Should I start 0.45% Normal Saline (NS) or 0.9% Normal Saline (NS) for fluid resuscitation in this patient?

Fluid Selection: 0.9% Normal Saline vs 0.45% Normal Saline

Use 0.9% normal saline for initial fluid resuscitation in most acute clinical scenarios, reserving 0.45% saline only for specific maintenance situations after initial resuscitation is complete and corrected serum sodium is normal or elevated. 1, 2

Initial Resuscitation: Start with 0.9% Normal Saline

For acute resuscitation scenarios, 0.9% normal saline is the appropriate initial choice in the following contexts:

• Sepsis and septic shock: Initial fluid bolus should be 0.9% saline (or preferably balanced crystalloids like lactated Ringer's), administered at 30 mL/kg within the first 3 hours 1
• Diabetic ketoacidosis (DKA): Begin with 0.9% NaCl at 15-20 mL/kg/hour during the first hour 2
• Hypovolemic states: Any patient requiring volume expansion for intravascular depletion should receive isotonic fluids initially 2
• Hyponatremia: Isotonic saline (0.9% NaCl) is first-line therapy, as hypotonic fluids will worsen hyponatremia 3

When to Transition to 0.45% Normal Saline

0.45% saline should only be used after initial resuscitation is complete and in specific circumstances:

• DKA maintenance phase: After the first hour of 0.9% saline, switch to 0.45% NaCl at 4-14 mL/kg/hour if corrected serum sodium is normal or elevated 2
• Continue 0.9% saline if corrected sodium remains low 2

Critical Caveat: Consider Balanced Crystalloids Over 0.9% Saline

While 0.9% saline is superior to 0.45% saline for resuscitation, emerging evidence strongly favors balanced crystalloids (lactated Ringer's, Plasma-Lyte) over 0.9% saline for most resuscitation scenarios:

• Sepsis: Lactated Ringer's solution was associated with improved survival (12.2% vs 15.9% mortality) and more hospital-free days compared to 0.9% saline 4
• DKA: Balanced fluids achieved faster DKA resolution (13 vs 17 hours) compared to normal saline 5
• Perioperative care: Buffered crystalloids reduce hyperchloremic acidosis, renal vasoconstriction, and major adverse kidney events compared to 0.9% saline 1
• Maternal sepsis: The Surviving Sepsis Campaign recommends balanced crystalloids instead of normal saline 1

Mechanism of Harm from 0.9% Saline

Large volumes of 0.9% saline cause:

• Hyperchloremic metabolic acidosis 1, 6
• Renal vasoconstriction and acute kidney injury 1
• Increased need for vasopressor therapy 1
• Higher rates of major adverse kidney events 1

Special Populations

Pregnant Patients with Sepsis

• Initial bolus: 1-2 L, escalating to 30 mL/kg within 3 hours if needed 1
• Prefer balanced crystalloids over 0.9% saline due to lower colloid oncotic pressure and higher pulmonary edema risk in pregnancy 1

CKD Patients with Hyponatremia

• Use 0.9% NaCl (not 0.45% saline which worsens hyponatremia) 3
• Monitor closely for volume overload given reduced renal excretion capacity 3
• Use lower infusion rates (4-14 mL/kg/hour) 3

Hypernatremia

• Never use 0.9% or 0.45% saline - both will worsen hypernatremia 7
• Use D5W (5% dextrose in water) as primary fluid 7

Practical Algorithm

1. Is this initial resuscitation for shock/hypotension?

   • Yes → Use balanced crystalloids (lactated Ringer's/Plasma-Lyte) as first choice 1, 4
   • If balanced crystalloids unavailable → Use 0.9% saline 2
   • Never use 0.45% saline 3

2. Is this DKA after the first hour of resuscitation?

   • Check corrected sodium:
     • Normal/elevated → Switch to 0.45% NaCl at 4-14 mL/kg/hour 2
     • Low → Continue 0.9% NaCl 2

3. Is the patient hyponatremic?

   • Use 0.9% saline (never 0.45% saline) 3

4. Is the patient hypernatremic?

   • Use D5W (never any saline concentration) 7

Common Pitfalls to Avoid

• Using 0.45% saline for initial resuscitation - this provides inadequate volume expansion and can worsen hyponatremia 3
• Continuing 0.9% saline when balanced crystalloids are available - this increases risk of acidosis and kidney injury 1, 4
• Using any saline concentration for hypernatremia - this paradoxically worsens the condition 7
• Failing to monitor electrolytes every 4-6 hours during active fluid resuscitation 2, 3