Abstract Pancreas and islet cell transplantation are used in the treatment of patients with diabetes complications. During static cold storage (SCS) ATP is depleted, and by-products of anaerobic respiration accumulate. Ischaemia-reperfusion injury (IRI) affects quantity and viability of islets and it is characterised by acinar necrosis, oedema, and endothelial disruption (graft pancreatitis). The introduction of HMPO2 in liver & kidney preservation has demonstrated a significant reduction in the consequences of IRI. Pancreas HMPO2 was shown to be feasible in pre-clinical studies. We aimed to compare a ‘continuous’ to an ‘end-ischaemic’ approach of HMPO2 in pancreases using a porcine circulatory death model. Porcine pancreases were either started on HMPO2 for the totality of the cold storage time (‘Continuous’ group, n=6) or for the last two hours of the cold storage time (‘End’ group, n=6). All pancreases then underwent normothermic reperfusion (NR) to mimic transplantation. Glucose stimulated insulin secretion (GSIS) was measured using Mercodia human insulin ELISA (n=3 in each group). Both groups had no significant change of wet-to-dry ratio during the experiment, despite an increase in gross weight. Flows were higher during NR in the Continuous group. Amylase, Lipase and LDH increased throughout the study for all pancreases and showed no statistically significant difference between both groups. The Continuous group had a significantly greater insulin secretion in response to glucose stimulation and followed a biphasic pattern. Continuous HMPO2 preserved islet function and was a superior mode of preservation, showing no statistically significant difference in oedema or markers of damage, with improved perfusion parameters.

Abstract Background The storage lesion is a multifactorial degradation effect seen in units of banked blood, and results in an increase in the time-constant of oxygen-unloading from red cells (tau). We have previously shown the relevance of this effect in observational data from a recent clinical trial of prolonged normothermic kidney perfusion as a preservation technique ahead of transplantation. We sought direct confirmatory evidence for diffusion-limited oxygen release by stored red cells (RBCs) during normothermic kidney perfusion in a precisely controlled experimental setting. Methods A novel twinned-circuit kidney perfusion system using common, recirculated dialysis was developed, achieving physical separation of RBCs of two conditions whilst allowing non-cellular components of the perfusate to equilibrate throughout. Using human kidneys unsuitable for transplantation, perfusion was alternated between stored and rejuvenated RBCs (treated with PIPA solution) originating from the same donation, to manipulate oxygen-unloading whilst holding all non-RBC parameters constant, without interrupting blood flow. Results Tau improved with PIPA treatment (1.6 to 1.1s). Rejuvenated oxygen-unloading kinetics reversibly and significantly improved the kidney’s oxygen diffusion capacity and increased cortical oxygen tension by 60%. This supported observations from our clinical trial, where increased tau was strongly associated with restricted renal oxygen consumption. Conclusions We have confirmed that oxygen delivery to tissues can become diffusion-limited during normothermic perfusion with stored blood. This effect is reversible by treatment with PIPA solution. This has important implications for ex-vivo normothermic organ perfusion, and potentially for other scenarios such as major hemorrhage, elective surgery with high blood product utilisation, and paediatric transfusion.

The volume of oxygen drawn from systemic capillaries down a partial pressure gradient is determined by the oxygen content of red blood cells (RBCs) and their oxygen-unloading kinetics, although the latter is assumed to be rapid and, therefore, not a meaningful factor. Under this paradigm, oxygen transfer to tissues is perfusion-limited. Consequently, clinical treatments to optimize oxygen delivery aim at improving blood flow and arterial oxygen content, rather than RBC oxygen handling. Although the oxygen-carrying capacity of blood is increased with transfusion, studies have shown that stored blood undergoes kinetic attrition of oxygen release, which may compromise overall oxygen delivery to tissues by causing transport to become diffusion-limited. We sought evidence for diffusion-limited oxygen release in viable human kidneys, normothermically perfused with stored blood. In a cohort of kidneys that went on to be transplanted, renal respiration correlated inversely with the time-constant of oxygen unloading from RBCs used for perfusion. Furthermore, the renal respiratory rate did not correlate with arterial O2 delivery unless this factored the rate of oxygen-release from RBCs, as expected from diffusion-limited transport. To test for a rescue effect, perfusion of kidneys deemed unsuitable for transplantation was alternated between stored and rejuvenated RBCs of the same donation. This experiment controlled oxygen-unloading, without intervening ischemia, holding all non-RBC parameters constant. Rejuvenated oxygen-unloading kinetics improved the kidney’s oxygen diffusion capacity and increased cortical oxygen partial pressure by 60%. Thus, oxygen delivery to tissues can become diffusion-limited during perfusion with stored blood, which has implications in scenarios, such as ex vivo organ perfusion, major hemorrhage, and pediatric transfusion. This trial was registered at www.clinicaltrials.gov as #ISRCTN13292277.

ABSTRACT A central dogma in physiology is that oxygen release at tissues is not diffusion-limited because gas exchange at capillaries is rapid. This assertion has influenced clinical care, which focuses on optimising oxygen delivery through improving blood flow and oxygen content, rather than oxygen unloading from red blood cells (RBCs). Since storage of blood causes profound changes that slow oxygen release from RBCs, transfusions could compromise tissue respiration. We investigated this in transplant human kidneys normothermically perfused with stored blood. During perfusions, renal respiration was measured from blood gases and RBCs were analysed for oxygen-unloading kinetics. Respiratory rate did not correlate significantly with the standard definition of oxygen delivery based on blood flow and oxygen content. However, a strong correlation was obtained after introducing a factor describing oxygen release from RBCs. Oxygen release to tissues can become diffusion-limited with transfused blood, and therefore the kinetic quality of RBCs should be considered.