Fluid boluses

What is a fluid bolus (and why do we give one)?

• A fluid bolus is a rapid, measured volume of IV fluid given to test whether the patient’s circulation improves (a “fluid challenge”).
• Aim: increase venous return (preload) and cardiac output if the patient is fluid responsive.
• Common reasons in anaesthesia: hypotension after induction, blood loss, vasodilation from neuraxial block, dehydration (fasting, bowel prep), sepsis/vasodilation (with senior input).
• A bolus should have a clear goal and a reassessment point (e.g., BP, heart rate, capillary refill, urine output, lactate trend, stroke volume if monitored).

Bolus vs maintenance vs replacement (simple definitions)

• Bolus: a quick, defined volume to improve haemodynamics now (e.g., 250 mL over 5–10 min).
• Maintenance: slower background fluid to cover normal daily needs (often much less intra-op than you think).
• Replacement: fluid given to replace losses (bleeding, third-space/evaporative losses, drains, high stoma output).
• Don’t use repeated boluses as “maintenance by stealth” without a plan.

Choosing the fluid (new-starter safe approach)

• Balanced crystalloid (e.g., Hartmann’s/Plasma-Lyte) is a common first choice for most peri-operative boluses.
• 0.9% saline may be appropriate in specific situations (e.g., hypochloraemic metabolic alkalosis, vomiting/gastric losses), but large volumes can cause hyperchloraemic acidosis and kidney stress.
• Colloid is not routine for most new-starter scenarios; follow local policy and senior guidance.
• Blood products are for haemorrhage with suspected/confirmed significant blood loss; don’t try to “bolus your way out” of major bleeding.

How much and how fast?

• Typical adult test bolus: 250 mL balanced crystalloid over 5–10 minutes, then reassess.
• If very small/elderly/frail or heart failure risk: consider 100–200 mL aliquots with closer reassessment.
• If clear hypovolaemia (e.g., fasting + vasodilation + low filling): you may repeat, but reassess each time and consider other causes of hypotension.
• In children: use weight-based bolus (commonly 10 mL/kg crystalloid; 20 mL/kg in some resuscitation contexts—follow local guidance and senior input).

When a bolus helps (and when it won’t)

• More likely to help: tachycardia with low BP, low urine output with other signs of low perfusion, bleeding/dehydration, dynamic signs of fluid responsiveness if available.
• Less likely to help: vasodilation (deep anaesthesia, spinal/epidural), cardiogenic shock/poor LV function, obstructive causes (tamponade, tension pneumothorax, massive PE).
• If BP is low due to vasodilation, vasopressors (e.g., metaraminol/phenylephrine) and reducing anaesthetic depth may be more effective than more fluid.
• If repeated boluses don’t produce a sustained improvement, stop and rethink the diagnosis.

Practical steps: giving a safe bolus

• Confirm access: working cannula, check for infiltration/extravasation; consider a larger cannula if rapid delivery is needed.
• Pick a clear dose and time: e.g., “250 mL Hartmann’s over 5–10 min”.
• Use appropriate equipment: pressure bag/rapid infuser if needed; ensure air is cleared from giving set.
• Reassess immediately after: BP trend, heart rate, capillary refill, mental status (if awake), ETCO2 trend (under GA), urine output, and any advanced monitor values.
• Document: indication, fluid type/volume, response, and next plan.

Common first-time scenarios in theatre

• Post-induction hypotension: check depth of anaesthesia, give vasopressor if vasodilated; consider a small bolus if likely relative hypovolaemia.
• Spinal anaesthesia hypotension: left uterine displacement in pregnancy, vasopressors early, small boluses if needed (avoid large volumes in pre-eclampsia/heart disease).
• Bleeding suspected: call for help early, estimate blood loss, check suction/canisters/swabs, send bloods (FBC, coag, fibrinogen), activate major haemorrhage protocol if appropriate; give blood products rather than repeated crystalloids.
• Sepsis/vasodilation: fluids may be needed but reassess frequently; early senior involvement and consider vasopressors and source control.

Monitoring response: what counts as “worked”?

• A useful response is improved perfusion: rising MAP, reduced tachycardia, improved capillary refill, improved urine output over time, improving lactate trend (not immediate).
• If you have cardiac output monitoring: a meaningful rise in stroke volume/cardiac output after a bolus suggests fluid responsiveness.
• Be cautious: a brief BP rise that quickly fades may indicate vasodilation rather than true volume depletion.
• Always look for harm: new crackles, rising oxygen requirement, falling SpO2, increasing airway pressures, worsening oedema.