What is the difference between colloid and crystalloid components of plasma?

Difference Between Colloid and Crystalloid Components of Plasma

Plasma contains two distinct families of intravenous fluids: crystalloids are solutions with small molecules (electrolytes and water) that distribute across fluid compartments, while colloids contain large molecules (proteins or synthetic polymers) that primarily remain in the intravascular space to generate oncotic pressure. 1

Fundamental Structural Differences

Crystalloids

• Contain small molecular weight substances including sodium, chloride, potassium, calcium, magnesium, and buffer molecules (lactate, acetate, gluconate) 1
• Freely cross capillary membranes and distribute throughout the extracellular fluid compartment (both intravascular and interstitial spaces) 2
• Osmolarity ranges from 277-309 mOsm/L depending on the specific formulation 1
• Examples include: 0.9% NaCl (normal saline), Ringer's lactate, Plasma-Lyte, and Isofundine 1

Colloids

• Contain high molecular weight substances that generate oncotic pressure and remain predominantly intravascular 1, 3
• Two main categories exist: synthetic colloids (hydroxyethyl starch, gelatin, dextran) and natural colloids (albumin, fresh frozen plasma) 1
• Larger molecules prevent easy passage across intact capillary membranes, thereby maintaining intravascular volume more effectively per unit volume administered 2, 3

Volume Distribution and Expansion Properties

Crystalloid Volume Kinetics

• Approximately 75-80% of administered crystalloid volume rapidly redistributes from the intravascular space into the interstitial compartment within 30-60 minutes 2
• Require 1.5 to 3 times the volume of colloids to achieve similar hemodynamic endpoints 4, 5
• The crystalloid-to-colloid volume equivalence ratio is approximately 1.5:1 (95% CI: 1.36-1.65) based on meta-analysis data 4

Colloid Volume Kinetics

• Remain in the intravascular compartment longer due to their oncotic properties, providing more sustained plasma volume expansion 2, 3
• Generate oncotic pressure that can draw fluid from the interstitial space into the plasma compartment 2
• Central venous pressure is significantly higher with colloids (albumin, HES, gelatin) compared to crystalloids (all p < .001) 5

Classification Systems

Crystalloid Classification

• By tonicity: Isotonic (280-310 mOsm/L), hypotonic (<280 mOsm/L like Ringer's lactate), or hypertonic (>310 mOsm/L like 3% or 7.5% NaCl) 1
• By chloride content: High-chloride (0.9% NaCl with 154 mmol/L) versus balanced crystalloids (98-127 mmol/L chloride) 1
• By ionic composition: Balanced solutions have electrolyte concentrations more similar to normal plasma than 0.9% NaCl 1

Colloid Classification

• Natural colloids: Albumin (4-5% iso-oncotic or 20% hyper-oncotic) and fresh frozen plasma 1
• Synthetic colloids: Hydroxyethyl starches (HES), gelatins, and dextrans (though dextrans are rarely used due to adverse effects) 1

Clinical Implications and Safety Profile

Crystalloid Safety

• Lower cost: Isotonic saline costs approximately 1.5 Euro/liter 4
• Fewer adverse effects: Lower risk of allergic reactions, coagulopathy, and renal dysfunction compared to synthetic colloids 4, 2
• Risk of hyperchloremic acidosis: Large volumes of 0.9% NaCl can cause renal vasoconstriction and acute kidney injury 1
• Tissue edema: Larger volumes required may lead to peripheral and pulmonary edema 2, 6

Colloid Safety Concerns

• Synthetic colloids (HES) increase mortality and renal failure risk in critically ill patients, particularly those with sepsis 1, 7
• HES increases need for renal replacement therapy (RR 1.30,95% CI 1.14-1.48) 1, 7
• Albumin costs approximately 140 Euro/liter, while HES costs approximately 25 Euro/liter 4
• Gelatins carry higher risk of anaphylactic reactions compared to other colloids 1
• Albumin shows no mortality benefit over crystalloids in most critically ill populations (RR 0.98,95% CI 0.92-1.06) 1, 7

Common Pitfalls to Avoid

• Do not use synthetic colloids (HES, gelatins) as first-line therapy in critically ill or septic patients due to increased renal failure risk 1, 8, 4
• Avoid hypotonic solutions like Ringer's lactate in traumatic brain injury patients to prevent fluid shift into damaged cerebral tissue 8
• Do not assume potassium-containing balanced crystalloids will cause hyperkalemia; studies show no increased risk even in at-risk patients 1
• Recognize that colloids do not improve mortality despite their theoretical advantages in volume expansion 1, 7