Maintenance IV Fluids and Calories: The Evidence Behind the Calculator

It is the second day after an emergency laparotomy. A 60-year-old woman weighing 90 kg, with a BMI of 35, is nil by mouth. Her sodium is 136 mmol/L, her potassium 3.6 mmol/L, and she is passing urine well. The fluid order on her chart, written at 3 in the morning, reads “NS @ 100 mL/hr”. Nobody has looked at it since. This is an illustrative case, not a real patient.

By the end of this article you will know what that order is giving her, what it is not giving her, and how to write a better one.

1. The Order Nobody Questions

“Normal saline is normal. What is wrong with it?” Let us do the sum. 100 mL an hour is 2.4 litres a day. Each litre carries 154 mmol of sodium and 154 of chloride, so she receives 369 mmol of each. Her daily requirement of sodium is about 70 mmol.

Everything above that is a salt load she has no use for. It buys hyperchloraemic metabolic acidosis, vasoconstriction of the renal efferent arteriole, interstitial oedema, and a longer stay. The word “normal” refers to its strength, close to the tonicity of plasma, not to what a patient needs in a day.

2. Read the Bag, Not the Label

Choose the fluid by what is in the bag, not by what it is called. Here is the composition table the tool used to carry as one (values approximate, per litre). Look first at the sodium column, then at the calories:

Three things stand out. The pure crystalloids (normal saline, Ringer’s lactate, Plasma-Lyte) carry no calories at all. Only Plasma-Lyte and Isolyte-P carry magnesium. And DNS has the full sodium of normal saline with dextrose added, so it does nothing to solve the salt problem.

3. The Glucose She Must Have

A patient kept nil by mouth beyond 12 hours needs obligatory glucose, to suppress lipolysis and stop the breakdown of protein. The minimum is 50 to 100 grams of dextrose a day, which is 200 to 400 kcal (NICE CG174, 2013). A litre of 5% dextrose gives 50 grams of glucose, or 200 kcal, so two litres cover the minimum.

“So two bags of dextrose and she is fed?” No. 200 to 400 kcal a day is not feeding anybody. It is enough to stop ketosis and no more, and a resting adult needs several times that. The tool deliberately stops at the obligatory glucose. If she will not be eating within about 48 hours, that is the point to involve the dietitian or the nutrition team and plan enteral feeding, rather than to add dextrose bags.

4. Potassium: Needed Every Day, Dangerous in Excess

“Her potassium is normal. Why add any?” Because the kidney keeps losing it. In the tool’s words: The kidney obligatorily excretes potassium even in hypokalaemic states. So a patient on potassium-free fluids drifts down day by day.

• Daily maintenance needs roughly 1 mmol/kg/day, typically 40 to 80 mmol/day, capped at 80 mmol for routine maintenance.
• Adding 20 mmol of KCl per litre is the standard safe concentration for a peripheral infusion. At 2 to 2.5 litres this gives 40 to 50 mmol, which may not meet the whole requirement; cover the shortfall orally, or with more IV KCl if she stays nil by mouth.

The safety rule. Never add KCl to IV fluids without a documented serum potassium and confirmed urine output (more than 0.5 mL/kg/hr). Do not add potassium if the serum potassium is above 5.0 mmol/L. If it is above 5.5, treat the hyperkalaemia first. Our patient has both a recent potassium of 3.6 and good urine output, so she may have it.

5. Hypotonic or Isotonic? Ask the Sodium

The choice between a hypotonic maintenance fluid (0.45% saline) and an isotonic one (0.9% saline) is made on the serum sodium. With hyponatraemia, a sodium below 130, hypotonic fluid worsens the dilution and can cause fatal cerebral oedema: these patients need isotonic fluid or fluid restriction, depending on the cause. With hypernatraemia, a sodium above 145, the patient needs free water, as 5% dextrose or 0.45% saline.

Our patient’s sodium is 136. She can have the usual hypotonic dextrose-saline.

6. The Obese Patient

“She weighs 90 kg, with a BMI of 35. Do I calculate on 90?” No, and the reason is simple. Adipose tissue needs less water per kilogram than lean mass does. Dose an obese patient (a BMI above 30) on actual body weight and the requirement comes out dangerously high.

Use the adjusted body weight instead: ABW = IBW + 0.4 × (actual weight - IBW), where IBW is the ideal body weight. If her ideal body weight is 50 kg, her adjusted weight is 50 + 0.4 × 40, which is 66 kg. Above a BMI of 30 the tool makes this substitution for you, and says so on the result.

7. Why Not Ringer’s Lactate?

A common mistake is to switch to Ringer’s lactate for maintenance once someone points out the chloride in saline. It is easy to see why: RL is “more physiological”. It is an excellent resuscitation fluid but a poor maintenance fluid. It carries 130 mmol/L of sodium, against a daily need of only about 1 mmol/kg. It provides no calories, so starvation ketosis is not covered. And it contains calcium, which is incompatible with blood products and with certain drugs in the same line. For maintenance, hypotonic dextrose-saline combinations are preferred.

Back to Our Patient

The 3 a.m. order gives her 369 mmol of sodium and no glucose. A better order uses 0.45% saline with 5% dextrose, which gives 77 mmol of sodium and 200 kcal per litre. Add 20 mmol of KCl per litre, now that her potassium and urine output are documented. With a BMI of 35, her volume is calculated on her adjusted body weight, not on 90 kg. And if she will not be eating by about 48 hours, the dietitian is called.

Try This

A 45-year-old man after bowel surgery has a serum sodium of 127 mmol/L. His intern writes 0.45% saline with 5% dextrose for maintenance, “because it is the standard”. Is it right? Illustrative case.

Answer. No. His sodium is below 130. Hypotonic fluid will worsen the dilution and can cause fatal cerebral oedema. He needs isotonic fluid or fluid restriction, depending on the cause of his hyponatraemia.

Take-Home Points

• Normal saline at 100 mL an hour gives 369 mmol of sodium a day against a need of about 70.
• A patient nil by mouth beyond 12 hours needs 50 to 100 g of dextrose a day; that is not nutrition.
• Add potassium only with a documented serum potassium and a urine output above 0.5 mL/kg/hr.
• Below a sodium of 130, never give hypotonic maintenance fluid.
• Above a BMI of 30, calculate on adjusted body weight.

For when the sodium or potassium is already wrong, see Acute Hyponatraemia: The Evidence Behind the Protocol and Acute Hypokalaemia: The Evidence Behind the Protocol; for feeding a starved patient, Refeeding Syndrome: The Evidence Behind the Pathway; and for water in health, Must We Really Drink Four Litres of Water a Day?

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Evidence tab on 5 November 2026, each from the source the tool names. The illustrative cases follow the tool’s rules; check the tool’s result for a real patient.

References

1. National Institute for Health and Care Excellence (NICE). Intravenous fluid therapy in adults in hospital (CG174). London: NICE; 2013 (Updated 2017).
2. Indian Council of Medical Research (ICMR). Standard Treatment Workflows (STW) for Common Conditions. New Delhi.
3. Association of Physicians of India (API). API Textbook of Medicine. Fluid and Electrolyte Balance.
4. Myburgh JA, Mythen MG. Resuscitation fluids. N Engl J Med. 2013;369(13):1243-1251.
5. Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650-655.
6. Lobo DN et al. Problems with solutions: drowning in the brine of an inadequate knowledge base. Clin Nutr. 2001;20(2):125-130.
7. Padhi S, Bullock I, Li L, Stroud M. Intravenous fluid therapy for adults in hospital: summary of NICE guidance. BMJ. 2013;347:f7073.