Hypokalaemia

Hypokalaemia in children

Hypokalaemia is a common electrolyte imbalance in children, defined as a plasma potassium level of less than 3.5 mmol/L. It is rarely due to a lack of dietary intake, and more commonly a result of increased losses or a shift of potassium from the extracellular to the intracellular space. The severity of hypokalaemia is classified based on the serum potassium level: mild (3.0-3.5 mmol/L), moderate (2.5-3.0 mmol/L), and severe (<2.5 mmol/L).

 

Aetiology

The cause of hypokalaemia is typically categorised by the mechanism of potassium loss or shift.

 

Increased Losses

  • Renal Losses: This is a common cause where the kidneys inappropriately excrete potassium. Causes include:

    • Medications: Loop and thiazide diuretics, corticosteroids, and aminoglycosides.

    • Renal Tubular Disorders: Conditions like Gitelman syndrome and Bartter syndrome are inherited tubular disorders that cause potassium and salt wasting.

    • Endocrine Disorders: Mineralocorticoid excess (e.g., Cushing’s syndrome, hyperaldosteronism) leads to increased potassium excretion.

  • Extra-renal Losses: This is often due to gastrointestinal issues.

    • Vomiting or Nasogastric Suction: Potassium is lost directly from gastric secretions.

    • Diarrhoea: Large amounts of potassium are lost in the stool.

    • Laxative Overuse.

 

Intracellular Shift

This is a redistribution of potassium into the cells, resulting in a low serum level, but with a normal total body potassium store.

  • Medications: Insulin infusion (e.g., in DKA management) and bronchodilators like salbutamol and theophylline.

  • Metabolic Alkalosis: A high pH promotes the movement of potassium into cells.

  • Re-feeding Syndrome: A severe electrolyte disturbance that can occur when malnourished patients are re-fed.

 

Clinical Presentation

Symptoms of hypokalaemia are often non-specific and are mainly related to muscle and cardiac function.

  • Muscular: Weakness, cramps, and constipation are common. Severe hypokalaemia (<2.5 mmol/L) can lead to a “neck flop” (neck muscle weakness) and paralytic ileus.

  • Cardiac: Hypokalaemia can cause life-threatening cardiac arrhythmias.

  • Other Signs: Hypokalaemia may present with other signs depending on the underlying cause. For example, the presence of hypertension may suggest an excess of aldosterone.

 

Investigations

  • Paired Bloods: Always repeat the serum potassium level with a “good sample” to confirm the hypokalaemia. Concurrently, check renal function, acid-base status, and magnesium levels, as hypomagnesaemia often coexists with and exacerbates hypokalaemia.

  • ECG: An ECG is mandatory for any child with moderate to severe hypokalaemia. ECG changes include flattened T waves, depressed ST segments, and prominent U waves (a small wave following the T wave). Severe hypokalaemia can cause life-threatening arrhythmias, including ventricular fibrillation.

 

Management

The management depends on the severity and underlying cause.

 

Mild to Moderate Hypokalaemia (K > 2.5 mmol/L)

  • Oral Potassium Replacement: For asymptomatic children, oral potassium is the preferred route. The dose is typically 2-4 mmol/kg per day, given in divided doses.

 

Severe Hypokalaemia (K < 2.5 mmol/L)

  • Intravenous (IV) Potassium Replacement: This is indicated for children with severe hypokalaemia or associated ECG changes.

    • Peripheral Line: The maximum concentration of KCl is generally 40 mmol/L. The rate should not exceed 10 mmol/hour. Higher concentrations can cause pain and phlebitis.

    • Central Line: Higher concentrations and faster infusion rates can be used, but this should only be done in a high-dependency (HDU) or PICU setting with continuous cardiac monitoring. The standard rate is 0.5 mmol/kg/hour.

 

Key Management Principles

  • Identify and Treat the Cause: Stop any causative drugs and treat the underlying condition.

  • Continuous Monitoring: Children receiving IV potassium must be on continuous cardiac monitoring.

  • Magnesium: Concurrent hypomagnesaemia should be corrected as it is often refractory to potassium replacement alone.

  • Avoid Rapid Correction: Potassium levels should be corrected slowly to avoid rebound hyperkalaemia and cardiac arrhythmias.