Normal Mg level

A normal range of the plasma magnesium concentration is 1.7 to 2.1 mg/dl.

Hypomagnesemia severity:

• Severe <= 1 mg/dl.
• moderate 1-1.5 mg/dl.
• mild 1.5-1.7 mg/dl.
• Other sources may indicate different but close values.

Clinical manifestations of hypomagnesemia 

• Neuromuscular manifestations, including neuromuscular hyperexcitability (eg, tremor, tetany, convulsions), weakness, apathy, delirium, and coma
• Cardiovascular manifestations include widening of the QRS and peaking of T waves with moderate magnesium depletion, and widening of the PR interval, diminution of T waves, and atrial and ventricular arrhythmias (torsades de pointes) with severe depletion.

The association with hypocalcemia and hypokalemia

• Hypokalemia occurs in 40-60% of hypomagnesemic patients simply because what causes potassium loss usually causes magnesium loss as well. Always check the K level in hypomagnesemia and vice versa.
• Hypocalcemia:
  • May develop in hypomagnesemic patients and if symptomatic it is almost always associated with severe hypomagnesemia (Mg < 1 mg/dl).
  • Occasionally, patients with normal plasma magnesium concentrations may have hypocalcemia (Normomagnesemic hypocalcemia) that improves with magnesium therapy, possibly due to cellular magnesium depletion.
  • The major factors resulting in hypocalcemia in hypomagnesemic patients are hypoparathyroidism, parathyroid hormone (PTH) resistance, and vitamin D deficiency.
  • Always check the Ca level in hypomagnesemic patients and vice versa.

Causes

• GI losses:
  • Diarrhea
  • PPI
  • Pancreatitis
• Renal losses
  • Diuretics
  • Osmotic diuresis
  • Volume resuscitation
  • Post-obstructive diuresis
  • Alcoholism
  • Hypercalcemia

Magnesium preparations

• Oral preparations
  • The bioavailability of oral preparations is assumed to be 33% in the absence of intestinal malabsorption.
  • Gastrointestinal discomfort and diarrhea are major side effects (Mg preparations are used as laxatives as well) and may limit their use.
  • Sustained-release preparations have the advantage that they are slowly absorbed and thereby minimize renal excretion of the administered magnesium and less diarrhea:
    •  Magnesium chloride contains 64 to 71.5 mg of elemental magnesium.
    •  Magnesium L-lactate contains 84 mg of elemental magnesium.
  • Instant release:
    • Magnesium oxide comes in 400 mg oral tablets (241 mg elemental magnesium).

Magnesium repletion:

• Important concepts:
  • The sudden rise in magnesium level after IV magnesium will remove the stimulus of magnesium reabsorption in the loop of Henle leading up to 50% of the infused magnesium to be excreted in the urine, the slower the infusion the less of the infused magnesium excreted in the urine (That’s why in stable patients IV magnesium should be infused over longer periods).
  • The serum magnesium level may appear artificially high if measured too soon after an IV magnesium dose is administered.
  • Significant magnesium depletion requires sustained correction (repetitive doses) of the hypomagnesemia.
  • Oral replacement is preferred in the treatment of mild-moderate asymptomatic hypomagnesemia, preferably with a sustained-release preparation, given the ability of magnesium to induce diarrhea.
  •  Plasma magnesium concentrations are regulated solely by renal excretion. Patients with kidney function impairment (creatinine clearance less than 30 mL/min/1.73 m2) are at risk for severe hypermagnesemia, thus we reduce the magnesium dose in such patients by 50 percent or more and closely monitor magnesium concentrations.

• Use IV magnesium for repletion in the following:
  •  Torsade de pointes or any kind of arrhythmias or EKG changes), 1 to 2 grams of magnesium sulfate can be given initially over 2 minutes. A repeat bolus can be given if needed.
  • In hemodynamically stable patients (No EKG changes or arrhythmia) with symptomatic hypomagnesemia (typically Mg ≤ 1 mg/dl), 1 to 2 grams of magnesium sulfate infused quickly over 30 minutes followed by 4-8 gm infused slowly over the next 12-24 hours.

• • For asymptomatic patients with severe hypomagnesemia: 4 to 8 grams of magnesium sulfate infused slowly over 12 to 24 hours. Repeat as necessary to keep magnesium level above 1 mg/dl.
  • Normomagnesemic patient with hypocalcemia, 4 gm of Mg sulfate is infused over 12-24 hours daily for3-5 days (keep monitoring Mg and Ca levels and stop once Ca level normalizes or hypermagnesemia develops).
• IV magnesium is also used in:
  • Severe asthma and COPD exacerbation: 2 gm infused over 20 minutes.
  • Eclampsia/preeclampsia with severe features, seizure prophylaxis, and treatment: Initial: 4 to 6 g loading dose over 15 to 30 minutes at the onset of labor or induction/cesarean delivery, followed by 1 to 2 g/hour continuous infusion for at least 24 hours after delivery; maximum infusion rate: 3 g/hour.

• Use oral magnesium in:
  • Hypomagnesemic patient with mild to moderate hypomagnesemia and no or minimal symptoms. However, many patients are unable to take oral magnesium or have side effects such as gastrointestinal discomfort and diarrhea. Thus, many hospitalized patients with hypomagnesemia are given IV rather than oral magnesium supplementation, even if symptoms are minimal or absent.
  • Sustained released preparation is preferred over instant release ones.
    • Mg chloride and Mg lactate (sustained release), can be given once daily.
    • 2-4 tablets may be sufficient for mild, asymptomatic disease.

• If a sustained-release preparation is not available, magnesium oxide 800 to 1600 mg daily in divided doses may be used. Diarrhea frequently occurs with magnesium oxide therapy.
• Oral preparations can also be used as well as laxative and heartburn medications.

• Patients with concomitant hypokalemia or hypocalcemia should also receive potassium and calcium replacement, because these disorders may take several days to correct when treated with magnesium alone.

Monitoring

• Patients with hypomagnesemia should get a 12-lead EKG and placed on telemetry.
• The magnesium concentration should be measured 6 to 12 hours after each dose of IV magnesium, avoid measuring the level soon as Mg level may be artificially high.
• In stable hospitalized patients receiving magnesium therapy, the plasma magnesium concentration should be measured daily or more frequently if indicated. Repeat doses are given based on the follow-up measurements.
• Repeat doses are given based on the follow-up measurement.
• Plasma magnesium levels do not correlate well with total body stores, as the majority of magnesium is intracellular; plasma concentrations may be transiently elevated for a few hours after administration of an IV dose.
• Patients undergoing intravenous magnesium replacement should be monitored for evidence of acute hypermagnesemia (eg, respiratory depression, areflexia).
• Serum magnesium levels usually rise quickly with therapy, but intracellular stores take longer to replete. It is therefore advisable in patients with normal kidney function to continue magnesium repletion for at least one to two days after the serum magnesium concentration normalizes.