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

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 (N₂)-washout technique are higher than those obtained using an exogenous tracer gas such as sulfur hexafluoride. This study explored whether the pure oxygen (O₂) used for the N₂ washout could underlie these higher values.

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 O₂ 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 O₂ for the N₂ washout or with air for the tracer gas washout.

At normal ventilation/perfusion (V'/Q') ratios, the rate of washout of N₂ and exogenous tracer gas was almost indistinguishable. At low V'/Q', the N₂ washout lagged the tracer gas washout. At very low V'/Q', N₂ became trapped in the acinus. Under low V'/Q' conditions, breathing pure O₂ introduced a marked asymmetry between the inspiratory and expiratory gas flow rates that was not present when breathing air.

The use of pure O₂ to washout N₂ increases O₂ 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 N₂.