Hemodilution

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Hemodilution is a deliberately induced thinning of the blood , whereby the volume of the blood plasma is increased relative to the proportion of red blood cells ( erythrocytes ) in particular . Among other things , this reduces the viscosity of the blood, which makes it thinner. The colloquial use of the term blood thinning for anticoagulation ( anticoagulation ) is not correct.

Effects

Flow properties

The flow properties of the blood (hemorheology) are influenced by blood thinning. It flows better because the hematocrit , plasma viscosity and the aggregation of blood cells are reduced. The blood thinning has a positive effect on the hemodynamics , since the pumping resistance for the heart decreases due to the lower plasma viscosity. The effect of the improvement in the flow properties exceeds the effect of the reduced number of erythrocytes, i.e. oxygen carriers, when the blood is slightly thinned.

Blood clotting

A side effect is that hemodilution also reduces the blood's ability to clot. It depends on the substance class: Gelatin preparations only reduce the concentration of coagulation factors and blood platelets (thrombocytes) by increasing their volume . Starch preparations, on the other hand, also contribute pharmacologically to reducing the coagulability by attaching themselves to the cell surface of the platelets (“coating”).

Osmotic effect

The colloidal solutions used for hemodilution are osmotically effective. These colloid solutions are hyperoncotic : the molecules dissolved in high concentration attract water molecules due to the large number of their polar endings, just like a high ion concentration . On the other hand, as polymers , they are so large that they do not or only very slowly diffuse through cell and blood vessel walls. This creates an increased colloid osmotic pressure and the solutions withdraw water from the surrounding tissue, which is used as an anti- edematous effect. The increase in volume (plasma expansion) of the blood also exceeds the supplied volume of liquid. Only the polysaccharides hydroxyethyl starch (HES) and dextran have this osmotic effect .

Applications

If the colloid solutions are infused in exchange for withdrawn blood, one speaks of isovolemic or normovolemic hemodilution (also: isovolemic or normovolemic hemodilution , NH).

If the solutions are infused without a connection with a blood withdrawal, one speaks of hypervolemic hemodilution (also: hypervolemic hemodilution , HH)

Blood circulation improvement

Isovoleme and hypervolume hemodilution are used therapeutically to improve microcirculation in circulatory disorders of the brain , retina and inner ear . Without the osmotic effect, the hypervolemia would bring with it the risk of edema formation; with their osmotic properties, the colloid infusions counteract the formation of edema or cause edema reduction, which is important in the treatment of recent strokes . With this use of hemodilution - which is only slight with the usual dosage - a reduction in blood coagulation is desired, since it counteracts the formation of thrombi on calcium plaques , for example .

Avoiding blood transfusions

In operations, which experience shows are often associated with great blood loss, transfusions from other donors' blood (allotransfusion) can be avoided or at least reduced by donating blood (autotransfusion). If the time interval between the autologous blood donation and the operation is short, a colloidal infusion solution is given while the blood is being taken . This acute normovolemic hemodilution (ANH) causes the patient to lose fewer red blood cells ( erythrocytes ) if they bleed during the operation . With this application of hemodilution, the reduction in the ability to clot is an undesirable but unavoidable side effect.

Compensation for blood loss

The same colloidal solutions are also used to prevent or treat hemorrhagic shock in the event of major blood loss . Here a normal blood volume (normovolemia) is restored in the event of accidents or maintained during operations. In the literal sense of the word, acute normovolemic hemodilution also occurs here . The reason for this use is on the one hand the time factor, i.e. the impossibility of immediately transfusing suitable blood in an unexpected emergency, and on the other hand the fact that donor blood is not available in unlimited quantities and the risk of immune reactions and infections cannot be completely ruled out. When compensating for an acute volume deficiency, plasma expanders are of course rapidly infused and at the same time combined with isotonic saline solutions, usually Ringer's solution , the water of which is kept in the blood vessel system by the osmotic effect of the plasma expanders. Even with this use of plasma expanders, the reduction in the coagulability is an undesirable but unavoidable side effect. In the event of extreme blood loss, the administration of fresh plasma (FFP) should therefore also be started in good time , which is stored frozen but brought to almost body temperature for infusion and is thawed in the process.

Individual evidence

  1. ^ A b Ernst Mutschler , Gerd Geisslinger, Heyo K. Kroemer , Peter Ruth, Monika Schäfer-Korting : drug effects. Textbook of pharmacology and toxicology. 9th, completely revised and expanded edition. Wissenschaftliche Verlagsgesellschaft mbH, Stuttgart 2008, ISBN 978-3-8047-1952-1 .