The abnormality chosen is:
HYPOMAGNESEMIA, a serum magnesium level less than 1.4 mEq/L. A low serum magnesium level often induces hypokalemia and/or hypocalcemia. If the hypomagnesemia is not first corrected, then the low potassium and/or low calcium will be refractory to treatment. Symptoms may include lethargy, confusion, tremor, fasciculation, ataxia, nystagmus, tetany, or seizure activity. Patients with severe hypomagnesemia may manifest with Chvosteks sign (facial spasm induced by tapping over the facial nerve below the zygomatic arch and in front of the ear) or Trousseaus sign (carpal spasm induced by inflation of a sphygmomanometer above the systolic pressure for more than 3 minutes). Electrocardiographic changes include prolongation of the PR and/or QT intervals or the development of atrial or ventricular arrhythmias. Digitalis toxicity is markedly potentiated by low serum magnesium levels.
The first step in the workup of persistent hypomagnesemia is to determine if the cause is secondary to either a gastrointestinal (malabsorption, diarrhea, or steatorrhea) or renal cause. The most effective method to determine the source of magnesium loss is the determination of a 24-hour urine magnesium level. The level will be low when gastrointestinal losses with an adequate renal response are the underlying etiology. Conversely, when renal magnesium wasting is the cause, the 24-hour urinary magnesium level will be elevated or inadequately suppressed. Medications (diuretics, digoxin, aminoglycoside antibiotics, amphotericin B, etc.) are a common cause of excessive renal magnesium excretion.
Hypomagnesemia may be corrected with either intravenous magnesium sulfate or oral magnesium oxide (400 mg PO 4-6 times per day) or with gluconate (250-500 mg PO QD), but these should be given with extreme caution in patients with renal failure. When symptoms are not life-threatening, oral replacement is preferable and should be continued for 3 to 4 days to ensure adequate body stores. In emergent cases, magnesium sulfate 2 grams in 100 ml of either normal saline or D5W may be given over 10 minutes and then followed by continuous intravenous infusion therapy by giving 2-6 grams in one liter of fluid over the next 6 hours. Serial determinations of the serum magnesium level should be made during replacement therapy, especially in patients with renal failure.