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SOURCE

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Source: Brophy DF, Flurie RW. Disorders of potassium and magnesium homeostasis. In: DiPiro JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM, eds. Pharmacotherapy: A Pathophysiologic Approach. 10th ed. New York, NY: McGraw-Hill; 2017. http://accesspharmacy.mhmedical.com/content.aspx?bookid=1861&sectionid=134127639. Accessed March 31, 2017.

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DEFINITION

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  • Serum magnesium <1.4 mEq/L (<1.7 mg/dL, <0.70 mmol/L)

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ETIOLOGY

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  • Drugs or disorders that interfere with intestinal absorption or increase renal excretion of magnesium can cause hypomagnesemia.

    • Small bowel disease most common cause worldwide.

    • Commonly associated with alcoholism.

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PATHOPHYSIOLOGY

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  • Primary renal magnesium wasting caused by:

    • Defect in renal tubular magnesium reabsorption.

    • Inhibition of sodium reabsorption in which magnesium transport follows passively.

    • Thiazide and loop diuretics.

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EPIDEMIOLOGY

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  • Common problem in both ambulatory and hospitalized patients.

  • Up to 65% of intensive care unit patients are magnesium deficient.

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PREVENTION

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  • Include green vegetables such as spinach in diet.

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RISK FACTORS

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  • Alcoholism.

  • Malnutrition.

  • Excessive vomiting.

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CLINICAL PRESENTATION

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SIGNS AND SYMPTOMS
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  • Neuromuscular disturbances earliest manifestations.

    • Tremor.

    • Fasciculations.

    • Tetany.

    • Chvostek’s sign.

    • Trousseau’s sign.

    • Generalized convulsions.

  • Cardiovascular.

    • Palpitations.

    • Arrhythmias.

    • Sudden cardiac death.

    • Hypertension.

    • Electrocardiogram (ECG) changes:

      • Mild hypomagnesemia.

        • Widened QRS complex.

        • Peaked T waves.

      • Moderate to severe hypomagnesemia.

        • Prolonged PR interval.

        • Progressive widening of QRS complex.

        • Flattened T waves.

  • Other electrolyte disturbances.

    • Hypokalemia.

    • Hypocalcemia.

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DIAGNOSIS

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MEANS OF CONFIRMATION AND DIAGNOSIS
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  • Serum magnesium <1.4 mEq/L (<1.7 mg/dL, <0.70 mmol/L)

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LABORATORY TESTS
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  • Metabolic panel.

    • Serum calcium.

    • Serum potassium.

  • Serum magnesium.

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DIAGNOSTIC PROCEDURE
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  • ECG

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DIFFERENTIAL DIAGNOSIS
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DESIRED OUTCOMES

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  • Resolve signs and symptoms.

  • Restore normal magnesium concentrations.

  • Correct concomitant electrolyte abnormalities.

  • Identify and correct underlying cause of magnesium depletion.

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TREATMENT: PHARMACOLOGIC THERAPY

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  • Severity of magnesium depletion and presence of symptoms dictate route of magnesium supplementation (Table 1).

    • Diarrhea dose-limiting toxicity with oral therapy.

    • Intramuscular magnesium is painful and should be reserved for patients with severe hypomagnesemia and limited venous access.

    • IV bolus injection is associated with flushing, sweating, and sensation of warmth.

  • Optimal replacement regimen is unknown.

    • Serum magnesium >1 mEq/L (>0.5 mmol/L): use oral products.

      • Magnesium uptake is a slow process that may require prolonged administration.

      • Diarrhea is the most common dose-limiting side effect.

    • Serum magnesium <1 mEq/L (<0.5 mmol/L) or s/s present: use IV products.

      • 4–6 g in 50–100 mL should be administered in divided doses over 12–24 hours.

      • Repeat as needed to in order to maintain magnesium > 1 mEq/L (0.5 mmol/L)

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MONITORING

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  • Evaluate serum magnesium concentration hourly when treating symptomatic patients.

    • Once symptoms resolve, evaluate serum magnesium concentration every 6–12 hours for 24 hours.

  • 3–5 days required to fully replete body stores.

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