Oxygen delivery to tissues is dependent on blood flow, the oxygen carrying capacity of the RBCs (related to hemoglobin levels), and the oxygen saturation of the hemoglobin in a perfusion-limited process. Previous research has shown that the kinetics of oxygen unloading—which is rarely measured—is slower than previously thought and may lead to diffusion-limited oxygen delivery to tissues, especially when RBCs have been stored. To gain a better understanding of the kinetics of oxygen unloading, researchers in Europe studied stored (n=30; median storage time, 12.5 days) and freshly drawn (n=32) ABO-Rh matched RBCs flowing through kidneys ex vivo. This study was part of a phase 1 trial examining normothermic machine perfusion before transplant. A linear relationship between oxygen delivery and respiration was expected based on a perfusion-limited model of oxygen unloading. The stored RBCs, however, exhibited a range of oxygen unloading kinetics—mainly slower compared to freshly drawn blood and did not completely correlate with storage duration. Biochemical rejuvenation of RBCs increased the oxygen unloading rates compared to RBCs not rejuvenated and increased the renal cortex oxygenation by 60%, suggesting that oxygen unloading kinetics can affect oxygen delivery to tissues. Future research on the physiological consequences of a diffusion-limited model of oxygen unloading and the impact of RBC storage is needed.
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