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.