Hyperkalaemia is one of the most common electrolyte problems seen on the acute medical take. It can cause dangerous heart rhythm disturbances with little warning, which is why it appears regularly in the Acute Medicine SCE and MRCP. This overview focuses on recognition and causes.
Defining severity
Potassium of 6.5 mmol/L or more is generally regarded as severe. A falsely raised result is common when a blood sample is haemolysed or delayed in reaching the laboratory, so the context of the result matters.
ECG changes
The ECG reflects the effect of potassium on the heart. Recognised changes include:
- tall, peaked T waves;
- flattened or absent P waves;
- a prolonged PR interval;
- broadening of the QRS complex;
- in severe cases, a sine wave pattern and cardiac arrest.
A normal ECG does not rule out danger, and the changes do not correlate closely with the potassium level.
Causes
- Reduced excretion: acute kidney injury and chronic kidney disease are the commonest causes.
- Medicines: drugs that act on the renin-angiotensin-aldosterone system, potassium-sparing diuretics, trimethoprim, NSAIDs and potassium supplements are frequently involved.
- Release from cells: rhabdomyolysis, tumour lysis, haemolysis and acidosis, including diabetic ketoacidosis.
- Hormonal: adrenal insufficiency.
Who is at risk
Older patients with chronic kidney disease, heart failure or diabetes who take several of the medicines above are at particular risk, especially during an intercurrent illness such as vomiting or diarrhoea.
What examiners test
Questions often give an ECG description and a drug history and ask for the cause or the significance of the finding. Common traps include missing trimethoprim or spironolactone in the medication list and accepting a haemolysed sample at face value.
How to revise
Learn the ECG changes in order of severity, and build a short list of medicines and conditions that raise potassium. That combination answers most questions in this area.