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/FiO₂) 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/FiO₂ sequences remained indistinguishable. In V/Q heterogeneous lungs at high FiO₂, V_ILND generated low Q versus V_A/Q shoulders and some negative V_A units, while V_ALND preserved Q versus V_A/Q log normality by blood flow diversion from low V_I/Q units. We managed V_ILND-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: V_ALND > V_ILND (redistribution) > V_ILND (shunt). In V/Q heterogeneous lungs V_ALND and V_ILND (redistribution) regained near optimal oxygen transfer on 100% oxygen, while impairment persisted with V_ILND (shunt). Unlike V_ALND, V_ILND (redistribution) produced Q versus V_A/Q distributions in V/Q heterogeneity compatible with multiple inert gas (MIGET) reports. V_ILND (redistribution) is a physiologically-based MIGET-compatible alternative to West's original V_ALND lung modeling approach.