Expected pressure-volume heterogeneity within the lung

Using a model of the lung providing for pressure-volume (P-V) heterogeneity, we reproduced straight (allowing for some concavity or convexity) alveolar plateaus in the nitrogen washout curve with a slope identical to an experimentally measured one (1.2% N2/liter). In a two-unit lung, with one unit of high volume (H) at total lung capacity (TLC) and another of low volume (L) at TLC, three variables could be adjusted to produce the desired alveolar plateaus: the ratio of TLCL/TLCH; the ratio of functional residual capacity, FRCL/FRCH; and the ratio of the steepness constants of their sigmoid P-V curve, KL/KH. We searched for all possible combinations of the three variables that yielded the desired alveolar plateau. For any TLCL/TLCH ratio, there were two single-value solutions for the FRCL/FRCH ratio; thus, the family of solutions was represented by two surfaces in three-dimensional space. We extended the study to a smaller alveolar slope (0.6% N2/liter), assuming that the total slope (1.2% N2/liter) was partially caused by diffusion-convection interaction. We also studied a three-compartment lung as indicative of a multicompartmental lung. The results are discussed in relation to experimentally confirmed P-V heterogeneity. Granting our basic assumptions of P-V heterogeneity, the real physical lung characteristics may be a subsection of the range of theoretical solutions described herein.