Inert gas elimination characteristics of the normal and abnormal lung

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Abstract
An alternate approach to interpretation of inert gas elimination data is presented, using the direct analysis of raw retention and excretion ratios for each of several infused inert gases. These data allow the calculation of venous admixture, physiological dead space and arterial-alveolar difference for each inert gas. A computer model with pure shunt, anatomic dead space and variable degrees of ventilation-perfusion ratio (.ovrhdot.VA/.ovrhdot.Q) heterogeneity was used to demonstrate the influence of these factors on the arterial-alveolar difference over a wide range of gas solubilities. By plotting arterial-mixed expired difference vs. solubility and arterial-alveolar difference vs. solubility, an approximate distribution of units with low, intermediate or high .ovrhdot.VA/.ovrhdot.Q can be defined. The approach was tested comparing the models of oleic acid-induced edema to give low .ovrhdot.VA/.ovrhdot.Q units, infused venous inert gas bubble emboli to give high .ovrhdot.VA/.ovrhdot.Q units and lobar atelectasis to produce pure shunt. This approach allows precise quantitation of shunt and dead space and a semiquantitative estimate of .ovrhdot.VA/.ovrhdot.Q distribution. It contains the same amount of information as the numerical inversion technique without subjecting the analysis to potential mathematical limitations.