As an FY1 you will frequently see hypokalaemia and most trusts have guidelines that should be used in the first instance. The advice below is informal & based on broad day to day practice. It should never replace clinical judgement and escalation for senior support if indicated.

The UK Medicines Information group have provided information on how best to replace potassium orally or IV depending on the severity of hypokalaemia.

Definition:
Each lab has their own ranges. Generally, hypokalaemia exists if the K is less than 3.5.

Causes:
Remember that most potassium is intracellular and the extracellular potassium is very tightly controlled. The maintenance IV requirements of sodium and potassium are the same (1mmol/kg) (see Prescribing Fluids). Giving too little potassium in IV fluids is a common cause of hypokalaemia. Overall the most common cause is GI losses.
  • GI Losses: Vomiting, diarrhoea, malabsorption, high output stoma, laxative abuse, poor oral intake/TPN or feed inadequate
  • Renal Losses: Genetic syndromes (Gitelman Bartter Liddle), mineralocorticoid excess eg Conn’s or Cushing’s, Renal tubular acidosis Type I-III, Diuretics
  • Intercellular shift: alkalosis, insulin, adrenaline, beta2 agonists
Assessment: look for any of the causes above. The usual cause is GI and renal. If the patient is on digoxin and hypokalaemic, consider stopping digoxin to avoid precipitating digoxin toxicity.

Treatment
This depends really on the level of hypokalaemia.

Oral Replacement:
This is in the form of Sando-K which is an effervescent potassium tablet. This can be given in a range of doses: from one tablet once a day, to up to two tablets three times a day. The dose depends on how profoundly hypokalaemic the patient is. Always put a stop date of 2-3 days for oral potassium replacement so the patient does not remain on it indefinitely. Here is a possible prescribing plan:

K 3.4: one tablet twice a day.
K 3.3: two tablets twice a day.
K 3.2: two tablets three times per day.

IV Replacement:
Once the K starts reaching 3.1 or even lower, you really want to think about IV replacement.
Remember the maintenance requirements for potassium are 1 mmol/kg/day as above which will maintain (keep the value static) not replace. Therefore giving 60 mmol potassium to a 60 kg patient with a potassium of 2.9 may not be sufficient to increase it (depending on ongoing losses & how well the kidneys conserve it). That said if the patient is eating & drinking they will get their maintenance requirements through food so any IV preparations will supplement.

With this in mind, you really want to think about giving 40 mmol or more. IV potassium comes in pre-prepared preparations for nurses in 0.9% sodium chloride or dextrose - either 10, 20 or 40 mmol of potassium in 1L. You cannot add more potassium to other types of fluids eg Hartmann’s or Plasma-Lyte - which both only contain 5 mmol.

If the patient has profound hypokalaemia (e.g. less than 3) you might want to give oral (at max dose as above) & IV replacement. Some might not tolerate Sando-K as it can taste awful and in these patients you might consider IV replacement and depending on how low their potassium is you might wish to give lower amounts (i.e. less than 40 mmol) e.g. 0.9% sodium chloride with 20 mmol potassium.

How quickly should you give IV potassium? 
The typical prescription is 40mmol in 1L of 0.9% sodium chloride or glucose over 8 hours. The maximum rate at which you can give potassium on a ward is 10mmol/hour and if you need to give it faster than that, you will need cardiac monitoring and a central line (i.e. HDU environment). Therefore if the patient requires fluid resuscitation & potassium replacement, consider administering fluid resuscitation in parallel to the bag containing potassium i.e. 500 ml 0.9% sodium chloride bolus at the same time as an 8 hour bag of IV 40mmol KCl in 1L 0.9% sodium chloride.

If the potassium is below 3 mmol or you're concerned you need to give the infusion quicker than 10mmol/hour - involve your seniors so they can facilitate discussions with HDU & ITU.

Further Reading
https://www.uptodate.com/contents/evaluation-of-the-adult-patient-with-hypokalemia

Dr Shedeh Javadzadeh
CT1 in Medicine