Overview
Hypokalaemia
Topic Overview
Summary
Hypokalaemia is serum potassium under 3.5 mmol/L. It is common, often iatrogenic (diuretics), and causes muscle weakness, arrhythmias, and ECG changes. Severe hypokalaemia (under 2.5 mmol/L) is life-threatening. Treatment involves potassium replacement (oral or IV), addressing the underlying cause, and monitoring for cardiac effects. Concurrent hypomagnesaemia must be corrected, or hypokalaemia will be refractory.
Key Facts
- Definition: K+ under 3.5 mmol/L; severe if under 2.5 mmol/L
- Causes: Diuretics, vomiting/diarrhoea, renal tubular acidosis, hyperaldosteronism
- Symptoms: Muscle weakness, cramps, constipation, arrhythmias
- ECG changes: U waves, flattened T waves, ST depression, prolonged QT
- Treatment: Oral KCl preferred if mild; IV KCl if severe or unable to take orally
- Max IV rate: 10-20 mmol/hr via peripheral line; higher rates require central line and cardiac monitoring
Clinical Pearls
Always check magnesium — hypokalaemia is refractory to potassium replacement if magnesium is low
Patients on digoxin are at high risk of toxicity with hypokalaemia — replace K+ aggressively
U waves on ECG are pathognomonic for hypokalaemia
Why This Matters Clinically
Hypokalaemia is extremely common and often overlooked. Severe cases cause fatal arrhythmias and respiratory failure. Identifying and treating the cause (not just replacing potassium) prevents recurrence.
Visual Summary
Visual assets to be added:
- ECG showing U waves and flattened T waves
- Hypokalaemia causes flowchart
- Potassium replacement dosing chart
- Potassium homeostasis diagram
Epidemiology
Incidence
- Common: Up to 20% of hospitalised patients
- More common in elderly, patients on diuretics
- Often iatrogenic
Demographics
- All ages
- Particularly elderly on multiple medications
- Patients with eating disorders
Common Causes
| Category | Examples |
|---|---|
| GI losses | Vomiting, diarrhoea, NG suction, laxative abuse |
| Renal losses | Diuretics (thiazides, loop), hyperaldosteronism, RTA, hypomagnesaemia |
| Redistribution | Insulin, beta-agonists, alkalosis |
| Reduced intake | Anorexia, alcoholism |
Pathophysiology
Potassium Homeostasis
- Total body K+: ~50 mmol/kg (98% intracellular)
- Normal serum K+: 3.5-5.0 mmol/L
- Maintained by Na+/K+-ATPase, renal excretion, aldosterone
Mechanisms of Hypokalaemia
| Mechanism | Examples |
|---|---|
| GI losses | Vomiting, diarrhoea (direct loss + metabolic alkalosis) |
| Renal losses | Diuretics, mineralocorticoid excess, RTA |
| Transcellular shift | Insulin, beta-agonists, alkalosis |
| Reduced intake | Rare as sole cause |
Effects of Hypokalaemia
| System | Effect |
|---|---|
| Cardiac | Arrhythmias (especially with digoxin), prolonged QT |
| Muscular | Weakness, cramps, paralysis |
| GI | Ileus, constipation |
| Renal | Concentrating defect (polyuria) |
| Metabolic | Metabolic alkalosis (in primary hyperaldosteronism) |
Why Magnesium Matters
- Hypomagnesaemia impairs K+ reabsorption in kidney
- Hypokalaemia is refractory until Mg2+ is corrected
Clinical Presentation
Symptoms
Signs
Red Flags
| Finding | Significance |
|---|---|
| K+ under 2.5 mmol/L | Severe — risk of arrhythmia, respiratory failure |
| ECG changes | Cardiac risk |
| Paralysis | Severe hypokalaemia |
| On digoxin | Increased digoxin toxicity |
| Respiratory muscle weakness | Respiratory failure risk |
Often asymptomatic if mild
Common presentation.
Muscle weakness, cramps
Common presentation.
Fatigue
Common presentation.
Constipation
Common presentation.
Palpitations
Common presentation.
Clinical Examination
Neurological
- Muscle power (proximal and distal)
- Reflexes (reduced in severe cases)
- Respiratory effort
Cardiovascular
- Pulse (arrhythmia)
- Signs of digoxin toxicity
Abdominal
- Distension, reduced bowel sounds (ileus)
Investigations
Blood Tests
| Test | Purpose |
|---|---|
| Serum K+ | Confirm and quantify |
| Serum Mg2+ | Concurrent hypomagnesaemia common |
| U&E | Renal function, sodium, bicarbonate |
| VBG/ABG | pH (alkalosis causes redistribution) |
| Glucose | Hyperglycaemia/insulin shifts |
ECG — Essential
| Finding | Description |
|---|---|
| Flattened T waves | Early sign |
| U waves | Pathognomonic |
| ST depression | |
| Prolonged QT/QU | Arrhythmia risk |
| Arrhythmias | AF, VT, torsades de pointes |
Urine Tests (If Cause Unclear)
| Test | Interpretation |
|---|---|
| Spot urine K+ | Under 20 mmol/L = GI loss; over 40 mmol/L = renal loss |
| TTKG | Transtubular potassium gradient (less used now) |
Consider Further Tests
- Renin/aldosterone (hyperaldosteronism)
- Cortisol (Cushing's)
- Urine drug screen (laxatives, diuretics)
Classification & Staging
By Severity
| Severity | K+ Level | Features |
|---|---|---|
| Mild | 3.0-3.4 mmol/L | Often asymptomatic |
| Moderate | 2.5-2.9 mmol/L | Symptoms likely; ECG changes |
| Severe | Under 2.5 mmol/L | High risk of arrhythmia, paralysis |
Management
Mild Hypokalaemia (3.0-3.4 mmol/L, Asymptomatic)
| Approach | Details |
|---|---|
| Oral potassium | Sando-K 2 tablets TDS; or Slow-K |
| Dietary advice | Bananas, oranges, potatoes, tomatoes |
| Address cause | Review diuretics, treat diarrhoea |
Moderate Hypokalaemia (2.5-2.9 mmol/L)
- Oral potassium if able to take orally
- IV KCl if symptomatic or unable to take orally
- Cardiac monitoring if ECG changes
Severe Hypokalaemia (Under 2.5 mmol/L or Symptomatic)
| Intervention | Details |
|---|---|
| IV KCl | Max 10 mmol/hr via peripheral line; max 20-40 mmol/hr via central line |
| Cardiac monitoring | Continuous ECG |
| Check Mg2+ | Replace magnesium if low |
| Recheck K+ frequently | Every 2-4 hours |
IV Potassium Safety
| Parameter | Limit |
|---|---|
| Concentration | Max 40 mmol/L via peripheral |
| Rate | Max 10-20 mmol/hr peripheral; higher via central |
| Monitoring | Cardiac monitoring if over 10 mmol/hr |
Replace Magnesium
- If Mg2+ low: IV magnesium sulphate 20-40 mmol over 24h
- Essential — hypokalaemia refractory until Mg corrected
Treat Underlying Cause
- Stop offending drugs (diuretics, laxatives)
- Treat diarrhoea/vomiting
- Investigate if unexplained (hyperaldosteronism, RTA)
Complications
Cardiac
- Arrhythmias (AF, VT, VF, torsades de pointes)
- Digoxin toxicity (potentiated by hypokalaemia)
- Sudden cardiac death
Muscular
- Rhabdomyolysis (severe hypokalaemia)
- Respiratory failure (respiratory muscle weakness)
- Ileus
Prognosis & Outcomes
Prognosis
- Excellent with prompt treatment
- Fatal arrhythmias possible if untreated
- Recurrence common if cause not addressed
Long-Term
- Identify and treat cause
- Monitor patients on diuretics
- Consider potassium-sparing diuretics or supplements
Evidence & Guidelines
Key Guidelines
- No specific national guideline for hypokalaemia
- Management based on consensus and pharmacological principles
Key Evidence
- Magnesium replacement essential for refractory hypokalaemia
- IV potassium safe at higher rates with cardiac monitoring
Patient & Family Information
What is Hypokalaemia?
Hypokalaemia means low potassium in your blood. Potassium is important for your muscles and heart to work properly.
Causes
- Water tablets (diuretics)
- Vomiting or diarrhoea
- Some medical conditions
Symptoms
- Muscle weakness or cramps
- Tiredness
- Palpitations
Treatment
- Potassium tablets or potassium through a drip
- Treating the cause
- Eating potassium-rich foods (bananas, oranges)
Resources
References
Key Studies
- Gennari FJ. Hypokalemia. N Engl J Med. 1998;339(7):451-458. PMID: 9700180
- Crop MJ, et al. The influence of potassium-sparing diuretics on mortality in patients with heart failure. J Am Coll Cardiol. 2016;67(13):1556-1564. PMID: 27038491
Reviews
- Kardalas E, et al. Hypokalemia: a clinical update. Endocr Connect. 2018;7(4):R135-R146. PMID: 29540487