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The diffusing capacity of the lungs for carbon monoxide is measured as the ml. of carbon monoxide (CO) that can be transferred from the alveoli to the blood per mm. Hg pressure difference of CO between the alveoli and the blood. This measurement can be made by steady state techniques or by the single breath method. Two steady state methods have been described recently, which differ in the methods by which the mean alveolar CO concentration is measured. In one method (1) the mean alveolar CO concentration is calculated by assuming that COand CO2 have the same dead space and that the partial pressure of CO2 in the arterial blood is equal to the mean alveolar partial pressure of CO2. This method requires an arterial puncture and accurate estimation of the partial pressure of CO2 in the arterial blood so that its routine and repeated use in patients is limited. The other steady state method described by Bates, Boucot, and Dormer (2) uses the end tidal CO as a measure of the mean alveolar CO and, as it is a bloodless method, can more easily be used for repeated measurements on patients. These two methods give results which lie in a similar range although some of the values for normal subjects reported by Bates and Pearce (3) are low in relation to the usually accepted range. The single breath method of Krogh and Krogh (4), recently modified by Ogilvie, Forster, Blakemore, and Morton (5), is simple to carry out but comparison with the steady state method of Filley has shown (6) that the single breath method gives higher values in normal subjects; in disease the values obtained by the single breath method may be very much higher than those obtained by the steady state meth6d. Similar re-

A COMPARISON OF METHODS OF MEASURING THE DIFFUSING CAPACITY OF THE LUNGS FOR CARBON MONOXIDE. INVESTIGATION BY FRACTIONAL ANALYSIS OF THE ALVEOLAR AIR