Bringing flow into haemostasis diagnostics.

Br J Anaesth. 2013 Dec;111(6):864-7. doi: 10.1093/bja/aet289. Epub 2013 Oct 11.



Virchow first defined the importance of blood flow in haemostasis and thrombosis, but first received acknowledgement 100 years after his publication in 1856.1 Blood is a non-Newtonian fluid and its viscosity is dependent on the flow, flow velocity, type of flow (laminar or non-laminar), concentration of different types of blood cells, plasma protein size/concentration, and temperature.2,3 At higher flow rates, blood behaves as a Newtonian fluid with reduced viscosity. Flow transfers energy to the blood and the vascular wall and this is called shear stress. Shear rate is defined as the rate at which adjacent layers of fluid move with respect to each other. Shear rate/stress differs in the venous and arterial circulation, but also increases when the vascular lumen decreases, naturally when blood moves peripherally and with pathological constrictions like thrombus, plaques, mechanical valves, stents, and vascular anastomoses. High shear forces lead to platelet activation through both von Willebrand factor (vWF)-dependent and non-dependent mechanisms.2 Blood flow and shear rates often change dynamically in perioperative patients due to changes in arterial pressure or vascular tone, blood loss, and fluid replacements. Currently, available coagulation assays are mostly performed under static condition, and therefore complex interactions among erythrocytes, platelets, white cells, and coagulation factors under various shear rates cannot be appreciated.