Hypokalaemia
Exam Tips
- Severity bands: mild 3.0-3.4, moderate 2.5-2.9, severe <2.5 mmol/L; but symptoms depend on rate of fall and comorbidity.
- In exams, pair hypokalaemia with magnesium: persistent low K despite replacement strongly suggests concurrent Mg deficiency.
- ECG progression is high yield (T-wave flattening, ST depression, U waves, QT prolongation); see ECG electrolyte-disturbance figure in standard finals texts (e. g, Kumar & Clark electrolyte chapter).
- Differentiate renal vs extrarenal losses using urinary potassium and acid-base pattern.
- Never bolus IV potassium and avoid glucose-based infusions in acute correction scenarios.
Definition
Hypokalaemia is a serum potassium concentration below 3.5 mmol/L and is one of the commonest electrolyte abnormalities in UK clinical practice. It usually reflects increased renal or gastrointestinal potassium loss, or a transcellular shift into cells, and clinical risk depends on both potassium level and the speed of fall rather than biochemistry alone.
Pathophysiology
About 98% of total body potassium is intracellular, creating the membrane potential needed for nerve and muscle excitability. Potassium balance depends on intake, cellular shifts (via Na+/K+-ATPase, insulin, beta-adrenergic tone, and acid-base state), and renal excretion (mainly distal nephron under aldosterone control). Hypokalaemia develops most often from renal kaliuresis (for example loop/thiazide diuretics, hyperaldosteronism), gastrointestinal loss (vomiting/diarrhoea/laxatives), or intracellular shift (alkalosis, insulin, beta-agonists). Low extracellular potassium hyperpolarizes excitable tissues, causing muscle weakness, reduced gut motility, and cardiac electrical instability (including QT-related ventricular arrhythmia risk, especially with concomitant hypomagnesaemia). Chronic depletion can impair renal concentrating ability and promote hypokalaemic nephropathy.
Risk Factors
- Older age (polypharmacy, especially diuretic exposure)
- Hospital admission/acute illness
- Loop, thiazide, or thiazide-like diuretic treatment
- Vomiting, diarrhoea, laxative misuse, or bowel losses
- Alcohol dependence/malnutrition
- Cardiac disease, hypertension, cirrhosis, CKD
- Hyperaldosteronism or hypercortisolism
- Concomitant hypomagnesaemia
- High beta-adrenergic states (thyrotoxicosis, catecholamine excess)
- Intense exercise with inadequate electrolyte replacement
Clinical Features
Symptoms
- Often asymptomatic when mild and slowly developing
- Fatigue, lethargy, muscle cramps, myalgia
- Limb weakness (can progress to ascending paralysis)
- Paraesthesia or numbness
- Constipation, nausea, vomiting, abdominal distension
- Polyuria, polydipsia, nocturia
- Palpitations, presyncope/syncope
- Dyspnoea if respiratory muscle weakness
Signs
- Reduced muscle power and depressed reflexes
- Flaccid weakness in severe cases
- Cardiac rhythm disturbance (irregular pulse, tachyarrhythmia or bradyarrhythmia)
- Features of volume depletion or underlying endocrine disease
- Ileus with abdominal distension and reduced bowel sounds
- Severe red flags: profound weakness, rhabdomyolysis, respiratory compromise
Investigations
Management
Lifestyle Modifications
- Treat and stop ongoing losses (vomiting, diarrhoea, laxative misuse) and review causative medicines
- Increase dietary potassium intake (fruit/vegetables/legumes) when appropriate, but do not rely on diet alone in moderate-severe deficiency
- Alcohol reduction support and nutrition optimisation in malnourished patients
- Safety-net for red flags: worsening weakness, palpitations, syncope, breathlessness
Pharmacological Treatment
Oral potassium replacement (first-line if mild-moderate and stable)
- Potassium chloride modified-release tablets: typically 24-72 mmol/day in divided doses (BNF dosing adjusted to response and tolerance)
- Potassium chloride effervescent/sachet preparations (e. g, Sando-K 12 mmol per tablet): commonly 1-2 tablets 2-3 times daily, titrated to serum potassium
Preferred route when safe; monitor U&E within 24-72 hours initially. Contraindications/cautions: hyperkalaemia, severe renal impairment/oliguria, untreated Addisonian states, concomitant potassium-sparing agents, ACE inhibitors, ARBs, or aliskiren increase hyperkalaemia risk. GI irritation/ulceration risk with oral KCl.
Intravenous potassium replacement (severe/symptomatic or unable to take oral)
- Potassium chloride IV in sodium chloride 0.9%: common ward rate 10 mmol/hour via infusion pump
- Higher rates up to 20 mmol/hour via central line with continuous ECG monitoring in critical care
Never give IV potassium as bolus. Avoid dextrose-containing fluids (can worsen intracellular shift). Typical peripheral concentration limit is lower (often up to 40 mmol/L) to reduce phlebitis; use local protocol. Frequent serum potassium checks (every 2-4 hours in severe cases) and continuous rhythm monitoring if rapid infusion or arrhythmia risk.
Correct associated electrolyte/endocrine drivers
- Magnesium-L-aspartate oral (e. g, 10 mmol sachets, often 1 sachet twice daily) if hypomagnesaemia
- Magnesium sulfate IV (e. g, 20 mmol over several hours) in significant deficiency
- If mineralocorticoid excess: spironolactone 25-100 mg once daily (condition-specific, specialist-guided)
Refractory hypokalaemia often improves only after magnesium repletion. In CKD or concurrent RAAS blockade, use lower doses and closer monitoring to avoid rebound hyperkalaemia.
Complications
- Ventricular and supraventricular arrhythmias, including life-threatening dysrhythmia
- Rhabdomyolysis
- Respiratory failure from diaphragmatic weakness
- Ileus and severe gut hypomotility
- Impaired glucose tolerance
- Polyuria/nocturia from urinary concentrating defects
- Chronic hypokalaemic nephropathy and renal dysfunction
- Increased mortality risk, particularly with cardiovascular comorbidity
Prognosis
Most patients improve rapidly once the cause is identified and potassium (plus magnesium when needed) is corrected. Prognosis worsens with severe hypokalaemia, delayed treatment, structural heart disease, cirrhosis, QT-prolonging drugs, or persistent renal/endocrine causes; even mild chronic hypokalaemia may increase long-term cardiorenal morbidity.
Sources & References
🏥BMJ Best Practice(2)
💊BNF Drug References(37)
- Acetazolamide[contraindications]
- Aminophylline[cautions]
- Amiodarone hydrochloride[cautions]
- Bendroflumethiazide[contraindications]
- Bendroflumethiazide[cautions]
- Bumetanide[contraindications]
- Chlortalidone[cautions]
- Chlortalidone[contraindications]
- Co-amilofruse[contraindications]
- Co-tenidone[contraindications]
- Digoxin[cautions]
- Droperidol[contraindications]
- Fluticasone with vilanterol[cautions]
- Formoterol fumarate[cautions]
- Furosemide[contraindications]
- Furosemide with triamterene[contraindications]
- Guanfacine[cautions]
- Hydrochlorothiazide[cautions]
- Hydrochlorothiazide[contraindications]
- Indacaterol[cautions]
- Indapamide[cautions]
- Indapamide[contraindications]
- Metolazone[cautions]
- Metolazone[contraindications]
- Milrinone[cautions]
- Olodaterol[cautions]
- Pentamidine isetionate[cautions]
- Salbutamol[cautions]
- Salmeterol[cautions]
- Sodium bicarbonate[contraindications]
- Terbutaline sulfate[cautions]
- Torasemide[contraindications]
- Umeclidinium with vilanterol[cautions]
- Xipamide[cautions]
- Xipamide[contraindications]
✅NICE Guidelines(1)
- Hypokalaemia[overview]
📖Textbook References(1)
- [Williams, Bailey and Love's Short Practice of Surgery] Norman Williams, Christopher Bulstrode, P Ronan O'Connell - Bailey & Love's Short Practice of Surgery 26E (2013, CRC Press) - libgen.li.pdf(pp. 1204)[context]