Free Water Deficit Calculator

Free water deficit (FWD) is the volume of pure water needed to correct hypernatremia to a normal sodium level (140 mEq/L). Formula: FWD (L) = 0.6 × Weight (kg) × (1 – 140/Na). Use 0.5 for women (lower total body water).

FWD = TBW × (1 – 140/Na), where TBW = 0.6 × Wt (men) or 0.5 × Wt (women). For a 70 kg man with Na 155: TBW = 42 L. FWD = 42 × (1 – 140/155) = 42 × 0.097 = 4.1 L. Replace half over 24h, rest over 24–48h.

Normal serum sodium is 135–145 mEq/L. Hypernatremia is defined as Na > 145 mEq/L. Severe hypernatremia is Na > 160 mEq/L. Hypernatremia reflects a deficit of free water relative to total body sodium content.

Total body water (TBW) differs by sex: men have ~60% water content, women ~50% due to higher proportion of adipose tissue (which has less water than muscle). Use 0.45–0.5 for elderly and 0.3–0.4 for morbidly obese to avoid overcorrection.

Maximum correction rate: 0.5 mEq/L/hr (10–12 mEq/L/24h). Replace half of the calculated FWD in the first 24 hours, the remainder over the next 24–48 hours. In chronic hypernatremia (> 48h), correct more slowly (0.3–0.4 mEq/L/hr).

Rapid correction causes cerebral edema as water shifts into brain cells (which have accumulated organic osmolytes to adapt to hypernatremia). This can cause seizures, coma, brain herniation, and death. The brain takes 48–72 hours to eliminate these osmolytes.

Mild (Na 145–150): thirst, dry mucous membranes. Moderate (Na 150–160): lethargy, irritability, muscle weakness, fever. Severe (Na > 160): altered mental status, seizures, coma, intracerebral hemorrhage (from brain cell shrinkage).

Causes: insufficient water intake (elderly, infants, altered mental status), diabetes insipidus (central or nephrogenic), excessive sodium intake (hypertonic saline, sodium bicarbonate, salt ingestion), osmotic diuresis (hyperglycemia, mannitol), diarrhea, burns.

DI is a condition of excessive dilute urine output due to ADH deficiency (central DI) or renal ADH resistance (nephrogenic DI). Central DI: treat with desmopressin (DDAVP). Nephrogenic DI: treat with thiazides + amiloride, low-salt diet, NSAIDs.

Water deprivation test: after fluid restriction, administer DDAVP. Central DI: Urine osmolality rises significantly after DDAVP. Nephrogenic DI: No response to DDAVP. Also check plasma ADH levels (low in central, high in nephrogenic).

D5W (dextrose 5% in water) or 0.45% NS (half-normal saline). D5W provides free water without sodium. 0.45% NS provides 77 mEq/L Na (half isotonic). In patients with hypotension, use NS first then switch to hypotonic fluids.

Daily maintenance water = 30–35 mL/kg (adults) plus 1 mL/kg for each mEq/L above 145. For a 70 kg patient with Na 155: maintenance = 30×70 = 2100 mL/day + additional (155-145)×70 = 700 mL = total 2800 mL/day.

TBW determines the volume of distribution for sodium. A smaller TBW (women, elderly, malnourished) means less water is needed to correct the same sodium level. Using 0.6 for everyone overestimates FWD in women and the elderly.

Neonates: 75–80% water. Infants: 70%. Children: 65%. Young adults: 60% (men), 50% (women). Elderly: 50–55% (men), 40–45% (women). Use the age-appropriate TBW fraction for more accurate FWD calculations.

Use TBW = 0.6 for children > 5 years, 0.65 for 1–5 years, 0.7 for infants 1–12 months, 0.75 for neonates. For a 10 kg child (1 yr) with Na 158: TBW = 7 L. FWD = 7 × (1 – 140/158) = 7 × 0.114 = 0.8 L. Use D5 1/4NS to correct.

Fever increases insensible water losses by ~10% per 1°C above 37°C. Add 2–3 mL/kg/24h per degree of fever to the daily water needs. Tachypnea (ventilated patients) also increases insensible loss.

In HHS, the water deficit is significant (often 8–12 L). Use corrected Na to calculate FWD. For a 80 kg man with HHS (corrected Na 160): TBW = 48 L. FWD = 48 × (1 – 140/160) = 48 × 0.125 = 6 L. Replace half over 24h (3 L).

Use adjusted or ideal body weight rather than actual weight in obesity. TBW fraction is lower (0.5 for men, 0.4 for women). For a 120 kg man (IBW 70 kg): use TBW = 0.6 × 70 = 42 L (not 0.6 × 120 = 72 L which overestimates FWD).

DDAVP (desmopressin) is used to treat central diabetes insipidus. It replaces ADH, reducing excessive dilute urine output and allowing the administered water to correct the sodium. Dose: 1–2 mcg IV/SC or 10–20 mcg intranasally.

Burn patients have increased insensible losses (2–3 L/day above normal for large burns). The FWD formula may underestimate needs. Use the Parkland formula for the first 24h, then calculate ongoing replacement as: 1 mL/kg/%TBSA + maintenance.

Inadequate free water replacement (iatrogenic) is the most common cause in hospitalized patients. Patients on fluid restriction, NPO status, impaired thirst, or inability to access water. Lactulose therapy for hepatic encephalopathy also causes hypernatremia.

Hypernatremia (Na > 145) directly causes hyperosmolality. But hyperosmolality can also occur with normal Na if glucose or BUN are very high. Hypernatremia always means water deficit. Hyperosmolality can be due to hypernatremia, hyperglycemia, or uremia.

Diarrheal fluid is hypotonic (loses more water than sodium). This leads to free water loss and relative sodium excess. Severe diarrhea in infants can rapidly cause life-threatening hypernatremia. Replace with D5 1/4NS or ORS.

Admit to ICU. Calculate FWD using TBW. Replace with D5W at 0.3–0.5 mEq/L/hr. Check Na every 2 hours. Goal: reduce Na by no more than 10 mEq/L in 24h. Consider hemodialysis if Na > 200 or if renal failure is present.

Hypernatremia causes brain cell shrinkage (water moves out of cells). The brain adapts by producing organic osmolytes (myoinositol, glutamine, taurine) over 48–72 hours. Rapid correction causes water influx before osmolytes are cleared, leading to cerebral edema.

Severe hypernatremia (Na > 160) has 40–60% mortality in adults (higher in elderly and ICU patients). Mortality is related to the underlying cause (sepsis, dehydration, renal failure). Na > 180 has > 75% mortality.

Prolonged exercise with excessive sweating causes free water loss (sweat is hypotonic). Replace with water (not sports drinks) to dilute the sodium. Occurs in marathon runners, triathletes, and military personnel. Prevention: drink to thirst, avoid overhydration.

Hemodialysis is reserved for extreme hypernatremia (Na > 200), hypernatremia with renal failure, or when medical correction is not feasible. Dialysate sodium is set at 140–145 mEq/L. Correct slowly (max 10 mEq/L per session).

TPN provides concentrated glucose and amino acids that increase the water requirement. Add 1 mL/kcal of TPN to the daily water needs. For a patient receiving 2000 kcal TPN: add 2 L water. Monitor serum Na and osmolality daily.