Adult-Gerontology Acute Care Practice Guidelines
Definition
A.Serum magnesium less than 1.5 mg/dL (see Box 5.4).
Incidence
A.2% of general population.
B.10% to 20% hospitalized patients.
Pathogenesis
A.Magnesium is the second most abundant intracellular cation in the body after potassium.
B.It is critical in the functioning of neuromuscular, cardiac, and nervous system functions. Magnesium is vital to vascular tone, heart rhythm, bone formation, and muscle contraction, among many other critical functions.
Predisposing Factors
A.Starvation.
B.Alcohol use.
C.Diarrhea.
D.Vomiting.
E.Gastrointestinal fistulas.
Subjective Data
A.Common complaints/symptoms.
1.Neuromuscular symptoms similar to hypocalcemia.
a.Hyperreflexia.
b.Carpopedal spasm.
c.Tetany.
d.Seizures.
e.Positive Chvostek’s and Trousseau’s signs.
2.Severe hypomagnesemia: Possible EKG changes and life-threatening arrhythmias (including torsades de pointes).
Physical Examination
A.Reflexes.
B.EKG.
C.Blood pressure.
D.Cardiac examination.
Diagnostic Tests
A.Serum magnesium.
B.Total calcium and ionized calcium (hypomagnesemia is often accompanied by hypokalemia and/or hypocalcemia).
C.Potassium.
D.Serum creatinine and blood urea nitrogen (BUN).
Differential Diagnosis
A.Decreased intake.
1.Chronic alcoholism.
2.Prolonged fasting.
3.Protein-calorie malnutrition.
4.Inadequate supplementation in parenteral nutrition-dependent patients.
B.Gastrointestinal losses.
1.Inflammatory bowel disease.
2.Chronic diarrhea.
3.Laxative abuse.
4.Malabsorption syndromes.
5.Surgical bowel resection or small intestinal bypass surgery.
C.Renal losses.
1.Drugs.
a.Diuretics.
b.Amphotericin B.
c.Aminoglycosides.
d.Cyclosporine.
e.Tacrolimus.
f.Pentamidine.
g.Proton pump inhibitors.
h.Foscarnet.
i.Cetuximab.
j.Cisplatin.
2.High urinary output.
a.Post-obstructive or resolving acute tubular necrosis (ATN) diuresis.
b.Post-transplant polyuria.
c.Hypercalcemia.
3.Inherited hypomagnesemia.
a.Gitelman’s syndrome.
b.Bartter’s syndrome.
c.Phosphate depletion.
4.Primary hyperaldosteronism.
5.Chronic metabolic acidosis.
6.Idiopathic renal wasting.
Evaluation and Management Plan
A.Step 1—Initiate intravenous (IV) magnesium replacement 4 g for all patients with severe symptoms or eclampsia/preeclampsia.
1.Deliver first dose quickly (over 4–5 minutes) if followed by an infusion or repeat slow IV bolus (over 6–12 hours).
2.Infuse 4 to 6 g over 8 to 12 hours.
a.Magnesium distributes into tissues slowly, but is rapidly eliminated by the kidney.
B.Step 2—Replace magnesium.
1.Available in IV and oral forms.
a.Use IV replacement for patients with severe depletion, those who cannot tolerate oral replacement, those who have eclampsia/preeclampsia, and those who are symptomatic.
b.Use oral supplements for chronic hypomagnesemia and asymptomatic states.
i.Absorption is unpredictable with oral supplements, and diarrhea is common.
ii.Repletion of total body stores takes several days.
2.Recommended IV doses of magnesium sulfate (dose should be decreased by 50% in patients with renal impairment).
a.1 to 4 g if serum magnesium 1 to 1.5 mg/dL.
b.4 to 8 g if serum magnesium less than 1 mg/dL.
3.Oral supplements.
a.Magnesium oxide 400 mg BID.
C.Step 3—Treat other electrolyte and acid–base disorders.
D.Step 4—Assess cause and initiate a plan for prevention.
1.Potassium-sparing diuretics can reduce renal magnesium wasting.
Follow-Up
A.Follow-up depends on underlying cause.
B.Patients who have cardiac arrhythmias should be closely followed until magnesium levels are restored.
Consultation/Referral
A.Consult cardiology as needed for cardiac arrhythmias.
B.Consult nephrology for management of renal impairment.
Special/Geriatric Considerations
A.Geriatric patients are at high risk for cardiac arrhythmias and renal impairment.
B.Caution should be used to assure magnesium levels are optimized in this patient population.
Bibliography
Kraft, M. D., Btaiche, I. F., Sacks, G. S., & Kudsk, K. A. (2005, August). Treatment of electrolyte disorders in adult patients in the intensive care unit. American Journal of Health System Pharmacy, 62(16), 1663–1682. doi:10.2146/ajhp040300
Moe, S. M., & Daoud, J. R. (2014). Disorders of mineral metabolism: Calcium, phosphorous, and magnesium. In D. S. Gipson, M. A. Perazella, & M. Tonelli (Eds.), National Kidney Foundation’s primer on kidney diseases (6th ed., pp. 100–112). Philadelphia, PA: Elsevier Saunders.