Single-breath pulmonary diffusing capacity. Reference values and application in connective tissue diseases and in various lung diseases.

1.1. Diffusing capacity of the lungs was measured by using a Mark 4 Resparameter. The single-breath diffusing capacity (D) for carbon monoxide was found to be dependent on the lung volume (VA) during the breath-hold. The same applied to D/VA and to the time constant (tau) for carbon monoxide uptake. This confirms the theoretical considerations and the results of other investigators. This effect should be taken into account especially when making serial determinations of D on the same subject. 1.2. The repeatability of the method was found to be reasonably good; the coefficient of variation was about 4% in normal subjects, as well as in a series of patients with rheumatoid arthritis. 1.3. The effect of the correction for haemoglobin concentration was calculated and tabulated for a set of possible values of DM/Vc and haemoglobin. This correction did not reduce the scatter of D values in normal subjects, but it was adopted for clinical use for theoretical reasons. Caution in its use and interpretation of the correction is emphasized. 2.1. Healthy males and females, 20-69 years of age, were examined to establish reference values for the D measurement. Multiple linear regression equations were calculated stepwise, and equations based on age and height were then used in the clinical prediction equations. Similarly, prediction equations were calculated for clinical spirometry. 2.2. The prediction equations were compared with some published reference values. The equations provide a basis for the evaluation of the effect of various factors, such as age and smoking, in different populations. 3.1. Respiratory function, clinical symptoms and chest x-ray findings were examined in patients with connective tissue diseases. Three different investigations were thus formed: a) consecutive patients (free of lung disease other than that possibly due to a connective tissue disease) with definite rheumatoid arthritis (RA, 21 cases), systemic lupus erythematosus (SLE, 18 cases) or scleroderma (SCL, 6 cases) were subjected to detailed tests. b) 129 patients from the Rheumatism Foundation Hospital, Heinola, Finland, were subjected to measurement of diffusing capacity and vital capacity in addition to chest radiography and routine clinical assessment c) the histological findings on 12 patients subjected to a needle biopsy of the lung in order to exclude other conditions were compared with the results of diffusing capacity and x-ray examinations. 3.2. Restrictive impairment of the respiratory function was the general finding in all of these groups. The reduction in diffusing capacity was out of proportion to the reduction in lung volume, however, in most cases with abnormally low values. Low D values were encountered in about half of the RA and SLE and in all of the SCL patients in group a, and in 13% of the "average" RA patients studied in Heinola. The typical histological finding in patients with reduced D was a thickening of the alveolar wall...