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Module 3 : Prescribing



A Worked Example


Miss Imaginary is a well 60kg female who is nil by mouth and requires maintenance fluids for the next 24 hours.


Assuming this is her first day of maintenance fluids, and she is currently well hydrated with no unusual losses anticipated:


Required volume of water: approximately 3 litres

Requires sodium: 60 x 2 = 120 mmol sodium

Required potassium 60 x 1 = 60 mmol potassium


Possible regimens:




Typically, maintenance fluids are prescribed to be infused over a 24 hour period. This is to minimise the chance of harming a patient by infusing fluids to quickly (more information about this later in Prescribing : Benefits and risks), and to ensure that they stay well hydrated throughout the day. In the case of a 3L total to be infused, each 1 litre ‘bag’ might be set to run for a period of 8 hours.


Although both of the regimens suggested above are sensible and provide the patient with approximately her requirements for one day, you should note that neither provide exactly the correct electrolytes. It is very hard in practice to precisely calculate or meet a patient's fluid or electrolyte needs. However, it is important that the patient is constantly clinically and biochemically reassessed, to ensure your regimen is not causing harm.


Generally speaking, well patients such as in this worked example should not be kept nil by mouth for periods as long as 24hr e.g. in the lead up to elective surgery, as it is thought to slow recovery and may cause harm. However for the purposes of this simple example you can see how maintenance fluid and electrolyte requirements could be calculated.



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