Physiological measurement of the small airways

Noninvasive physiological measurements are reviewed that have been reported in the literature with the specific aim being to study the small airways in lung disease. This has mostly involved at-the-mouth noninvasive measurement of flow, pressure or inert gas concentration, with the intent of deriving one or more indices that are representative of small airway structure and function. While these measurements have remained relatively low-tech, the effort and sophistication increasingly reside with the interpretation of such indices. When aspiring to derive information at the mouth about structural and mechanical processes occurring several airway generations away in a complex cyclically changing cul-de-sac structure, conceptual or semi-quantitative lung models can be valuable. Two assumptions that are central to small airway structure-function measurement are that of an average airway change at a given peripheral lung generation and of a parallel heterogeneity in airway changes. While these are complementary pieces of information, they can affect certain small airways tests in confounding ways. We critically analyzed the various small airway tests under review, while contending that negative outcomes of these tests are probably a true reflection of the fact that no change occurred in the small airways. Utmost care has been taken to not favor one technique over another, given that most current small airways tests still have room for improvement in terms of rendering their content more specific to the small airways. One way to achieve this could consist of the coupling of signals collected at the mouth to spatial information gathered from imaging in the same patient.