Figure 420

Acute Mg replacement for life-threatening events such as seizures or potentially lethal cardiac arrhythmias has been described [8-12,19]. Acute increases in the level of serum Mg can cause nausea, vomiting, cutaneous flushing, muscular weakness, and hyporeflexia. As Mg levels increase above 6 mg/dL (2.5 mmol/L), electrocardio-graphic changes are followed, in sequence, by hyporeflexia, respiratory paralysis, and cardiac arrest. Mg should be administered with caution in patients with renal failure. In the event of an emergency the acute Mg load should be followed by an intravenous (IV) infusion, providing no more than 1200 mg (50 mmol) of Mg on the first day. This treatment can be followed by another 2 to 5 days of Mg repletion in the same dosage, which is used in less urgent situations. Continuous IV infusion of Mg is preferred to both intramuscular (which is painful) and oral (which causes diarrhea) administration. A continuous infusion avoids the higher urinary fractional excretion of Mg seen with intermittent administration of Mg. Patients with mild Mg deficiency may be treated with oral Mg salts rather than parenteral Mg and may be equally efficacious [8]. Administration of Mg sulfate may cause kaliuresis owing to excretion of the nonreabsorbable sulfate anion; Mg oxide administration has been reported to cause significant acidosis and hyperkalemia [19]. Parenteral Mg also is administered (often in a manner different from that shown here) to patients with preeclampsia, asthma, acute myocardial infarction, and congestive heart failure.


Life-threatening event, eg, seizures and cardiac arrhythmia

I. 2-4 g MgSO4 IV or IM stat (2-4 vials [2 mL each] of 50% MgSO^ Provides 200-400 mg of Mg (8.3-16.7 mmol Mg) Closely monitor: Deep tendon reflexes Heart rate Blood pressure Respiratory rate

IV drip over first 24 h to provide no more than 1200 mg (50 mmol) Mg/24 h

Subacute and chronic Mg replacement

I. 400-600 mg (16.7-25 mmol Mg daily for 2-5 d) IV: continuous infusion IM: painful

Oral: use divided doses to minimize diarrhea

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