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Colloids vs Crystalloids
💧 Key Point: Crystalloids remain the first-line fluids in the treatment of shock and hypovolaemia.
💧 Crystalloids
- Examples: 0.9% saline, 5% dextrose, 0.45% saline + 5% dextrose, Hartmann’s (Ringer’s lactate).
- The dextrose component is rapidly metabolised by the liver.
- Giving 5% dextrose is essentially equivalent to giving free water (isotonic on infusion, but distributes as free water once metabolised).
- If sodium is not provided ➝ risk of hyponatraemia.
- Typical daily maintenance (adult): ~2 × 1 L 5% dextrose + 1 L 0.9% saline, each with 20 mmol KCl (adjust to patient needs).
🧪 Colloids
- Examples: albumin, dextran, gelatin-based fluids.
- Much more expensive than crystalloids.
⚙️ Mechanism
- Contain high molecular weight molecules ➝ exert oncotic pressure to retain fluid in the vascular compartment.
- Theoretical advantage: smaller volume required compared with crystalloids for same haemodynamic effect.
- However, rapid intravascular expansion may be undesirable (e.g. ongoing haemorrhage).
- Large RCTs and systematic reviews show no consistent outcome benefit of colloids over crystalloids in most situations.
⚠️ Side Effects of Colloids
- Anticoagulant, antiplatelet, and fibrinolytic effects (bleeding risk).
- Allergic/anaphylactoid reactions (especially with gelatins or dextrans).
- Renal impairment (notably with hydroxyethyl starch, now avoided).
📚 References
- BNF – IV Fluids
- NICE NG29: Intravenous fluid therapy in adults in hospital.
- Cochrane Review: Colloids vs crystalloids for fluid resuscitation in critically ill patients.
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