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

Abstract

Background: Evaluation of donor lung function relies on the arterial oxygen partial pressure to inspired oxygen fraction ratio (PaO2/FiO2) 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 PaO2/FiO2 in the brain-dead donor and during Ex-Vivo Lung Perfusion (EVLP). Results: High FiO2 and increased mixed venous oxygen saturation, caused by increased delivery and reduced consumption of oxygen, raise the PaO2/FiO2 despite substantial intrapulmonary shunt. Anemia does not modify the PaO2/FiO2—intrapulmonary shunt relationship. During EVLP, the reduced artero-venous difference in oxygen content increases the PaO2/FiO2 without this corresponding to an optimal graft function, while the reduced perfusate oxygen-carrying capacity linearizes the PaO2/FiO2—intrapulmonary shunt relationship. Conclusions: Adopting PaO2/FiO2 to evaluate graft suitability for transplantation should account for extrapulmonary factors affecting its interpretation.