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Estimation of ventilation-perfusion (VA/Q) inequality by the multiple inert gas elimination technique requires knowledge of arterial, mixed venous, and mixed expired concentrations of six gases. Until now, arterial concentrations have been directly measured and mixed venous levels either measured or calculated by mass balance if cardiac output was known. Because potential applications of the method involve measurements over several days, we wished to determine whether inert gas levels in peripheral venous blood ever reached those in arterial blood, thus providing an essentially noninvasive approach to measuring VA/Q mismatch that could be frequently repeated. In 10 outpatients with chronic obstructive pulmonary disease, we compared radial artery (Pa) and peripheral vein (Pven) levels of the six gases over a 90-min period of infusion of the gases into a contralateral forearm vein. We found Pven reached 90% of Pa by approximately 50 min and 95% of Pa by 90 min. More importantly, the coefficient of variation at 50 min was approximately 10% and at 90 min 5%, demonstrating acceptable intersubject agreement by 90 min. Since cardiac output is not available without arterial access, we also examined the consequences of assuming values for this variable in calculating mixed venous levels. We conclude that VA/Q features of considerable clinical interest can be reliably identified by this essentially noninvasive approach under resting conditions stable over a period of 1.5 h.

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Gas exchange was investigated in normal anesthetized dogs during high-frequency, low-tidal volume ventilation (HFV) using the multiple inert gas elimination method. The pattern of inert gas elimination was initially normal during conventional mechanical ventilation. During HFV there was an increase in the difference between the excretion values of acetone and its less soluble neighboring gases, enflurane and ether, but elimination was independent of molecular weight. This pattern was consistent with a major degree of parallel ventilation-perfusion inequality with 49.4 +/- 1.7% of alveolar ventilation being distributed to lung units with VA/Q ratios greater than 20. Additional experiments, however, showed insufficient change in pulmonary blood flow distribution during HFV to account for these apparently poorly perfused lung units. Instead, it was found that the flux from the lung of the most soluble gas, acetone, per unit concentration difference along the airways was approximately twice that for other gases. Experiments using a simple airway model suggested that this enhanced transport of high-solubility gases during HFV is dependent on the wet luminal surface of conducting airways. A reciprocating exchange of gas between the lumen and airway lining layer is proposed as the most likely explanation for these results.