A Theoretical and Experimental Analysis of Anomalies in the Estimation of Pulmonary Diffusing Capacity by the Single Breath Method

The theoretical basis of the single breath method of estimating pulmonary diffusing capacity is re‐examined and the mathematical analysis is extended to include the behaviour of an ideal model during inspiration and expiration. Equations derived, describing the changes of carbon‐monoxide concentration in the alveolar gas during inspiration, breath holding and expiration, are consistent with data from experiments on a normal subject. The analysis is applied to the problem presented by the observation of Forster and co‐workers using their modification of Krogh's single breath technique that the estimate of pulmonary diffusing capacity was dependent on experimental procedure, varying with length of breath holding, the time taken to inspire the test mixture and the manner in which the sample of alveolar gas was taken. The theoretical and experimental evidence presented shows that these anomalies can arise from aneglect of the effects of alveolar volume changes inescapably associated with the test and may be eliminated by a simple allowance for the times of inspiration of test gas and expiration of sample. Practical conclusions include the desirability of using small alveolar gas samples, collected as early as possible during expiration. A method is described in which 85‐ml. samples are collected in evacuated tonometers. The relevance of the findings to the estimation of pulmonary capillary blood volume and the diffusing capacity of the pulmonary membrane is also discussed.