What blood temperature for an ex vivo extracorporeal circuit?

Ex vivo circuits are commonly used to evaluate biomaterials or devices used for extracorporeal blood purification. However, various aspects of the ex vivo circuit, apart from the circuit materials, may affect inflammation and coagulation. One such aspect is temperature. The aim of this study was to evaluate the influence of different blood temperature conditions on inflammation parameters in an ex vivo circuit. Blood was collected from 20 healthy volunteers and run through three different experimental conditions for 4h: a miniaturized ex vivo extracorporeal circuit equipped with a blood warmer set to 37°C, the same circuit without the warmer (23°C), and a tube placed in an incubator at 37°C (no circuit). We measured the granulocyte macrophage colony-stimulating factor, the tumor necrosis factor, and the interleukin (IL)-1β, IL-6, IL-8, and IL-10 concentrations at baseline, 15, 60, 120, and 240min. Human leukocyte antigen (HLA)-DR, CD11b, CD11a, CD62L, tumor necrosis factor alpha converting enzyme, annexin V expression, and NFkB DNA binding were measured in monocytes and polymorphonuclear neutrophils (PMNs) using flow cytometry at baseline, 120 min, and 240min. While cytokine production over time was very slight at room temperature, levels increased by more than 100-fold in the two heated conditions. Differences in the expression of some surface markers were also observed between the room temperature circuit and the two heated conditions (CD11b PMN, P<0.0001; HLA-DR Mono, P=0.0019; and CD11a PMN, P<0.0001). Evolution of annexin V expression was also different over time between the three groups (P=0.0178 for monocytes and P=0.0011 for PMNs). A trend for a greater NFkB DNA binding was observed in the heated conditions. Thus, for ex vivo studies using extracorporeal circuits, heating blood to maintain body temperature results in significant activation of inflammatory cells while hypothermia (room temperature) seems to suppress the leukocyte response. Both strategies may lead to erroneous conclusions, possibly masking some specific effects of the device being studied. Investigators in this field must be aware of the fact that blood temperature is a crucial confounding parameter and the type of "background noise" they will face depending on the strategy adopted.