ECG changes in hypomagnesemia: Mechanism
Hypomagnesemia seldom occurs in an isolated situation so that it is difficult to document ECG changes in hypomagnesemia (isolated). It is often associated with other electrolyte imbalances like hypokalemia and hypocalcemia which confound the ECG changes. Nevertheless, there have been case reports of isolated hypomagnesemia with ECG changes.
Tsai TF and colleagues in their report of isolated hypomagnesemia1 noted global T wave inversions and prolonged QTc (corrected QT interval). The person had history of heavy alcohol use and presented with syncope. There was no associated hypokalemia or hypocalcemia. ECG resolved after correction of hypomagnesemia which was initially at 1.1 mg.dL. Coronary angiography did not reveal any obstructive coronary artery disease in him.
Magnesium is an important cofactor for adenosine triphosphate powered processes in the body, including renal potassium reabsorption. That is why hypomagnesemia seldom occurs in isolation and is almost invariably associated with renal potassium loss and hypokalemia. Ninety five percent of body magnesium is intracellular and serum magnesium levels are a poor indicator of total body magnesium level. Both the sodium-potassium pump and calcium pump need magnesium as a cofactor.
Magnesium acts as a physiological calcium antagonist in the myocardial cell
, with low magnesium levels causing calcium release from sarcoplasmic reticulum and high levels blocking this. It is well known that torsades de pointes or polymorphic ventricular tachycardia in the setting of QT interval prolongation responds to magnesium infusion. This suppression of torsade de pointes is due to blockade of L type calcium channels by magnesium and torsades get controlled even without shortening of the prolonged QT interval.
The effect of hypomagnesemia on cardiac arrhythmias is thought to be related to the impairment of sodium potassium adenosine triphosphatase (Na-K ATPase) in the cell membrane involved in the transmembrane transport of sodium and potassium. All sorts of cardiac arrhythmias have been described in the setting of hypomagnesemia. But it is not clear how much is the actual contribution of hypomagnesemia as it is often associated with other electrolyte abnormalities like hypokalemia.
- Global T-wave inversions with isolated hypomagnesemia. J Emerg Med. 2013;45:e107-11.