HYPERNATREMIA
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This is rarely associated with hypervolemia, where the association is typically iatrogenic, e.g., administration of hypertonic sodium bicarbonate. More commonly, hypernatremia is the result of a combined H2O and volume deficit, with losses of H2O in excess of Na+. Elderly individuals with reduced thirst and/or diminished access to fluids are at the highest risk of hypernatremia due to decreased free H2O intake. Common causes of renal H2O loss are osmotic diuresis secondary to hyperglycemia, postobstructive diuresis, or drugs (radiocontrast, mannitol, etc.); H2O diuresis occurs in central or nephrogenic diabetes insipidus (DI) (Chap. 168). In pts with hypernatremia due to renal loss of H2O, it is critical to quantify ongoing daily losses in addition to calculation of the baseline H2O deficit (Table 1-2).
| H2O Deficit |
1. Estimate TBW: 50–60% body weight (kg) depending on body composition 2. Calculate free-water deficit: [(Na+ – 140)/140] × TBW 3. Administer deficit over 48–72 h |
| Ongoing H2O Losses |
4. Calculate free-water clearance, CeH2O: where V is urinary volume, UNa is urinary [Na+], UK is urinary [K+], and SNa is serum [Na+]. |
| Insensible Losses |
| 5. ~10 mL/kg per day: less if ventilated, more if febrile |
| Total |
| 6. Add components to determine H2O deficit and ongoing H2O loss; correct the H2O deficit over 48–72 h and replace daily H2O loss. |
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