Multi-compartment V/Q lung modeling: Log normal distributions of inspired or expired alveolar gas?

Using a 50-compartment Python-coded mathematical lung model, we compared mixed venous blood flow (Q) distributions and arterial oxygen tension/inspired oxygen fraction (PaO2/FiO(2)) relationships in lungs modeled with log normal distributions (LND) of inspired (V-I) versus expired (V-A) alveolar gas volumes. In lungs with normal V/Q heterogeneity, Q versus V-A/Q and Q versus V-I/Q distributions were similar with either approach, and PaO2/FiO(2) sequences remained indistinguishable. In V/Q heterogeneous lungs at high FiO(2), VILND generated low Q versus V-A/Q shoulders and some negative V-A units, while VALND preserved Q versus V-A/Q log normality by blood flow diversion from low V-I/Q units. We managed VILND-induced negative V-A units either by shunt conversion (V-I decreased to 0) or V-I redistribution simulating collateral ventilation (V-I increased till V-A = 0). Comparing oxygen transfer: VALND > VILND (redistribution) > VILND (shunt). In V/Q heterogeneous lungs VALND and VILND (redistribution) regained near optimal oxygen transfer on 100% oxygen, while impairment persisted with VILND (shunt). Unlike VALND, VILND (redistribution) produced Q versus V-A/Q distributions in V/Q heterogeneity compatible with multiple inert gas (MIGET) reports. VILND (redistribution) is a physiologically-based MIGET-compatible alternative to West's original VALND lung modeling approach.