Colloids

Question 1

What is a colloid?

Answer 1

A fluid containing a large molecule which exerts an oncotic pressure across the capillary membrane. These molecules are suspended in a crystalloid solution.

Question 2

What colloids are you aware of?

What are their advantages and disadvantages?

Answer 2

Natural:
1. Blood products (e.g. RBC)
+ can correct other abnormalities, best in haemorrhage
- Can have cell/antigen mediated reactions/ infection transmission
2. Albumin
+ Naturally occurring, can replace deficiency
- Pooled from multiple patients (potential infection transmission)

Synthetic:
1. Gelatins (35kDa)
\+ Long shelf life
- rapidly excreted by kidney
- anaphylaxis potential
2. Hydroxyethyl starches
\+ Longer plasma half-life than other gelatins
- increased incidence of renal injury

Question 3

What is albumin?

Answer 3

• A polypeptide with MW of 69kDa.
• Highly negatively charged
• Repelled by negatively charged glycocalyx of the endothelium.
• Extends its intravascular half life to 5-10 days.

Question 4

In what form do we use albumin?

Answer 4

• Human albumin solution
• From plasma, serum, placenta
• Pooled from thousands of donors (potential CJD risk)
• 4.5% and 20% preparations

Question 5

Where is albumin produced?

Answer 5

• Liver: dependent upon neuroendocrine influences and plasma oncotic pressure.
• It is released into the plasma on production and is the predominant plasma protein.
• It is an acute phase protein and synthesis is suppressed in critical illness/physiological stress.

Question 6

What are the physiological functions of native albumin?

Answer 6

1. Transport molecule for:
   - cations
   - hormones (e.g. T4), steroids
   - unconjugated bilirubin
   - bile salts
   - acidic drugs (barbiturates, warfarin, NSAIDs)
2. Maintenance of oncotic pressure:
   - contributes 80% of colloid oncotic pressure.
3. Acid-base balance:
   - acts as a buffer

Question 7

When do we use albumin in critical care?

Answer 7

1. Fluid resuscitation: Surviving sepsis recommends it following crystalloid (30ml/kg) have been given
2. Management of hepato-renal syndrome.
3. Large volume paracentesis.
4. Replacement fluid in plasmapheresis.

Question 8

What evidence is there regarding the use of albumin in critical care patients?

Answer 8

1. SAFE trial (NEJM 2004):
   - Australian RCT
   - 4% HAS vs saline ICU fluid resus.
   - Mortality equivalent
   - TBI subgroup had significantly higher mortality
2. ALBIOS trial (NEJM 2014):
   - Italian RCT
   - 20% HAS vs crystalloid
   - mortality equivalent

Question 9

What are the disadvantages of using albumin?

Answer 9

1. Worse outcomes at 28 days in TBI
2. More expensive than crystalloids
3. May worsen third-space loss in glycocalyx compromise in sepsis/trauma.
4. Theoretical risk of CJD transmission

Question 10

By what mechanism may albumin leak into the extravascular space?

Answer 10

1. Basal membrane impairment in gap junctions caused by pro-inflammatory cytokines.
2. Glycocalyx compromise in sepsis/trauma.