An analysis of factors affecting the measurement of pulmonary diffusing capacity by the single breath method.

The diffusing capacity of the lungs (DL) is defined as the rate of a gas transferred, divided by the difference in the mean partial pressure of that gas across the alveolar-capillary "membrane." Direct determination of the diffusing capacity for oxygen (DLo2) has been limited by technical difficulties and by the problematical calculation of the mean oxygen pressure on the capillary side of the pulmonary membrane. Measurement of DL for carbon monoxide (DLCO) has been preferred because the affinity of CO for hemoglobin is so great that plasma tension or "back pressure" has been considered negligible and therefore disregarded (1). The technique which Krogh (2) introduced for determination of DLCO involved measurement of alveolar CO pressure (PAcO) at the beginning and end of breath-holding. Gas containing CO was inhaled and a small portion exhaled immediately for determination of the initial CO pressure (PAcoi). A second sample was delivered at the end of the breath-holding period for measurement of the final CO pressure (PACof). Also determined were the volume of the lungs at which diffusion occurred (VA) and the barometric pressure (B). If PACo decays exponentially with time (t) during breath-holding, and if the plasma back pressure is zero, then: