Hypokalaemia

Potassium is the most abundant cation in the body and hypokalaemia is the most common electrolyte abnormality found in patients within the hospital setting.

Hypokalaemia is defined as a serum concentration of potassium < 3.5mmol/L (normal range 3.5-5.3mmol/L). Severity is further classified into

  • Mild hypokalaemia = 3.1-3.5mmol/L.
  • Moderate hypokalaemia = 2.5-3.0mmol/L.
  • Severe hypokalaemia = <2.5mmol/L.

Most cases of hypokalaemia (>95%) are mild and can be corrected simply by the use of suitable electrolyte replacement.

However, even small drops in potassium levels can increase the risk of cardiac arrhythmias, especially in post-surgical patients, so all cases of hypokalaemia should be acted upon and monitored accordingly.


Causes

Most cases of hypokalaemia occur from either diuretic use or excess GI losses, however there are other causes that should also be considered

  • Medication
    • Diuretics (common)
      • Most commonly thiazide and loop diuretics
    • Steroids
    • Excessive insulin administration
    • Beta-2 agonists, such as salbutamol
  • Excess loss
    • Diarrhoea and vomiting (common)
      • Including bowel obstruction, fistula formation, pyloric stenosis, or laxative abuse
    • Hyperaldosteronism, such as Conn’s syndrome
    • Burns or excessive sweating
  • Decrease intake
    • Inadequate replacement in IV fluids whilst nil-by-mouth (common)
    • Malnutrition
  • Miscellaneous
    • Chronic alcoholism
    • Cushing’s Syndrome
    • Renal tubular acidosis
    • Hypomagnesaemia
    • Chronic peritoneal dialysis

Presentation

Hypokalaemia is generally asymptomatic in mild cases. However, in more severe cases, patients can present with muscle weakness, paraesthesia, constipation / ileus or pseudo-obstruction, hypotonia, hyporeflexia, muscle cramps, tetany, and even respiratory failure (rare), alongside potential cardiac symptoms due to arrhythmias developing.

ECG Changes in Hypokalaemia

Hypokalaemia causes cardiac hyperexcitability, resulting in re-entrant loops to form and can result in arrhythmias developing.

There are several ECG changes that can occur:

  • Elongated PR interval
  • T wave flattening* or T wave inversion
  • Prominent U wave*
  • ST segment depression

*Flattening of the T wave with the presence of the U wave may appear as a prolonged QT interval, however the true QT is actually unchanged.

If uncorrected, this can eventually develop into threatening arrhythmias such as VT or VF


Investigation

Patients with hypokalaemia should be investigated and managed appropriately due to the risks associated with cardiac arrhythmias.

Much of the history and examination* in most cases will reveal the underlying diagnosis, however several initial investigations should also be performed, including:

  • ECG, checking for any cardiac involvement
    • If any are noted (or the patient requires aggressive IV potassium replacement), the patient should be put on a cardiac monitor
  • Bloods, especially FBC, U&Es, PO42-, and Mg2+
    • Magnesium levels are often associated with hypokalaemia; magnesium deficiency exacerbates potassium wasting by increasing potassium secretion

A venous blood gas (VBG) can be used to rapidly check potassium levels following intervention

*Ensure to assess the fluid status of the patient, which will aid diagnosis of underlying cause and of management options, and consider a catheter if appropriate


Management

Management of hypokalaemia should involve treatment of the underlying cause alongside correction with suitable replacement, especially in the hospital setting.

Specific management will depend on the underlying cause. Importantly, any patients who are fluid overloaded, on significant diuretic therapy, or complex in diagnosis or management, advice should always be sought from the renal physicians or suitable specialists prior to any action.

In mild cases (and if without cardiac involvement), where the patient is able to eat and drink normally, oral supplements (such as SandoK, and food high in potassium like bananas) as replacement should suffice in most circumstances and potassium levels normalise.

In patients with moderate to severe hypokalaemia, have ongoing losses, or unable to take supplements orally (amongst others), IV replacement* will likely be indicated. IV potassium replacement (40mmol K+ in a litre of saline) can be given, however if rapid rates or higher concentrations are required then a central line and admission to a monitored bed is necessary due to associated cardiac risks.

Whilst on replacements, daily bloods should be performed to monitor levels. Any hypomagnesaemia should be concurrently corrected if present (as will facilitate a more rapid correction of hypokalaemia)

*Dextrose-free solution should be given, to minimise insulin stimulation and causing a worsening hypokalaemia

Key Points

  • Hypokalaemia is defined as serum [K+] <3.5mmol
  • Most causes are through diuretic use, inadequate replacement in IV fluids, or diarrhoea or vomiting
  • Most cases are asymptomatic, yet even mild cases can cause arrhythmias
  • Ensure to check other electrolytes, especially Mg2+ levels
  • Management involves providing suitable replacement and treating the underling cause

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