Hypomagnesemia
ataxia, and seizures may be encountered. Hypomagnesemia is frequently associated with both hypocalcemia (impaired PTH secretion) and hypokalemia (due to renal K+ wasting). Cardiac manifestations include electrical irritability and potentiation of digoxin toxicity; both factors are aggravated by hypokalemia. Hypomagnesemia is associated with an increased incidence of atrial fibrillation. Prolongation of the P-R and QT intervals may also be present.
Asymptomatic hypomagnesemia can be treated orally or intramuscularly. Serious manifestations such as seizures should be treated with intravenous magnesium sulfate, 1-2 g (8-16 mEq or 4-8 mmol) given slowly over 15-60 min.
Although no specific anesthetic interactions are described, coexistent electrolyte disturbances such as hypokalemia, hypophosphatemia, and hypocalcemia are often present and should be corrected prior to surgery. 
Isolated hypomagnesemia should be corrected prior to elective procedures because of its potential for causing cardiac arrhythmias. Moreover, magnesium appears to have intrinsic antiarrhythmic properties and possibly cerebral protective effects (see Chapter 26). It is frequently administered preemptively to lessen the risk of postoperative atrial fibrillation in patients undergoing cardiac surgery.
Inadequate intake
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Reduced gastrointestinal absorption
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Increased renal losses
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Decreased Intake or Absorption. Malabsorption, chronic GI losses, deficient intake (alcoholics), TPN without adequate supplementation
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Increased Loss. Diuretics, other medications (gentamicin, cisplatin, amphotericin B, others), RTA, DM (especially DKA), alcoholism, hyperaldosteronism, excessive lactation
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Other. Acute pancreatitis, hypoalbuminemia, vitamin D therapy
is often associated with deficiencies of other intracellular components such as potassium and phosphorus. It is commonly found in patients undergoing major cardiothoracic or abdominal operations, and its incidence among patients in intensive care units may exceed 50%