The Relationship Between Shear Rate and Vessel Diameter

To the Editor: In the study published by Eckmann et al. (1) there are interesting data about the alterations of blood viscosity under various conditions during surgery. However, the information concerning the relation between shear rate and vessel diameter is confusing, especially to the readers who are interested in hemorheology. It is well known that hemorheology is the science dealing with flow and deformation of blood and blood vessels (2). Its major field of interest is blood viscosity (BV). BV is determined by plasma viscosity, hematocrit, erythrocyte deformability, and aggregability. These factors are interrelated in a complex manner. Because of the erythrocytes ability to deform and aggregate, blood is a fluid with non-Newtonian behavior. That is to say, blood viscosity varies with the force applied to it. BV is dependent on shear rate. This means viscosity decreases when shear rate increases. The rheological term “shear rate” is almost synonymous with velocity gradient. Shear rate is determined by the diameter of vessels. The high shear rate is present when flow is fast and the vessel diameter small, and low shear rate is present when flow is slow and the vessel has a large diameter. The non-Newtonian behavior of blood is due to the tendency of erythrocytes to aggregate at low shear rates. Furthermore, when shear rate is high, the erythrocytes are deformed to optimally adapt to flow conditions. In normal circumstances, in capillaries, high shear rates occur and blood viscosity is low (2–4). Although in their introduction the relation between shear rate and vessel diameter correlated positively, the low flow is reported to result in low shear rates, which is evidence of the inverse relation between shear rate and the vessel diameter shown in the discussion section of their study. This paradox can cause confusion for the readers. We think that basic information should be used carefully in scientific investigations. M. Ercan, PhD C. Koksal, MD