Fluids, electrolytes and nutrition in the surgical patient

• Fluid and electrolyte balance is a treatment in its own right after surgery: sodium, water and potassium losses must be replaced, but excess free water and sodium are both harmful and can prolong ileus.
• Balanced crystalloids have chloride close to plasma levels (about 98 to 110 mmol/l, compared with 154 mmol/l in 0.9 % saline) and are preferred as maintenance and replacement fluids; 0.9 % saline is reserved for specific needs, such as chloride depletion or hypochloraemia. ^[3]
• NICE CG174 bases routine maintenance on water 25 to 30 ml/kg/day, sodium, potassium and chloride about 1 mmol/kg/day, and 50 to 100 g/day of glucose, with 20 to 25 ml/kg/day in older, frail or malnourished patients. ^[1]
• Third-space losses after surgery are not a routine replaceable volume: fluid moves into the interstitium, is retained while vasopressin is high, and usually re-mobilises on the second or third postoperative day; giving large volumes on day 1 produces oedema and weight gain.
• Early enteral feeding is feasible for most patients once swallowing is safe, and it is associated with earlier return of gut function; parenteral nutrition is reserved for patients who are malnourished and cannot meet needs by the gut.
• Refeeding must be started at low energy with phosphate, potassium, magnesium and thiamin provided from the outset, because the metabolic switch after starvation can be fatal.

By the end of this page you should be able to:

1. Write a standard maintenance fluid prescription for a surgical patient, adjust it for age, obesity, renal or cardiac impairment, and refeeding risk, using NICE CG174. ^[1]
2. Decide when to use a balanced crystalloid, when 0.9 % saline is justified, and explain why chloride matters.
3. Explain why third-space fluid is not routinely replaced and how perioperative hormones move water across the first five postoperative days.
4. Describe the prevention and supportive management of postoperative ileus, including fluid balance.
5. Screen for malnutrition, state the energy, protein and micronutrient needs of a surgical patient, and choose the enteral-versus-parenteral route within NICE CG32.
6. Start feeds safely in a malnourished patient using the refeeding protocol, including thiamin, phosphate, potassium and magnesium.
7. Apply the ERAS and ESPEN recommendations on early feeding and early oral intake.

The first skill is to decide how much fluid a patient needs over the next 24 hours, and for what reason. NICE CG174 divides fluid prescribing into five categories, often remembered by the 5 Rs: resuscitation, routine maintenance, replacement, redistribution and reassessment.^[1] A surgical patient often needs more than one at the same time, and the IV fluid management plan should state the 24-hour prescription, the monitoring plan, and who is responsible.^[1]

Resuscitation is for hypovolaemia. NICE recommends crystalloids containing sodium in the range 130 to 154 mmol/l, given in 500 ml boluses over less than 15 minutes, with reassessment after each bolus.^[1] Balanced fluids fit this range and are a sensible default, but the patient's electrolytes, age and cardiac state matter more than the label on the bag. A single formula is not enough: transfusion, colloid, blood products, drugs and enteral intake all count toward the daily volume, and every source should be written into the fluid balance chart.^[1]

Routine maintenance is for the patient who cannot eat or drink. NICE limits the initial prescription to water 25 to 30 ml/kg/day, approximately 1 mmol/kg/day each of sodium, chloride and potassium, and approximately 50 to 100 g/day of glucose to limit starvation ketosis.^[1] Sodium chloride 0.18 % in 4 % glucose with 27 mmol/l potassium is one way of delivering this, and NICE warns that prescribing more than 2.5 litres per day increases the risk of hyponatraemia.^[1] The 50 to 100 g of glucose is worth noting: it is enough to stop ketosis, but it is not nutrition. A patient on maintenance fluid alone will lose protein mass and will need feeding organised separately.^{[1] [5]}

Maintenance needs change with the patient. For obese patients the prescription should be based on ideal body weight, total volumes rarely need to exceed 3 litres per day, and expert help should be sought when the body mass index exceeds 40 kg/m2.^[1] Older or frail patients, those with renal impairment or cardiac failure, and those malnourished or at risk of refeeding syndrome need less fluid, in the range of 20 to 25 ml/kg/day.^[1] Potassium must never be added to IV fluid bags at ward level: it should be prescribed in standard, pharmacy-prepared solutions, with weight-based doses rounded to the nearest available bag.^[1]

Replacement and redistribution are the surgical parts of the plan. Replacement covers existing deficits and ongoing abnormal losses, taking the quantity and composition of each loss into account. Gastrointestinal losses from vomiting, nasogastric aspiration, fistulae and high-output stomas are salt-rich and usually need a crystalloid richer in sodium and chloride than maintenance fluid. Urinary sodium may be helpful in patients with high-volume gastrointestinal losses, because a urinary sodium below 30 mmol/l can indicate total body sodium depletion even when the plasma sodium is normal.^[1] NICE advises expert help for complex redistribution problems, including postoperative fluid retention and redistribution and refeeding issues.^[1]

A fluid balance chart that is filled in but not reviewed is of no value. NICE recommends at least daily reassessment of clinical fluid status, urea, creatinine and electrolytes, and fluid balance chart, with weight measurement at least twice weekly; patients receiving replacement or redistribution fluids need more frequent monitoring.^[1] After surgery the combination of weight, urine output and serum sodium tells you more than the daily fluid balance alone. NICE also recommends reporting incidents of clear fluid mismanagement, such as unnecessary prolonged dehydration or inadvertent fluid overload, as critical incidents.^[1]

The chloride concentration is the key difference. Human plasma chloride is 98 to 106 mmol/l, whereas 0.9 % saline contains 154 mmol/l.^[3] A review of crystalloid composition shows that balanced or buffered solutions approximate plasma chloride much more closely: for example compound sodium lactate 109 mmol/l, Ringer's lactate 109 mmol/l, Ionosteril 110 mmol/l, Sterofundin ISO 127 mmol/l, and Plasma-Lyte 148 98 mmol/l.^[3] No solution is truly perfect, but the gap between saline and balanced fluids is large.^[3]

Why this matters around surgery. Large volumes of 0.9 % saline produce a hyperchloraemic metabolic acidosis, because saline has a strong ion difference of zero against a plasma strong ion difference of about 40 mEq/l.^[3] More importantly for the kidney, the excess chloride load causes renal vasoconstriction and reduces renal cortical perfusion.^[3] Saline expands the extracellular (interstitial) fluid more than an equivalent balanced solution, so 0.9 % saline is more likely to produce fluid overload and interstitial oedema.^[3] An experimental link between acidosis and impaired gastropyloric motility has also been reported.^[3]

Balanced fluids have their own limits. Ringer's lactate and Hartmann's solution are hypotonic, with an in-vivo osmolality of about 254 mosmol/kg water, compared with 286 for 0.9 % saline.^[3] Perioperative administration of hypotonic fluid is a significant free-water load that may not be excreted while anti-diuretic hormone is high after surgery, producing weight gain and oedema.^[3] These fluids are also contraindicated in patients with or at risk of cerebral oedema, and Ringer's lactate cannot be mixed with blood products because of its calcium content, or with ceftriaxone because of precipitation.^[3]

NICE does not name a single fluid, but it restricts resuscitation to crystalloids with sodium 130 to 154 mmol/l, and tells clinicians to monitor serum chloride daily when fluids containing more than 120 mmol/l chloride, such as 0.9 % saline, have been given, reassessing the prescription and acid-base status if hyperchloraemia or acidaemia develops.^[1] These rules are the ward-level translation of the balanced-versus-saline debate. Observational data in patients having elective abdominal surgery found major complications, transfusion and dialysis more common in patients who received saline exclusively, although these were not randomised and confounding cannot be excluded.^[3] Randomised trials collated in the same review show that saline reliably lowers pH and raises serum chloride, generally without consistent differences in short-term renal outcomes; the SPLIT trial (2262 patients) found no difference in acute kidney injury or day-90 survival between saline and Plasma-Lyte 148, and a surgical study of 1100 patients found no difference in acute kidney injury or complications but a transient hyperchloraemic metabolic acidosis with saline on postoperative day 1.^[3] The balanced view is that a balanced or chloride-restricted crystalloid is the safer default for maintenance and replacement, and that 0.9 % saline remains correct when the aim is chloride repletion, or in resuscitation where hyperchloraemia is not expected to persist. ESPEN and ERAS-aligned reviews place this fluid choice inside the broader perioperative pathway, because a poor fluid strategy undermines early feeding and mobilisation even when every other ERAS element is correct.^[5]

The idea that the surgical patient loses a large fixed volume into a third space, which must then be replaced with crystalloid on top of maintenance, is no longer accepted. Fluid does move into the interstitium after tissue injury, and a degree of weight gain is unavoidable, but this is not a hidden, predictable deficit that can be added to the daily calculation. NICE makes no routine allowance for a third space; it asks the prescriber to adjust maintenance for measured deficits, excesses, ongoing losses and abnormal distribution, including postoperative redistribution.^[1] The third space is better thought of as part of the postoperative inflammatory phase than as a volume to be filled.

Postoperative hormones explain the timing. Surgical stress raises arginine vasopressin (anti-diuretic hormone) sharply, and in a prospective liver resection study it remained above baseline for at least three days, with the elevation most pronounced after major resection; aldosterone rose only briefly and had returned toward baseline by postoperative day 2.^[7] The dominant effect is free-water retention: net weight gain peaking on postoperative days 2 and 3, transient hyponatraemia, reduced urinary sodium excretion (a median value around 70 mmol/l against a baseline around 114 mmol/l) and a return of weight toward baseline by postoperative day 5.^[7] These observations support two clinical habits: a restrictive fluid strategy in the first 48 hours, and patience with the postoperative weight that looks high on day 3. A postoperative weight gain of 3.5 kg or more has been identified as an independent risk factor for complications.^[7]

Fluid overload is not harmless. It impairs gut motility and can convert a physiological postoperative ileus into a prolonged one, and it contributes to pulmonary and interstitial oedema. NICE therefore advises 20 to 25 ml/kg/day in older, frail or malnourished patients, and NICE CG174 also flags gross postoperative fluid retention as a reason to seek expert help.^[1] The correct response to a rising weight on day 1 is usually to reduce maintenance, not to give more diuretic.

Postoperative ileus is defined practically as the slow return of bowel transit after surgery. Reported incidence ranges from 8 to 21 % after colorectal surgery and 10 to 30 % after abdominal surgery, largely because there is no single accepted definition and thresholds in the literature range from 3 to 7 days.^[4] Prolonged ileus increases length of stay, morbidity and cost, and begets further fluid administration, so prevention is a fluid-and-nutrition problem as much as a surgical one.^[4]

Risk factors are partly unmodifiable: male sex, older age, cardiac comorbidity, previous abdominal surgery, longer operations and significant blood loss.^[4] Several are modifiable: open surgical approach, excessive bowel handling, delayed mobilisation, excessive intraoperative fluid and postoperative opioid dose.^[4] A retrospective study linked higher intraoperative fluid volumes to a higher risk of ileus, and a meta-analysis of colorectal surgery reported an odds ratio of 0.32 for ileus with intraoperative lidocaine, supporting multimodal analgesia.^[4]

The inflammatory phase of ileus begins three to four hours after the first incision, when manipulation activates peritoneal mast cells and macrophages and releases TNF-alpha and IL-6 into the bowel wall.^[4] Because this is the prolonged phase, prevention at the time of surgery matters more than treatment afterwards. The practical prevention package is familiar: minimally invasive surgery where appropriate, gentle handling and avoidance of unnecessary bowel manipulation, no routine postoperative nasogastric tube or intra-abdominal drain, judicious fluid replacement, regional or opioid-sparing analgesia, early mobilisation and early oral intake with nausea and vomiting prophylaxis.^[4]

Treatment is supportive. Clear the secondary causes (sepsis, electrolyte disturbance, anastomotic or abdominal catastrophe), rest the bowel with nil by mouth if vomiting is significant, rehydrate with a suitable crystalloid, replace magnesium, potassium and phosphate, and decompress with a nasogastric tube when output is high.^[4] Prokinetics such as metoclopramide and erythromycin have inconsistent evidence for shortening ileus, and their value is symptomatic rather than curative; alvimopan, a peripherally acting mu-opioid antagonist, has better evidence in specific procedures such as cystectomy but is not universally available.^[4] Simpler adjuncts supported by limited evidence include postoperative chewing gum, coffee and early ambulation.^[4]

Fluid management in established ileus has a specific trap. A patient who is vomiting and oliguric may look volume-depleted but can be fluid-overloaded from the ongoing IV infusion at the same time: weight and electrolytes must be used together, and replacement of the measured gastrointestinal loss with a salt-rich crystalloid is safer than a standard maintenance bag.

Malnutrition is the entry point rather than the endpoint of surgical nutrition care. About 17 to 20 % of elective surgical patients meet malnutrition risk criteria, and many of them have a body mass index above 25 kg/m2, so routine body mass index assessment is not enough.^[5] Screening should use a validated tool such as MUST or NRS-2002, repeated weekly in hospital, and it should look for unintentional weight loss, poor intake, muscle wasting and functional decline.^[2][5]

NICE CG32 sets clear indications for nutrition support. It should be considered when any of the following holds: a body mass index below 18.5 kg/m2; unintentional weight loss above 10 % over 3 to 6 months; a body mass index below 20 kg/m2 with weight loss above 5 % in the same interval; or eating little or nothing for more than five days, or a poor absorptive capacity with high nutrient losses or increased needs.^[2] Malnutrition in this sense is not the same as overnutrition, and obesity does not exclude it.

The choice of route follows three questions.

1. Can the patient swallow safely? If not, enteral tube feeding or parenteral nutrition is needed.
2. Is the gastrointestinal tract functional and accessible? If yes, enteral feeding is preferred; if not, parenteral nutrition is the alternative.
3. Is the patient malnourished and facing major surgery? If so, preoperative optimisation and, when needed, perioperative parenteral supplementation come into play.

NICE CG32 recommends that malnourished surgical patients be considered for preoperative enteral tube feeding before major abdominal procedures, and that general surgical patients do not have enteral tube feeding within 48 hours of surgery unless they meet the indication criteria.^[2] Perioperative oral nutrition support should be considered for malnourished surgical patients who can swallow safely, and oral intake should be offered within 24 hours after abdominal surgery when gut function is intact.^[2] ESPEN-aligned review evidence finds early postoperative oral or enteral feeding feasible in many elective gastrointestinal operations, accelerating gastrointestinal recovery without increasing major complications when the route is chosen correctly and advanced in steps.^[5] Supplementary perioperative parenteral nutrition is reserved for malnourished surgical patients who cannot meet needs by the gut, and parenteral nutrition should be used to supplement or replace enteral feeding when intestinal tolerance persistently limits it.^[2]

Intravenous fluids and feeds must be added up together. A patient on maintenance fluid plus enteral feed plus parenteral nutrition is not on three separate regimens; the combined sodium, water and energy intakes must be calculated once and reviewed daily.^[2]

For a patient who is not severely ill and not at risk of refeeding, NICE CG32 suggests a total intake of 25 to 35 kcal/kg/day of energy (including protein energy), 0.8 to 1.5 g/kg/day of protein, and 30 to 35 ml/kg/day of fluid, plus adequate electrolytes, minerals, micronutrients and fibre.^[2] Energy values can be lowered in overweight patients, and hot or febrile patients need more. Prehabilitation trials in high-risk colorectal patients used protein targets around 1.5 g/kg/day with supplemental whey protein, and reported lower complication rates.^[5]

During preoperative optimisation, oral nutritional supplements are the common vehicle. ESPEN recommends energy- and protein-dense oral nutritional supplements for patients whose regular oral intake cannot meet requirements, often given in 150 to 300 ml portions two to five times daily by sip feeding, with the composition matched to tolerance and need.^[5] Screening and optimisation of iron, vitamin D, folate and other micronutrients should run alongside macronutrient support, because deficiencies are common before surgery and each carries its own anaesthetic and recovery cost.

Enteral nutrition is the default. It uses the gut, maintains mucosal barrier function, reduces septic morbidity and is more physiological. Gastric feeding through a nasogastric tube is the standard access in general ward and intensive care patients with upper gastrointestinal function; post-pyloric access is reserved for upper gastrointestinal dysfunction. Percutaneous gastrostomy is used when enteral feeding is expected to last four weeks or more.^[2]

Parenteral nutrition is appropriate when the gut cannot be used, or cannot be used sufficiently: inadequate or unsafe oral and enteral intake, non-functional or inaccessible bowel, perforation, obstruction, high-output fistula, short bowel or perioperative states where enteral tolerance will not allow progress.^[2] Route-specific parenteral routes follow standard practice: central access for more concentrated solutions and longer courses, peripheral access for shorter courses; continuous infusion preferred in severe illness; cyclical infusion later on for longer courses.^[2]

ERAS refinements of this decision are practical. Clear fluids are usually allowed up to 2 hours preoperatively, and solid food up to 6 hours in many programmes, so that the patient is not starved beyond the fasting window. Carbohydrate loading (usually a clear carbohydrate drink) is widely used as metabolic preparation. Intraoperatively, ERAS teams target a near neutral fluid balance and multimodal analgesia, and postoperatively the default is early oral or enteral intake with early mobilisation.^[6] Each of these elements is evidence-based in its own right; their benefit comes from compliance with the whole bundle rather than from any single item.^[6]

Refeeding syndrome is the most dangerous error in surgical nutrition. After more than five days of little or no intake, introducing energy too quickly drives insulin release, which moves phosphate, potassium and magnesium into cells and precipitates a potentially fatal arrhythmia, respiratory failure or fluid overload. The clinical hallmarks are hypophosphataemia, hypokalaemia, hypomagnesaemia, hyperglycaemia and acute fluid overload, and the patients at highest risk are those with body mass index below 16 kg/m2, weight loss above 15 % over 3 to 6 months, intake little or nothing for more than 10 days, or pre-feed low potassium, phosphate or magnesium.^[2]

NICE CG32 gives a specific starting protocol. For a patient who has eaten little for more than 5 days, feed at no more than 50 % of requirements for the first 2 days before increasing if monitoring shows no refeeding problem.^[2] For a patient at high risk (meeting the Box 1 criteria), start at a maximum of 10 kcal/kg/day and reach or exceed full needs by day 4 to 7; in extreme cases (body mass index below 14 kg/m2 or negligible intake for more than 15 days) use only 5 kcal/kg/day and monitor cardiac rhythm continuously.^[2] At the same time, restore circulatory volume and monitor fluid balance closely, provide full requirements of fluid, electrolytes, vitamins and minerals from the outset, and give thiamin 200 to 300 mg daily plus vitamin B co strong 1 or 2 tablets three times daily (or a full-strength intravenous vitamin B preparation if necessary) with a balanced multivitamin or trace element supplement once daily, before and during the first 10 days of feeding.^[2] Phosphate, potassium and magnesium supplements should be provided proactively, with likely requirements 0.3 to 0.6 mmol/kg/day of phosphate, 2 to 4 mmol/kg/day of potassium, and 0.2 mmol/kg/day of magnesium intravenously or 0.4 mmol/kg/day orally, unless pre-feeding levels are high. Pre-feeding correction of low levels is not required.^[2] Full energy and nitrogen targets are reached only over 4 to 7 days, and only when clinical and biochemical monitoring shows no refeeding problem.^[2]

The sequencing matters. Full micronutrient and electrolyte provision from the outset, with calorie restriction, is the opposite of the intuitive ward habit of giving a full feed and waiting for bloods. That habit is the commonest way refeeding is missed. Fluid is also controlled: refeeding-phase overload is a recognised cause of death.

ERAS is a multimodal bundle, not a checklist to tick after the fact, and the ERAS Society was built on the observation that actual bedside practice drifts away from the protocol unless it is audited continuously.^[6] Across specialties it delivers an average two-day reduction in hospital stay and roughly a 30 % reduction in complications.^[6] ESPEN (the European Society for Clinical Nutrition and Metabolism) underwrites the nutrition elements inside ERAS: screening, protein and energy targets, and early feeding.^[5] The guideline editions used here are NICE CG174 (published 10 December 2013, last updated 5 May 2017) and NICE CG32 (published 22 February 2006, last updated 4 August 2017), with ESPEN recommendations as summarised in peer-reviewed ERAS reviews (2024-2026), the ERAS Society publications of 2026, and drug and fluid composition data from 2016 crystalloid reviews.^{[1][2][3][5][6]} Current clinical practice should be cross-checked against the latest editions of each guideline available locally.

• The two cheapest effective tools are a daily weight and a fluid balance chart reviewed every day. A weight gain of more than about 3.5 kg after surgery should prompt a review of fluid and sodium balance.^[7]
• In settings without balanced crystalloids, 0.9 % saline can still be given, but the volume should be limited, chloride monitored, and hyponatraemia and oedema watched for. Use 0.18 % saline in 4 % glucose for maintenance where appropriate, and avoid excess free water in patients with head injury.
• Enteral feeding is usually available even where parenteral nutrition is not. A nasogastric tube, a kitchen diet fortified with protein, and a trained volunteer or carer can deliver most of the benefit of a commercial feeding regimen.
• Refeeding prophylaxis does not require expensive parenteral preparations: oral thiamin and a multivitamin, careful calorie control and daily electrolyte checks are effective and inexpensive.
• In patients where hospital stays are unavoidable, early oral intake, early ambulation and early removal of drains usually recover more time and cost than any drug.
• Any fluid or nutrition decision should be written into a single, visible plan so that every shift of staff knows the 24-hour target.

Preoperative assessment and perioperative care (surgery/perioperative-care); postoperative fever and complications (surgery/postoperative-fever-and-complications); postoperative pain and analgesia (surgery/postoperative-pain-and-analgesia); acid-base disorders (physiology/acid-base-disorders); dehydration and paediatric fluids (paediatrics/dehydration-and-paediatric-fluids); hyperkalaemia, hypokalaemia, hyponatraemia, hypernatraemia (renal/electrolyte topics).