Resolution of the multiple inert gas method for estimating V̇a/Q̇ maldistribution

During steady-state infusion of a mixture of dissolved inert gases, their elimination by the lung depends on the distribution of ventilation/perfusion (V̇a/Q̇) ratios. Thus, certain features of the V̇a/Q̇ distribution can be inferred from inert gas measurement. Because of the: (1) complexity of the lung, and (2) experimental errors, the ability of such a technique to describe the shape and position of the V̇a/Q̇ distribution accurately is limited. In this report we present an analysis of the resolution of the method for 9 representative sets of inert gas data, taking account of both of the above factors. These 9 sets span the range of commonly observed data, both in health and in diseases such as asthma, interstitial fibrosis, chronic obstructive lung disease and respiratory distress syndromes. Both error-free and error-containing data are studied and by linear programming methods, bounds are placed on maximum and minimum possible perfusion in several regions of the V̇a/Q̇ spectrum. Modality is also studied by linear programming. The results show that the resolving power of the method depends greatly on the specific case under study. When groups of units are separated in V̇a/Q̇ by a decade, this can be determined with considerable confidence. Shunt and low V̇a/Q̇ areas can generally be well resolved, but when distributions are very broad, resolution is limited.