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 & ‘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).