Crystalloids vs colloids

What are they? (simple definitions)

• Crystalloids: water with small dissolved salts/sugars that move freely between body fluid spaces (e.g. 0.9% saline, Hartmann’s/Plasma-Lyte, 5% dextrose).
• Colloids: fluids containing larger molecules designed to stay in the bloodstream longer and pull water into the circulation (e.g. albumin, synthetic starches/gelatins where still used).
• Key idea: most infused fluid does not stay in the blood vessels—distribution depends on the type of fluid and the patient’s physiology.

Where does the fluid go? (distribution &amp, ‘intravascular effect’)

• Crystalloids distribute into the extracellular space, only a fraction remains intravascular after equilibration (especially if capillary leak is present).
• Colloids tend to expand plasma volume more per mL than crystalloids in stable patients, but this advantage reduces in sepsis/major inflammation due to capillary leak.
• Practical takeaway: if you need a rapid intravascular effect, start with a crystalloid bolus and reassess, consider blood products early if haemorrhage is likely.

Common crystalloids (what to know on day 1)

• Balanced crystalloids (Hartmann’s, Plasma-Lyte): closer to plasma electrolyte composition, often first choice for peri-operative resuscitation/boluses.
• 0.9% saline: high chloride, large volumes can cause hyperchloraemic metabolic acidosis and may worsen renal perfusion—use thoughtfully.
• 5% dextrose: behaves like free water once glucose is metabolised, not for resuscitation (poor intravascular expansion). Useful for maintenance in selected patients and treating hypernatraemia (with senior input).

Common colloids (what to know on day 1)

• Albumin (e.g. 4–5%): human-derived colloid, sometimes used in ICU or specific situations (e.g. large-volume resuscitation where sodium/chloride load is a concern).
• Synthetic colloids (e.g. hydroxyethyl starch): associated with increased risk of kidney injury and bleeding in critical illness, generally avoided/restricted—follow local policy.
• Gelatins: can cause anaphylaxis and may affect coagulation, practice varies—check departmental guidance.

Choosing a fluid: practical peri-operative approach

• Ask: what problem am I treating? (maintenance, replacement of losses, resuscitation, or blood loss).
• Resuscitation for hypotension/hypoperfusion: small boluses of balanced crystalloid (e.g. 250 mL) with frequent reassessment (BP, HR, capillary refill, urine output, lactate/trends).
• Suspected haemorrhage: do not ‘chase’ with litres of crystalloid—activate major haemorrhage pathway early, give blood products as indicated.
• Maintenance: avoid excessive volumes, consider patient size, fasting time, ongoing losses, and comorbidities (heart failure, CKD).
• Electrolyte/acid–base context matters: choose balanced solutions when large volumes are expected, avoid inappropriate free water in raised ICP or severe hyponatraemia.

Adverse effects and safety checks

• Fluid overload: pulmonary oedema, worsening oxygenation, raised JVP, peripheral oedema—risk higher in elderly, heart failure, renal impairment.
• Electrolyte/acid–base issues: saline can cause hyperchloraemic acidosis, dextrose solutions can worsen hyponatraemia if free water excess, balanced solutions can raise potassium slightly but are usually safe in typical peri-op use.
• Coagulation: large-volume crystalloid can dilute clotting factors, some colloids can impair coagulation more directly.
• Allergy/anaphylaxis: more associated with some colloids (especially gelatins), be prepared to treat anaphylaxis promptly.

First-time scenarios you will meet

• Spinal anaesthesia hypotension: treat with vasopressor (per local protocol) and a crystalloid bolus, reassess frequently rather than giving large volumes blindly.
• Fast AF with hypotension: fluids may help if hypovolaemic, but avoid overload, consider cardioversion/vasopressors and senior review—do not assume ‘needs litres’.
• Oliguria intra-op: check catheter/kinks, haemodynamics, surgical factors, a small crystalloid bolus may be appropriate if hypovolaemia likely, avoid repeated boluses without response.
• Sepsis: early antibiotics, source control, and cautious fluid boluses with reassessment, balanced crystalloids are commonly used, consider early vasopressors in persistent hypotension (senior/ICU input).

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