Quick Links :



Module 3 : Prescribing





Crystalloid fluids all contain chemicals of a simple ionic or crystalline structure, which are capable of passing across a semi-permeable membrane. By osmosis, the water content of these fluids follows this crystalline component.


Because crystalloid fluids are able to cross a semi-permeable membrane, they are able to freely move from the intravascular to the extravascular compartment. Depending on their exact composition, they are also able to enter the intracellular compartment to a greater or lesser extent.


Below is a table outlining properties of three of the most commonly prescribed crystalloids, although it is important to realise that there are a great number of crystalloid products available.






Crystalloid fluids are commonly used to provide maintenance water and electrolytes to patients who are dehydrated or nil by mouth. They are well suited to this task as their constituents can move freely from the intravascular compartment into the rest of the body.


Glucose solutions


These solutions are particularly useful for providing maintenance or rehydrating patients, as they distribute quickly across all fluid compartments in the way that pure water would. At least two-thirds will end up in the intracellular space, helping to hydrate the patient. However, they have the disadvantage of containing no electrolytes and so will decrease the patient's blood osmolality by dilution.



  Test Yourself !  
  If we want to rehydrate someone, why don't we just give IV water instead of glucose solutions? Think about it and click the button to reveal the answer.

How did you do?

It is very dangerous to give pure water intravenously, without any additives to increase its tonicity. There is a risk of erythrocyte lysis as the water crosses into these cells by osmosis. Large volumes would also be likely to dilute the electrolyte content of the blood, although electrolyte dyscrasias may occur if large volumes of any fluid are given IV. Interestingly, glucose is actually more efficient at rehydrating the intracellular compartment than pure water alone would be. This is because glucose is metabolised in the cells by the respiration reaction to become carbon dioxide and water. For each molecule of glucose dissolved in solution the cell will generate 6 molecules of water!







Note: You may hear some clinicians talking about ‘dextrose’ solutions. This is the old name for glucose solutions, and is not to be confused with dextrans (a type of colloid). The name has now been superseded and you will no longer find dextrose in the ward’s stockroom.


Saline solutions


Saline contains the important blood electrolyte sodium, of which we need approximately 1-2mmol/day (see Physiology : Electrolytes in health). However, it distributes less evenly across the fluid compartments of the body, and stays mostly in the extravascular space.



  Test Yourself !  
  Theoretically, sodium chloride solution should be able to cross the semi-permeable cell membrane into the intracellular space. However in practice much more of this fluid is distributed to the extracellular space compared to glucose solutions. Think about why this is, then click the button to reveal the answer.

How did you do?

Saline will distribute across the extracellular space in the normal proportions of the fluid compartment model (see Physiology : Fluid compartments to revise this). This is because the small sodium chloride and water molecules can move freely across semipermeable membranes and because its tonicity is the same as that of the extracellular fluid. However, for the most part the fluid does not enter the cells because a sodium gradient is established by ion-pumps in the cell membrane. These pumps actively transport sodium out of the cells in exchange for potassium. As the sodium is therefore effectively blocked from entering the cells, the water is unable to follow. Hence the fluid distributes only in the extracellular compartment.





Sodium chloride 0.9% solution is often called ‘Normal Saline’. It is important to understand that this expression is used as it has an osmolality similar to that of extracellular fluid (it is ‘isotonic’). It does not contain the same electrolytes in the same concentrations as the extracellular fluid does. It contains a high concentration of sodium and chloride (154mmol/L of each, compared to the normal 135-145mmol/L and 95-105mmol/L respectively in the extracellular fluid), and none of the other vital electrolytes.


Although normal saline is a ‘staple’ maintenance fluid, and you are likely to see it prescribed often on the wards, it is important to be aware that it may cause hypernatraemia if used in excess. There is also increasing evidence that it may also lead to a potentially dangerous state of hyperchloraemic acidosis in susceptible patients. In many situations, physiological solutions are now being used as an alternative.


Physiological solutions


The ‘physiological’ fluids (Hartmanns  and Ringer’s being the most common), attempt to provide a greater range of electrolytes, in a composition more similar to that of plasma. They may be safer to use than saline and they are now becoming inexpensive. Like saline, these are ‘isotonic’ fluids and so distribute mostly across the extravascular space.


These fluids are used routinely to rehydrate patients in theatres, and are becoming more commonly used as maintenance fluids also.



Previous Next



This tab has icon in it.

Suspendisse blandit velit eget erat suscipit in malesuada odio venenatis.