Like calcium, magnesium can also affect the QT interval on the ECG. Hypomagnesaemia (low serum magnesium) mainly causes a long QT interval. It is also associated with ectopic beats and atrial tachyarrhythmias. It can coexist with hypokalemia. If not treated, there is a risk of developing Torsades de Pointes.

Torsades de Pointes

Torsades de Pointes is a form of Polymorphic Ventricular Tachycardia that is associated with a long QT interval. It has the appearance of twisting (Torsades) around the baseline. To be Torsades, there must be both evidence that the VT is polymorphic and that there was a long QT preceding it.

Torsades de Pointes can be caused by lots of different drugs that prolong the QT interval. Apart from a low magnesium it can also be caused by a low potassium. Regardless of the cause, the immediate treatment is IV magnesium.

What about high magnesium levels?

Hypermagnesaemia can cause similar changes to hyperkalemia, including a long QT, wide QRS, peaked T waves, flat P waves, bradycardias and arrest.

If it looks so similar, how do I know when to check the Mg?

Electrolyte disorders often coexist, for example hypomagnesaemia frequently occur together with hypokalemia and/or hypocalcaemia. The clinical context provides important clues to the underlying electrolyte disorder(s). For example, you might suspect multiple electrolyte disorders with conditions such as alcoholism, malnutrition, vomiting / diarrhoea or renal impairment.

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ECG Library (5)

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In the ECGquest archives, this ECG has been tagged with: - 12-Lead Long QT Narrow QRS Regular Tachycardia Sinus tachycardia Hypokalemia Hypomagnesaemia Dr Smith's ECG Blog CC BY-NC
Alcohol withdrawal and sinus tachycardia 18338

This ECG is from a patient of unknown age who presented with alcohol withdrawl.

In the ECGquest archives, this ECG has been tagged with: - Diarrhoea 12-Lead Standard Calibration Inferior ST depression Lateral ST depression Long QT Narrow QRS Normal Axis Normal PR Normal rate Regular Normal Sinus Rhythm Hypocalcaemia Hypomagnesaemia ECG of the Week CC-BY-NC-SA
ECG of the Week – 29th January 2018 – Interpretation 17731

This ECG is from a man in his 60s who presented with several weeks of diarrhoeal illness following overseas travel. He had a past medical history of ischaemic cardiomyopathy. His medications included warfarin, a beta-blocker, a PPI, an ACE inhibitor and diuretics.