Chapter 36. Electrolyte Abnormalities (Na, K, Ca, Mg, PO₄)

Perioperative Management

• Hypernatremia: Na ≥145 mEq/L
  • Critical value: Na ≥ 160 mEq/L
  • Common causes: Hyperaldosteronism (excess mineralocorticoid), Cushing syndrome (excess glucocorticoid), excessive hypertonic saline or sodium bicarbonate administration, gastrointestinal losses, renal excretion, osmotic diuresis, diabetes insipidus
  • Signs and symptoms: Intense thirst, confusion, irritability, hyperreflexia, lethargy, coma, twitching, seizures
  • Notes:
    • Hypernatremia is always associated with a primary gain in Na or excess loss of water. Diagnosing the etiology hinges on assessment of the patient’s volume status (see Figure 36-1)
    • A rapid rate of rise in Na is associated with worse neurologic outcomes (central pontine myelinolysis):
      • Myelin sheath destruction in pons
      • Always iatrogenic
      • Very high mortality
      • Earliest symptom is difficulty speaking and swallowing
      • MRI is diagnostic; however, most cases are diagnosed at autopsy
    • Gradual increases in Na, even to levels ≥160 mEq/L, are generally well tolerated
  • Treatment: Calculate free water deficit (L) = {([measured Na] − 140)/140} × body weight (in kg) × 0.6 (men) or 0.5 (women)
    • If Na is moderately elevated:
      • Administer H2O enterally provided the gut is functional; otherwise judiciously infuse isosmotic, hyponatremic IV solution (D5W) with close monitoring
    • If treating acute central diabetes insipidus (DI):
      • Begin vasopressin IV at 2 U/h and titrate to reduce urine output to ≤0.5 mL/kg/h
      • Administer 0.45% NaCl 1 mL IV for each 1 mL urine output
  • Caution:
    • A decrease in serum sodium of >0.5 mEq/L/h can lead to cerebral edema. If the patient has elevated ICP, correct hyponatremia very slowly if at all
    • If the abnormality develops rapidly (over hours), the rate of correction can match the rate of acquisition even if it exceeds 0.5 mEq/L/h
• Hyponatremia: Na ≤135 mEq/L:
  • Critical value: Na ≤125 mEq/L
  • Common causes: Burns, sweating, vomiting, diarrhea, pancreatitis, diuretics, salt-wasting nephropathy, cerebral salt wasting, mineralocorticoid deficiency (Addison’s disease), congestive cardiac failure, cirrhosis with ascites, nephrotic syndrome, chronic renal failure, syndrome of inappropriate antidiuretic hormone secretion (SIADH), hypothyroidism, hypopituitarism (glucocorticoid deficiency), primary polydipsia, iatrogenic (excessive administration of parenteral hypotonic fluids, post-transurethral prostatectomy)
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