Influence of ventilation, exercise, and body position on techniques for determining steady state diffusing capacity

Three different methods of measuring steady state diffusing capacity were analysed regarding the effect of using either the physiological dead space for CO2 (DLCOss1), the measured anatomical dead space (DLCOss2), or an assumed anatomical dead space of the lungs (DLCOss3) for the determination of alveolar CO. The material consisted of 28 healthy men and 91 patients with various pulmonary diseases. DLCOss1 and DLCOss2 were compared in the total material (Section 1) using determinations both at rest and during exercise. As was expected DLCOss1 was almost always higher than DLCOss2. The difference between the two methods followed the curve that could be predicted from the mathematically obtained relation between DL, VD/VT, and fractional CO uptake, published by West in a diagram 1962. In determinations with a large sum of the latter two variables, DLCOss1 was highly dependent on the validity of the PaCO2 used. The validity of an end-tidal sample under different ventilatory patterns is investigated in Section 2 and the relation between DLCOss and alveolar ventilation in Section 3. Relative hypoventilation gave low values for DLCOss regardless of method. There was a rapid increase in DLCOss with increasing alveolar ventilation in the normal ventilation range. During hyperventilation DLCOss1, like FuCO, decreased with increasing ventilation. The composition of the end-tidal sample was frequently distorted under forced breathing with large VT so that erroneously high VDa and DLCOss2 were computed. Consequently, DLCOss2 appeared positively correlated to VE and VT over the whole ventilation range. On the other hand, the uncertainty of the size of the anatomical dead space--assumed in DLCOss3--at high flow rates in the bronchial tree reduces the usefulness of this method during pronounced hyperventilation.