Pulmonary diffusing capacity: a comparison of breath-holding and steady state methods using carbon monoxide.

Three methods are in current use for measuring the diffusing capacity of the lungs for carbon monoxide (DL). Two of these are steady state methods, and they differ in the methods used to calculate the mean alveolar carbon monoxide tension. Filley, MacIntosh and Wright (1) calculate the alveolar Pco indirectly, assuming the physiological dead space for carbon dioxide to be the same as that for carbon monoxide, while Bates, Boucot and Dormer (2) use the end-tidal Pco as the mean alveolar Pco. The third method is the single breath method of Krogh and Krogh (3) as modified by Ogilvie, Forster, Blakemore and Morton (4). Each method in theory measures the total resistance to gas diffusion offered by the structures that lie between the gas in the alveoli and the hemoglobin within the red cells of the pulmonary capillaries. The results obtained by each method differ in normal subjects; and in disease states, such as emphysema, the differences may be great. While the different results obtained in similar subjects by the two steady state methods can largely be explained by the different methods of estimating the mean alveolar Pco (5), the breath-holding method consistently gives higher results than either of the steady state methods (6). Marshall (7) has recently compared the breath-holding method of measuring DL with a steady state method, using end-tidal samples in a small group of normal subjects and patients with emphysema. He found that, while in normal subjects the end-tidal sample satisfactorily reflected the mean alveolar Pco, in emphysema with impaired intrapulmonary gas mixing, the end-tidal sample overestimates the mean alveolar Pco and therefore gives falsely low results for the diffusing capacity. These results