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Module 3 : Prescribing





As well as water, it is also important to provide daily replacements of the electrolytes lost.

Probably the two most important electrolytes to consider when prescribing maintenance fluids are:


1. Sodium

2. Potassium


Relatively large amounts of sodium are lost in urine each day and so sodium is present in some concentration in almost all crystalloid fluids. Traditionally, potassium was added to each bag as required in the form of concentrated KCL. However, in order to minimise errors  during administration it is now more common to find bags of crystalloid with potassium pre-added.





Potassium is now routinely added to many crystalloid fluids. This picture shows one litre of glucose solution with 20 mmol added potassium


Remember, potassium over- (or under-) dose can be very dangerous, if you are in any doubt about a patient's potassium ensure you take regular blood or ABG samples, and check the patient’s ECG for signs of potassium excess or deficiency.


There is a maximum concentration of potassium that is safe to infuse intravenously, at at a maximum rate. Please see local trust policy and guidelines about this.



  Test Yourself !  
  What are the ECG signs of potassium excess?

Write a list and then check your answer below.

How did you do?

There can be many different changes. Here are the main ones you should know about:



Flattened P-waves

Prolonged PR interval, , flattened P-waves.

Wide QRS

ST segment sloped

Tall T waves ('tented')





ECG showing lead II for a patient with hyperkalaemic ECG changes. Notice the markedly tented T-waves and broad QRS complexes of unusual shape.






Healthy people need approximately 1-2mmol/kg/day of sodium per day, and 1mmol/kg/day of potassium. As bags of crystalloid contain fixed amounts of these it may be difficult or impractical to exactly match a patient's requirements. This is unlikely to be a problem in the short term, but patients receiving fluids for any prolonged period of time should have their bloods checked daily for derangement.


Usually, patients will lose electrolytes through urine and insensible losses, and these can be fairly accurately determined. However, some patients may have higher than normal losses and it is particularly important to monitor electrolytes in these patients. In particular:


Vomiting and diarrhoea cause excess potassium loss

Burns, fistulae and surgical drains cause excess sodium loss



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