Lucan · Conditions

Hypokalaemia

Australian guidance: Therapeutic Guidelines (eTG) kidney and urinary: Hypokalaemia in adults

1. Definition and epidemiology

Hypokalaemia is a serum potassium concentration below the lower limit of the reference interval used by the captured eTG topic, which gives the normal range as 3.5 to 5.2 mmol/L. The topic bands severity as mild at 3.1 to 3.4 mmol/L, moderate at 2.5 to 3 mmol/L and severe at less than 2.5 mmol/L. It describes hypokalaemia as one of the most commonly encountered electrolyte disturbances, with acute and chronic causes, and groups those causes into increased potassium loss, movement of potassium into cells, and reduced potassium intake.

The captured topic does not carry a prevalence or incidence figure for hypokalaemia, either in the community or among hospital inpatients, and it does not quantify the proportion attributable to diuretics. No numerical epidemiology is therefore given here. It does record that chronic hypokalaemia occurs in people with chronic disease such as chronic kidney disease, heart failure and diabetes, and that it is associated with a poor prognosis.

2. Pathophysiology

The captured topic explains hypokalaemia through the pattern of potassium loss rather than through molecular transport. It separates causes into increased potassium loss, transcellular shift and reduced intake, and its investigation logic turns on whether urine losses sit with metabolic alkalosis or acidosis, and on the patient's volume status and blood pressure.

Where urine losses occur with metabolic alkalosis and hypovolaemia or a normal to low blood pressure, the listed causes are diuretics, especially loop and thiazide diuretics, vomiting, aminoglycosides, high-dose penicillin, ampicillin and amphotericin, hypomagnesaemia, and Bartter and Gitelman syndromes.

Where the same alkalosis occurs with hypervolaemia or elevated blood pressure, the renin and aldosterone pattern narrows the cause. Elevated renin with elevated aldosterone suggests a renin-secreting tumour, renal artery stenosis or accelerated hypertension; reduced renin with elevated aldosterone suggests primary aldosteronism, usually from Conn syndrome; and reduced renin with reduced aldosterone suggests apparent mineralocorticoid excess from a genetic disorder or high liquorice intake, congenital adrenal hyperplasia, or Liddle syndrome.

Where urine losses occur with metabolic acidosis, renal tubular acidosis is the listed cause. Excessive gastrointestinal losses are attributed to diarrhoea and to nonmedical use of laxatives.

Potassium shift into cells is attributed to metabolic alkalosis, high-dose insulin, intravenous insulin such as that used for diabetic ketoacidosis, hypokalaemic periodic paralysis, beta-adrenergic drugs such as salbutamol, phaeochromocytoma, an anabolic state and excessive caffeine intake. Reduced intake is attributed to inadequate dietary potassium and poor nutrition.

The captured topic does not carry the molecular mechanism of the transcellular shift. It names insulin, beta-adrenergic agonists, alkalosis and an anabolic state as causes, but it does not describe sodium-potassium ATPase activity, insulin or beta-adrenergic receptor signalling, or the renal tubular transporters that mediate aldosterone-driven potassium secretion. It also does not quantify the relationship between serum potassium and total body potassium deficit. Those mechanisms are not carried by the captured source and are not supplied here.

3. Clinical presentation and diagnostic criteria

Mild hypokalaemia may be asymptomatic or produce only non-specific symptoms, and the captured topic does not list those symptoms, so weakness, cramps, constipation and polyuria are not described here. It does state that hypokalaemia can cause life-threatening arrhythmias, specifically ventricular fibrillation, ventricular tachycardia and torsades de pointes, and that these are more common in severe hypokalaemia.

The most common electrocardiogram changes in severe hypokalaemia are listed as T wave flattening, ST depression, widening of the QT interval, U waves and ventricular ectopic beats.

The lower limit of the reference interval is 3.5 mmol/L, and the topic's severity bands describe concentrations below that limit. Before treatment, the topic requires confirmation by repeating the serum potassium measurement on a second blood sample. Cause assessment begins with the clinical history and physical examination; if the cause is not evident, the investigations in Figure 23.14 are suggested, and assessment of acid-base status is described as helpful because metabolic acidosis and metabolic alkalosis point to different causes. A normal acid-base status does not exclude a cause associated with acidosis or alkalosis, and specialist advice should be sought if the interpretation is in doubt.

The captured topic does not reproduce Figure 23.14, so the stepwise investigation algorithm it depicts is not carried, and no urine electrolyte or serum magnesium cutoff from that figure is stated here. It also does not grade the electrocardiogram changes by severity, describe a sine wave pattern, or define symptomatic hypokalaemia beyond the presence of symptoms or electrocardiogram changes.

4. Investigations and AU-specific cutoffs

The reference interval of 3.5 to 5.2 mmol/L is the Australian harmonised chemical pathology interval, cited by eTG to the Royal College of Pathologists of Australasia, and all thresholds in the topic are expressed in mmol/L. The severity bands of 3.1 to 3.4, 2.5 to 3 and less than 2.5 mmol/L are the cutoffs that determine whether oral or intravenous replacement is appropriate.

Confirmation requires a second serum potassium measurement on a separate sample before treatment starts. Acid-base assessment is the next discriminator, because the cause patterns are grouped by metabolic alkalosis and metabolic acidosis, and a normal result does not exclude either pattern.

When urine losses occur with metabolic alkalosis and hypervolaemia or elevated blood pressure, the renin and aldosterone pattern separates the three patterns described in section 2: both elevated, renin reduced with aldosterone elevated, or both reduced.

The topic lists electrocardiogram changes as a feature of severe hypokalaemia and as an indication for intravenous replacement, and it specifies electrocardiogram monitoring during intravenous infusion. It lists hypomagnesaemia as a cause and advises suspecting other causes, including hypomagnesaemia, when hypokalaemia does not respond to treatment, but it does not state a serum magnesium threshold or a magnesium replacement regimen.

Beyond the reference interval, the captured topic does not carry an age-specific or sex-specific cutoff, a separate threshold for chronic kidney disease or heart failure, or a laboratory-specific Australian decision limit. The treatment target it gives is the reference interval itself, 3.5 to 5.2 mmol/L, with individualised targets for patients at ongoing risk.

5. Management

Management is guided by the cause, the severity and the urgency of correction, after confirmation on a repeat sample. Mild or asymptomatic moderate hypokalaemia is usually managed with oral potassium replacement and treatment of the cause, whereas severe, symptomatic or electrocardiogram-associated hypokalaemia usually requires intravenous replacement; intravenous replacement may also be considered for asymptomatic moderate hypokalaemia when more urgent correction is warranted, for example before surgery or because of comorbidity. Intravenous replacement, particularly for acute hypokalaemia, should be given in hospital where the infusion and the patient can be closely monitored.

For most patients, replacement can stop once the serum potassium concentration normalises, but individualised targets are needed for those with ongoing risk such as poor oral potassium intake or long-term diuretic therapy. If hypokalaemia does not respond, other causes of potassium loss should be suspected, including nonmedical use of laxatives, acid-base disturbance and hypomagnesaemia. Potassium-sparing drugs have no place in acute hypokalaemia and are reserved for chronic hypokalaemia when oral replacement is inappropriate.

Oral replacement is given as potassium chloride modified-release 1200 mg (16 mmol) daily, adjusted to the serum potassium concentration, with a usual range of 600 to 3600 mg (8 to 48 mmol) daily in divided doses, or as potassium effervescent 548 to 1096 mg (14 to 28 mmol) dissolved in water and taken two or three times daily. Modified-release tablets must be swallowed whole, and effervescent tablets are the alternative when swallowing is difficult. Serum potassium must be monitored during oral replacement to confirm normalisation and to guide dose and duration, with the interval determined by the clinical context.

Intravenous replacement is given as potassium chloride 10 to 20 mmol in a premixed infusion bag over 1 to 2 hours, with electrocardiogram monitoring as appropriate. Continuous cardiac monitoring is mandatory when the infusion rate exceeds 10 mmol per hour, and the dose may be repeated every 2 to 4 hours to reach a target of 3.5 to 5.2 mmol/L. The usual rate is 10 mmol per hour; if urgent correction is required, 20 to 40 mmol per hour may be given in a critical care setting with continuous cardiac monitoring and frequent serum potassium measurement.

Intravenous potassium must be given from premixed bags, because adding ampoules to intravenous fluid can produce inadequate mixing and a lethal concentration. The maximum concentration for a peripheral vein is 40 mmol/L, except for the isotonic premixed minibag of potassium chloride 10 mmol in 100 mL of 0.29% sodium chloride solution, which may also be given peripherally. Glucose-containing fluids should be avoided because glucose triggers insulin release and shifts potassium into cells; in diabetic ketoacidosis, hypokalaemia treatment should start before intravenous insulin and glucose.

For chronic hypokalaemia, spironolactone is started at 12.5 mg daily and increased gradually to a maximum of 100 mg daily according to the serum potassium concentration, using the lowest effective dose. The captured topic does not carry a magnesium replacement dose, a dietary potassium prescription, a defined duration of oral therapy, or a protocol for refractory hypokalaemia beyond investigating the cause.

6. Pharmacology and AU PBS notes

The oral formulations available at the time of writing are potassium chloride modified-release 600 mg tablets, containing 8 mmol of potassium ions, and potassium 548 mg effervescent tablets, containing 14 mmol of potassium ions. The modified-release product is dosed at 1200 mg (16 mmol) daily to start, within a usual range of 600 to 3600 mg (8 to 48 mmol) daily in divided doses, and the effervescent product at 548 to 1096 mg (14 to 28 mmol) two or three times daily, with both adjusted to the serum potassium concentration. The effervescent tablet contains potassium chloride, potassium bicarbonate and potassium carbonate, and it is the option to consider when a patient cannot swallow modified-release tablets whole.

Intravenous potassium is supplied as premixed infusion bags in various concentrations and volumes; the choice depends on the amount of potassium required, whether access is central or peripheral, and how much fluid the patient can tolerate. Practice in critical care settings may differ, and local protocols should be followed there.

Spironolactone is used only for chronic hypokalaemia, starting at 12.5 mg daily and titrated to a maximum of 100 mg daily, with review every 1 to 2 weeks until the serum potassium concentration is at target and stable, to avoid overcorrection and hyperkalaemia.

The captured topic does not carry PBS listing status, restriction codes, brand names, pack sizes or prices for potassium chloride, potassium effervescent tablets or spironolactone. It also does not carry the mechanism of action, onset or half-life of these agents, so those details are absent here rather than supplied from memory.

7. Complications and monitoring

Life-threatening arrhythmias, namely ventricular fibrillation, ventricular tachycardia or torsades de pointes, are the acute complication the topic highlights, and the risk is greater in severe hypokalaemia. Rapid intravenous infusion of potassium may cause arrhythmias and sudden cardiac death; faster rates are restricted to critical care with continuous cardiac monitoring. Chronic hypokalaemia in people with chronic kidney disease, heart failure or diabetes is associated with a poor prognosis.

Serum potassium should be checked regularly during treatment, with the frequency guided by the clinical context. Measurement every 2 hours is specified for acute hypokalaemia and for patients receiving a higher dose or a rate exceeding 10 mmol per hour, while less frequent measurement may be appropriate for maintenance replacement in chronic hypokalaemia. During oral replacement, monitoring confirms normalisation and guides dose and duration. Spironolactone requires review every 1 to 2 weeks until the serum potassium concentration is at target and stable, because overcorrection can produce hyperkalaemia.

After intravenous replacement, the need for oral replacement is reassessed according to the cause. The captured topic does not carry a mortality rate for hypokalaemia, a defined duration of electrocardiogram monitoring beyond "as appropriate", or a monitoring interval for stable oral replacement, so no such figures are given here.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.