Limitations of arterial partial pressure of oxygen to fraction of inspired oxygen ratio for the evaluation of donor lung function

BACKGROUND: Evaluation of donor lung function relies on the arterial oxygen partial pressure to inspired oxygen fraction ratio (PaO(2)/FiO(2)) measurement. Hemodynamic, metabolic derangements, and therapeutic intervention occurring during brain dead observation may influence the evaluation of gas exchange. METHODS: We performed a mathematical analysis to explore the influence of the extrapulmonary determinants on the interpretation of PaO(2)/FiO(2) in the brain‐dead donor and during Ex‐Vivo Lung Perfusion (EVLP). RESULTS: High FiO(2) and increased mixed venous oxygen saturation, caused by increased delivery and reduced consumption of oxygen, raise the PaO(2)/FiO(2) despite substantial intrapulmonary shunt. Anemia does not modify the PaO(2)/FiO(2)—intrapulmonary shunt relationship. During EVLP, the reduced artero‐venous difference in oxygen content increases the PaO(2)/FiO(2) without this corresponding to an optimal graft function, while the reduced perfusate oxygen‐carrying capacity linearizes the PaO(2)/FiO(2)—intrapulmonary shunt relationship. CONCLUSIONS: Adopting PaO(2)/FiO(2) to evaluate graft suitability for transplantation should account for extrapulmonary factors affecting its interpretation.