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