NICE CG174 is the guideline for intravenous fluids in adults in hospital and it gives you two separate prescriptions. Resuscitation is a 500 ml bolus of a crystalloid containing 130 to 154 mmol/L of sodium over less than 15 minutes, given when a defined set of indicators says the patient is hypovolaemic, and repeated only after reassessment. Routine maintenance is 25 to 30 ml/kg/day of water, approximately 1 mmol/kg/day each of potassium, sodium and chloride, and approximately 50 to 100 g/day of glucose to limit starvation ketosis. Candidates who conflate the two, or who prescribe fluid without saying which of the two they are doing, lose the station.

How do I assess this patient?

Immediate management first: ABCDE, with circulation the focus. CG174 lists the indicators that a patient may need urgent fluid resuscitation and you should quote them.

  • Systolic blood pressure less than 100 mmHg
  • Heart rate more than 90 beats per minute
  • Capillary refill time more than 2 seconds, or peripheries cold to touch
  • Respiratory rate more than 20 breaths per minute
  • NEWS of 5 or more
  • Passive leg raise suggesting fluid responsiveness

Then the full assessment, which CG174 frames as the five Rs: resuscitation, routine maintenance, replacement, redistribution and reassessment. History covers intake, thirst, and the volume and type of abnormal losses: vomiting, nasogastric output, stoma, fistula, drains, diarrhoea. Examination covers pulse, blood pressure, capillary refill, jugular venous pressure, oedema and postural hypotension. Monitoring covers the NEWS trend, the fluid balance chart and weight. Bloods are a full blood count and urea, creatinine and electrolytes, with lactate or pH if you are resuscitating.

What are the key investigations and findings?

Urea and electrolytes at least daily while on intravenous fluids, and more often if you are replacing potassium or the sodium is moving. A blood gas for lactate and base excess in any patient being resuscitated. A urinary sodium can help in high-volume gastrointestinal losses, where a value below 30 mmol/L suggests total body sodium depletion even with a normal plasma sodium. Keep an hourly urine output in any patient you are worried about.

What is the management?

Resuscitation

500 ml of a balanced crystalloid or 0.9% sodium chloride over less than 15 minutes, then reassess by ABCDE, monitoring respiratory rate, pulse, blood pressure and perfusion continuously and measuring venous lactate or arterial pH and base excess. Repeat if the indicators persist. For suspected sepsis, CG174 now defers to NG253, which uses 250 ml boluses over 10 to 15 minutes up to 1,000 ml in total before senior advice is sought. Seek expert help early in patients with complex redistribution problems or significant comorbidity.

Routine maintenance

  • 25 to 30 ml/kg/day water, about 1 mmol/kg/day potassium, sodium and chloride, about 50 to 100 g/day glucose
  • Consider 20 to 25 ml/kg/day for patients who are older or frail, who have renal impairment or cardiac failure, or who are malnourished and at risk of refeeding
  • For obese patients, prescribe to ideal body weight, use the lower end of the range, and seek expert help if BMI is over 40; patients rarely need more than 3 litres a day
  • Prescribing more than 2.5 litres a day increases the risk of hyponatraemia
  • Never add potassium to bags on the ward; use ready-mixed bags

Replacement and redistribution

Replace ongoing losses litre for litre with a fluid that matches what is being lost, and adjust for redistribution in sepsis, after major surgery, and in patients with oedema or hypoalbuminaemia. Reassess and rewrite the prescription rather than repeating yesterday's.

When do I escalate and to whom?

Escalate to the registrar if the indicators of hypovolaemia persist after a bolus, if the patient is bleeding, if the lactate is rising, or if the fluid balance is confusing. Involve critical care when the patient needs more fluid than you can safely give on a ward, when there is a risk of overload in a patient with cardiac or renal disease, or when vasopressors may be needed. Ask a senior about any patient with a complex electrolyte problem, and the renal team about a sodium or potassium that is moving quickly.

What is the interviewer listening for?

  • The explicit split between resuscitation and maintenance, and which one you are prescribing
  • The CG174 indicators and the 500 ml bolus over less than 15 minutes, followed by reassessment
  • The maintenance figures said as numbers per kilogram, and the groups who need less
  • Recognition that fluid is a drug with harms: hyponatraemia, overload, hyperchloraemic acidosis
  • A plan that includes monitoring: daily electrolytes, fluid balance, weight

What might they ask next?

  • The patient has a high-output ileostomy. What are you replacing and with what?
  • The sodium is 128 on day 3 of maintenance fluids. What went wrong?
  • Why does CG174 warn about more than 2.5 litres a day?
  • When would you choose a balanced crystalloid over 0.9% sodium chloride?

The Core Surgery Interview question bank has a scored scenario on fluid prescribing in a post-operative patient: /Question-Bank/Sample-Questions.