The Effects of red Blood Cell Transfusion on Tissue Oxygenation and the Microcirculation in the Intensive Care Unit: A Systematic Review

•Novel techniques can be used to assess tissue oxygenation and the microcirculation.•Overall, the majority (11 of 17) of studies investigating the effects of red blood cell transfusion on tissue oxygenation concluded that transfusion failed to improve tissue oxygenation.•However, 9 studies revealed a consistent pattern-patients with abnormal tissue oxygenation prior to transfusion improved with transfusion, whereas those with normal pre-transfusion indices did not.•Future clinical studies addressing the “transfusion threshold” should focus on parameters of tissue oxygenation rather than hemoglobin concentrations.

The transfusion of red blood cells (RBCs) is a common intervention in intensive care unit (ICU) patients, yet the benefits are far from clear in patients with moderate anemia (eg, hemoglobin (Hb) levels of 7-10 g/dL). Determining which of these patients benefit, and how to even define benefit, from transfusion is challenging. As the intended physiological benefit underpinning RBC transfusion is to improve tissue oxygenation, several studies utilizing a wide range of assessment techniques have attempted to study the effects of transfusion on tissue oxygenation and microcirculatory function. The objective of this systematic review was to determine whether RBC transfusion improves tissue oxygenation/microcirculatory indices in the ICU population, and to provide an introduction to the techniques used in these studies. Eligible studies published between January 1996 and February 2017 were identified from searches of PubMed, Embase, Cinahl, ScienceDirect, Web of Science, and The Cochrane Library. Seventeen studies met inclusion criteria, though there was significant heterogeneity in study design, patient population, assessment techniques and outcomes reported. Overall, the majority of studies (11 of 17) concluded that transfusion did not generally improve tissue oxygenation or microcirculation. Inter-individual effects were highly variable, however, and closer review of sub-groups available in 9 studies revealed that patients with abnormal tissue oxygenation or microcirculatory indices prior to transfusion had improvement in these indices with transfusion, irrespective of assessment method. This finding suggests a new strategy for future trials in the ICU: utilizing tissue oxygenation/microcirculatory parameters to determine the need for transfusion rather than largely arbitrary hemoglobin concentrations.