TEMPORAL EFFECTS IN THE ESTIMATION OF PULMONARY DIFFUSING CAPACITY

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Abstract
Steady state estimates of the pulmonary diffusing capacity for CO (DLCO) [in humans] require measurement of the uptake and the average alveolar partial pressure of CO (PaCO). The expired alveolar sample obtained by different experimental methods and/or breathing patterns rarely represents the actual PaCO. Nonuniform distribution of ventilation, diffusion and perfusion causes discrepancies in the measurement of diffusing capacity. An additional source of error in choosing PaCO arises in the sampling time chosen by the experimental method. A theoretical study of a ramp-with-pause and a square breathing pattern demonstrates that the sample-time error exists even in the homogeneous lung. The study shows for the homogeneous lung that the correct fractional concentration of alveolar CO (FAV) occurs at a time (TAV), 1/2 of a breathing period after the effective inspiration time (TI) for the 2 very different breathing patterns. TI is well-defined in relation to any breathing pattern which can be approximated by ramps and pauses. If TAV and the sample time chosen by the experimental method are known, then the measured DLCO can be corrected to the actual diffusing capacity (DL). The theory agrees with experimental results and computer simulations of inhomogeneous lungs from the literature. The theory for the homogeneous lung is also relevant to the inhomogeneous lung.