What is isotonic saline?

What is Isotonic Saline?

Isotonic saline is 0.9% sodium chloride solution (normal saline), containing 154 mEq/L of both sodium and chloride, with an osmolality approximately equal to plasma (308 mOsmol/kg), making it the only commonly available isotonic crystalloid solution. 1

Composition and Physical Properties

• 0.9% saline contains 9 grams of NaCl per liter (0.9 g per 100 mL) 2
• The solution provides 154 mEq/L of sodium and 154 mEq/L of chloride 1, 3
• The osmolality is approximately 308 mOsmol/kg, which matches plasma osmolality 1
• 0.9% saline is the only commonly available isotonic crystalloid when real osmolality (mosmol/kg) rather than theoretical osmolality (mosmol/L) is measured 1

Why "Isotonic" Matters Clinically

• Isotonic solutions prevent fluid shifts between intracellular and extracellular compartments because they match plasma osmolality 1
• In brain injury, intravenous fluids must be isotonic in terms of osmolality (not osmolarity) to prevent increases in brain water 1
• Solutions like Ringer's lactate and Ringer's acetate are hypotonic when real osmolality is determined and should be avoided in brain injury 1

Distribution in the Body

• When isotonic saline is infused, approximately 25% remains in the intravascular space while 75% distributes to the interstitial space 4
• This distribution pattern makes it effective for volume expansion, though less efficient than colloids which remain intravascular 4
• The glomerular filtration rate increases in response to isotonic saline infusion, though less than with balanced crystalloids like Ringer's acetate 5

Clinical Applications

• Isotonic saline is the crystalloid of choice for brain-injured patients to maintain cerebral blood flow and prevent cerebral edema 1
• For maintenance intravenous fluids in children 28 days to 18 years, isotonic solutions significantly decrease the risk of developing hyponatremia compared to hypotonic fluids 1
• Isotonic saline is recommended for volume expansion in patients at increased risk for contrast-induced acute kidney injury 1
• In emergency laparotomy, isotonic fluids should be titrated by bolus based on objective measures of hypovolemia 1

Important Limitations and Adverse Effects

• Large volumes of 0.9% saline can cause hyperchloremic metabolic acidosis, hypernatremia, and acute kidney injury due to its supraphysiologic chloride content (154 mEq/L vs. 98-106 mEq/L in plasma) 1, 6
• Saline-induced hyperchloremia can decrease kidney perfusion, increase vasopressor requirements, and cause electrolyte derangements 1
• In hemorrhagic shock, 0.9% saline causes higher incidence of metabolic acidosis, coagulopathy, and greater volume requirements compared to balanced solutions like lactated Ringer's 1
• Isotonic saline may impair coagulation and cause fluid overload when used in excessive volumes 4

When to Avoid or Use Cautiously

• Patients with edematous states (heart failure, cirrhosis, nephrotic syndrome) have impaired ability to excrete sodium and water, risking volume overload with typical maintenance rates 1
• In these patients, isotonic saline at maintenance rates will likely be excessive and should be restricted with close monitoring 1
• Balanced crystalloids (like Ringer's lactate or PlasmaLyte) may result in improved outcomes and reduced morbidity compared to 0.9% saline, particularly in high-risk patients requiring significant fluid resuscitation 1

Comparison to Other Solutions

• Unlike hypotonic solutions (0.45% or 0.2% saline), isotonic saline does not increase the risk of hospital-acquired hyponatremia in children 1, 7
• Unlike hypertonic saline (3%, 7.5%, or 23%), isotonic saline does not create osmotic gradients that pull fluid from intracellular to extracellular spaces 1, 2
• Unlike balanced crystalloids, 0.9% saline lacks buffer equivalents and has a chloride concentration far exceeding physiologic levels 1, 6