AN ANALYSIS OF FACTORS AFFECTING THE MEASUREMENT OF PULMONARY DIFFUSING CAPACITY BY THE SINGLE BREATH METHOD*

Abstract

Variations between multiple tests were due to changes in the volume of the lungs (VA) at which the breath was held. A quantitative relationship between the single breath diffusing capacity (SB DL) and Va was demonstrated: in 14 normals a 92% increase of VA increased DL by 47%; and in 15 patients this increase averaged 60%. Both in normals and in patients the SB DL was invariably larger than the "physiologic deadspace" steady state DL (SS DL). Effective CO "back pressure" caused an apparent reduction of SB Dl of 0.24 ml/min/mm Hg for each 1% rise of COHb saturation. Effective back pressure was 1.9 times greater than estimates obtained from Haldane''s relationship. Neglect of back pressure caused underestimation of SB DL by 3.1% and of SS DL by 21.1%. Back pressure explained alinearity in relationship of duration of breath holding to in of alveolar CO. Increase of functional residual capacity (FRC) induced by body plethysmograph failed to increase the SS DL. "Instantaneous Dl" determinations showed that variations of pulmonary capillary blood volume were largely responsible both for the larger value of SB DL and for variations with VA. Inequalities of ventilation-perfusion ratios explained the greater discrepancy between SS DL and SB DL in patients with pulmonary disease than in normal subjects: in normals the SB value was 1.6 times larger than the SS value, in patients with "alveolar-capillary block" it was 2.3 times larger, and in emphysema it was 3.4 times larger. A relationship between this discrepancy and the degree of uneven gas distribution was demonstrated. The two tests did not sample the "same lung". For multiple, serial or follow-up studies the SB tests is useful only when related to VA. An abnormally low SB Dl implies serious impairment of apparent DL while a normal value, particularly in patients with uneven lungs, does not rule out serious "effective" impairment of the lungs to diffuse O2 or CO. SS DLCO or DL02 values cannot be estimated from the SB DLCO.