Colloids

Question 1

Colloids (plasma substitutes): gelatins, albumin

Common indications!

Answer 1

• Colloids are used to expand circulating volume in states of circulatory compromise (including shock). However, we prefer compound sodium lactate or sodium chloride 0.9% for this indication.
• In cirrhotic liver disease, albumin is used to prevent effective hypovolaemia in large-volume paracentesis (ascitic fluid drainage).

Question 2

Mechanisms of action?

Answer 2

• A colloid is a solution containing a large, osmotically active molecule,such as albumin or modified gelatin. In principle, the large molecules cannot readily diffuse out of vessels, and their osmotic effect ‘holds’ the infused fluid in the plasma.
• For example, under experimental conditions, 70–80% of a gelatin-based fluid remains in the plasma. Their effect in expanding circulating volume is therefore potentially greater than that of a crystalloid (e.g. sodium chloride), of which around 20% remains in the plasma after distribution.
• In practice, however, most patients requiring volume expansion (e.g. in severe sepsis) have relatively ‘leaky’ capillaries, and it is likely that some of the gelatin is lost into the interstitium.

Question 3

Important adverse effects?

Answer 3

• Excessive administration of colloid fluids may cause a fall in cardiac output and precipitate cardiac failureby increasing left ventricular filling beyond the point of maximal contractility on the Starling curve.
• Most colloids contain a significant amount of sodium (e.g. 154 mmol/L in the case of Gelofusine®) and this may produce oedema. Gelatins may cause hypersensitivity reactions, including anaphylaxis – another reason to prefer crystalloids, which are non-allergenic.

Question 4

Practical prescribing?

Prescription?

Answer 4

• Colloids are prescribed in the ‘infusions’ section of the drug chart.
• Synthetic colloids are generally prescribed by brand name.
• You need to specify the volume to be infused and the rate at which it is to be given.
• The rate may be described either in mL per hour or as the intended duration for infusion of the total volume.
• For example, if deemed appropriate to use a colloid in circulatory compromise or shock,you might prescribe 250 mL of Gelofusine® to be given over 10 minutes (equivalent to 1500 mL/hr).
• In the context of large-volume paracentesis, you should consult with specialist colleagues regarding the need for albumin.
• A common regimen is to give 100 mL of HAS 20% for every 2 L of ascitic fluid drained.