Magnesium is a powerful cell membrane stabilizing agent. It is well recognized to act on the cerebral motor cortical cells and suppress seizure , especially in eclampsia of pregnancy .
Mg SO4 is still the drug of choice for seizures of pregnancy. It was soon realised , the molecular basis of cellular excitability is similar for every cell . And thus , we got this great antiarrhythmic drug !
- Magnesium is a cofactor in the enzyme Na /K ATPase in the myocyte cell membrane
- Integrity of this enzyme is essential for proper maintenance of the intracellular potassium levels.
- Many times hypokalemia can not be fully corrected by administration of K + alone .
- Co- administration of magnesium increase the intracellular K + and hyperpolarize the cells and make it less excitable.
- Further , magnesium competes with ca++ ions to enter the cells and thus it is a natural calcium blocker. This property also helps in controlling refractory calcium dependent cardiac arrhythmia.
Indications for magnesium
- Torsades de pointes . Note: Magnesium does not shorten the QT interval significantly but still effective in torsades.
- Any refractory VT especially , post MI.
- Digoxin induced , hypokalemia dependent atrial tachycardias, MAT
It is administered 1-2mg boluses of 2-3 boluses.
Where we should not use magnesium ?
Routine Use of magnesium in recurrent non sustained VT following MI is not recommended .(Courtesy ISIS -4 trial )
Reference
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1368485/
Magnesium : Nature’s own calcium blocker
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1368485/
Some think ISIS 4 was a delibrate attempt to defame magnesium !
Dr.S.Venkatesan MD 
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