01Core principlesThe concepts and mechanisms needed to understand the subject.
Intravenous fluid is a medicine with an indication, composition, dose, rate, duration and monitoring plan. Perioperative prescriptions often fail when resuscitation and routine maintenance are conflated. A shocked patient needs a rapid, reassessed intervention; a stable fasting patient needs only their net daily physiological requirement; a patient with drains or vomiting needs measured replacement in addition; oedema or sepsis may produce maldistribution despite total-body fluid excess.
Assessment combines history, examination, trends and laboratory data. Restricted intake, thirst, vomiting, diarrhoea, drains, urine, fever and bleeding define inputs and losses. Pulse, blood pressure, capillary refill, jugular venous pressure, oedema and postural change inform distribution and perfusion. NEWS, weight, fluid balance, creatinine, urea and electrolytes reveal trajectory. Passive leg raising can test whether transient preload augmentation improves haemodynamics but must be interpreted with the whole clinical picture.
The adult CG174 resuscitation prescription is a 500 mL bolus of a crystalloid containing sodium 130–154 mmol/L over less than 15 minutes. The critical next step is reassessment of airway, breathing, circulation, disability and exposure, perfusion, observations and lactate or acid-base measures. More fluid follows only if the response and diagnosis support it. Cardiac failure, severe renal dysfunction and small or frail patients may tolerate less.
Routine maintenance covers normal water, electrolyte and limited glucose needs when oral or enteral routes are insufficient. It does not replace a pre-existing deficit or abnormal external loss. The initial adult estimate is weight-based, then adjusted for age, frailty, organ function, intake and ongoing losses. Glucose limits starvation ketosis but does not provide nutrition. Prescriptions are revised from response rather than copied indefinitely.
Key points
- Start with the five Rs: Resuscitation, Routine maintenance, Replacement, Redistribution and Reassessment. State which indication each prescribed bag is treating.
- For adult resuscitation under CG174, use crystalloid containing sodium 130–154 mmol/L, give 500 mL over less than 15 minutes, then reassess before any further bolus.
- For routine maintenance alone, begin with 25–30 mL/kg/day water, about 1 mmol/kg/day each of sodium, potassium and chloride, and 50–100 g/day glucose; use less in older, frail, renal or cardiac patients.
- Add replacement for measured abnormal losses separately from maintenance and account for deficits, excesses, third-space redistribution and oral or enteral intake.
- Review clinical status, fluid balance, urea, creatinine and electrolytes at least daily, with more frequent review during resuscitation or unstable replacement; weigh at least twice weekly when therapy continues.
- High-chloride fluids can cause hyperchloraemia and acidaemia. If chloride content exceeds 120 mmol/L, monitor serum chloride daily and reconsider the prescription if either develops.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Look for thirst, reduced intake, fluid loss, tachycardia, low pressure, prolonged capillary refill, cool peripheries, postural change, reduced urine and a response to passive leg raising.
Breathlessness, oxygen requirement, crackles, raised venous pressure, oedema, rapid weight gain and positive balance suggest excess or maldistribution and change the safety of further fluid.
Write whether the prescription treats shock, normal fasting requirements, a measured deficit, ongoing gastrointestinal or urinary loss, or redistribution from sepsis and inflammation.
Estimate blood loss separately, measure drain and stoma output, include vomiting and urine, and avoid unsupported replacement for presumed insensible or third-space losses.
Pregnancy, severe renal or liver disease, burns, diabetic ketoacidosis and paediatric care require their own algorithms and must not inherit the general adult maintenance calculation.
Ongoing hypotension, deteriorating perfusion, rising lactate or new pulmonary oedema after a bolus indicates an unsafe trajectory and requires immediate senior diagnostic review.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Clinical fluid assessment and passive leg raise - Why
- Estimate perfusion, congestion and potential fluid responsiveness before and after an intervention.
- Interpretation and limitations
- Haemodynamic improvement within about 30–90 seconds supports possible volume responsiveness; breathlessness or deterioration suggests overload, but neither response identifies the cause of shock alone.
- 02
Fluid balance and body weight - Why
- Compare all routes of intake with urine, drains and other losses and detect cumulative retention or depletion.
- Interpretation and limitations
- Trends are more informative than one total. An inaccurate chart should not justify precision; reconcile unrecorded theatre fluid, blood, oral intake and output.
- 03
Urea, creatinine and electrolytes - Why
- Detect kidney injury and sodium, potassium, chloride or bicarbonate consequences of illness and prescribing.
- Interpretation and limitations
- Review at least daily during continuing therapy and more often when unstable. Creatinine may lag acute change; interpret chloride and bicarbonate together when high-chloride fluid has been used.
- 04
Lactate and acid-base assessment - Why
- Track impaired perfusion and identify metabolic consequences during resuscitation.
- Interpretation and limitations
- A falling lactate can support improvement but is not specific to fluid responsiveness; persistent elevation prompts reassessment of haemorrhage, sepsis, cardiac output and oxygen delivery.
- 05
Urine output and urinary sodium in selected losses - Why
- Monitor renal response and, in high-volume gastrointestinal loss, help assess total-body sodium depletion.
- Interpretation and limitations
- Low urine output has many causes. Urinary sodium below 30 mmol/L may suggest sodium depletion but becomes unreliable with renal impairment or diuretic therapy.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked case: postoperative hypotensionLow pressure after abdominal surgeryAn adult in recovery is tachycardic and hypotensive with cool peripheries after an operation where blood loss may have been underestimated.+
- 1Perform ABCDE assessment, call senior anaesthetic and surgical help, review theatre fluid and blood loss, inspect drains and wound, and obtain haemoglobin, lactate, acid-base and coagulation data as indicated.
- 2If hypovolaemia remains likely, give 500 mL crystalloid containing sodium 130–154 mmol/L over less than 15 minutes while arranging haemorrhage control and matched blood support if bleeding is significant.
- 3Reassess observations, perfusion, lungs, venous pressure, mental state, urine and lactate immediately; do not prescribe the next bolus before deciding whether response supports further fluid.
- 4If instability persists, escalate source control, haemorrhage management, vasopressor or cardiac assessment according to cause instead of treating every low pressure with crystalloid.
- 5Once stable, recalculate maintenance and measured replacement separately and document review timing, biochemical monitoring and limits for stopping fluid.
02Routine maintenance pathwayStable adult temporarily unable to drinkThe patient is haemodynamically stable, has no major deficit or abnormal loss and cannot meet needs enterally for the next day.+
- 1Use current weight to estimate 25–30 mL/kg/day water, around 1 mmol/kg/day each sodium, potassium and chloride, and 50–100 g/day glucose.
- 2Reduce initial water to roughly 20–25 mL/kg/day when older, frail, malnourished or affected by cardiac or renal impairment, then include other IV, oral and enteral intake.
- 3Review next-day balance, examination, weight trend and biochemistry before rewriting the prescription; stop IV fluid when oral or enteral intake is sufficient.
03Replacement pathwayHigh-output stoma with maintenance needsA stable postoperative adult has ongoing measured gastrointestinal losses in addition to inability to drink.+
- 1Measure the output and assess its likely water and electrolyte composition, clinical deficit, renal response and current maintenance requirement.
- 2Prescribe normal maintenance separately, then replace ongoing loss with an appropriate fluid and rate, accounting for any existing deficit and all other intake.
- 3Use frequent fluid balance, weight, urine, sodium, potassium, magnesium, chloride and renal trends to adjust; seek specialist help for high or complex losses.
05Relevant medicines and safetySpecific regimens and precautions where medicines are relevant.
Sodium-containing crystalloid for resuscitation
Give 500 mL containing sodium 130–154 mmol/L over less than 15 minutes, then reassess.Use smaller or slower challenges when cardiac or renal reserve is limited; repeated unassessed boluses risk pulmonary oedema and delayed source control.
Routine-maintenance IV fluid
Initially provide 25–30 mL/kg/day water with about 1 mmol/kg/day each sodium, potassium and chloride.Reduce water for frailty or cardiac and renal impairment; supply 50–100 g/day glucose, avoid adding potassium directly to bags, and monitor.
06Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- During resuscitation, continuously follow respiratory rate, pulse, blood pressure and perfusion and repeat ABCDE assessment after each bolus.
- For ongoing therapy, assess fluid status, balance, urea, creatinine and electrolytes initially at least daily and measure weight at least twice weekly.
- Increase monitoring frequency for instability, resuscitation, high-output losses, redistribution, renal change or active electrolyte correction.
- Check serum chloride daily after fluids containing more than 120 mmol/L chloride and reassess composition if hyperchloraemia or acidaemia develops.
- At every transfer of care, reconcile all fluid, blood, medicine infusions, oral intake, urine and drains and rewrite the indication and next review time.
07Special situationsVariants, exceptions and circumstances that change the usual approach.
A bag has one job
Label the dominant indication for each bag. Mixing resuscitation, maintenance and replacement logic usually creates hidden sodium, chloride or water excess.
Response precedes repetition
The diagnostic value of a bolus comes from immediate reassessment; an automatic series of bags converts a testable intervention into uncontrolled exposure.
Glucose is not nutrition
The maintenance glucose allowance limits starvation ketosis but provides too little energy, protein, micronutrient or nitrogen to meet nutritional requirements.
Creatinine can lag
A normal early creatinine does not prove renal perfusion is adequate; use trajectory, urine, haemodynamics and the clinical cause together.
Oedema can coexist with depletion
Inflammation, hypoalbuminaemia and venous congestion redistribute fluid, so swollen tissues do not reveal whether the effective circulating volume will respond safely.
08Common pitfallsFrequent interpretation and management errors.
- 01
Calling every postoperative low blood pressure dehydration without assessing bleeding, anaesthetic effect, sepsis or cardiac causes.
- 02
Repeating 500 mL boluses without documenting physiological response and signs of overload.
- 03
Using the maintenance calculation to replace stoma, drain, vomiting or blood losses.
- 04
Giving 0.9% sodium chloride repeatedly without checking chloride and acid-base status.
- 05
Treating oliguria alone with fluid despite congestion or established kidney injury.
- 06
Importing CG174 figures into children, pregnancy, burns, DKA or severe renal and liver disease.