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

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)-washout technique are higher than those obtained using an exogenous tracer gas such as sulfur hexafluoride. This study explored whether the pure oxygen (O) used for the N 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 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 for the N washout or with air for the tracer gas washout.

Results: At normal ventilation/perfusion ('/') ratios, the rate of washout of N and exogenous tracer gas was almost indistinguishable. At low '/', the N washout lagged the tracer gas washout. At very low '/', N became trapped in the acinus. Under low '/' conditions, breathing pure O 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 to washout N increases O uptake in low '/' 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.