Hypomagnesaemia causing long QTc (510ms)


  • Normal serum magnesium = 0.8 – 1.0 mmol/L.
  • Hypomagnesaemia = <0.8 mmol/L

ECG changes

  • The primary ECG abnormality seen with hypomagnesaemia is a prolonged QTc.
  • Atrial and ventricular ectopy, atrial tachyarrhythmias and torsades de pointes are seen in the context of hypomagnesaemia, although whether this is a specific effect of low serum magnesium or due to concurrent hypokalaemia is uncertain.
  • Nevertheless, correction of serum magnesium to >1.0 mmol/L (with concurrent correction of serum potassium to >4.0 mmol/L) is often effective in suppressing ectopy and supraventricular tachyarrhythmias, while a rapid IV bolus of magnesium 2g is a standard emergency treatment for torsades de pointes.

Related Topics

Further Reading


  • Cameron P, Jelinek G, Kelly AM, Murray L, Brown AFT. Textbook of Adult Emergency Medicine. Elsevier 2009.
  • Chan TC, Brady WJ, Harrigan RA, Ornato JP and Rosen PR. ECG in Emergency Medicine and Acute Care. Elsevier 2005.
  • Murray L, Daly FFS, Little M, and Cadogan M. Toxicology Handbook (2nd edition), Elsevier Australia 2011. [Google Books Preview].
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
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