Fluid Bolus for Hypovolemic Hyponatremia
For a patient with hypovolemic hyponatremia, administer isotonic saline (0.9% NaCl) to restore intravascular volume, with careful monitoring to ensure sodium correction does not exceed 8 mmol/L in 24 hours. 1, 2, 3
Initial Management Approach
Isotonic saline (0.9% NaCl) is the fluid of choice for hypovolemic hyponatremia because the primary problem is volume depletion with sodium loss 1, 2, 3. The Hepatology society explicitly recommends discontinuing diuretics and administering isotonic saline for volume repletion in hypovolemic hyponatremia 1.
Key Diagnostic Features to Confirm Hypovolemia
Before administering saline, verify true hypovolemia by checking for:
- Urine sodium <30 mmol/L (positive predictive value 71-100% for response to saline) 1
- Clinical signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1
- Low urine osmolality (<100 mOsm/kg) with high urine osmolality relative to serum suggests volume depletion 1
Infusion Strategy
Start with isotonic saline at 15-20 mL/kg/h initially, then reduce to 4-14 mL/kg/h based on clinical response 1. Normal saline contains 154 mEq/L sodium with osmolarity of 308 mOsm/L, making it truly isotonic 1.
Critical Safety Parameters
Correction Rate Limits
Never exceed 8 mmol/L sodium correction in 24 hours to prevent osmotic demyelination syndrome 1, 4, 5. For high-risk patients (advanced liver disease, alcoholism, malnutrition, severe hyponatremia), limit correction to 4-6 mmol/L per day 1.
Monitoring Protocol
- Check sodium every 2-4 hours during active correction 1
- Calculate sodium deficit: Desired increase (mEq/L) × (0.5 × ideal body weight in kg) 1
- Watch for signs of overcorrection: if sodium rises too quickly, immediately switch to D5W and consider desmopressin 1
Common Pitfalls to Avoid
Do not use hypotonic fluids (like lactated Ringer's with 130 mEq/L sodium) as these can worsen hyponatremia through dilution 1. Lactated Ringer's is slightly hypotonic (273 mOsm/L) and was not studied in hyponatremia trials 1.
Do not use hypertonic saline (3%) unless the patient has severe neurological symptoms (seizures, coma, altered mental status) 1, 4, 5. Hypertonic saline is reserved for severely symptomatic patients and requires ICU monitoring 1.
Distinguish from other causes: If the patient appears hypovolemic but has urine sodium >20 mmol/L despite volume depletion, consider cerebral salt wasting (especially in neurosurgical patients), which still requires volume replacement but may need hypertonic saline and fludrocortisone 1.
Once Euvolemia is Achieved
After volume repletion, reassess the underlying cause 1. If sodium levels improve with isotonic fluids, continue until euvolemia is achieved 1. If hyponatremia persists despite adequate volume repletion, the patient likely has a different etiology (SIADH or hypervolemic hyponatremia) requiring different management 1, 2, 3.