Regional pulmonary perfusion, blood volume, and their relationship change in experimental early ARDS

Regional pulmonary perfusion (Q) has been investigated using blood volume (F_b) imaging as an easier-to-measure surrogate. However, it is unclear if changing pulmonary conditions could affect their relationship. We hypothesized that vascular changes in early acute respiratory distress syndrome (ARDS) affect Q and F_b differently. Five sheep were anesthetized and received lung protective mechanical ventilation for 20 h while endotoxin was continuously infused. Using dynamic ¹⁸F-FDG and ¹³NN Positron Emission Tomography (PET), regional F_b and Q were analysed in 30 regions of interest (ROIs) and normalized by tissue content (F_bn and Q_n, respectively). After 20 h, the lung injury showed characteristics of early ARDS, including gas exchange and lung mechanics. PET images of F_bn and Q_n showed substantial differences between baseline and lung injury. Lung injury caused a significant change in the F_bn-Q_n relationship compared to baseline (p < 0.001). The best models at baseline and lung injury were F_bn = 0.32 + 0.690Q_n and F_bn = 1.684Q_n-0.538Q_n², respectively. Endotoxine-associated early ARDS changed the relationship between F_b and Q, shifting from linear to curvilinear. Effects of endotoxin exposure on the vasoactive blood flow regulation were most likely the key factor for this change limiting the quantitative accuracy of F_b imaging as a surrogate for regional Q.