How is a free water deficit treated?

Treatment of Free Water Deficit

Free water deficit is treated primarily with 5% dextrose in water (D5W) administered intravenously, with the rate of correction carefully controlled to avoid exceeding 8-10 mEq/L per day to prevent osmotic demyelination. 1

Calculating the Water Deficit

• Use the formula: Water deficit = Total body water × [(Current Na⁺/Desired Na⁺) - 1], where Total body water (TBW) = 0.6 × weight in kg for adult males 1
• For example, in a 102 kg male: TBW = 0.6 × 102 = 61.2 liters 1
• If current sodium is 155 mEq/L and desired is 145 mEq/L: Water deficit = 61.2 × [(155/145) - 1] = approximately 4.2 liters 1

Fluid Selection: Critical Decision Point

D5W is the primary IV fluid for hypernatremic dehydration—never use normal saline (0.9% NaCl) as it will paradoxically worsen hypernatremia. 1

• D5W delivers no renal osmotic load, allowing controlled correction without adding sodium burden 1
• Normal saline has a tonicity approximately 3-fold higher than typical urine osmolality in hypernatremic states and will exacerbate the problem 1
• In severe cases with concurrent DKA/HHS, combination therapy with D5W, ringer's lactate, and free water via nasogastric tube may be necessary 2

Rate of Administration

Calculate initial D5W rate by dividing total water deficit by desired correction time (typically 48 hours): For example, 6.1 L ÷ 48 hours = 127 mL/hour. 1

• The induced change in serum osmolality should not exceed 3 mOsm/kg H₂O per hour 3
• Correction rate must not exceed 8-10 mEq/L per day to prevent central demyelinating lesions 1
• For adults, baseline physiological fluid requirement is 25-30 ml/kg/24h, which should be added to deficit replacement 1

Monitoring Protocol

Check serum sodium every 4-6 hours during initial correction and adjust the D5W rate based on these measurements. 1

• Monitor hemodynamic status (blood pressure improvement), fluid input/output, and clinical examination 3
• Assess for signs of fluid overload, particularly in patients with renal or cardiac compromise 1
• More frequent monitoring of serum osmolality and mental status is required in patients with renal or cardiac disease 1
• Continue monitoring until osmolality normalizes to <300 mOsm/kg (or calculated osmolarity <295 mmol/L) 3

Concurrent Electrolyte Management

• Address potassium abnormalities concurrently with sodium correction, as free water deficit often accompanies other electrolyte disturbances 1
• Once renal function is assured, consider adding 20-30 mEq/L potassium to IV fluids 3

Special Considerations for Severe Cases

In patients with combined hypernatremia and hyperglycemic crises (DKA/HHS), desmopressin and free water administration via nasogastric tube can be helpful adjuncts. 2

• This rare combination requires careful fluid selection to address both the hyperglycemia and severe hypernatremia simultaneously 2
• Mental status may not improve until both the glucose and sodium are adequately corrected 2

Critical Pitfalls to Avoid

• Never use 0.9% NaCl as primary fluid—it worsens hypernatremia by providing excessive osmotic load 1
• Never correct sodium too rapidly—exceeding 8-10 mEq/L per day risks osmotic demyelination syndrome, which can cause permanent neurological damage or death 1
• Do not fail to adjust therapy based on frequent sodium measurements 1
• Avoid overlooking volume status assessment in patients with compromised renal or cardiac function 1

Transition to Maintenance

• Once serum sodium begins normalizing and approaches 145-150 mEq/L, transition to maintenance fluid regimen with appropriate sodium concentration 1
• Continue monitoring periodically alongside excellent support for oral fluid intake once patient can tolerate it 3