gas was almost indistinguishable. At low  V ′/ Q ′, the N 2  washout lagged the tracer gas washout. At very low  V ′/ Q ′, N 2  became trapped in the acinus. Under low  V ′/ Q ′ conditions, breathing pure O 2  introduced a marked asymmetry between the inspiratory and expiratory gas flow rates that was not present when breathing air.    Discussion  The use of pure O 2  to washout N 2  increases O 2  uptake in low  V ′/ Q ′ units. This generates a background gas flow into the acinus that opposes flow out of the acinus during expiration, and so delays the washout of N 2 .

The differing physiology of nitrogen and tracer gas multiple-breath washout techniques