Hysteresis in the relation between diffusing capacity of the lung and lung volume.

The diffusing capacity of the lung for carbon monoxide (DLCO) varies directly with lung volume (VA) when measured during a breath-holding interval. DLCO measured during a slow exhalation from total lung capacity (TLC) to functional residual capacity (FRC) does not vary as VA changes. Since VA is reached by inhaling during breath holding and by exhaling during the slow exhalation maneuver, we hypothesized that the variability in the relation between DLCO and VA was due to hysteresis. To test this hypothesis, breath-holding measurements of DLCO were made at three lung volumes, both when VA was reached by inhaling from residual volume (RV) and when Va was reached by exhaling from TLC. At 72% TLC, DLCO was 22% higher when VA was reached by exhalation compared to inhalation (P < 0.02). At 52% TLC, DLCO was 19% higher when VA was reached by exhalation compared to exhalation (P < 0.005). DCLO measured during a slow exhalation fell on the exhalation limb of the CLCO/VA curve. these data indicate that there is hysteresis in DLCO with respect to lung volume.