Maintenance fluids

What are “maintenance fluids” (and what they are not)

• Maintenance fluids = replacement of normal daily requirements (water + electrolytes + small amount of glucose) when oral intake is inadequate.
• They are NOT for resuscitation (shock/sepsis/bleeding) and NOT for replacing ongoing losses (e.g. high NG output, diarrhoea, stoma).
• Always ask: is the patient euvolaemic (normal circulating volume)? If not, treat that first (usually with boluses) before thinking about maintenance.

Daily requirements (adult) — the numbers to remember

• Water: ~25–30 mL/kg/day (use 25 mL/kg/day in older/frail, heart failure, CKD, or at risk of fluid overload).
• Sodium: ~1 mmol/kg/day.
• Potassium: ~1 mmol/kg/day (only if kidneys working and K+ not high).
• Glucose: ~50–100 g/day to reduce starvation ketosis (does not provide full nutrition).
• Typical total volume for a 70 kg adult: ~1.75–2.1 L/day (often less in frail/overloaded patients).

A simple practical approach to prescribing

• Step 1: Check if IV maintenance is needed: can they drink? are they NBM? are they nauseated/drowsy? are they safe to swallow?
• Step 2: Check volume status: HR, BP, capillary refill, JVP, lung bases, peripheral oedema, urine output, recent weights, fluid balance chart.
• Step 3: Check bloods: U&E (Na+, K+, urea/creatinine), glucose; consider Mg2+, phosphate if prolonged poor intake.
• Step 4: Choose a balanced plan for the next 24 h: usually a mix of crystalloid with sodium + potassium plus some glucose-containing fluid.
• Step 5: Document indication, target volume, and monitoring plan; review daily (or more often if unstable).

What fluids are commonly used (and why)

• Balanced crystalloids (e.g. Hartmann’s/Plasma-Lyte) are often preferred for general use because they are closer to plasma composition than 0.9% saline.
• 0.9% saline contains high chloride; large volumes can contribute to hyperchloraemic metabolic acidosis and fluid retention.
• Glucose solutions (e.g. 5% glucose) provide free water and some calories but contain no sodium; on their own they can worsen hyponatraemia.
• Potassium is usually added to maintenance (e.g. 20–40 mmol KCl per litre) ONLY if urine output is adequate and K+ is not high.
• Avoid hypotonic saline (e.g. 0.18% NaCl + 4% glucose) in most adults due to hyponatraemia risk unless specifically indicated and closely monitored.

Example starter prescriptions (adjust to patient and local policy)

• Fit adult, normal U&E, NBM for 24 h: aim ~25–30 mL/kg/day total, include sodium + potassium + some glucose (e.g. balanced crystalloid with added K+ plus one bag of 5% glucose over 24 h).
• Older/frail or heart failure risk: aim ~20–25 mL/kg/day (or less), avoid sodium overload, consider slower rates, and review lungs/weights frequently.
• If Na+ low: avoid large volumes of free water (e.g. 5% glucose alone); treat cause and seek senior input if symptomatic or severe.
• If Na+ high: free water deficit may be present; correct slowly and plan with senior/medical team (often requires careful hypotonic fluid strategy).
• If K+ low and kidneys ok: add KCl cautiously (common: 20–40 mmol/L), recheck K+ and ECG if significant derangement.
• If K+ high or AKI/oliguria: do NOT add potassium; get senior review and treat hyperkalaemia/AKI as appropriate.

Monitoring: what to check and how often

• Clinical: pulse, BP, respiratory rate, oxygen requirement, lung auscultation, oedema, mental state.
• Urine output: aim roughly ≥0.5 mL/kg/h in many adults (interpret in context; low output may be pre-renal, renal, or post-renal).
• Fluid balance chart: inputs, outputs, drains/NG/stoma losses; daily weight is very helpful if available.
• Bloods: U&E and glucose at baseline; then at least daily while on IV maintenance (more frequently if unstable, elderly, renal impairment, or significant electrolyte abnormalities).
• Cannula care: check site regularly; consider need for ongoing IV access.

Common first-time scenarios in theatre and peri-op

• Short elective case: patients often do not need full “maintenance” intra-op; focus on replacing deficits/third-space is usually overestimated—avoid routine large volumes.
• Post-op NBM: prescribe a 24 h plan rather than “one bag”; ensure potassium is considered and glucose included if prolonged fasting.
• Epidural/spinal hypotension: treat as haemodynamic issue (vasopressors, bolus if indicated) rather than simply running maintenance faster.
• High NG output or stoma: this is NOT maintenance—replace losses with appropriate fluid (often sodium-rich) and add potassium as guided by bloods; involve seniors early.