In a drowsy adult with chronic obstructive pulmonary disease and severe hypernatremia (serum sodium ≈167 mmol/L), how many liters of 0.45 % saline (half‑normal saline) are required to lower the serum sodium by roughly 10 mmol/L?

Calculating Hypotonic Saline Volume for Sodium Correction in Severe Hypernatremia

In a drowsy adult with COPD and severe hypernatremia (serum sodium ≈167 mmol/L), approximately 6–8 liters of 0.45% saline are required to lower serum sodium by 10 mmol/L, though the exact volume depends on body weight and ongoing losses. 1

Understanding the Calculation

The Sodium Deficit Formula

The volume of hypotonic fluid needed is calculated using the Adrogue-Madias formula, which estimates total body water (TBW) and the sodium change per liter of infused fluid 1:

• Total Body Water (TBW) = 0.5 × body weight (kg) for adults 1
• For a 70-kg adult: TBW = 0.5 × 70 = 35 liters 1

Sodium Content of 0.45% Saline

• 0.45% NaCl (half-normal saline) contains 77 mEq/L of sodium 2
• This is hypotonic compared to plasma and provides substantial free water for correction 2

Calculating the Required Volume

To lower sodium by 10 mmol/L from 167 to 157 mmol/L 1:

Change in sodium per liter of 0.45% saline = (77 - 167) / (TBW + 1) 1 = -90 / 36 = approximately -2.5 mmol/L per liter infused 1

Volume needed = Desired change ÷ Change per liter 1 = 10 mmol/L ÷ 2.5 mmol/L per liter = approximately 4 liters 1

However, this is a theoretical minimum that does not account for ongoing losses, insensible losses, or the patient's clinical status 1.

Critical Correction Rate Limits

Maximum Safe Correction Speed

The serum sodium must not be reduced by more than 10–12 mmol/L in any 24-hour period to prevent cerebral edema from rapid osmotic shifts. 1, 3, 4

• For chronic hypernatremia (>48 hours duration), limit correction to 8–10 mmol/L per day 3
• The preferred rate is 0.5 mmol/L per hour or less to minimize neurological complications 5, 4
• Corrections faster than 48–72 hours in severe hypernatremia increase the risk of pontine myelinolysis 1

High-Risk Patient Considerations

Patients with COPD, advanced age, malnutrition, or chronic illness require even more cautious correction rates (4–6 mmol/L per day maximum). 1, 6

• COPD exacerbations with electrolyte disturbances carry significantly increased mortality risk 6
• Drowsiness indicates severe hypernatremia requiring ICU-level monitoring 7, 4

Practical Infusion Strategy

Initial Fluid Selection and Rate

Begin with 0.45% NaCl at an initial rate calculated to achieve 0.5 mmol/L per hour reduction, typically 100–150 mL/hour for a 70-kg adult. 1, 3

• Avoid isotonic saline (0.9% NaCl) as it will worsen hypernatremia by delivering excessive sodium load 2
• Consider 5% dextrose in water (D5W) as an alternative hypotonic fluid with no sodium content 2

Monitoring Protocol

Check serum sodium every 2–4 hours during active correction to ensure the rate does not exceed safe limits. 1, 4

• Adjust infusion rate based on serial sodium measurements 4
• Monitor for signs of cerebral edema (worsening mental status, seizures) if correction is too rapid 7, 3
• Track urine output and ongoing losses that may require additional free water replacement 4

Addressing Underlying Causes

COPD-Specific Considerations

In COPD patients with hypernatremia, assess for dehydration from increased insensible losses, inadequate fluid intake, and medication effects (diuretics). 6

• Hyponatremia is more common than hypernatremia in COPD exacerbations, so hypernatremia suggests severe dehydration or diabetes insipidus 6
• Correct any volume depletion with initial isotonic fluids before switching to hypotonic solutions if true hypovolemia exists 7, 4

Diabetes Insipidus Evaluation

If hypernatremia persists despite adequate free water replacement, consider central or nephrogenic diabetes insipidus 3:

• Urine osmolality <300 mOsm/kg with hypernatremia suggests diabetes insipidus 3
• Desmopressin (Minirin) may be required for central diabetes insipidus 3

Common Pitfalls to Avoid

Never correct chronic hypernatremia faster than 10 mmol/L in 24 hours, as this causes cerebral edema and can be fatal. 1, 3, 4

Do not use isotonic saline (0.9% NaCl) for hypernatremia correction, as it delivers 154 mEq/L sodium and will worsen the condition. 2

Avoid relying solely on formulas without accounting for ongoing losses, insensible losses (increased in COPD), and urine output. 1, 4

Do not delay treatment while pursuing extensive diagnostic workup—begin correction immediately while investigating the cause. 7, 4

Realistic Clinical Estimate

Total Volume Over 24 Hours

For a 70-kg adult with sodium of 167 mmol/L, expect to infuse 6–8 liters of 0.45% saline over 24 hours to safely lower sodium by 10 mmol/L, accounting for ongoing losses and the need for gradual correction. 1, 4

• This translates to approximately 250–350 mL/hour of 0.45% saline 1
• Adjust based on serial sodium measurements every 2–4 hours 4
• Additional free water may be needed if urine output is high or insensible losses are significant 4

Alternative Approach with D5W

If using 5% dextrose in water (D5W) instead of 0.45% saline 2:

• D5W provides pure free water with no sodium content 2
• Approximately 3–4 liters of D5W may achieve similar correction, but requires more careful monitoring 2
• D5W is preferred when sodium needs to be lowered more aggressively without adding any sodium 2