Prescribing IV Fluids: The Five Rs and the Daily Maintenance
It is the commonest prescription on a surgical ward and one of the most carelessly written. A bag of fluid is signed off in seconds, often copied from the last chart, rarely reconsidered on the ward round. Yet a litre of the wrong fluid at the wrong rate can drown a failing heart, tip the sodium off a cliff, or acidify the blood — real harm from something everyone treats as harmless salt water. Intravenous fluid is a drug: it has a dose, an indication, and side effects. Prescribing it well means answering five questions before you sign the bag.
It is the post-take ward round, and three charts tell the same lazy story: 0.9% saline, one litre, eight-hourly, day after day. The first patient is a 78-year-old man two days after a bowel operation; his ankles are now pitting and his lung bases crackle — he is being quietly drowned. The second, a young woman kept nil-by-mouth for vomiting, has had three litres of plain saline and a sodium creeping upward with a chloride to match; her blood gas shows a mild metabolic acidosis nobody ordered. The third, an elderly woman given litre after litre of 5% dextrose, is confused and hyponatraemic — the water without the salt has diluted her sodium. Not one of these fluids was wrong in the abstract. Each was wrong for the patient in front of it. The consultant stops at the first bed and asks the question that should have been asked before any bag was hung: why is this patient getting fluid at all?
Fluid is a drug — start with the five Rs
NICE built the whole framework around one habit: decide the indication before the bag. The NICE guidance on IV fluids (CG174) reduces a messy clinical problem to five questions, each beginning with R. Resuscitation: is the patient shocked and needing rapid volume now? Routine maintenance: does a patient who simply cannot drink need their day's water and electrolytes? Replacement: are there ongoing abnormal losses — vomit, diarrhoea, drain output — that must be matched? Redistribution: is fluid sitting in the wrong compartment, as in sepsis or low albumin, so that giving more will not help the circulation? And Reassessment: is what you prescribed yesterday still right today? The order matters. You resuscitate first, maintain second, and never write the same chart on autopilot. To understand where each fluid actually goes once it is in the vein, this article leans on the Fluid-compartments chapter — the physiology of the intravascular, interstitial and intracellular spaces is what makes the five Rs make sense.
Resuscitation: filling the tank, fast
The shocked, hypovolaemic patient — bleeding, septic, or dry from losses — needs volume in the circulation now. The move is a rapid bolus of a balanced crystalloid: give a fixed aliquot fast (a typical adult bolus is around 500 mL over roughly ten to fifteen minutes, smaller and slower in the frail or the heart-failure patient) and then reassess. Look again at the pulse, the blood pressure, the capillary refill and the urine output, and repeat the bolus if the patient is still under-filled. There is a hard rule hiding here: do not resuscitate with dextrose. Once 5% dextrose is infused, the glucose is metabolised and you are left with free water that distributes across every compartment — only a small fraction stays in the vessels where you need it. Balanced crystalloids (compound sodium lactate — Hartmann's — or a balanced solution such as Plasma-Lyte) stay intravascular far better because their sodium content holds them there. If the patient keeps needing bolus after bolus, that is the signal to escalate — call for senior help, consider that the losses are ongoing (haemorrhage) and look toward critical care and vasopressors rather than simply hanging another bag.
Think of the circulation as a bucket with a mesh side, not a sealed drum. A balanced crystalloid is like pouring in thick, salted water that clings to the mesh and lingers in the bucket for a while. Pure dextrose is like pouring in clean water: the sugar dissolves away and what is left seeps straight through the mesh into the surrounding ground — the interstitium and the cells — leaving barely a splash in the bucket you were trying to fill. That is why you fill a collapsing circulation with salt-rich fluid, and why plain water dressed up as 5% dextrose is useless for resuscitation.
Routine maintenance: the day's ration for the nil-by-mouth patient
The patient who cannot drink still needs a predictable daily minimum — no more, no less. For someone kept nil-by-mouth but not volume-depleted, maintenance simply replaces what a normal person eats and drinks in a day. NICE gives round numbers worth memorising: water about 25–30 mL/kg/day; sodium, potassium and chloride each about 1 mmol/kg/day; and glucose about 50–100 g/day, not for calories but to blunt starvation ketosis. Run those numbers for a 70 kg adult and you land near two litres of water, roughly 70 mmol of sodium and 70 mmol of potassium. Notice what that is not: it is not three bags of 0.9% saline, which would deliver over 460 mmol of sodium — a massive salt load the kidneys must then labour to excrete. The maintenance answer is closer to a hypotonic fluid such as 0.18% saline in 4% dextrose ("dextrose-saline"), or 5% dextrose alternating with a smaller amount of saline, with potassium added to each bag. You are prescribing to the patient's actual needs and weight, not hanging a default bag. This is exactly where fluid prescribing meets the Potassium chapter: maintenance without added potassium quietly starves the patient of it, because there is almost no potassium in plain saline or dextrose.
Targets: ~2000 mL water, ~70 mmol sodium, ~70 mmol potassium, ~50–100 g glucose. A workable regimen: three bags over 24 hours, weighted toward dextrose-based fluid with potassium chloride added (commonly around 20–40 mmol per bag, always pre-mixed and rate-limited — never a bolus). Compare the naive alternative — three bags of 0.9% saline — which hits the water target but delivers roughly seven times the sodium the patient needs and essentially no potassium and no glucose. Same volume, very different prescription. The lesson: match the ions, not just the litres.
- Prescribe fluid by the five Rs: Resuscitation, Routine maintenance, Replacement, Redistribution, Reassessment.
- Resuscitate with a rapid balanced-crystalloid bolus (~500 mL), then reassess and repeat if still under-filled.
- Never resuscitate with 5% dextrose — the free water leaves the circulation almost immediately.
- Maintenance needs: water ~25–30 mL/kg/day, Na+/K+/Cl- ~1 mmol/kg/day each, glucose ~50–100 g/day.
- Maintenance is usually dextrose-based with added potassium — not three bags of plain 0.9% saline.
- Prescribe to the patient's actual weight and needs; potassium is added to bags, never pushed.
Replacement: matching the fluid to what is being lost
On top of maintenance sits a separate account: ongoing abnormal losses. A patient with high-volume vomiting, profuse diarrhoea, a leaking fistula, a high-output drain, a fever driving up insensible losses, or third-space sequestration after major surgery is losing fluid the maintenance ration never accounted for. The principle is to replace like with like — match the composition of the fluid to the composition of what is being lost. This is where the ion content matters. Gastrointestinal secretions are rich in potassium, so heavy vomiting or diarrhoea depletes it, and replacement must include generous potassium (again, the Potassium chapter). Losses that are essentially isotonic — a high-output ileostomy, diarrhoea — are best matched with a balanced crystalloid rather than plain water, because replacing salt-rich losses with salt-free dextrose invites hyponatraemia. Measure or estimate the losses over each shift and add that volume, of the right kind, on top of the day's maintenance.
Redistribution: when fluid is in the wrong compartment
Sometimes the problem is not too little fluid in the body but fluid in the wrong place. In sepsis, inflammation makes capillaries leaky, so fluid escapes the vessels and pools in the interstitium — the patient looks oedematous yet the circulation is still empty. In hypoalbuminaemia (liver disease, nephrotic syndrome, severe illness), the fall in oncotic pressure lets water drift out of the vessels into the tissues; the total-body water may be high while the effective circulating volume is low. Redistribution is the R that catches you out: the oedematous patient can be simultaneously overloaded in the interstitium and under-filled in the vessels, and pouring in more fluid worsens the swelling without reliably fixing the circulation. Recognising redistribution means treating the cause — the sepsis, the low albumin — rather than reflexively chasing the oedema with diuretics or the low pressure with endless boluses. The compartment physiology behind all of this is set out in the Fluid-compartments chapter.
Reassessment: the R everyone skips
Yesterday's prescription is a hypothesis, not a standing order. Fluids are drugs, and no drug should run unreviewed. Reassessment means looking, every day, at the same handful of things: the patient's weight and fluid balance chart, the trend in urine output, the state of the peripheries and the lung bases, and the daily electrolytes. If the patient is starting to eat and drink, the intravenous fluid should be scaled back and stopped — de-escalating to oral is itself a clinical decision, and continuing IV fluids into recovery is a common cause of quiet overload. Reassessment is also where you catch the harm early: a rising sodium and chloride flag too much saline, a falling sodium flags too much hypotonic fluid, and a climbing weight with new crackles flags overload before it becomes an emergency. The single most useful habit on any ward round is to ask of every running bag: does this patient still need this, in this amount, of this fluid?
The most dangerous fluid order is the one nobody reconsiders. "0.9% saline, one litre, eight-hourly" written on autopilot is the single commonest way to harm a patient with fluid: it overloads the salt-sensitive, acidifies with chloride, and never delivers the potassium or glucose of true maintenance. Treat every bag like a drug dose. If you would not re-prescribe an antibiotic for days without review, do not do it to a bag of saline.
Getting it wrong: overload, hyperchloraemic acidosis, hyponatraemia
Three signature harms follow careless prescribing, and each maps onto a fluid you over-gave. Fluid overload comes from too much volume — too fast, or in a patient who cannot handle it. Salt-and-water accumulate, and the excess shows first as peripheral pitting oedema and then, dangerously, as pulmonary oedema: breathlessness, crackles, falling oxygen saturations. The vulnerable are the patients with heart failure, chronic kidney disease and the elderly, whose kidneys and hearts cannot offload a load a young patient would shrug off — which is exactly why the Cardiovascular chapter pairs fluid caution with diuretics, the drugs used to pull that excess back off. Hyperchloraemic metabolic acidosis is the quieter harm of large volumes of 0.9% saline: "normal" saline is not physiological — it carries 154 mmol/L of chloride, well above plasma, and flooding the blood with chloride displaces bicarbonate and drops the pH. It is one of the reasons balanced crystalloids, whose chloride is closer to plasma, are now preferred for resuscitation. And iatrogenic hyponatraemia is the mirror-image error: give too much hypotonic fluid (5% dextrose, or dextrose-saline in excess) and the free water dilutes the plasma sodium. This is a genuinely dangerous, sometimes fatal, complication — and it must be corrected slowly, because raising a chronically low sodium too fast risks osmotic demyelination of the brainstem. The mechanism and the correction limits are the whole subject of the Sodium chapters, which this prescribing chapter feeds directly into.
Three ward-round patterns you can name at a glance. Sodium and chloride both drifting up, with a mild metabolic acidosis on the gas: too much 0.9% saline — switch toward a balanced crystalloid and reduce the volume. Sodium falling day by day in a patient on litres of 5% dextrose: dilutional hyponatraemia — cut the free water, and if the drop was slow, correct it slowly. Rising weight, new pitting oedema and crackling lung bases in an elderly or heart-failure patient: overload — stop or slow the fluid and consider a diuretic. Same ward, three different mistakes, all written on a fluid chart.
- Replacement is a separate account from maintenance — add the day's abnormal losses on top, matched in composition.
- GI losses are potassium-rich; replace vomiting/diarrhoea/fistula losses with generous potassium.
- Redistribution: in sepsis and low albumin, fluid leaks to the interstitium — more fluid worsens oedema without filling the vessels.
- Reassess daily: weight, fluid balance, urine output, chest, and electrolytes; de-escalate to oral as soon as possible.
- Overload hits heart failure, CKD and the elderly first — pulmonary oedema is the emergency; diuretics pull it back.
- Excess 0.9% saline → hyperchloraemic acidosis; excess hypotonic fluid → dilutional hyponatraemia (correct slowly).
- Resuscitating a shocked patient with 5% dextrose instead of a balanced crystalloid — the free water leaves the circulation and the patient stays under-filled.
- Prescribing three bags of 0.9% saline a day as "maintenance" — a huge sodium load with no potassium or glucose, driving overload and hyperchloraemic acidosis.
- Correcting a chronic, dilutional hyponatraemia too quickly — raising sodium faster than ~8–10 mmol/L in 24 h risks osmotic demyelination (central pontine myelinolysis).
A 70 kg man is kept nil-by-mouth after abdominal surgery but is haemodynamically stable and not fluid-depleted. Which prescription best meets his 24-hour maintenance needs?
- Treat IV fluid as a drug and prescribe by the five Rs — Resuscitation, Routine maintenance, Replacement, Redistribution, Reassessment.
- Resuscitate with a rapid balanced-crystalloid bolus (~500 mL) then reassess; never with 5% dextrose, whose free water leaves the circulation.
- Maintenance ≈ 25–30 mL/kg/day water, 1 mmol/kg/day each of Na+/K+/Cl-, and 50–100 g/day glucose — usually dextrose-saline with potassium, not plain saline.
- The three ways to get it wrong: fluid overload (pulmonary oedema in heart/renal failure and the elderly), hyperchloraemic acidosis from excess 0.9% saline, and iatrogenic hyponatraemia from excess hypotonic fluid.
- NICE Clinical Guideline CG174 — Intravenous fluid therapy in adults in hospital.
- Rang & Dale's Pharmacology — fluid, electrolyte and acid–base balance.
- BNF (British National Formulary) — fluids and electrolytes; potassium chloride prescribing and infusion cautions.
- NICE Clinical Guideline CG32 — Nutrition support for adults (oral, enteral and parenteral).
- Ganong's Review of Medical Physiology — body fluid compartments and osmolality.
- Moritz ML, Ayus JC. Maintenance intravenous fluids in acutely ill patients. New England Journal of Medicine.
