Fluid and Drug Prescribing in Children

Introduction

Prescribing for children is not adult prescribing scaled down. Physiology, pharmacokinetics and the consequences of error all differ, and the margin between a therapeutic and a harmful dose is proportionally narrower. Medication errors are several times more common in paediatric practice than in adult practice, and the great majority are calculation errors rather than errors of clinical judgement.

Several features of childhood explain why:

• Total body water is higher - around 75% of body weight in a neonate against 60% in an adult - with a proportionally larger extracellular compartment, so fluid shifts happen faster
• Surface area to weight ratio is much greater, so insensible losses are proportionally larger and a small child dehydrates quickly
• Metabolic rate per kilogram is higher, which raises both fluid and calorie requirements per kilogram
• Renal function is immature - glomerular filtration rate does not reach adult values corrected for surface area until around 2 years - so the ability to excrete a water, sodium or drug load is limited
• Hepatic enzyme systems are immature in neonates, altering the metabolism of many drugs and prolonging half-lives
• Doses are weight-based, so a decimal point in the wrong place produces a tenfold error rather than an obviously implausible number

The topic is heavily examined because it is one of the few areas where a foundation doctor is expected to get an arithmetic answer exactly right on their first day. It rewards learning a small number of rules properly.

Getting the weight right

• Weigh the child. An actual, recently measured weight is used for every calculation wherever it is possible to obtain one, and a weight recorded weeks ago in a growing infant is not good enough.
• In an emergency, use a length-based resuscitation tape, which is more accurate than any age-based formula
• If neither is available, use the APLS formulae as a working estimate
• In obesity, some drugs are dosed on actual body weight and others on ideal body weight - check the individual monograph rather than assuming
• Record the weight used on the prescription chart, with the date it was measured

Maintenance fluids

Maintenance fluid replaces normal ongoing losses - urine, stool and insensible losses through skin and respiration - in a child who cannot drink. It is calculated using the Holliday-Segar method, usually remembered as the 4/2/1 rule.¹

A worked example

A 26 kg child requires: (10 x 100) + (10 x 50) + (6 x 20) = 1,000 + 500 + 120 = 1,620 mL over 24 hours, which is about 68 mL/hour. The same answer using the hourly rule: (10 x 4) + (10 x 2) + (6 x 1) = 40 + 20 + 6 = 66 mL/hour. The small discrepancy between the two methods is expected and clinically irrelevant.

• Cap the total at roughly adult maintenance - conventionally around 2,000 mL/day for girls and 2,500 mL/day for boys
• Restrict to 50-80% of calculated maintenance where there is a reason to expect high antidiuretic hormone activity or fluid overload - postoperatively, in meningitis, in bronchiolitis, or in cardiac or renal disease
• Neonates are different: requirements start at around 50-60 mL/kg/day on day 1 and increase daily over the first week, and 10% glucose is used initially because of limited glycogen stores
• Include all sources in the total - drug infusions, flushes and any oral intake all count

Which fluid

• Potassium requirement is roughly 1-2 mmol/kg/day. Use pre-mixed bags - potassium must never be added to a bag on the ward.
• Peripheral infusions are usually limited to 40 mmol/L; higher concentrations require central access and cardiac monitoring
• Check U&Es before starting potassium-containing fluid, and confirm the child is passing urine
• Measure U&Es and glucose at least daily, and more often if abnormal, if large volumes are being given, or if the child is unwell
• Weigh daily and keep a strict fluid balance chart
• Stop intravenous fluids as soon as enteral intake is adequate - the safest fluid is the one the child drinks

Resuscitation, deficit and ongoing losses

A complete fluid prescription answers four questions: does this child need resuscitating, how dehydrated are they, what are they still losing, and what are their maintenance needs?

Resuscitation

• 10 mL/kg of a balanced crystalloid or 0.9% sodium chloride over less than 10 minutes, then reassess^1,6
• Repeat as required, reassessing heart rate, capillary refill, conscious level and perfusion after each bolus
• Escalate to a senior clinician and paediatric intensive care after 40-60 mL/kg, which signals a need for inotropes and often intubation⁵
• 20 mL/kg is used in term neonates and in trauma with haemorrhage, where blood products should be considered early
• Do not use colloids or starches; albumin has only niche indications
• Check glucose in every shocked child - hypoglycaemia is common, treatable, and easily missed

Assessing dehydration

Calculating and replacing the deficit

Deficit (mL) = percentage dehydration x weight in kg x 10. A 12 kg child assessed as 7% dehydrated has a deficit of 7 x 12 x 10 = 840 mL.

• Replace the deficit over 24 hours, in addition to maintenance
• Replace over 48 hours in diabetic ketoacidosis and in hypernatraemic dehydration, where rapid correction risks cerebral oedema⁷
• Subtract any resuscitation boluses already given from the deficit in a shocked child
• Replace ongoing losses millilitre for millilitre with an appropriate fluid - nasogastric losses with 0.9% sodium chloride plus potassium, and stoma or drain losses similarly
• Reassess the calculation at least daily rather than running the same bag for three days

Principles of drug prescribing

• Use the BNF for Children, not the adult BNF. The two differ in dose, indication, formulation and safety advice.²
• Prescribe in mg/kg, then calculate the actual dose and write both - for example paracetamol 15 mg/kg = 180 mg orally, four to six hourly
• Never exceed the maximum adult dose, however the weight-based calculation comes out. A 60 kg 14-year-old does not get 900 mg of paracetamol.
• Check whether the dose is per dose or per day - a frequent source of error when reading a monograph quickly
• Check the concentration of the formulation. Paracetamol suspension comes as 120 mg/5 mL and 250 mg/5 mL, and prescribing a volume rather than a dose invites a twofold error.
• Round to a measurable volume and supply an oral syringe; household spoons are not a measuring device
• Consider palatability, formulation and route - a correctly calculated dose the child spits out is not a treatment. Dispersible tablets, sugar-free preparations and taste all affect adherence.
• Off-label and unlicensed prescribing is common and legitimate in paediatrics, because many drugs were never trialled in children. Document the rationale and tell the parents.
• Have high-risk calculations independently double-checked by a second competent person

Drugs to avoid or use with caution

Doses worth having at your fingertips

Errors and how systems prevent them

Most serious paediatric medication incidents are not failures of knowledge but failures of arithmetic and transcription. Knowing the common patterns makes you far more likely to catch one.

• Tenfold errors from a misplaced or missing decimal point - the single commonest serious paediatric error
• Milligram and microgram confusion, particularly with adrenaline, digoxin and opioids
• Using an adult reference source, or an adult dose in an adolescent who happens to be large
• Using an out-of-date weight in a rapidly growing infant
• Confusing formulations - two strengths of the same suspension, or an immediate-release with a modified-release preparation
• Prescribing a daily dose as a single dose, or vice versa
• Failing to adjust for renal or hepatic impairment, or for prematurity - where corrected gestational age, not chronological age, determines dosing
• Interruption during calculation, which is a recognised and preventable contributor

It is also worth being explicit with families. Parents administer most paediatric medicines, and errors at home are common: check they know the dose in millilitres, that they have an oral syringe rather than a spoon, that they understand the maximum in 24 hours, and that they know not to give a second preparation containing the same drug - paracetamol is present in many combination products.

Putting it together

A worked example brings the components together. A 15 kg 4-year-old presents with gastroenteritis, is assessed as 7% dehydrated but not shocked, and cannot keep down oral fluids despite a nasogastric attempt.

1. Maintenance: (10 x 100) + (5 x 50) = 1,000 + 250 = 1,250 mL over 24 hours
2. Deficit: 7 x 15 x 10 = 1,050 mL, to be replaced over 24 hours
3. Total over 24 hours: 1,250 + 1,050 = 2,300 mL, which is approximately 96 mL/hour
4. Fluid: 0.9% sodium chloride with 5% glucose, with potassium added once U&Es are back and the child is passing urine
5. Plus ongoing losses replaced millilitre for millilitre if diarrhoea continues
6. Monitor: U&Es and glucose at least daily, daily weight, strict fluid balance, and reassessment of hydration at least twice daily
7. Stop the drip and revert to oral rehydration as soon as the child will drink

Three habits will keep you out of trouble in practice and score well in an exam: always start from a documented weight, always state which component of the fluid prescription you are calculating, and always sanity-check the final number against what a child of that size could plausibly need. A rate of 400 mL/hour in a toddler is arithmetically possible and clinically absurd, and noticing that is the skill the whole topic is really testing.^1,2