Viscosity, blood

Fiber dimensions have been studied for hemodialysis. When blood is circulated through the fiber lumen (m vivo), a significant reduction in apparent blood viscosity may occur if the flow-path diameter is below 100 p.m (11). Therefore, current dialy2ers use fibers with internal diameters of 180—250 p.m to obtain the maximum surface area within a safe range (see Dialysis). The relationship between the fiber cross section and the blood cells is shown in Figure 5. In many industrial appUcations, where the bore fluid is dialy2ed under elevated pressure (>200 kPa or 2 atm), fibers may burst at points of imperfection. Failure of this nature is especially likely for asymmetric fibers that display a large number of macro voids within the walls. 

The longer the vessel, the more the blood comes into contact with the vessel wall and the greater the resistance is. However, vessel length in the body remains constant. Therefore, as with blood viscosity, it is not a variable factor causing changes in resistance. 

Ohm s law may be rewritten to include the three factors that affect vascular resistance blood viscosity (q), vessel length (L), and vessel radius (r). The following equation is known as Poiseuille s law ... 

Osterode W, Holler C, Ulberth F (1996) Nutritional antioxidants, red cell membrane fluidity and blood viscosity in type 1 (insulin dependent) diabetes mellitus. Diabet Med 13(12) 1044-1050... 

Risks associated with infusion of blood products include transfusion-related reactions, virus transmission (rare), hypocalcemia resulting from added citrate, elevations in serum potassium and phosphorus concentrations from use of stored blood that has hemolyzed, increased blood viscosity from supranormal hematocrit elevations, and hypothermia from failure to appropriately warm solutions before administration. 

Venous stasis is slowed blood flow in the deep veins of the legs resulting from damage to venous valves, vessel obstruction, prolonged periods of immobility, or increased blood viscosity. Conditions associated with venous stasis include major medical illness (e.g., heart failure, myocardial infarction), major surgery, paralysis (e.g., stroke, spinal cord injury), polycythemia vera, obesity, or varicose veins. 

Polymerization allows deoxygenated hemoglobin to exist as a semisolid gel that protrudes into the cell membrane, distorting RBCs into sickle shapes. Sickle-shaped RBCs increase blood viscosity and encourage sludging in the capillaries and small vessels. Such obstructive events lead to local tissue hypoxia and accentuate the pathologic process. 

Shand et al. (2002) have shown that, compared with placebo-treated subjects, long-term raloxifene treatment in postmenopausal women, at a dose of either 60 or 120 mg/d, was not associated with adverse changes in hemorheological factors (determinants of blood viscosity) that may contribute to venous thromboembolism. 

Massive use of diuretics entails a hazard of adverse effects (A) (1) the decrease in blood volume can lead to hypotension and collapse (2) blood viscosity rises due to the increase in eryth-ro- and thrombocyte concentration, bringing an increased risk of intravascular coagulation or thrombosis. 

Therapy is directed at reducing whole blood viscosity where this is signiflcantly raised. Small volume venesections, in which 250 ml as opposed to 500 ml of whole blood, are carried out at 2- or 3-week intervals. It should be noted that studies have demonstrated impairment of cerebral blood flow and a shortened survival in these individuals so that such intervention is appropriate in the severe cases. 

Ginkgo leaf extract appears to act primarily as a mild cerebral vasodilator that increases cerebral blood flow and reduces blood viscosity. Ginkgolides inhibit platelet activating factor, and this may improve microcircula-tory blood flow in atherosclerotic disease with slightly increased risk of bleeding. There appears to be an antioxidant effect that may be neuroprotective. Although some studies suggested a monoamine oxidase inhibitor (MAOI) effect, this is considered to have questionable clinical relevance. 

Mechanism of Action A blood viscosity-reducing agent that alters the flexibility of RBCs inhibits production of tumor necrosis factor, neutrophil activation, and platelet aggregation. Therapeutic Effect Reduces blood viscosity and improves blood flow. Pharmacokinetics Well absorbed after oral administration. Undergoes first-pass metabolism in the liver. Primarily excreted in urine. Unknown if removed by hemodialysis. Half-life 24-48 min metabolite, 60-90 min. 

They are most commonly used plasma expanders. It is polysaccharide isolated from beet sugar which is formed by the action of Leuconstec mesenteroides. It is available in mainly two forms depending upon the molecular weight. Dextran 70 (mol. wt. 70,000) available in 6% solution and Dextran 40 (mol. wt. 40,000) available in 10% solution. They are infused intravenously in the treatment of shock. Dextran 40 acts more rapidly than dextran 70. It decreases the blood viscosity and prevents the sludging of RBC s. Dextran 70 remains in circulation for longer period (upto 24 hrs) and is slowly excreted by glomerular filtration. 

Methylxanthines decrease blood viscosity and may improve blood flow under certain conditions. The mechanism of this action is not well defined, but the effect is exploited in the treatment of intermittent claudication with pentoxifylline, a dimethylxanthine agent. However, no evidence suggests that this therapy is superior to exercise conditioning. 

In animal models and some human studies, ginkgo has been shown to increase blood flow, reduce blood viscosity, and promote vasodilation, thus enhancing tissue perfusion. Enhancement of endogenous nitric oxide (see Chapter 19) and antagonism of platelet-activating factor may be involved. 

The first reason is theoretical. The demonstrated effects of the new substances and combinations on lipids and carbohydrates do not have any major relevance to the thromboembolic process. The latter is linked primarily to changes in the hemostatic system and blood coagulation, involving platelet aggregation, coagulation factors, fibrinogen concentrations, and blood viscosity. 

An antihypertensive substance is one which does not affect blood volume, blood viscosity, or the function of the heart it lowers blood pressure to normal levels in hypertensive states by generalized arteriolar dilatation, including those of the kidneys. Vasodilators (such as histamine), which lower arterial pressure at the expense of renal blood flow, are not antihypertensive. Furthermore, an ideal substance should affect the blood pressure in normal states to little or no extent. It can be predicted, however, that when true antihypertensive substances are found, they will not increase or maintain renal blood flow in the face of lowered arterial pressure when renal arterioles have lost the ability to dilate because of pathological changes (see Figure 3). 

Mechanism of action that provides symptom relief with pentoxifylline is poorly understood but is thought to involve red blood cell deformability as well as a reduction in fibrinogen concentration, platelet adhesiveness and whole blood viscosity (75). The recommended dose of pentoxifylline is 400 mg three times daily with meals. Pentoxifylline causes a marginal but statistically significant improvement in pain-free and maximal walking distance (a net benefit of 44 m in the maximal distance walked on a treadmill (95% Cl, 0 14 to 0 74) based on meta-analyses of randomized, placebo-controlled, double-blind clinical trials (76). At the same time pentoxifylline does not increase the ABI at rest or after exercise (56). Pentoxifylline may be used to treat patients with intermittent claudication however, it is likely to be of marginal clinical importance (56,77). Medical therapies... 

There are various factors that can influence the distribution of analytes in a dried blood spot. Water-soluble chemicals uniformly coated on DBS cards would redistribute when the blood was spotted. The redistribution of chemicals may depend on their properties, viscosity of blood, the volume spotted, and the technique used for spotting. Another factor is the viscosity of the blood. Viscosity is normally dependent on the blood composition (hematocrit, protein, lipid levels), and it can affect the physical spread of the blood spot in that the same volume of a less viscous blood will form a larger diameter spot than that of a more viscous blood sample. Viscosity, combined with the chemical redistribution on the sample cards, will increase the complexity of the analyte distribution. 

Erythropoietin [ery throw PO eetin] is a glycoprotein, normally made by the kidney, that regulates red cell proliferation and differentiation in bone marrow. Human erythropoietin, produced by recombinant DNA technology, is effective in the treatment of anemia caused by end-stage renal disease, anemia associated with HIV-infected patients, and anemia in some cancer patients. Supplementation with iron may be required to assure an adequate response. The protein is usually administered intravenously in renal dialysis patients, but in others the subcutaneous route is preferred. Side effects such as iron deficiency and an elevation in blood pressure occur. [Note The latter may be due to increases in peripheral vascular resistance and/or blood viscosity.]... 

Sweeney JD, Labuzzetta JW, Michielson CE, Fitzpatrick JE (1989) Whole blood aggregation using impedance and particle counter methods. Am J Clin Pathol 92 794-797 Turitto VT (1982) Blood viscosity, mass transport, and throm-bogenesis. Prog Hemost Thromb 6 139-177... 

Blood viscosity rises owing to the increase in erythrocyte and thrombocyte concentrations, bringing an increased risk of intravascular coagulation or thrombosis. 

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