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Purpose Hemoglobin (Hb) transfusion thresholds are established in intensive care units. A restrictive transfusion threshold (Hb 70–75 g/l) is recommended in septic patients, and a liberal transfusion threshold (Hb 90 g/l) for cardiogenic shock. It is unclear whether these historically adopted transfusion thresholds meet the challenges of individual patients. Methods We evaluated microvascular flow index (MFI) and proportion of perfused vessels (PPV) in the sublingual microcirculation with CytoCam-IDF microscopy and near-infrared spectroscopy (NIRS). A study team-independent, treating intensivist assigned a total of 64 patients to 1 of 2 two transfusion thresholds, 43 patients to the Hb 75 g/l threshold and 21 patients to the Hb 90 g/l threshold, at a surgical intensive care unit. We performed microcirculatory measurements 1 h before and 1 h after transfusion of 1 unit of red blood cells. Results Microcirculatory flow variables correlated negatively with pre-transfusion flow variables (ΔMFI: ρ = − 0.821, p <  0.001; ΔPPV: ρ = − 0.778, p <  0.001). Patients with good initial microcirculation (cutoffs: MFI > 2.84, PPV > 88%) showed a deteriorated microcirculation after red blood cell transfusion. An impaired microcirculation improved after transfusion. At both transfusion thresholds, approximately one third of the patients showed an initially impaired microcirculation. In contrast, one third in every group had good microcirculation above the cutoff variables and did not profit from the transfusion. Conclusion The data suggest that the established transfusion thresholds and other hemodynamic variables do not reflect microcirculatory perfusion of patients. Blood transfusion at both thresholds 75 g/l and 90 g/l hemoglobin can either improve or harm the microcirculatory blood flow, questioning the concept of arbitrary transfusion thresholds.

This review describes the evidence relating to fluid resuscitation in sepsis, septic shock and massive haemorrhage. Beside the scientific evidence based on clinical trials, possible effects on the microcirculation and, therefore, organ function will be illustrated and areas of future research highlighted. The critical appraisal of the existing evidence should enable the reader to choose the optimal volume substitution for an individual patient.

Introduction Sublingual microcirculation monitoring is suitable for bedside use in critically ill patients. We present a case in which severely impaired sublingual microcirculation was the first alarming sign of an early deterioration of the patient’s medical situation. Case presentation This is the case of a 58-year-old white woman admitted to our intensive care unit after the removal of parts of her small intestine due to a volvulus. Her microcirculation was checked the day after surgery in terms of an ongoing study and predicted a massive deterioration of her clinical situation. Conclusions This case highlights the potential value of monitoring the microcirculation in critically ill patients. Two full hours could have been saved for diagnostic workup and earlier treatment had we considered the impaired microcirculation alone as a warning sign. Regardless of the supposed cause, impaired microcirculation should alert the responsible physician and should be followed by a diagnostic workup. Sublingual microcirculation monitoring can be useful in intensive care units to detect a deteriorated microcirculation earlier than with standard monitoring. Electronic supplementary material The online version of this article (10.1186/s13256-019-2118-4) contains supplementary material, which is available to authorized users.