Hypermagnesemia

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General role of magnesium
Causes of Hypermagnesemia
Clinical manifestations of hypermagnesemia
Treatment of hypermagenesemia
Anesthetic considersations

GENERAL ROLE OF MAGNESIUM

Intracellular cation involved in:
cofactor in many enzymatic pathways

Total body magnesium distribution:
67% of total body magnesium content within bone
31% of total body magnesium content located intracellular
1 2 % of total body magnesium content located within the extracellular fluid

Balance of magnesium involves:

Intake:
20 – 30 mEq/day
240 – 370 mg/day
30 – 40% of magnesium is absorbed within the distal portion of small bowel

Elimination:
renal excretion is the primary route of elimination of magnesium
renal excretion  6 –12 mEq/day
efficient reabsorption of magnesium by the kidneys
25% of filtered magnesium is reabsorbed in the PCT
50-60% of filtered magnesium is reabsorbed by the thick limb of the LOH

Factors which increase magnesium reabsorption by the kidneys include:
-hypomagnesemia
-parathyroid hormone (PTH)
-hypocalcemia
-extracellular fluid depletion
-metabolic acidosis

Factors which increase renal excretion of magnesium include:
-hypermagnesemia
-acute volume expansion
-hyperaldosteronism
-hypercalcemia
-ketoacidosis

CAUSES OF HYPERMAGNESEMIA

-excessive intake of magnesium
ex. magnesium containing antacids (laxatives)

-renal impairment of magnesium
ex. GFR < 30 ml/min

-magnesium sulfate therapy
ex. therapy for gestational hypertension

Other causes of hypermagnesemia include:
-adrenal insufficiency
-hypothyroidism
-rhabdomyolysis
-lithium administration

CLINICAL MANIFESTATIONS OF HYPERMAGNESEMIA

Central nervous system:
-hyporeflexia
-sedation

Cardiovascular system:
magnesium levels > 10 mg/dL:
-decreased MAP
magnesium levels > 24 mg/dL:
-myocardial depression
-vasodilation
-bradycardia
ecg changes may include:
-prolonged PR interval
-widening of QRS complex

Respiratory system:
-respiratory arrest with marked hypermagnesium

Neuromuscular system:
-skeletal muscle weakness

TREATMENT OF HYPERMAGNESEMIA
-discontinue magnesium intake
-intravenous calcium gluconate 1 gm may anatagonize the effects hypermagnesemia temporarily
-loop diuretics + ½ normal saline with dextrose may enhance urinary magnesium excretion
-dialysis for marked renal impairment

ANESTHETIC CONSIDERATIONS IN HYPERMAGNESEMIA

May require close monitoring of:
-blood pressure
-electrocardiogram
-neuromuscular junction function
-urinary foley catherization
-muscle relaxants should be reduced by 20 – 25% of regular dosing