Information content of multiple inert gas elimination measurements

The multiple inert gas elimination technique provides a fundamental assessment of the distribution of ventilation-perfusion (VA/Q) ratios in the lung. The resolution of the finer structure of this distribution is limited however. This study examines the theoretical basis of this limitation and presents an objective method for evaluating the independence of inert gas measurements. It demonstrates the linear dependence of the inert gas kernels and their filtering characteristics to be the factors most limiting information content. The limited number of gases available for measurement and experimental error are lesser limitations. At usual levels of experimental error, no more than seven different inert gases having partition coefficients between those of SF6 and acetone will provide independent information, and information content will be maximized by choosing gases with partition coefficients spaced equally on a logarithmic scale. A fivefold reduction in experimental error will not significantly alter the information content of the measurements. The analysis applies equally to other methods of multiple inert gas elimination data interpretation.