Journal of Applied Physiology, Vol 63, Issue 6 2438-2449, Copyright © 1987 by American Physiological Society
Pulmonary diffusing capacity in the presence of ventilation inhomogeneity
G. M. Saidel and M. Modarreszadeh
Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio.
A model has been developed to quantify the effectiveness of
alveolar-capillary transport in the presence of ventilation inhomogeneity.
The exhalation dynamics of carbon monoxide (CO), argon (Ar), and lung
volume from a single-breath experiment are analyzed simultaneously. A
membrane transport coefficient (MTCO) that does not vary with lung volume
is evaluated by a two-stage optimization procedure and related to diffusing
capacity. Also, the model allows for a decrease in membrane transport rate
associated with reduced lung volume. The model is tested by simulation
studies and experiments with human subjects having normal or diseased
(mainly obstructed) lungs. The MTCO provides a clear distinction between
normal and obstructed lungs with respect to alveolar-capillary transport,
whereas the semilog slope of the Ar alveolar plateau characterizes the
ventilation inhomogeneity. Only when the diffusing capacity is corrected by
the Ar slope, DLCO(Ar), do the breathing maneuvers performed from different
preinflation volumes (residual volume or functional residual capacity)
yield the same results for lungs with ventilation inhomogeneity. The
uncorrected DLCO overestimates the effectiveness of alveolar-capillary
transport in the presence of ventilation inhomogeneity.
