For an adult patient (~70 kg) with chronic hypernatremia and a serum sodium of 169 mEq/L, what is the free‑water deficit and what D5W (5 % dextrose in water) infusion rate should be used to correct it?

Calculating Free Water Deficit and D5W Infusion Rate for Chronic Hypernatremia (Sodium 169 mEq/L)

For an adult patient (~70 kg) with chronic hypernatremia and serum sodium of 169 mEq/L, calculate the free water deficit using the formula: Water deficit (L) = 0.6 × body weight (kg) × [(current Na ÷ 140) – 1], which yields approximately 12.2 liters, and initiate D5W at 100–150 mL/hour to achieve a safe correction rate of 8–10 mEq/L per 24 hours. 1

Step 1: Calculate the Free Water Deficit

• Use the standard water deficit formula: Water deficit (L) = 0.6 × body weight (kg) × [(current Na ÷ 140) – 1] 1, 2
• For a 70 kg adult with sodium 169 mEq/L: Water deficit = 0.6 × 70 × [(169 ÷ 140) – 1] = 0.6 × 70 × 0.207 = 8.7 liters 1
• This formula assumes total body water is 60% of body weight in men and 50% in women; adjust the multiplier to 0.5 for women 2
• Critical caveat: This calculation grossly underestimates total body water loss by 40–50% but provides a reasonable starting point for free water replacement 2

Step 2: Determine the Safe Correction Rate

• Maximum safe correction rate for chronic hypernatremia (>48 hours duration) is 8–10 mEq/L per 24 hours to prevent cerebral edema from rapid osmotic shifts 1, 3, 4
• For sodium 169 mEq/L, target reduction to 159–161 mEq/L in the first 24 hours 1, 3
• Never exceed 10 mEq/L correction in 24 hours regardless of symptom severity in chronic cases 1, 4
• Acute hypernatremia (<48 hours) can be corrected more rapidly at 1 mEq/L per hour for the first 6–8 hours, but this scenario is rare in adults 3, 4

Step 3: Calculate D5W Infusion Rate

• D5W provides pure free water after glucose metabolism and is the preferred fluid for correcting hypernatremia without adding sodium load 1, 5, 3, 4
• To decrease sodium by 8–10 mEq/L in 24 hours, infuse approximately 3–4 liters of D5W over 24 hours (125–167 mL/hour) 1, 3
• Practical starting rate: 100–150 mL/hour of D5W allows for safe correction while accounting for ongoing insensible losses and urinary output 1, 3, 6
• For a 70 kg patient with sodium 169 mEq/L: Start D5W at 125 mL/hour (3 liters/24 hours) and adjust based on serial sodium measurements 1, 3

Step 4: Account for Ongoing Losses

• Add estimated ongoing free water losses to the calculated deficit to determine total fluid requirements 1, 4
• Insensible losses average 500–1000 mL/day (higher with fever, tachypnea, or burns) 4
• Urinary losses depend on renal concentrating ability; patients with diabetes insipidus may lose 5–10 liters/day 1, 4
• For nephrogenic diabetes insipidus specifically, D5W is preferred over 0.45% saline because the dilute urine (very low sodium) aligns with D5W's sodium-free composition 1

Step 5: Monitoring Protocol

• Check serum sodium every 2–4 hours during initial correction phase to ensure the rate does not exceed 10 mEq/L per 24 hours 1, 3, 4
• Measure urinary output hourly; consider urinary catheter placement for accurate monitoring 1
• Assess volume status continuously (vital signs, skin turgor, mucous membranes) to guide fluid rate adjustments 1, 4
• If sodium decreases faster than 10 mEq/L in 24 hours, reduce D5W rate by 25–50% to prevent cerebral edema 3, 4
• Once sodium reaches 150–155 mEq/L, slow correction to 5–8 mEq/L per day 1, 4

Special Considerations for Chronic Hypernatremia

• Chronic hypernatremia (>48 hours) allows brain cells to generate idiogenic osmoles that protect against cell shrinkage; rapid correction causes these osmoles to draw water into cells, precipitating cerebral edema 4
• Patients with cirrhosis, malnutrition, or advanced age require even slower correction (6–8 mEq/L per 24 hours maximum) due to impaired adaptive mechanisms 7, 4
• Avoid isotonic saline (0.9% NaCl) as initial therapy because it delivers 154 mEq/L sodium and worsens hypernatremia by adding osmotic load 1, 4
• In patients with renal concentrating defects (e.g., nephrogenic diabetes insipidus), isotonic fluids will maintain or worsen hypernatremia; hypotonic fluids are mandatory 1

Alternative Fluid Options (When D5W is Contraindicated)

• 0.45% saline (half-normal saline) provides 77 mEq/L sodium and can be used if D5W causes hyperglycemia, but correction will be slower 1
• 0.18% saline (quarter-normal saline) contains 31 mEq/L sodium and provides more free water than 0.45% saline 1
• Oral free water via nasogastric tube (if patient cannot drink) can supplement IV therapy at 200–400 mL every 4 hours 6
• Desmopressin (DDAVP) 1–2 mcg IV/SC every 12 hours can reduce urinary free water losses in central diabetes insipidus but is ineffective in nephrogenic DI 6

Critical Pitfalls to Avoid

• Never give D5W as a rapid bolus because it causes precipitous sodium decline and cerebral edema; always infuse as continuous drip 1
• Never use isotonic saline (0.9% NaCl) to correct hypernatremia unless the patient is in hypovolemic shock requiring immediate volume resuscitation 1, 4
• Never correct chronic hypernatremia faster than 10 mEq/L in 24 hours regardless of symptom severity 1, 3, 4
• Failing to account for ongoing losses (insensible, urinary, gastrointestinal) will result in inadequate correction 1, 4
• In patients receiving continuous renal replacement therapy (CRRT), standard isotonic dialysate will overcorrect hypernatremia; infuse calculated D5W prefilter to prevent this 5

Example Calculation for 70 kg Adult with Sodium 169 mEq/L

1. Water deficit = 0.6 × 70 × [(169 ÷ 140) – 1] = 8.7 liters 1, 2
2. Target correction = 8–10 mEq/L in 24 hours (sodium 159–161 mEq/L) 1, 3
3. D5W rate = 125 mL/hour (3 liters/24 hours) 1, 3
4. Add ongoing losses = 500 mL insensible + urinary output (measure hourly) 1, 4
5. Total fluid requirement = 3 liters D5W + 0.5 liters insensible + urinary losses 1, 4
6. Adjust rate every 4 hours based on sodium measurements and clinical response 1, 3