What are considered first-line isotonic crystalloids for fluid resuscitation?

Isotonic Crystalloids for Fluid Resuscitation

Isotonic crystalloids are defined as solutions with osmolarity of 280-310 mOsm/L, and the first-line options for fluid resuscitation are 0.9% normal saline, Plasma-Lyte, and buffered isotonic solutions like Ringer's lactate (though RL has important limitations in specific populations). 1, 2

Definition and Classification

Isotonic crystalloids are solutions that maintain osmolarity between 280-310 mOsm/L, which prevents fluid shifts across cell membranes and avoids cerebral edema formation. 1, 2

The primary isotonic crystalloids include:

• 0.9% Normal Saline (NS): The only commonly available truly isotonic crystalloid solution with osmolarity of approximately 308 mOsm/L 2
• Plasma-Lyte: A buffered isotonic solution with osmolarity of 280-310 mOsm/L 1
• Isofundine: Another isotonic option with osmolarity in the 280-310 mOsm/L range 1
• Ringer's Lactate (RL): While commonly used, this is technically hypotonic by real osmolality and should be avoided in certain populations, particularly traumatic brain injury patients 2, 3

First-Line Recommendations by Clinical Context

General Resuscitation and Shock

For initial resuscitation in adults with septic shock, administer at least 30 mL/kg of isotonic crystalloid in the first 3 hours. 4 For pediatric septic shock, give 20 mL/kg as a rapid bolus, up to 40-60 mL/kg during initial resuscitation. 4

Isotonic crystalloids are recommended as the initial resuscitation fluid for infants and children with any type of shock, with no specific isotonic crystalloid demonstrating superiority over others in terms of mortality. 4

Brain Injury and Neurosurgical Patients

In acute brain injury, 0.9% normal saline is the crystalloid of choice because it is the only commonly available truly isotonic solution that prevents cerebral edema formation. 2, 3

Alternative buffered isotonic solutions like Plasma-Lyte (osmolarity 280-310 mOsm/L) are acceptable alternatives and may avoid hyperchloremic metabolic acidosis associated with prolonged 0.9% saline use. 1

Critical: Ringer's Lactate must be avoided in traumatic brain injury patients despite its common availability—it is hypotonic by real osmolality and has been associated with increased mortality in TBI patients compared to 0.9% saline. 2, 3

Acute Kidney Injury Prevention

For patients at risk for acute kidney injury or with established AKI, isotonic crystalloids are recommended over colloids (albumin or starches) as initial management for intravascular volume expansion. 4

Balanced isotonic solutions may offer advantages over 0.9% saline in preventing acute kidney injury, as the high chloride content of normal saline can lead to hyperchloremic acidosis and potentially worsen renal function. 4, 5

Solutions to Avoid

Hypotonic solutions (<280 mOsm/L) must be avoided in all resuscitation scenarios due to risk of cerebral edema formation. 1, 2, 3 This includes:

• Gelatins 1
• Standard Ringer's lactate in brain injury patients 2, 3
• Ringer's acetate 1

Colloids should not be used as first-line therapy:

• Albumin is contraindicated in traumatic brain injury (increased mortality with RR 1.63, p=0.003) 1, 2, 3
• Synthetic colloids (HES, gelatins) are associated with worse neurological prognosis and should be avoided in brain injury 1, 2, 3
• Hydroxyethyl starch increases mortality and need for renal replacement therapy in severe sepsis 4

Common Pitfalls and Caveats

Terminology confusion: The term "isotonic" is often used imprecisely in clinical practice. Not all solutions labeled as "isotonic" are sufficient volume expanders or truly isotonic by osmolarity. 6 Always verify the actual osmolarity (should be 280-310 mOsm/L) rather than relying on traditional labeling.

Hyperchloremic acidosis: Prolonged use of 0.9% saline (which contains 153 mEq/L of chloride) induces hyperchloremic metabolic acidosis and may worsen renal function. 4, 5 Consider switching to balanced solutions like Plasma-Lyte for maintenance therapy after initial resuscitation.

Volume overload: Maintain euvolemia rather than pursuing aggressive fluid loading. Both hypovolemia and hypervolemia are detrimental, particularly in brain injury patients where positive fluid balance is associated with higher mortality and worse functional outcomes. 1

Hypertonic saline is NOT for routine resuscitation: Reserve 3% or 7.5% hypertonic saline exclusively for acute intracranial pressure crises or impending herniation, not for primary fluid resuscitation. 2, 3