In a 50-year-old female weighing 65 kg with a serum sodium of 154 mEq/L, what is the free‑water deficit and what are the safe correction rates using free water and 0.45 % sodium chloride (half‑normal saline)?

Free Water Deficit Calculation and Correction Strategy for Hypernatremia

For this 50-year-old female (65 kg) with serum sodium 154 mEq/L, the free water deficit is approximately 2.6 liters, and correction should be achieved over 48 hours using D5W (dextrose 5% in water) as the primary fluid, with a maximum sodium reduction of 8–10 mEq/L per 24 hours to prevent cerebral edema. 1, 2

Free Water Deficit Calculation

Calculate total body water (TBW):

• For adult females: TBW = 0.5 × body weight (kg)
• TBW = 0.5 × 65 kg = 32.5 liters 1

Calculate free water deficit using the formula:

• Free water deficit = TBW × [(Current Na⁺ / Desired Na⁺) - 1]
• Free water deficit = 32.5 × [(154/140) - 1]
• Free water deficit = 32.5 × 0.10 = 3.25 liters 1

Alternative calculation targeting 145 mEq/L (safer initial target):

• Free water deficit = 32.5 × [(154/145) - 1]
• Free water deficit = 32.5 × 0.062 = 2.0 liters 1

Correction Rate Guidelines

Maximum safe correction limits:

• Never exceed 8–10 mEq/L decrease in serum sodium per 24 hours to prevent cerebral edema from overly rapid correction 1, 3, 2
• For chronic hypernatremia (>48 hours duration): aim for 8–10 mEq/L per day 4, 2
• For acute hypernatremia (<48 hours): may correct at 1 mEq/L per hour for the first 6–8 hours, then slow to 8–10 mEq/L per day 4, 2
• The induced change in serum osmolality should not exceed 3 mOsm/kg/h 5

Free Water Replacement Using D5W (5% Dextrose in Water)

D5W is the preferred fluid for hypernatremia correction:

• D5W delivers no renal osmotic load, allowing controlled correction of water deficit without adding sodium burden 1
• D5W provides free water once the dextrose is metabolized 1, 2

Initial D5W infusion rate calculation:

• To decrease sodium by 9 mEq/L over 24 hours (from 154 to 145 mEq/L):
• Administer approximately 2.0 liters D5W over 24 hours = 83 mL/hour 1
• Over 48 hours: 3.25 liters ÷ 48 hours = 68 mL/hour 1

Practical approach:

• Start D5W at 75–100 mL/hour initially 1
• Check serum sodium every 2–4 hours during active correction 6, 2
• Adjust rate to ensure sodium decreases no faster than 0.4 mEq/L per hour (≈10 mEq/L per 24 hours) 1, 6
• Continue until serum sodium reaches 145 mEq/L, then reassess 2

Correction Using 0.45% Sodium Chloride (Half-Normal Saline)

0.45% NaCl contains 77 mEq/L sodium with osmolarity ≈154 mOsm/L:

• This is a hypotonic solution appropriate for moderate hypernatremia correction 1
• Provides both free water and some sodium replacement 1

When to use 0.45% NaCl instead of D5W:

• Patient has concurrent hypovolemia requiring volume expansion 5, 2
• Patient needs electrolyte replacement in addition to free water 5
• After initial volume resuscitation with isotonic saline (0.9% NaCl) in hypovolemic patients, switch to 0.45% NaCl 5

0.45% NaCl infusion rate:

• For this patient: start at 100–150 mL/hour 5
• Provides approximately 2.4–3.6 liters over 24 hours
• Monitor serum sodium every 2–4 hours and adjust rate accordingly 6, 2

Critical warning for 0.45% NaCl:

• Use with great care in patients with congestive heart failure, severe renal insufficiency, or edema with sodium retention 7
• Risk of fluid overload is directly proportional to electrolyte concentration 7
• In patients with diminished renal function, may result in sodium retention 7

Initial Assessment and Volume Status

Before starting correction, determine volume status:

• Hypovolemic hypernatremia: Start with isotonic saline (0.9% NaCl) at 15–20 mL/kg/h until hemodynamically stable, then switch to D5W or 0.45% NaCl 5, 3
• Euvolemic hypernatremia: Use D5W as primary fluid 1, 2
• Hypervolemic hypernatremia: Use D5W and consider loop diuretics to remove excess sodium 2

Monitoring Protocol

Frequent biochemical monitoring is essential:

• Check serum sodium every 2–4 hours during initial correction phase 6, 2, 8
• Monitor serum potassium, chloride, magnesium, glucose, BUN, creatinine, and plasma osmolality every 2–4 hours 1
• Assess clinical status including neurological examination, fluid balance, and body weight 5
• Once sodium is stable and decreasing appropriately, can extend monitoring intervals to every 6 hours 2

Common Pitfalls to Avoid

Do not use isotonic saline (0.9% NaCl) as primary correction fluid:

• Normal saline contains 154 mEq/L sodium, which is higher than the patient's current level 1
• Isotonic fluids will worsen hypernatremia in patients unable to excrete free water appropriately 1
• Avoid normal saline in patients with renal concentrating defects (e.g., nephrogenic diabetes insipidus) 1

Do not correct too rapidly:

• Rapid correction (>10 mEq/L per 24 hours) increases risk of cerebral edema, seizures, and death 1, 3, 2
• In neonates and preterm infants, corrections faster than 48–72 hours markedly increase risk of pontine myelinolysis 1

Do not forget ongoing losses:

• Calculate and replace ongoing free water losses from urine, insensible losses, and other sources 6
• Patients with diabetes insipidus require ongoing hypotonic fluid administration to match excessive losses 1

Special Considerations

Identify and treat underlying cause:

• Assess for diabetes insipidus, inadequate water intake, excessive losses (diarrhea, burns), or medications causing hypernatremia 6, 9, 2
• Ensure patient has access to free water once able to drink 6, 9

Adjust for concurrent conditions:

• Patients with cardiac or renal compromise require more frequent monitoring of volume status and osmolality 5
• Avoid hypotonic fluids in patients at risk for cerebral edema from other causes 7