This is a common cause of angst for final years approaching foundation training.
Essentially - Colloids (starch expanders designed to be 'like blood') vs Crystalloids are the two categories.
Colloids have gone out of fashion somewhat and so most requisites for doctors are to prescribe suitable crystalloid regimes. (Think of it as when you boil a bag of saline down you'd be left with 'crystals' of the salts!)
Colloids have an unfortunate proclivity for causing anaphylactoid reactions, they also 'don't really do what blood does' and so are often relegated to specific emergency situations.
If you are EVER prescribing colloids - think 'why am I doing this - is the patient very very hypotensive? Do they need something ELSE like ICU?' and ALWAYS consult a senior doctor - we do NOT use them routinely. You might even find they are not stocked on your ward. Enough said... multiple trials have found there were worse outcomes with colloid... but some more recent ones have found little to no difference in ICU patients. (see: Systematic Review and JAMA Trial RCT)
Luckily for us - there are good guidelines from NICE about Maintenance Fluid Replacement IV in Medical Patients which ought to be followed. I really think it should be memorised - it's great for exams and in real life scenarios. This was published in 2013 so it's recent. See here: Entire Guidance (NICE)
You should know about... Hartmann's Solution, 0.9% Saline, 5% Dextrose (all the sugar in it is often metabolised first pass - 5g of glucose/100ml).
You must also be aware of potassium addition to solutions.
You must DEFINITELY be aware of hyponatraemia - Patients who cannot meet their daily needs of fluids and electrolytes through oral or enteral routes but are otherwise euvolaemic often need IV fluid therapy for maintenance. The most common complications of this therapy are hyponatraemia (if excessive IV water is administered), volume overload (if excessive sodium and water are administered) and volume depletion and/or acute kidney injury (if inadequate sodium and water are administered). There are no published trials considering what the optimal IV fluid regimen for maintenance is.
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