THE ARlTERIAL-ALVEtOLAR INERTI GAS ("N2") D)IFFERENCE IN NORMAL AND EMPIHYSEMATOUS SUBJE(TIS, AS INI)ICATED) BY THEI ANALYSIS OF URINE *

Canfield and Rahn (2) have theorized that an arterial-alveolar difference in inert gas pressure is established whenever various units of the lung have unequal ventilation-perfusion ratios, even though the net inert gas exchange is zero. ' The magnitude of this difference is expected to serve as an index of the variance of ventilation-perfu-sion ratios among the alveoli. The existence of such a difference has been shown in anesthetized dogs (2, 3). Similar measurements have not yet been made on man. The major difficulty in these determinations is the lack of a sufficiently, accurate and reproducible method for measuring the partial pressure of inert gas in the arterial blood. It can be argued (see below) that urine inert gas tension reflects arterial inert gas tension. With this in mind, a method for determining the inert gas content of urine (4) has been applied to human urine in a manner believed to enable the calculation of the arterial inert gas tension within ±42 mm Hg. Since the inert gas tension of mean alveolar air can be calculated with simi-lar accuracy, a study of the arterial-alveolar inert gas pressure difference has been possible. This difference is +3 mnm Hg in normal labora-tory personnel and can be over +20 mm in patients with diffuse, obstructive emphysema. METHODS Seven male laboratory personnel and seven male sub-jects with severe, diffuse, obstructive emphysema were * A preliminary report of this work was presented at th