Inhomogeneity of ventilation leads to unpredictable errors in measured DLCO

We evaluated the effects of inhomogeneity of ventilation on single-breath (SB), rebreathing (RB) and open circuit (OC) DLCO using a mathematical model consisting of two alveolar compartments and a common dead space. Inhomogeneity in ventilation was studied by altering inspired volume, initial alveolar volume and compartment size independently. When distribution of inspired volume between alveolar compartments was inhomogeneous (9:1), DLCO was underestimated by 35% for SB, 25% for RB, and 16% for OC, and there was an underestimation in VA of 9%, 15% and 9%, respectively. With inhomogeneity in initial alveolar volume there was an overestimation in DLCO of 13%, 7% and 11% for SB, RB and OC techniques and an underestimation of VA of 7%, 12% and 9%. Finally inhomogeneity of compartment size led to an underestimation of DLCO of 18%, 35% and 36% with no change in measured VA. These results suggest DLCO measurements are sensitive to inhomogeneity of ventilation, and importantly, all techniques were at times, significantly in error.