Gas washout techniques

Abstract

The peripheral, or small, airways occupy >95% of the lung volume and are thought to be a key site of pathophysiology in various obstructive respiratory diseases. A lung function test such as spirometry cannot properly monitor these peripheral lung regions, and overlooks a crucial aspect, namely lung structural heterogeneity that may be modified by disease. Inert gas washout techniques, first described >75 years ago, were introduced to quantify ventilated lung volume and ventilation heterogeneity by monitoring a “tracer” gas of interest. In this chapter we outline the basic principles of inert gas washout using single-and multiple-breath tests, starting with the most basic question: why perform a washout test at all? We review the various analysis steps, the resulting indices and their interpretation. We then discuss the evidence of clinical utility in CF, asthma, COPD and bronchiolitis obliterans. Finally, we outline some important limitations of washout techniques.