Prescribing potassium containing infusions?

Hi,

so I have the Prescribing exam coming up and I'm struggling to understand when K is given with NaCl as KCL 0.3% for example or when it's prescribed as mmol's e.g.
1000mls 0.9% NaCl with 10mmol of K over 4 hours, for example?

I really struggle with fluids so would appreciate anyone explaining this to me in dummy style :P

As you are probably aware, replacement fluids should be 1 mmol/kg/day Na, K, Cl; 25-30 ml/kg/day water and 50-100 g/day glucose.
There are premade forms of 0.9% saline, 5% dextrose, etc. that have 20 mmol or 40 mmol already added. 0.3% KCl contains 40 mmol/L K, 0.15% contains 20 mmol/L, however, the bag is usually written as 20 or 40 mmol rather than 0.3%.

An average 70 kg man will need 1750-2100 ml water; 70 mmol Na, K, Cl; and 50-100 g glucose. Therefore, a fluid regimen may be 1 bag 1L 5% dextrose with 40 mmol K over 8 hours; 1 bag 500 ml 0.9% NaCl with 20 mmol K over 8 hours; 1 bag 500 ml 5% dextrose with 20 mmol K over 8 hours.
In total, the patient would have received 2L water, 77 mmol Na, 80 mmol K and 100g glucose, in line with the daily requirements. However, if the patient has a K deficit then they will need more. If a patient has low K, you may want to give 40 mmol K over 4 hours (10 mmol/hour is the maximum rate of K infusion without ECG monitoring). Realistically you will be checking U&Es regularly and adjusting accordingly.

By the way, I am also a medical student so don't take what I say as gospel, there are plenty of doctors on tsr who will have a better idea, but I think I have a relatively good grasp of fluids.

Incredibly helpful, thank you! Do u have any resources that you used to understand?