Overview
Hypokalaemia describes a reduced serum potassium ion concentration.
Potassium is regulated using:
- Aldosterone:
- Leads to sodium retention and potassium excretion
- Acid-base homeostasis:
- Acidosis (increased H+ ions) displace potassium in cells leading to potassium leaving cells and entering the bloodstream
- Insulin:
- Insulin increases the uptake of potassium into cells
Classification
It is classified as follows:
- Mild: 3.1 – 3.5 mmol/L
- Moderate: 2.5 – 3.0 mmol/L
- Severe: <2.5 mmol/L
Effect on the cardiac action potential
Hypokalaemia leads to incomplete inactivation of the sodium ion channels, making triggering action potentials less likely. This can lead to delayed repolarisation and can result in arrhythmias.
Causes
Decreased intake
Hypokalaemia is rarely seen due to decreased intake:
- Anorexia nervosa
- Diuretic therapy
Increased potassium moving into cells
- Metabolic or respiratory alkalosis:
- Hydrogen ions leave the cells and potassium enters, reducing serum levels
- Insulin and glucose administration
- Insulin drives potassium into cells
- Hypothermia
Increased renal excretion
- Thiazide and loop diuretics – can cause a hypokalaemic metabolic alkalosis
- Acetazolamide – can cause a hypokalaemic metabolic acidosis
- Hyperaldosteronism
- Type 1 and type 2 renal tubular acidosis
- Hypomagnesaemia
Gastrointestinal tract losses
- Vomiting – can cause a hypokalaemic metabolic alkalosis
- Diarrhoea – can cause a hypokalaemic metabolic acidosis
Other causes
- Excessive sweating
- Burns
- Chronic alcohol abuse
Presentation
Patients with mild hypokalaemia may be asymptomatic. Some features may be:
- Fatigue
- Weakness
- Muscle pain
- Constipation
- Severe paralysis and respiratory failure
- Ileus
- Paraesthesia
Investigations
- Consider repeat blood test – if the result is unexpected
- ECG – is essential as hypokalaemia can cause arrhythmia. May show:
- Flat T waves
- PR prolongation
- ST depression
- U waves – small deflections immediately following the T wave
- Long QT
Management
- Management generally involves potassium replacement and treating the underlying cause.
- The maximum rate of replacement should be 0.5 mmol/kg/hour with the weight capped at 80 kg (i.e. 40 mmol/hour maximum in an 80 kg adult).
Complications
- Cardiac arrhythmia
- Muscle weakness
- Rhabdomyolysis
