Pharmacology for Anaesthesia and Intensive Care

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20 Intravenous fluids

molecules. The weight average is a more complicated calculation, usually larger and

usually quoted by the manufacturer.

Kinetics

Morethan 95% of infused gelatin is excreted unchanged in the urine. Unlike hydrox-

yethyl starches, gelatins are not stored in the reticulo-endotheial system. Their vol-

ume of dristribution varies greatly (minimum 120 ml.kg–1)depending on the nature

of the capillaries that alter in states of sepsis.

Side effects

The only signficant side effect is anaphylactic and anaphlactoid reactions, which

occur in approximately 1 in 10 000 administrations.

The evidence on disturbances of coagulation is inconclusive.

Hydroxyethyl starches

Hydroxyethyl starch (HES) is a derivative of amylopectin that is a highly branched

starch compound. When anhydroglucose residues are substituted with hydroxyethyl

groups two important changes occur: first, increased solubility and second, reduced

metabolism, which increases its half-life.

Formulation

There are many more preparations of HES than there are of gelatins. The types

of preparation are based around concentration, molecular weights, the degree of

substitution as described above and the C2:C6 ratio, which reflects the position of

the substitution.

Kinetics

The high molecular weight HES solutions provide the longest plasma-volume-

expanding effect but are also associated with unwanted effects. The low molecu-

lar weight HES solutions only have very limited duration of action, whereas the

medium molecular weight HES solutions provide the best balance. However,

high molecular weight HES solutions are now available in balanced electrolyte

formulations, which are reported to reduce side effects.

Side effects


Coagulation–the effects on coagulation are variable and depend on the molec-

ular weight and degree of substitution of the HES. High molecular weight HES

and/or those with a high degree of substitution may cause a type I von Willebrand-

like syndrome and post-operative bleeding. Balanced electrolye preperations may

eliminate these side effects.


Renal–HES may be involved in the generation of renal tubular swelling as a

result of reabsorption of macromolecules, which may lead to tubular obstruction,