MedCourseFinder

Rhythm

Hypokalaemia

Source-checked · Content last updated · 1 min read ·

This is the date the draft was written, not a medical review date.

Hypokalaemia causes ST depression, T-wave flattening or inversion and prominent U waves, with apparent QT prolongation (in fact QU). It predisposes to atrial and ventricular arrhythmias, including torsades de pointes, particularly with other QT-prolonging factors.

ECG criteria

  • ST-segment depression, usually downsloping or sagging.
  • Flattened or inverted T waves.
  • Prominent U waves, often taller than the T wave, best seen in the mid-precordial leads (for example V2 to V4).
  • Apparent QT prolongation due to fusion of T and U waves (QU interval); measure carefully.
  • Prolonged PR interval, increased P-wave amplitude and ectopy (atrial, ventricular) may be present; the rhythm is otherwise usually sinus.

Mechanism

Low extracellular potassium hyperpolarises resting cells yet paradoxically delays ventricular repolarisation and prolongs the action potential. This produces the U wave and increases the risk of early afterdepolarisations, which can trigger torsades de pointes.

Common causes

  • Gastrointestinal losses: vomiting, diarrhoea, laxative abuse.
  • Renal losses: loop and thiazide diuretics, hyperaldosteronism, hypomagnesaemia, renal tubular disorders.
  • Transcellular shift: insulin, beta-agonists, alkalosis, thyrotoxic periodic paralysis.
  • Poor intake; frequently coexists with hypomagnesaemia.

Management principles

Management is to identify and correct the cause and to replace potassium and magnesium by an appropriate route and rate, with ECG monitoring when severe or when arrhythmia is present. Follow local guidelines and protocols, taking care with renal function and rate of replacement.

High-level principles for study. Treatment decisions follow local guidelines and the clinical situation.

Pitfalls and mimics

  • A U wave fused with the T wave can be misread as a long QT interval; measure the QT only to the end of the true T wave or use an alternative method.
  • Prominent U waves also occur with bradycardia, left ventricular hypertrophy, hypercalcaemia and drugs (for example class Ia and III antiarrhythmics).
  • Hypokalaemia with unresponsive correction should prompt a check for low magnesium.

Practise in the ECG trainer — Recognise this pattern on generated tracings.

References

  1. Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
  2. Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. Wolters Kluwer, 12th edition (2013) — Tier 3 (textbook)
  3. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. Elsevier, 14th edition (2020) — Tier 3 (textbook)

References are listed only where the article cites them. See the sources page for the full policy. How sources are chosen and credited