Magnesium: Side Effects


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Oct 15, 2005
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From Baby Miller...Ch. Obstetrics

Magnesium is effective in pregnant women with pre-eclampsia by "decreasing the irritability of the CNS"??, which decreases the likelihood of seizures.

Magnesium also

1. decreases hyperactivity at the NMJ by presumably decreasing the presynaptic release of Ach

2. decrases the sensitivity of postjunctional membranes to Ach

3. Relaxes uterine and vascular smooth muscle, increasing uterine blood flow

Therapeutic levels to prevent seizures are between 4-6 mEq/L. Loss of DTR are a sign of impending toxicity

Levels higher than 6 mEq/L can lead to
a) Severe skeletal muscle weakness-->paralysis
b) Hypoventilation, lethargy, somnolence
c) Prolonged PR and QT intervals-->Cardiac arrest

Antidote for Mg toxicity-->IV Calcium 5-10 mEq, saline, diuretics

Magnesium is excreted by the kidneys, so it has to be titrated carefully in patients with impaired renal function, as in pregnant pts with preeclampsia.

-Since Mg decreases the release of presynaptic Ach at the NMJ, neuromuscular blockers can have longer lasting effects on pts on Mg.
-Baby Miller als states Mg can enhance the effects of opioids and sedatives
-Preeclampsia may be associted with decreases in plasma cholinesterase potentiating the effects of succinylcholine. Barash states that ?magnesium decreases the release of potassium in response to succinylcholine?
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Jul 12, 2006
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Hypotension: Mg can be used in pheochromocytoma removal:

"Anesth Analg. 2004 Sep;99(3):680-6, table of contents. Links
Pheochromocytoma crisis: the use of magnesium sulfate.James MF, Cronjé L.
Department of Anesthesia, University of Cape Town Faculty of Health Sciences, Anzio Road, Observatory 7925, Cape Town, Western Cape, South Africa. [email protected]

Pheochromocytoma crisis is a rare life-threatening event that may appear with a variety of clinical symptoms. We present three cases of life-threatening crisis in which magnesium sulfate was particularly beneficial in controlling symptoms and signs when more conventional forms of therapy had failed. Two patients presented with hypertensive encephalopathy, and the third presented with catecholamine-induced cardiomyopathy. All three patients successfully underwent tumor excision with magnesium sulfate used as the sole drug for control of hemodynamic disturbances during surgery. The problems of pheochromocytoma crisis and the potential benefits of magnesium sulfate in this condition are reviewed."

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