Series dead space volume assessed as the mean value of a distribution function.

Series dead space (VdS) is assumed to be represented by that volume exhaled until alveolar gas is observed. Phase II of the single breath CO2-diagram contains the (flow, concentration and sequence weighted) distribution off all stationary interfaces (SI) expired before phase III. We describe a new method to estimate the mean value of VdS based on the differentiation of phase II. This approximation of VdS is called the 'Pre Interface Expirate' (PIE) and is compared in this study with the integrative approach of Langley. Tidal volume (Vt) and PEEP were varied from 71 to 123% and from 0 to 6 cmH2O respectively. The estimation of VdS by differentiation of phase II (PIE) shows excellent reproducibility and depends only on phase II - not on phase III and IV as does VdS-Langley. PIE does not depend on Vt and PEEP per se but reflects the distension of convective airways due to elevated end-inspiratory airway pressure. Our results confirm the predictions of Paiva's model calculations in that the size of VdS is determined by the distension of airways rather than by the altered position of the SI.