In this section : Electrolyte Disturbances
Hyperkalaemia
Hypomagnesaemia
Hypophosphataemia
Hypernatraemia
Hypokalaemia
Hyponatraemia
Hypocalcaemia
Hypercalcaemia
Hypokalaemia
Last updated 1st March 2021
Content created by Tina Grant
Introduction
- Although the physiology is complex and textbooks tend to emphasise the rare syndromes the cause of hypokalaemia is usually obvious from clinical assessment
Need to Know Physiology
- Normal dietary K intake 50-100 mmol per day
- 90% of body’s 3500mmol K is intracellular and <1% intravascular
- Na/K pumps on cell membranes pump K into cells in exchange for Na under influence of insulin, beta agonists, alkalosis and aldosterone
- 90% excretion of K occurs through kidneys and only 10% through colon unless patient has diarrhoea
- Nearly all filtered K is reabsorbed and nearly all urine K is secreted by cells in the cortical collecting duct: aldosterone and alkalosis both lead to loss of K by these cells
- Hypokalaemia Is defined as serum K <3.5 mmol/L
- Hypomagnesaemia causes renal potassium loss and often coexists with hypokalemia. Recommend check serum Mg if serum K <3mmol/l.
Common Causes of Hypokalaemia
- Diuretics except K sparing diuretics – renal K loss
- Vomiting – mechanism is by alkalosis leading to transcellular shift and renal K loss
- Diarrhoea – mechanism is by direct GI loss
- Acute illness – inadequate intake and by adrenaline response leading to transcellular shift
- Alcoholism – combination of poor diet, vomiting and coexistent magnesium depletion
- Refeeding syndrome – K and PO4 driven into cells when body switches back to CHO metabolism after a period of starvation
- NB Surreptitious vomiting and laxative abuse – neither of which may be volunteered
Uncommon Causes of Hypokalaemia
- Renal losses with normal BP – Bartter’s and Gitelman’s syndromes (both alkalotic), RTA (acidotic)
- Renal losses with hypertension – Conn’s syndrome (low renin, high aldo), renovascular (both high), Liddle’s syndrome (both low)
Consequences
- Cardiovascular – arrhythmias include sinus bradycardia, ectopic beats, paroxysmal atrial tachycardia, AV block, VT or VF
- Neuromuscular – muscle weakness, myalgia, cramps, paralysis, rhabdomyolysis.
- Gastrointestinal – constipation, ileus
- Renal – polyuria,
Investigations
- If cause is obvious eg diuretic, vomiting, diarrhoea – no further tests required.
- Measure BP
- Hypertension raises possibility of Conn’s Syndrome or renovascular disease. May then want to check renin and aldosterone
- Blood for U&E, bicarbonate, PO4 and Mg
- Bicarbonate – most cases will be alkalotic. Hypokalaemia with acidosis raises possibility of renal tubular acidosis
- Serum PO4 – often low in refeeding syndrome
- Serum Magnesium – often low in hypokalaemia, especially when due to alcohol.
- Urine for potassium:creatinine ratio (KCR). Send10 ml in plain tube to biochem). KCR >1.5 suggests renal K wasting. Best to do this before starting replacement
- ECG – typical changes when K <3mmol/L = flat T waves, ST depression and prominent U waves. Other changes include QT prolongation and arrhythmias
General Management Points
- Potassium replacement
- Potassium salts should preferably be given as effervescent tablets (Sando-K- 12mmol/tab), rather than modified release tablets (Slow K) which can cause oesophageal ulceration.
- Potassium preservation
- K sparing diuretics – Amiloride (better tolerated) and Spironolactone.
- ACEI/ARB – inhibit aldosterone release
- Magnesium replacement
- Potassium repletion may not be effective unless magnesium depletion is corrected – see below.
Mild Hypokalaemia Serum K 3.0-3.5 mmol/L
- Approx deficit 200mmol K
- Give Sando K 2 tabs TDS (72mmol K).
- Or try potassium preservation with eg amiloride 5 mg daily or ACEI/ARB – useful if hypokalaemia is diuretic induced, hypertensive or heart failure
- Check U&E twice weekly in hospital
Moderate Hypokalaemia 2.5-2.9 mmol/L
- Approx deficit 200-400mmol K
- Give Sando K 3 tabs TDS times daily (108mmol K) if able to take orally and if GI tract functions normally.
- If not then give IV (see below) aiming for an additional 100mmol/day
- Monitor K daily until serum K >2.9mmol/l then manage as above
Severe Hypokalaemia <2.5 mmol/L or Arrhythmia
- Approx deficit >400mmol
- IV potassium likely to be required
- Use pre-prepared bags of 40mmol KCl in 500ml saline 0.9%
- The rate of infusion should not normally exceed 125ml/hr (10mmol K/hour). This is safe to give through a peripheral vein
- Higher concentrations should not be given on general wards unless in exceptional circumstances and under close supervision
- 20mmol/hour can be given if required through large peripheral vein with ECG monitoring
- 40mmol/hour is occasionally required for life threatening dysrthythmia or paralysis – should only be given in Critical Care using central vein with ECG monitoring
- If hypomagnesaemic, give 4ml magnesium sulphate 50% (8mmol) diluted to 10ml with 0.9% saline over 20 mins then start first 40mmol KCl infusion, followed by magnesium replacement as per hypomagnesaemia guideline.