Hypermagnesemia is a greater-than-normal serum concentration of
magnesium. A serum magnesium level can appear falsely ele-vated when blood
specimens are allowed to hemolyze or are drawn from an extremity with a
tourniquet that was applied too tightly.
By far the most common
cause of hypermagnesemia is renal failure. In fact, most patients with advanced
renal failure have at least a slight elevation in serum magnesium levels. This
condition is aggravated when such patients receive magnesium to control
seizures or inadvertently take one of the many commercial antacids that contain
Hypermagnesemia can occur in a patient with untreated di-abetic
ketoacidosis when catabolism causes the release of cellu-lar magnesium that
cannot be excreted because of profound fluid volume depletion and resulting
oliguria. An excess of mag-nesium can also result from excessive magnesium administered
to treat hypertension of pregnancy and to lower serum magne-sium levels.
Increased serum magnesium levels can also occur in adrenocortical
insufficiency, Addisonâ€™s disease, or hypother-mia. Excessive use of antacids
(eg, Maalox, Riopan, Mylanta)and laxatives (Milk of Magnesia) also increases
serum magne-sium levels.
Acute elevation of the serum magnesium level depresses the cen-tral
nervous system as well as the peripheral neuromuscular junc-tion. At mildly
elevated levels, there is a tendency for lowered blood pressure because of
peripheral vasodilation. Nausea, vom-iting, soft tissue calcifications, facial
flushing, and sensations of warmth may also occur. At higher magnesium
concentrations, lethargy, difficulty speaking (dysarthria), and drowsiness can
occur. Deep tendon reflexes are lost, and muscle weakness and paralysis may
develop. The respiratory center is depressed when serum magnesium levels exceed
10 mEq/L (5 mmol/L). Coma, atrioventricular heart block, and cardiac arrest can
occur when the serum magnesium level is greatly elevated and not treated.
On laboratory analysis, the serum magnesium level is greater than 2.5
mEq/L or 3.0 mg/dL (1.25 mmol/L). ECG findings may in-clude a prolonged PR
interval, tall T waves, and a widened QRS. ECG findings demonstrate a prolonged
QT interval and atrio-ventricular blocks.
Hypermagnesemia can be prevented by avoiding the administra-tion of
magnesium to patients with renal failure and by carefully monitoring seriously
ill patients who are receiving magnesium salts. In patients with severe
hypermagnesemia, all parenteral and oral magnesium salts are discontinued. In
emergencies, such as res-piratory depression or defective cardiac conduction,
ventilatory support and IV calcium are indicated. In addition, hemodialysis
with a magnesium-free dialysate can reduce the serum magnesium to a safe level
within hours. Loop diuretics and 0.45% sodium chloride (half-strength saline)
solution enhance magnesium ex-cretion in patients with adequate renal function.
IV calcium glu-conate (10 mL of a 10% solution) antagonizes the neuromuscular
effects of magnesium.
Patients at risk for hypermagnesemia are identified and assessed. When
hypermagnesemia is suspected, the nurse monitors the vital signs, noting
hypotension and shallow respirations. The nurse also observes for decreased
patellar reflexes and changes in the level of consciousness. Medications that
contain magnesium are not given to patients with renal failure or compromised
renal function, and patients with renal failure are cautioned to check with
their health care providers before taking over-the-counter medications. Caution
is essential when preparing and administering magnesium-containing fluids
parenterally because available parenteral mag-nesium solutions (eg, 2-mL
ampules or 50-mL vials) differ in concentration.