Hypokalaemia

Potassium is an electrolyte which is essential for regulating nerve and muscle function, including cardiac muscle function. Disturbances in serum (blood) potassium affect the activity of Na/K – ATP pumps in the muscle tissue, leading to inappropriate muscle contractions.

The potassium concentration in the serum is around 3.5-5.2 mmol/L. This range describes the values between which 95% of the healthy population’s levels are expected to be. It does not necessarily mean that patients with results outside the reference range are at risk of complications. It also does not mean that all patients with levels within the reference ranges have an optimal concentration.

Hypokalaemia is a concentration of potassium in the blood below the reference range. Mild cases of hypokalaemia with serum potassium levels of 3-3.5 mmol/L can be asymptomatic, while severe cases with serum potassium of <2.5 mmol/L can lead to life threatening complications.

Healthy young patients with potassium levels slightly outside the reference ranges rarely experience problems. Older patients with acute cardiac conditions like rapid atrial fibrillation or acute myocardial infarction may require tighter potassium concentrations than the standard range to achieve optimal outcomes.

Laboratories can have different reference ranges due to different techniques used to collect and analyse specimens. This should be considered when reviewing results.

In the general population hypokalaemia is estimated to occur in 1-3% of people. People with malnutrition or on diuretics have a higher risk of developing hypokalaemia.

Symptoms of hypokalaemia include muscle weakness or cramps, lethargy, constipation, palpitations, nausea or vomiting, tingling or numbness in the limbs. In severe cases hypokalaemia can cause cardiac arrhythmias and cardiac arrest.

Common causes of hypokalaemia include

• Increased aldosterone levels caused by primary hyper aldosteronism or untreated heart failure. Aldosterone is the primary hormone regulating renal potassium excretion.
• Medicines, including loop and thiazide diuretics, nebulised or oral beta agonists and amphotericin B.

Mild cases of hypokalaemia in young patients without cardiac complications can often be managed with oral potassium supplements. Intravenous potassium supplementation could be required when the potassium concentration is <3 mmol/L with associated paralysis, hypokalaemia is associated with a cardiac rhythm disturbance, or oral supplementation is not possible. Concomitant oral and IV potassium supplementation should be considered when the patient’s potassium concentration is <3 mmol/L.

The actual increase in serum potassium from supplements is variable and depends on several factors like kidney disease or heart failure, and the presence of medicines such as diuretics, ACE Inhibitors and angiotensin receptor blockers.

Intravenous potassium supplementation should be administered at a rate of no greater than 20 mmol/hr and ideally at a rate of no greater than 10 mmol/hr when administered via a peripheral cannula. Faster rates can be administered via central lines terminating in high flow veins, such as the vena cava, in monitored settings such as an ICU.

Most people need 1mmol/kg of potassium per day to replace physiological losses. In patients who are unable to meet daily potassium requirements (e.g. patients who are nil by mouth) or when there are ongoing potassium losses (e.g. patients on large doses of loop diuretic), supplementation doses should account for this.

Aldosterone antagonists, such as spironolactone and eplerenone, are not used for management of acute hypokalaemia. They are however useful in the ongoing management of hypokalaemia secondary to hyperaldosteronism due to their aldosterone antagonist effects. They are also used in patients with recurrent hypokalaemia from loop diuretics. This includes patients with heart failure and cirrhotic liver disease.

Hypomagnesaemia can cause potassium wasting in the kidneys. Hence patients with hypokalaemia resistant to potassium supplementation should have magnesium levels assessed, and magnesium supplementation initiated where necessary.

Many foods e.g. bananas, baked potatoes, edamame, raisins and salmon are rich in potassium. Individuals with chronic hypokalaemia should have dietitian input to increase dietary potassium.