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Hypomagnesemia

Definition

  • Serum Mg++ <1.8 mg/dL

Etiology:

  • Malnutrition (assc. with heavy chronic alcohol use causing renal wasting)
  • Malabsorption or diarrhea/GI loss
  • PPI induced
  • Renal losses (polyuria and high tubular flow, e.g., osmotic diuresis, post-ATN diuresis)
  • Hypercalcemia (e.g. hyperparathyroidism)
  • Loop and thiazide diuretic associated
  • Proximal tubular toxins (e.g. aminoglycosides, amphotericin, cisplatin)
  • Calcineurin inhibitors (tacrolimus > cyclosporine)
  • Volume expansion (reduced Mg++ reabsorption due to reduced Na and H2O reabsorption)
  • Gitleman and Bartter syndromes
  • Uncontrolled DM, post-parathyroidectomy (Hungry bone syndrome)

    Clinical manifestations:

    • NM hyperexcitability (tremor, tetany, convulsions), weakness, delirium, coma
    • CV (widening QRS, peaked T waves, wide PR interval, atrial and ventricular arrhythmias)
    • Hypocalcemia, hypoparathyroidism, PTH resistance, and decreased calcitriol synthesis

    Evaluation:

    • Order CMP, serum Mg++ and Phosphorus level
    • Review patient history, clinical circumstances, nutritional status and medications

    Management: PowerChart “Electrolyte Replacement Protocol”

    Approach to Rx:

    Route and dose based on severity of clinical manifestations and degree of hypomagnesemia

    Patients with no or minimal symptoms:

    • PO repletion recommended if able to tolerate
    • Can give IV if unable or have GI side effects (discomfort, diarrhea)
    • Typical daily PO dose in patients with normal renal function is 240-1000mg (20-40 mEq of elemental Mg++ in divided doses

     Patients with severe symptoms: tetany, arrhythmias, seizure

    • 1-2 grams (8-16 mEq) Magnesium sulfate bolus over 2-15mins if hemodynamically unstable (including those with arrhythmias consistent with torsade de pointes or hypomagnesemic hypokalemia). Repeat bolus if remains hemodynamically unstable
    • If hemodynamically stable, give 1-2 grams Magnesium sulfate in 50-100mL of D5W over 5-60mins followed by infusion of 4-8 grams MgSulfate slowly over 12-24 hrs
    • Adjust dose in AKI and CKD due to risk for severe hypermagnesemia
    • Measure serum Mg 6-12 hrs after each IV dose and adjust dose accordingly

    For routine IV or maintenance repletion, use the following estimated repletion doses:

    • If the plasma Mg++ is < 1 mg/dL, give 4 to 8 grams (32 to 64 mEq of magnesium sulfate over 12 to 24 hours and repeat as needed.
    • If the plasma Mg++ is 1 to 1.5 mg/dL, give 2 to 4 grams (16 to 32 mEq of magnesium sulfate over 4 to 12 hours.
    • If the plasma Mg++ is 1.6 to 1.9 mg/dL, give 1 to 2 grams (8 to 16 mEq of magnesium sulfate over 1 to 2 hours.

    (Conversion relationships: 1 mmol = 2 mEq = 24 mg of elemental magnesium = 240 mg magnesium sulfate.)

    Key Points:

    • Correct the underlying disease
    • Correct Mg++ based on severity of hypomagnesemia and symptoms if any
    • Great caution should be exercised when treating hypomagnesemia in AKI and CKD patients due to the increased risk for severe hypermagnesemia
    • Replacement therapy with IV magnesium in patients with arrhythmias or NM symptoms

     

     

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