Colloids

Definition: colloids are substances that do not form true solutions and do not pass through semipermeable membranes. Colloid infusions increase osmotic pressure, draining water out of the interstitial space into the intravascular compartment. They stay in the intravascular compartment for varying lengths of time depending on their molecular weight.

Different colloids have very different properties. The most commonly used colloids are:

Table 5.5 Contents of common crystalloids in mmol litre 1 (mg dl

Crystalloids

Na;

K+

Ca 2

CI

HCO;

Osmolality

pH

Other

Plasma

140

4

2-3 (9-2)

100

26

285-295

7-0

Sodium chloride 0-9%

154

0

0

154

0

308

5-0

Sodium chloride 0-45%

77

0

0

77

0

154

5-0

Sodium chloride 3%

513

0

0

513

0

1026

5-0

Dextrose 5%

0

0

0

0

0

252

4-0

Dextrose 50 g I-1

Dextrose 4% saline 0-18%

30

0

0

0

30

255

4-0

Dextrose 40 g I-1

Hartmann's

131

5

2 (8)

111

29

278

6-5

29 lactate

Ringer's lactate

147

4

2-2 (8-8)

156

0

302

28 lactate

Sodium bicarbonate 1-26%

150

0

0

0

150

300

8

Sodium bicarbonate 8-4%

1000

0

0

0

1000

2000

8

• Gelatines (for example, Gelofusine and Haemaccel) - gelatine is a degradation product of animal collagen and is inexpensive and readily available. Different brands vary in electrolyte content. The calcium content of Haemaccel means that it may form clots if mixed with stored blood (which contains citrate) in the same intravenous line.

• Dextrans - these are glucose polymers available in different molecular weights (for example, Dextran 70 = 70 000 Daltons). These are rarely used because of side effects.

• Hydroxyethyl starch (HES) - this is derived from amylopectin, a plant polymer and contains no electrolytes. Unmodified starch is unsuitable as a plasma substitute since it is broken down rapidly by amylase. The hydroxyethylation of starch protects the polymer against breakdown. Different products with different mean molecular weights exist. Larger particles have a higher degree of protection from metabolism and give a more prolonged effect (days). HES is therefore useful in patients with capillary leak. Some research suggests that HES reduces capillary leak by an unknown mechanism. It cannot be used alone if greater than 30% plasma volume replacement is needed - water should be given as well, otherwise an osmotic nephrosis and acute renal failure can occur.

• Human albumin solution (HAS) - albumin is the fraction of plasma that provides the main part of the circulation's osmotic pressure and has therefore been used as a plasma substitute. It is derived from human plasma and is heat sterilised so it is virtually disease-free; 4.5% HAS reflects normal plasma; 20% HAS has water and salt removed -hence it is sometimes called "salt-poor albumin". HAS was mainly used to replace fluid losses in burns where high albumin loss is also a problem. The major limitations to the use of HAS are high production costs and limited supplies. Albumin has a number of other functions which include:

• transport of molecules

• free radical scavenging

• binding of toxins

• inhibition of platelet aggregation.

Colloids are either monodispersed like albumin if the molecular size and weight are uniform throughout the product, or polydispersed if there are a variety of different molecular sizes and shapes like starches. The molecular weight determines the retention time and duration of colloidal effect in the circulation. Lower molecular weight particles have a higher osmotic effect but are rapidly excreted by the kidneys in contrast to larger particles.

Most colloid solutions are subject to maximum limits suggested by the manufacturers on the basis of available research data. Most UK colloids are suspended in sodium chloride but colloids in 5% dextrose are available in the USA.

The retention of both crystalloids and colloids in the circulation is variable. In dehydration, retention time may be increased but, in critical illness with its associated capillary leak, retention times are shorter. Generally, crystalloids stay in the circulation for less than an hour. Gelatines stay in the circulation for a few hours. Albumin has two half lives, the first being only 4 hours. Starch solutions have a half life of several days.

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