Blood Viscosity

Red blood cells (RBCs) are the most abundant cells in human blood. Remarkably RBCs deform and bridge together to form aggregates under very low shear rates. The theory and mechanics behind aggregation are, however, not yet completely understood. The main objective of this work is to quantify and characterize RBC aggregates in order to enhance the current understanding of the non-Newtonian behaviour of blood in microcirculation. Suspensions of human blood were flowed and observed in vitro in poly-di-methyl-siloxane (PDMS) microchannels to characterize RBC aggregates. These microchannels were fabricated using standard photolithography methods. Experiments were performed using a micro particle image velocimetry (μPIV) system for shear rate measurements, coupled with a high-speed camera for flow visualization. RBC aggregate sizes were quantified in controlled and measurable shear rate environments for 5, 10 and 15% hematocrit. Aggregate sizes were determined using image processing techniques, while apparent viscosity was measured using optical viscometry. For the samples suspended at 5% H, aggregate size was not strongly correlated with shear rate. For the 10% H suspensions, in contrast, lowering the shear rate below 10 s-1 resulted in a significant increase of RBC aggregate sizes. The viscosity was found to increase with decreasing shear rate and increasing hematocrit, exemplifying the established non-Newtonian shear-thinning behaviour of blood. Increase in aggregation size did not translate into a linear increase of the blood viscosity. Temperature was shown to affect blood viscosity as expected, however, no correlation for aggregate size with temperature was observed. Non-Newtonian parameters associated with power law and Carreau models were determined by fitting the experimental data and can be used towards the simple modeling of blood's non-Newtonian behaviour in microcirculation. This work establishes a relationship between RBC aggregate sizes and corresponding shear rates and one between RBC aggregate sizes and apparent blood viscosity at body and room temperatures, in a microfluidic environment for low hematocrit. Effects of hematocrit, shear rate, viscosity and temperature on RBC aggregate sizes have been quantified.

BackgroundCardiovascular protection following empagliflozin therapy is not entirely attributable to the glucose lowering effect. Increased hematocrit might influence the shear stress that is the main force acting on the endothelium, regulating its anti-atherogenic function.ObjectiveWe designed the study with the aim of investigating the effect of empagliflozin on blood viscosity and shear stress in the carotid arteries. A secondary endpoint was the effect of empagliflozin on carotid artery wall thickness.MethodsThe study was a non-randomized, open, prospective cohort study including 35 type 2 diabetic outpatients who were offered empagliflozin or incretin-based therapy (7 liraglutide, 8 sitagliptin) in combination with insulin and metformin. Blood viscosity, shear stress and carotid wall thickness were measured at baseline and at 1 and 3 months of treatment. Blood viscosity was measured with a viscometer, and shear stress was calculated using a validated formula. Intima-media thickness (IMT) of the carotid artery was detected by ultrasound and was measured with dedicated software.ResultsBlood viscosity (4.87 ± 0.57 vs 5.32 ± 0.66 cP, p ConclusionsThis study is the first to describe a direct effect of empagliflozin on blood viscosity and wall shear stress. Furthermore, IMT was markedly reduced early on in the Empagliflozin group.

BackgroundThis study describes a new method for determining site-specific vascular shear stress using dynamic measures of shear rate and blood viscosity (BV) in the carotid arteries, and examines characteristics of carotid arterial shear stress among patients with lacunar infarction.MethodsVascular shear stress measurements were conducted in 37 patients (17 lacunar infarction patients and 20 control subjects) using duplex ultrasonography. Vessel wall diameters and velocities were measured in each arterial segment at peak-systolic (PS) and end-diastolic (ED) phases, for calculation of PS/ED shear rates. PS/ED shear stresses [dyne/cm²] were determined with PS/ED shear rates and shear-rate dependent BV values. For comparison, both values of hematocrit-derived BV and BV measurements at 300 s⁻¹ were used for calculation of shear stress.ResultsAll cardiovascular disease (CVD) risk factors including BV values were similar between the two groups. In both common carotid arteries, PS and ED shear stresses were significantly lower in the patients with lacunar infarction than in controls in multivariate models that included age, sex, and other major CVD risk factors. PS and ED shear stresses using the shear rate specific BV were 4.5% lower and 7.3% higher than those using the two other BVs, respectively.ConclusionLacunar infarction was associated with reduced carotid arterial shear stress. The use of estimated BV for calculating carotid arterial shear stress provides more accurate assessment of the hemodynamic contribution of shear stress than previous models that have arbitrarily assigned a constant value to this dynamic flow property.

Whole blood viscosity is a test for blood stasis and is an ideal evidence-based pathology parameter that is largely undervalued and retrogressing in clinical utilization. Coagulation profiles as indices of haemostasis are available but limited to central or referral laboratories and often involve long turn-around time. It is therefore important to study the correlation between the index of stasis and indices of haemostasis.ObjectiveTo investigate the correlation of index of stasis with indices of haemostasis.MethodThe clinical laboratory observational research method, using archived pathology data. Indices of haemostasis including activated partial thromboplastin time (APTT) and prothrombin time (PT), the international normalization ratio (INR), and plasma D-dimer were evaluated. On the other hand, the index of blood stasis used was the estimated whole blood viscosity (eWBV) and derived haematocrit and serum protein levels. All (N = 193) tests were collected within a calendar year from the same pathology service, and further, for the correlation, each set of variables from the same blood sample collection was used.ResultsThe haemostasis data are skewed (skewness > 2.0), while eWBV and platelet count are normal (skewness p p ConclusionThere are limitations to the possible correlation between eWBV and haemostasis indices. However, haemostasis indices have inverse associations with eWBV, and the latter can aid in the evaluation of haemostasis hence could be utilized as an alternative or complementary test to haemostasis tests. Research may normalize skewed data to obtain better correlation; therefore, further study is required to advance discourse, giving cognizance to clinical practice.