Zero expiratory pressure and low oxygen concentration promote heterogeneity of regional ventilation and lung densities

BACKGROUND: It is not well known what is the main mechanism causing lung heterogeneity in healthy lungs under mechanical ventilation. We aimed to investigate the mechanisms causing heterogeneity of regional ventilation and parenchymal densities in healthy lungs under anesthesia and mechanical ventilation. METHODS: In a small animal model, synchrotron imaging was used to measure lung aeration and regional‐specific ventilation (sV̇). Heterogeneity of ventilation was calculated as the coefficient of variation in sV̇ (CV (sV̇)). The coefficient of variation in lung densities (CV(D)) was calculated for all lung tissue, and within hyperinflated, normally and poorly aerated areas. Three conditions were studied: zero end‐expiratory pressure (ZEEP) and F(I)O (2) 0.21; ZEEP and F(I)O (2) 1.0; PEEP 12 cmH(2)O and F(I)O (2)1.0 (Open Lung‐PEEP = OLP). RESULTS: The mean tissue density at OLP was lower than ZEEP‐1.0 and ZEEP‐0.21. There were larger subregions with low sV̇ and poor aeration at ZEEP‐0.21 than at OLP: 12.9 ± 9.0 vs. 0.6 ± 0.4% in the non‐dependent level, and 17.5 ± 8.2 vs. 0.4 ± 0.1% in the dependent one (P = 0.041). The CV (sV̇) of the total imaged lung at PEEP 12 cmH(2)O was significantly lower than on ZEEP, regardless of F(I)O (2), indicating more heterogeneity of ventilation during ZEEP (0.23 ± 0.03 vs. 0.54 ± 0.37, P = 0.049). CV(D) changed over the different mechanical ventilation settings (P = 0.011); predominantly, CV(D) increased during ZEEP. The spatial distribution of the CV(D) calculated for the poorly aerated density category changed with the mechanical ventilation settings, increasing in the dependent level during ZEEP. CONCLUSION: ZEEP together with low F(I)O (2) promoted heterogeneity of ventilation and lung tissue densities, fostering a greater amount of airway closure and ventilation inhomogeneities in poorly aerated regions.