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Module 3 : Prescribing

 

 

Colloid

 

Colloid fluids are so named because they all contain large chemicals such as proteins which are incapable of passing across  a semi-permeable membrane. Because of this, they increase the colloid oncotic pressure of the intravascular fluid (blood). See Physiology : Fluid compartments in disease states to revise this.

 

 

  Test Yourself !  
   
  If you give a patient a litre of a colloid fluid, how would you expect the water content to distribute in the first 15 mins after giving it?

That's correct, well done!
 

Mostly in the intravascular space – This is the correct answer. As the water content will be drawn to an area of high oncotic pressure by osmosis, water given in a colloid solution will stay in the intra-vascular space as the colloid cannot cross the semi-permeable membrane which is the capillary vessel walls. All colloids have this common important property: they are all ‘plasma expanders’ (ie they increase the intravascular volume). They won't stay in the blood forever though, depending on the stability of the colloid it will be metabolised and excreted over time.

 

Mostly in the interstitial space – Incorrect answer. Most of the colloid will not cross the capillary membrane to this area.

 

Evenly across the extracellular space - Incorrect answer. Most of the colloid will not cross the capillary membrane to this area.

 

Mostly in the intracellular space - Incorrect answer. The colloid cannot cross wither the capillary membrane or the cell wall, and therefore the fluid will not distribute here.

 

Equally across all the fluid compartments - Incorrect answer. Only ‘pure’ water (or a chemical which is metabolised to become ‘pure water’) would distribute evenly across all the fluid compartments.

 

Sorry, that's incorrect!
 

Mostly in the intravascular space – This is the correct answer. As the water content will be drawn to an area of high oncotic pressure by osmosis, water given in a colloid solution will stay in the intra-vascular space as the colloid cannot cross the semi-permeable membrane which is the capillary vessel walls. All colloids have this common important property: they are all ‘plasma expanders’ (ie they increase the intravascular volume). They won't stay in the blood forever though, depending on the stability of the colloid it will be metabolised and excreted over time.

 

Mostly in the interstitial space – Incorrect answer. Most of the colloid will not cross the capillary membrane to this area.

 

Evenly across the extracellular space - Incorrect answer. Most of the colloid will not cross the capillary membrane to this area.

 

Mostly in the intracellular space - Incorrect answer. The colloid cannot cross wither the capillary membrane or the cell wall, and therefore the fluid will not distribute here.

 

Equally across all the fluid compartments - Incorrect answer. Only ‘pure’ water (or a chemical which is metabolised to become ‘pure water’) would distribute evenly across all the fluid compartments.

 

 

 

 

The type of colloid you would chose to prescribe in any given situation may depend on a number of properties. Below is a table of some commonly prescribed colloids and their uses:

 

 

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Colloids are less frequently prescribed than crystalloids, and they tend to have quite specific uses.

Human protein solutions are usually prescribed due to the specific proteins they contain. For example, platelets may be given to a thrombocytopaenic patient to increase their platelet count, or human albumin solution may be given to patients with liver failure who are unable to produce their own albumin affectively.

 

Synthetic colloids are used almost exclusively for their action as a volume expander for patients who are intravascularly depleted.In practice however, it is unclear as to whether synthetic colloids are any better than crystalloid for volume expansion, in that in disease states, the vascular compartment may become ‘leaky’ to colloids, and colloids are known to have other adverse reactions e.g. allergy. This is still the topic of much research.

 

Often, a colloid may be selected for more than one of its properties. For example, blood may be appropriate for a patient who has experienced a large haemorrhage as it will both replace lost haemoglobin and act as a volume expander. More specific indications for some commonly prescribed colloids will be discussed later in this module.

 

 

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