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

Regional pulmonary perfusion (Q) has been investigated using blood volume (Fb) 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 Fb differently. Five sheep were anesthetized and received protective mechanical ventilation for 20 hours while endotoxin was continuously infused. Using dynamic 18F-FDG and 13NN Positron Emission Tomography (PET), regional Fb and Q were analysed in 30 regions of interest (ROIs) and normalized by tissue content (Fbn and Qn, respectively). After 20 hours, the animals lung injury showed characteristics of early ARDS, including gas exchange and lung mechanics. PET images of Fbn and Qn showed substantial differences between baseline and lung injury. Lung injury caused a significant change in the Fbn-Qn relationship compared to baseline (p<0.001). The best models at baseline and lung injury were Fbn=0.32+0.690Qn and Fbn=1.684Qn-0.538Qn2, respectively. Early ARDS changed the relationship between Fb and Q 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 Fb imaging as a surrogate for regional Q.