Intravenous Fluids
This briefing document synthesizes critical insights regarding intravenous (IV) fluid therapy, covering the physiological distribution of body water, the chemical and clinical profiles of crystalloid and colloid solutions, and the ongoing debate regarding optimal resuscitation strategies.
Executive Summary
Intravenous fluid therapy is a cornerstone of modern critical care, yet the selection of fluid is often governed by tradition rather than physiological precision. The source material highlights several critical takeaways:
The Misnomer of "Normal" Saline: 0.9% NaCl is neither chemically nor physiologically normal. Its high chloride content is linked to hyperchloremic metabolic acidosis and acute kidney injury (AKI).
Fluid Distribution Realities: Isotonic crystalloids primarily expand the interstitial space rather than the plasma volume. Only about 25–27% of infused "normal" saline remains in the intravascular compartment.
The Dextrose Risk: 5% dextrose solutions (D5W) largely expand intracellular volume, risking cell swelling. In hemodynamically compromised patients, dextrose can significantly increase lactate production and exacerbate hyperglycemia-related complications.
Colloid Efficiency: Colloids (e.g., albumin, starches) are roughly three times more effective than crystalloids at expanding plasma volume, yet they lack a proven survival benefit in general populations and carry specific risks like impaired hemostasis.
A Tailored Approach: The "colloid-crystalloid conundrum" is best resolved by matching fluid properties to specific clinical needs—such as using balanced crystalloids for dehydration and colloids for life-threatening hypovolemia.
I. Physiological Foundations of Fluid Distribution
Effective IV therapy requires understanding how water distributes throughout the body's compartments. Total body water (TBW) accounts for 60% of lean body weight in males and 50% in females.
Body Fluid Compartmentalization
The goal of IV therapy is typically to support plasma volume, which represents a very small fraction of total body water.
Osmotic and Oncotic Forces
Osmotic Activity: Determined by the number of solute particles. Sodium is the primary determinant of extracellular fluid volume.
Colloid Osmotic Pressure (Oncotic Pressure): Generated by large molecules (primarily albumin) that do not cross the vascular endothelium. This pressure holds water within the intravascular compartment.
II. Crystalloid Fluids
Crystalloids are aqueous solutions of small molecules, primarily electrolytes, that diffuse freely between intravascular and interstitial spaces.
0.9% Sodium Chloride (Normal Saline)
Despite its name, 0.9% saline is physiologically abnormal compared to human plasma.
Chemical Disparity: It contains 154 mEq/L of both Sodium and Chloride, whereas plasma contains 140 mEq/L and 103 mEq/L, respectively. Its pH is 5.7, significantly more acidic than the blood's 7.4.
The "Evils of Chloride": High-volume saline infusions cause hyperchloremic metabolic acidosis. Furthermore, chloride-mediated renal vasoconstriction has been associated with a higher incidence of acute kidney injury (AKI).
Volume Effect: Infusing 1 liter of 0.9% saline adds only 275 mL to plasma volume, while 825 mL shifts into the interstitial space, promoting edema.
Balanced Salt Solutions
These fluids are designed to more closely mimic plasma composition.
Ringer’s Lactate (RL): Contains potassium and calcium with lactate as a buffer. Lactate is converted by the liver into bicarbonate, consuming hydrogen ions in the process. RL does not produce hyperchloremic acidosis because its chloride concentration (109 mEq/L) is near-physiological.
Ringer’s Acetate: Uses acetate instead of lactate as a buffer. Because acetate is metabolized in muscle rather than the liver, it is preferred for patients with liver failure.
Normosol® and Plasma-Lyte®: These are considered the closest to "ideal" replacement fluids. They use non-lactate buffers (acetate and gluconate), contain magnesium instead of calcium (making them safe for blood transfusions), and have a pH of 7.4.
III. 5% Dextrose Solutions (D5W)
Once used for "protein-sparing" caloric support, D5W is now largely discouraged for routine resuscitation in the critically ill.
Intracellular Swelling: As dextrose is metabolized, the remaining water moves into the cells. Only about 10% of a D5W infusion stays in the plasma; two-thirds enters the intracellular space, causing undesirable cell swelling.
Lactate Production: In hemodynamically compromised patients, up to 85% of glucose metabolism can result in lactate production, potentially worsening lactic acidosis.
Hyperglycemia: Dextrose infusions can exacerbate hyperglycemia, which is linked to immune suppression, increased infection risk, and worsened ischemic brain injury.
IV. Colloid Fluids
Colloids contain large insoluble particles that remain in the vascular compartment, creating oncotic pressure to retain water.
Albumin
Albumin is responsible for 80% of natural plasma oncotic pressure.
5% Albumin: Isoncotic with plasma. It is 3 times more effective than saline at expanding plasma volume (700 mL of plasma expansion per 1 L infused).
25% Albumin: Hyperoncotic (COP of 70 mm Hg). It draws fluid from the interstitium into the plasma. It is used to reduce edema and increase blood pressure in hypoalbuminemic patients but should not be used for simple volume loss resuscitation.
Safety Note: Albumin is associated with higher mortality in patients with traumatic brain injury (TBI).
Hydroxyethyl Starches (HES)
HES are synthetic polysaccharides classified by molecular weight (MW) and molar substitution ratio (MSR).
Properties: High MW and MSR (e.g., Hetastarch) result in longer duration of action but higher toxicity. Newer generations (e.g., Tetrastarch/Voluven) have lower MW/MSR and reduced risk.
Adverse Effects: HES can impair hemostasis by inhibiting Factor VIII and von Willebrand factor. They have also been linked to AKI and death in critically ill patients, and can cause a "hyperamylasemia" (elevated serum amylase) that does not reflect pancreatitis.
Dextrans
Dextrans are glucose polymers with high oncotic pressure but are less popular due to side effects:
Hemostatic Defects: They impair platelet aggregation and enhance fibrinolysis.
Clinical Complications: They can interfere with blood cross-matching and, rarely, cause anaphylaxis or renal injury.
V. The Resuscitation Debate: Crystalloid vs. Colloid
The "Colloid-Crystalloid Conundrum" stems from a lack of consensus on a universal resuscitation fluid.
Recommendations for a Tailored Approach
Rather than a "one size fits all" strategy, the source suggests selecting fluids based on the specific clinical scenario:
Life-Threatening Hypovolemia: Use colloids (e.g., 5% albumin) for rapid plasma restoration.
Dehydration/Uniform Fluid Loss: Use balanced crystalloids (e.g., Ringer’s Lactate).
Edema with Hypoalbuminemia: Use hyperoncotic colloids (e.g., 25% albumin) to shift fluid from the interstitium to the plasma.
Traumatic Brain Injury: Use hypertonic saline (3% to 25%) to reduce intracranial pressure, as it may maintain cerebral perfusion better than mannitol and avoids albumin's mortality risk in this context.
