EFFECT OF VENTILATION INHOMOGENEITY ON INTRABREATH MEASUREMENTS OF DIFFUSING-CAPACITY IN NORMAL SUBJECTS

In normal seated subjects we increased single-breath ventilation inhom ogeneity by changing both the preinspiratory lung volume and breath-ho ld time and examined the ensuing effects on two different techniques o f measuring the diffusing capacity of the lung for carbon monoxide (DL (CO)). We measured the mean single-breath DL(CO) using the three-equat ion method (DL(CO)SB-3EQ) and also measured DL(CO) over discrete inter vals during exhalation by the ''intra-breath'' method (DL(CO)exhaled). We assessed the distribution of ventilation using the normalized phas e III slope for helium (S(N)). DL(CO)SB-3EQ was unaffected by preinspi ratory lung volume and breath-hold time. DL(CO)exhaled increased with increasing preinspiratory lung volume and decreased with increasing br eath-hold time. These changes correlated with the simultaneously obser ved changes in ventilation inhomogeneity as measured by SN (P < 0.01). We conclude that measurements of DL(CO)exhaled do not accurately refl ect the mean DL(CO). Intrabreath methods of measuring DL(CO) are based on the slope of the exhaled CO concentration curve, which is affected by both ventilation and diffusion inhomogeneities. Although DL(CO)exh aled may theoretically provide information about the distribution of C O uptake, the concomitant effects of ventilation nonuniformity on DL(C O)exhaled may mimic or mask the effects of diffusion nonuniformity.