A Clinical Review of the Single Breath Method of Measuring the Diffusing Capacity of the Lungs

Abstract

A critical review of the single breath method of measuring diffusing capacity has been made on 81 patients studied routinely over a two year period. Failure of the patient to inspire and expire fully while performing the test was not a serious source of error. It is suggested that the residual volume calculated from the helium dilution obtained while performing the single breath diffusing capacity (effective residual volume), is the correct volume to use, rather than the residual volume calculated by the closed circuit helium dilution method (true residual volume), since this is the volume to which CO is exposed during the test. Differences between effective and true residual volume correlate roughly with the evenness of alveolar gas distribution. Breath holding times on the average were measured with an acceptable error, through occasional unavoidable inaccuracies suggest that time corrections should be made. Inadequate dead space washout was shown to be the most serious source of error in this series. This was especially so in those with reduced lung function since they had a smaller volume to expire after breath-holding, the alveolar sample was smaller, and any contaminating dead space gas in the sample was relatively undiluted, resulting in an erroneously low diffusing capacity. There are several potential areas in which technical error may occur in the single breath diffusing capacity performed in a routine service laboratory. These can be overcome by proper patient instruction, care in timing, and correction in timing, with special attention being paid to the adequacy of dead space washout.