SN1 SHORT NOTE 5M

Describe Crystalloids vs Colloids in detail.

General Surgery · Final Professional

The Fundamental Distinction

Intravenous fluids fall into two families, governed by one physical principle — the size of the dissolved particles decides whether the fluid can cross the capillary wall, and therefore where it ends up and how long it stays in the circulation. Crystalloids contain small molecules (electrolytes ± glucose) that pass freely across the capillary membrane; colloids contain large molecules that are retained within the vessels, where they exert an oncotic pull. Recall the compartments: total body water is ~60% of weight, two-thirds intracellular and one-third extracellular, and of the extracellular fluid only about a quarter (~3.5 L) is intravascular.

Crystalloids

Isotonic crystalloids (0.9% saline, Ringer lactate, Plasma-Lyte) distribute through the whole extracellular fluid, so only ~25% stays intravascular — needing ~3× the volume of blood lost to refill the circulation; these are the resuscitation/replacement fluids. Hypotonic fluids (0.45% saline, dextrose-saline) give more free water for maintenance, and hypertonic saline (3%) is reserved for raised intracranial pressure and symptomatic hyponatraemia. 5% dextrose is effectively free water — it spreads across all body water and is never used for resuscitation.

Colloids

Colloids are natural (albumin, FFP) or synthetic (gelatins, dextrans, hydroxyethyl starch). Their large molecules hold water intravascularly, so they expand plasma volume with a smaller infused volume and last longer — but they are costly and carry specific risks (dextrans → anaphylaxis and platelet dysfunction; starches → kidney injury and coagulopathy, and are now largely withdrawn).

Composition of Common Fluids

Fluid	Na⁺	Cl⁻	Note
Plasma (reference)	135–145	95–105	—
0.9% saline	154	154	High Cl⁻ → hyperchloraemic acidosis in large volumes
Ringer lactate	131	111	Balanced; contains K⁺, Ca²⁺, lactate
5% dextrose	0	0	Free water only — not for resuscitation

CLINICAL PEARL

Clinical pearl: Large trials (e.g. Safe) show no survival benefit of colloids over crystalloids in most resuscitation, and some colloids cause harm — so the cheaper, safer crystalloid is first-line, with balanced solutions preferred over large-volume normal saline to avoid hyperchloraemic acidosis. Titrate to endpoints of perfusion (urine output > 0.5 mL/kg/h, clearing lactate), not to a fixed volume.

Prescribing Fluids in Practice

The prescription depends on the purpose. For maintenance, an adult needs about 25–30 mL/kg/day of water with ~1 mmol/kg/day each of sodium and potassium plus some glucose; in children the '4-2-1' rule gives the hourly rate (4 mL/kg for the first 10 kg, 2 for the next 10 kg, 1 per kg thereafter). For resuscitation, isotonic crystalloid is given as rapid boluses (250–500 mL in adults, 10–20 mL/kg in children), reassessing after each. Specific settings have their own regimens — ~30 mL/kg in early septic shock, the Parkland formula in burns, and normal saline in diabetic ketoacidosis.

Replacing Specific Losses

Abnormal losses should be replaced with a fluid resembling what is lost. Gastric losses (vomiting, nasogastric aspirate) are rich in hydrogen, chloride and potassium — so prolonged vomiting causes a hypochloraemic, hypokalaemic metabolic alkalosis, corrected with saline and added potassium. Small-bowel, biliary and pancreatic losses are rich in sodium and bicarbonate, and diarrhoea or a high-output stoma loses large volumes — all measured and matched rather than guessed.

Endpoints & Complications

Fluids are drugs and harm in either direction. Under-resuscitation leaves organs hypoperfused; over-resuscitation causes pulmonary and peripheral oedema and impairs wound and gut healing, while excess normal saline adds a hyperchloraemic metabolic acidosis. Therapy is therefore titrated to objective endpoints of restored perfusion — a falling heart rate, warm peripheries, capillary refill under 2 seconds, urine output above 0.5 mL/kg/h and a clearing lactate — with frequent reassessment rather than a fixed prescription. A useful discipline is to prescribe every fluid by its Drug (which fluid), Dose, Duration and De-escalation, exactly as for any other drug.

Crystalloids remain first-line in most resuscitation.

KEY POINT

Key points TO remember

Molecule size decides distribution: crystalloids cross capillaries; colloids are held in vessels by oncotic pressure.
Crystalloids: isotonic (resuscitation — only ~25% stays intravascular, need ~3× loss), hypotonic (maintenance), hypertonic (raised ICP); 5% dextrose = free water, never for resuscitation.
Colloids: natural (albumin, FFP) / synthetic (gelatins, dextrans, HES — HES withdrawn: AKI/coagulopathy).
Crystalloid first-line (no survival benefit of colloids); prefer balanced solutions to large-volume saline; titrate to perfusion.

EXAM TIP

Sources: Bailey & Love's Short Practice of Surgery; safe study.

All questions in Fluids, Electrolytes, Shock & Blood Transfusion

Every General Surgery chapter →