How does magnesium acts as an antiarrhythmic drug ?

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/

ISIS 4

Some think ISIS 4 was a delibrate attempt to defame magnesium !