Hypokalaemia

Content created by Tina Grant

Introduction

1. 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

1. Normal dietary K intake 50-100 mmol per day
2. 90% of body’s 3500mmol K is intracellular and <1% intravascular
3. Na/K pumps on cell membranes pump K into cells in exchange for Na under influence of insulin, beta agonists, alkalosis and aldosterone
4. 90% excretion of K occurs through kidneys and only 10% through colon unless patient has diarrhoea
5. 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
6. Hypokalaemia Is defined as serum K <3.5 mmol/L
7. Hypomagnesaemia causes renal potassium loss and often coexists with hypokalemia. Recommend check serum Mg if serum K <3mmol/l.

Common Causes of Hypokalaemia

1. Diuretics except K sparing diuretics – renal K loss
2. Vomiting – mechanism is by alkalosis leading to transcellular shift and renal K loss
3. Diarrhoea – mechanism is by direct GI loss
4. Acute illness – inadequate intake and by adrenaline response leading to transcellular shift
5. Alcoholism – combination of poor diet, vomiting and coexistent magnesium depletion
6. Refeeding syndrome – K and PO₄ driven into cells when body switches back to CHO metabolism after a period of starvation
7. NB Surreptitious vomiting and laxative abuse – neither of which may be volunteered

Uncommon Causes of Hypokalaemia

1. Renal losses with normal BP – Bartter’s and Gitelman’s syndromes (both alkalotic), RTA (acidotic)
2. Renal losses with hypertension – Conn’s syndrome (low renin, high aldo), renovascular (both high), Liddle’s syndrome (both low)

Consequences

1. Cardiovascular – arrhythmias include sinus bradycardia, ectopic beats, paroxysmal atrial tachycardia, AV block, VT or VF
2. Neuromuscular – muscle weakness, myalgia, cramps, paralysis, rhabdomyolysis.
3. Gastrointestinal – constipation, ileus
4. Renal – polyuria,

Investigations

1. If cause is obvious eg diuretic, vomiting, diarrhoea – no further tests required.
2. Measure BP
   • Hypertension raises possibility of Conn’s Syndrome or renovascular disease.  May then want to check renin and aldosterone
3. 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.
4. 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
5. 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

1. 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.
2. Potassium preservation
   • K sparing diuretics – Amiloride (better tolerated) and Spironolactone.
   • ACEI/ARB – inhibit aldosterone release
3. 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

1. Approx deficit 200mmol K
2. Give Sando K 2 tabs TDS (72mmol K).
3. Or try potassium preservation with eg amiloride 5 mg daily or ACEI/ARB – useful if hypokalaemia is diuretic induced, hypertensive or heart failure
4. Check U&E twice weekly in hospital

Moderate Hypokalaemia 2.5-2.9 mmol/L

1. Approx deficit 200-400mmol K
2. Give Sando K 3 tabs TDS times daily (108mmol K) if able to take orally and if GI tract functions normally.
3. If not then give IV (see below) aiming for an additional 100mmol/day
4. Monitor K daily until serum K >2.9mmol/l then manage as above

Severe Hypokalaemia <2.5 mmol/L or Arrhythmia

1. Approx deficit >400mmol
2. IV potassium likely to be required
3. Use pre-prepared bags of 40mmol KCl in 500ml saline 0.9%
4. The rate of infusion should not normally exceed 125ml/hr (10mmol K/hour). This is safe to give through a peripheral vein
5. Higher concentrations should not be given on general wards unless in exceptional circumstances and under close supervision
6. 20mmol/hour can be given if required through large peripheral vein with ECG monitoring
7. 40mmol/hour is occasionally required for life threatening dysrthythmia or paralysis – should only be given in Critical Care using central vein with ECG monitoring
8. 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.

Links

• Hypokalaemia on patient.co.uk (for Medical Professionals)
• DynaMed – Hypokalaemia – Approach to the Patient
• BMJ Best Practice – Assessment of Hypokalaemia