Sunday February 26, 2006
Hypomagnesemia and IV Magnesium (Mg) infusion
Hypomagnesemia has been reported in upto 60% of ICU patients and sometimes can be clinically very significant like in recovery phase of DKA (diabetic ketoacidosis). Symptoms of severe hypomagnesemia (less than 1 mEq/L) include respiratory failure, hyperactive deep-tendon reflexes, muscular fibrillations, mental status changes, tetany, seizures, positive Chvostek and Trousseau signs. EKG manifestations are prolong PR interval, widened QRS complex, ST depression, altered T waves and last but not the least is loss of voltage. About 33% of serum magnesium is protein-bound but unfortunately wide-spread test for free or active (ionized) magnesium is not available. It is a common practice to write IV Mg orders in grams or mls.
1 gram of IV Mg contains 8.12 meq of Mg and 1 meq of Mg provides 12 mg of elemental Mg.
One ml MgSO4 50% Solution = 4 meq Magnesium
One ml MgSO4 10% Solution = 8 meq Magnesium
Rapid IV administration can induce life threatening cardiac dysrhythmias, hypotension, flushing, sweating, sensation of warmth and hypocalcemia. In non-emergent cases, general rule of thumb is to infuse 1 gram per 1 hour. In risky situations, like impending arrhythmia, 2 grams of IV Magnesium sulfate may be given over 20 minutes. In extremely emergent cases 2 grams (16 mEq) of IV MgSO4 may be administered over 5 minutes and actually may be given as IV push if there is no permission of time.
In Preeclampsia, load IV 4-6 grams of MgSO4 in 100 ml of D5W over 20-30 minutes and maintenance is 2-3 grams/hour with close monitoring of target level (goal of 4-7 mEq/L) and clinical manifestations like decrease deep tendon reflexes. It is not a bad idea to keep IV calcium at bedside during massive IV magnesium infusion as in preeclampsia. IV calcium is an antidote for magnesium overdose.
In kidney dysfunction, IV magnesium dose should be reduced by about 50%.