Serum magnesium

a. < 5 mg/dL. Produces mild weakness and hypotension only.

b. 5-10 mg/dL. Causes muscle weakness, hyporeflexia, and hypotension.

c. > 10 mg/dL. Increases risk for complete heart block or paralysis.

3. Serum calcium. Look for hypocalcemia, resulting from suppression of parathyroid hormone (PTH) secretion. C. Radiographic and Other Studies. Obtain ECG to look for arrhythmia, prolonged PR interval, increased QRS complex, increased qT wave, or heart block.

V. Plan. In patients with normal renal function, stopping the magnesium source is usually adequate. In those with severe hypermagnesemia, IV calcium can be used to block the effect of magnesium, and dialysis may be necessary if concomitant renal failure is present.

A. Severe Hypermagnesemia With Cardiac or Respiratory Failure. Block effects of high serum magnesium with IV calcium.

1. Neonates. Administer calcium gluconate, 100 mg/kg per dose (elemental calcium, 9 mg/kg per dose) IV over 20 minutes. Can repeat if necessary.

2. Older children and adolescents. Give 100-200 mg elemental calcium IV over 5-10 minutes to acutely antagonize effect of magnesium.

B. Hypermagnesemia With Renal Failure. Hemodialysis or peritoneal dialysis may be necessary.

VI. Problem Case Diagnosis. The 2-day-old infant had hypermagnesemia secondary to treatment of preeclampsia in the mother. Supportive care was provided; 1 week later, hypermagnesemia had completely resolved.

VII. Teaching Pearl: Question. Is hypermagnesemia or hypomagne-semia associated with suppressed PTH secretion and hypocalcemia in neonates?

VIII. Teaching Pearl: Answer. Paradoxically, both hypermagnesemia and hypomagnesemia may suppress PTH secretion and cause hypocal-cemia in neonates, although by different mechanisms.

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