Hypomagnesemia
GENERAL PRINCIPLES
• A serum magnesium 2 mEq (or >24 mg) or a fractional excretion of magnesium of >2% during hypomagnesemia suggests increased renal excretion. The fractional excretion of magnesium is calculated by:
f Urirι≤ Mg1' /Vririe Crj ÷ [(Serum Mg1' ? 0.7jy⅛erαm Cr^∣ ? 100
• Hypocalcemia and/or hypokalemia can often be found as a result of hypomagnesemia-induced derangements in mineral homeostasis.
• ECG abnormalities may include a prolonged PR and QT interval with a widened QRS. Torsades de pointe s is the classically associated arrhythmia.
TREATMENT
• In patients with normal renal function, excess magnesium is readily excreted, and there is little risk of causing hypermagnesemia with recommended doses. However, magnesium must be given with extreme care in the presence of renal insufficiency.
• The route of magnesium administration depends on whether clinical manifestations from magnesium deficiency are present.
î Asymptomatic hypomagnesemia can be treated orally. Numerous preparations exist, including Mag-Ox 400 (240 mg elemental magnesium per 400-mg tablet), UroMag (84 mg per 140-mg tablet), and sustained-release Slow-Mag (64 mg per tablet). Typically, approximately 240 mg of elemental magnesium is administered daily for mild deficiency, whereas more severe hypomagnesemia may require up to 720 mg/d of elemental magnesium. The major side effect is diarrhea.
î Severe symptomatic hypomagnesemia should be treated with 1-2 g magnesium sulfate (1 g magnesium sulfate = 96 mg elemental magnesium) IV over 15 minutes. To account for gradual redistribution to severely depleted intracellular stores, replacement therapy may need to be maintained, often for 3-7 days. Serum magnesium should be measured daily and the infusion rate adjusted to maintain a serum magnesium level of of respiratory alkalosis should focus on identifying and treating the underlying disease.
• In intensive care unit patients, this may involve changing the ventilator settings to decrease ventilation (see Chapter 8, Critical Care).