7.3 Colloids

The Mw determines the viscosity and Mn indicates the oncotic pressure. Albumin is said to be monodisperse because all molecules have the same molecular weight (so Mw = Mn). Articial colloids are all polydisperse with molecules of a range of molecular weights.

The properties of an ideal colloid solution for use as a plasma volume expander are outlined in the table. An oncotic pressure similar to plasma will permit replacement of plasma volume without distribution to other fluid compartments and this is the key element that makes a solution a colloid solution.

Table 7.3 The Properties of an Ideal Colloid

General

• Distributed to intravascular compartment only
• Readily available
• Long shelf life
• Inexpensive
• No special storage or infusion requirements
• No special limitations on volume that can be infused
• No interference with blood grouping or cross-matching
• Acceptable to all patients & no religious objections to its use

Physical Properties

• Iso-oncotic with plasma
• Isotonic
• Low viscosity
• Contamination easy to detect

Pharmacokinetic Properties

• Half-life should be 6 to 12 hours
• Should be metabolised or excreted & not stored in the body

Non-Toxic & No Adverse Affect on Body Systems

• No interference with organ function even with repeated administration
• Non-pyrogenic, non-allergenic & non-antigenic
• No interference with haemostasis or coagulation
• Not cause agglutination or damage blood cells
• No affect on immune function including resistance to infection
• No affect on haemopoiesis
• Not cause acid-base disorders
• Not cause or promote infection (bacterial, viral or protozoal)

The dextrans cause more severe anaphylactic reactions than the gelatins or the starches. The reactions are due to dextran reactive antibodies which trigger the release of vasoactive mediators. Incidence of reactions can be reduced by pretreatment with a hapten (Dextran 1).

Dextran 70 has a duration of action of 6 to 8 hours. Interference with crossmatching occurs so the laboratory should be notified that dextrans have been used. Dextran interferes with haemostasis; it induces an acquired von Willebrand’s state. Consequently, there is a maximal dosage recommendation of 20 mls/kg (about 1,500 mls in an adult).

Dextran40 is used to improve microcirculatory flow in association with certain procedures (eg microsurgical reimplantations).

Gelatin is the name given to the proteins formed when the connective tissues of animals are boiled. They have the property of dissolving in hot water and forming a jelly when cooled. Gelatin is thus a large molecular weight protein formed from hydrolysis of collagen.

Gelatin solutions were first used as colloids in man in 1915. The early solutions had a high molecular weight (about 100,000). This had the advantage of a significant oncotic effect but the disadvantages of a high viscosity and a tendency to gel and solidify if stored at low temperatures. Reducing the molecular weight reduced the tendency to gel but smaller molecular weight molecules could not exert a significant oncotic effect. The need was for a modified gelation product that had a moderate molecular weight (for oncotic pressure) but a low gel melting point. (It is difficult to infuse a jelly).

Several modified gelatin products are now available; they have been collectively called the New-generations Gelatins. There are 3 types of gelatin solutions currently in use in the world:

Polygeline (‘Haemaccel’ Hoechst) is available in Australia. The gelatin is produced by the action of alkali and then boiling water (thermal degradation) on collagen from cattle bones. The resultant polypeptides (MW 12,000 - 15,000 ) are urea-crosslinked using hexamethyl di-isocyanate. The branching of the molecules lowers the gel melting point. The MW ranges from 5,000 to 50,000 with a weight-average MW of 35,000 and a number-average MW of 24,500.

Properties
Polygeline is supplied as a 3.5% solution of ‘degraded gelatin polypeptides cross-linked via urea bridges’ with electrolytes (Na+ 145, K+ 5.1, Ca++ 6.25 & Cl- 145 mmol/l). It is sterile, pyrogen free, contains no preservatives and has a recommended shelf-life of 3 years when stored at temperatures less than 30C.

Handling by the Body
It is rapidly excreted by the kidney. Following infusion, its peak plasma concentration falls by half in 2.5 hours. Distribution (as a percent of total dose administered) by 24 hours is 71% in the urine, 16% extravascular and 13% in plasma The amount metabolised is low: perhaps 3%.

Indications
The major use of Polygeline is the replacement of intravascular volume eg correcting hypovolaemia due to acute blood loss. It is also used in priming heart-lung machines.