What is the appropriate rate of 0.9% normal saline (NS) infusion per hour for a patient with hyponatremia (serum sodium level less than 135 mEq/L)?

Appropriate Rate of Normal Saline Infusion in Hyponatremia

For hyponatremia (sodium <135 mEq/L), there is no single fixed "mL per hour" rate for normal saline—the approach depends entirely on the patient's volume status, symptom severity, and underlying cause. Normal saline (0.9% NaCl) is only appropriate for hypovolemic hyponatremia, and even then, correction must not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 2.

Critical Framework: Volume Status Determines Treatment

The fundamental error is assuming normal saline is universally appropriate for hyponatremia—it is not. 1, 3

Hypovolemic Hyponatremia (True Volume Depletion)

• Normal saline IS indicated when urine sodium <30 mmol/L and clinical signs of hypovolemia exist (orthostatic hypotension, dry mucous membranes, decreased skin turgor) 1, 3
• Infusion rate is determined by hemodynamic status, but must respect the 8 mmol/L per 24-hour correction limit 1, 2
• This translates to approximately 0.33 mmol/L per hour maximum correction rate 2
• Monitor serum sodium every 2-4 hours during active correction 1, 2

Euvolemic Hyponatremia (SIADH)

• Normal saline will WORSEN hyponatremia in SIADH patients 1, 3
• First-line treatment is fluid restriction to 1000 mL/day (approximately 40 mL/hour total fluid intake) 1, 2, 4
• If fluid restriction fails, add oral sodium chloride 100 mEq three times daily 1
• Consider urea or vaptans for resistant cases 5, 4

Hypervolemic Hyponatremia (Cirrhosis, Heart Failure)

• Normal saline is contraindicated—it worsens fluid overload 1, 3
• Implement fluid restriction to 1000-1500 mL/day for sodium <125 mmol/L 1, 2
• Temporarily discontinue diuretics if sodium <125 mmol/L 1
• Consider albumin infusion in cirrhotic patients 1
• Avoid hypertonic saline unless life-threatening symptoms present, as it worsens ascites and edema 1

Severe Symptomatic Hyponatremia (Emergency)

For severe symptoms (seizures, coma, altered mental status), normal saline is inadequate—3% hypertonic saline is required. 1, 5, 3

• Administer 100 mL of 3% saline over 10 minutes, repeatable up to 3 times at 10-minute intervals 1, 2
• Target: increase sodium by 6 mmol/L over 6 hours or until symptoms resolve 1, 2
• Maximum correction: 8 mmol/L in 24 hours 1, 2, 5
• Check serum sodium every 2 hours during initial correction 1, 2

Critical Safety Limits

The correction rate is measured by change in serum sodium concentration, NOT volume infused. 2

Standard Risk Patients

• Maximum: 4-8 mmol/L per 24 hours, not exceeding 10-12 mmol/L 1, 2

High-Risk Patients (Cirrhosis, Alcoholism, Malnutrition)

• Maximum: 4-6 mmol/L per 24 hours, not exceeding 8 mmol/L 1, 2
• These patients have significantly higher risk of osmotic demyelination syndrome 1, 5

Calculating Sodium Deficit

When normal saline IS appropriate (hypovolemic hyponatremia):

Sodium deficit = Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg) 1

• This calculation guides total sodium needed, not infusion rate
• Infusion rate must still respect the 8 mmol/L per 24-hour limit 2

Common Pitfalls to Avoid

• Using normal saline in SIADH worsens hyponatremia by providing free water 1, 3
• Using normal saline in hypervolemic states worsens fluid overload without improving sodium 1
• Correcting faster than 8 mmol/L in 24 hours risks osmotic demyelination syndrome, which can cause quadriparesis, dysarthria, or death 1, 5
• Failing to distinguish between acute (<48 hours) and chronic (>48 hours) hyponatremia—acute can be corrected more rapidly, chronic requires strict adherence to limits 1, 6
• Inadequate monitoring during correction—check sodium every 2-4 hours initially 1, 2

Management of Overcorrection

If sodium increases >8 mmol/L in 24 hours:

• Immediately discontinue all sodium-containing fluids 1, 2
• Switch to D5W (5% dextrose in water) 1, 2
• Consider desmopressin to terminate water diuresis 1, 2
• Target: bring total 24-hour correction to ≤8 mmol/L from starting point 1

Special Populations

Neurosurgical Patients

• Distinguish cerebral salt wasting (CSW) from SIADH—treatment approaches are opposite 1
• CSW requires volume and sodium replacement (normal saline or hypertonic saline) 1
• Never use fluid restriction in subarachnoid hemorrhage patients at risk of vasospasm 1
• Consider fludrocortisone for CSW 1

Cirrhotic Patients

• Hyponatremia increases risk of spontaneous bacterial peritonitis (OR 3.40), hepatorenal syndrome (OR 3.45), and hepatic encephalopathy (OR 2.36) 1
• Sodium restriction (not fluid restriction) results in weight loss as fluid follows sodium 1
• Tolvaptan carries higher risk of gastrointestinal bleeding (10% vs 2% placebo) 1