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(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).