Blood viscosity modulates tissue perfusion – sometimes and somewhere

SUMMARY Each organ possesses specific properties for controlling microvascular perfusion. Such specificity provides an opportunity to design transfusion fluids that target thrombo‐embolic or vasospasm‐induced ischemia in a particular organ or that optimize overall perfusion from systemic shock. The role of viscosity in the design of these fluids might be underestimated, because viscosity is rarely monitored or considered in critical care decisions. Studies linking viscosity‐dependent changes of microvascular perfusion to outcome‐relevant data suggest that whole blood viscosity is negligible as a determinant of microvascular perfusion under physiological conditions when autoregulation is effective. Because autoregulation is driven to maintain oxygen supply constant, the organism will compensate for changes in blood viscosity to sustain oxygen delivery. In contrast, under pathological conditions in the brain and elsewhere, increases of overall viscosity should be avoided – including all the situations where vascular autoregulatory mechanisms are inoperative due to ischemia, structural damage or physiologic dysfunction. As latter conditions cannot be identified with high certainty, the risks that accompany therapeutic correction of blood viscosity outweigh the benefits. The ability to monitor blood viscosity at the bedside and to link changes in viscosity to outcome parameters in various clinical conditions would provide more solid foundation for evidence‐based clinical management.