Paediatric Medication Calculation
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Direct answer
Fourteen kilograms of toddler, paracetamol ordered at 15 mg per kg — the arithmetic (14 × 15 = 210 mg) is the easy half of paediatric medication safety; the hard half is weight accuracy, unit discipline (micrograms versus milligrams) and the independent double-check that catches the decimal slip before it reaches a child. Most paediatric prescribing is weight-based (dose per kg per day divided into frequencies), with age-based formulas — Dilling's (age divided by 20 times the adult dose), Young's (age divided by age plus 12), Clark's (weight in pounds divided by 150) — retained for exams and resource-poor settings. Resuscitation runs on formulas worth memorising: maintenance fluids by the 4-2-1 rule, endotracheal tube size (age in years divided by 4, plus 4 uncuffed), defibrillation 2 then 4 joules per kg, adrenaline 0.01 mg per kg, fluid boluses 20 mL per kg.
What you must remember
- Weight-based core: dose = ordered mg per kg × weight in kg, checked against frequency — paracetamol 15 mg per kg per dose every 4 to 6 hours is the classic example, amoxicillin 40 to 90 mg per kg per day in divided doses; re-read the per-day versus per-dose wording.
- Age rules for adult-dose fractions: Dilling's — age divided by 20 (the Indian textbook favourite); Young's — age divided by (age + 12); Clark's — weight in pounds divided by 150.
- Body surface area: from a nomogram or Mosteller's formula (square root of height in cm × weight in kg divided by 3600); chemotherapy doses as mg per metre squared.
- Maintenance fluids (Holliday-Segar 4-2-1 rule): 4 mL per kg per hour for the first 10 kg, 2 for the next 10, 1 for every kg above 20 — a 26 kg child needs 66 mL per hour (40 + 20 + 6).
- Resuscitation formulas: uncuffed ETT internal diameter = (age in years ÷ 4) + 4, cuffed + 3.5; adrenaline in arrest 0.01 mg per kg IV (0.1 mL per kg of 1 in 10,000); defibrillation 2 then 4 J per kg; crystalloid bolus 20 mL per kg.
- Weight estimation without scales: APLS formula — (age in years + 4) × 2 for 1 to 10 years; Broselow tape by length in emergencies.
- Safety rituals: an independent double-check for every high-alert drug (insulin, heparin, opioids, chemotherapy), pump rates verified against the order, never a trailing zero (5 mg, not 5.0 mg) nor a naked decimal (0.5 mg, not .5 mg).
A ward calculation from prescription to pump
A 6-year-old, 20 kg, is ordered ceftriaxone 80 mg per kg per day IV in two doses for pneumonia. The calculation: 80 × 20 = 1600 mg per day, so 800 mg per dose. The vial is 1 g in 10 mL of reconstituted solution (100 mg per mL), so the nurse draws 8 mL. Before pushing, the second-nurse check walks the chain aloud — today's weight (not last week's), per-day versus per-dose wording, units, concentration, final volume — because each link is where errors live. For an infusion the arithmetic extends: vancomycin 60 mg in 50 mL over 60 minutes programmes as 50 mL per hour; a 4-2-1 check confirms maintenance at 60 mL per hour for this child. In the emergency corner the discipline compresses: a seizing 16 kg child needs lorazepam 0.1 mg per kg — 1.6 mg — drawn as 0.32 mL of the 2 mg per mL ampoule; a second nurse confirms while the clock runs. Every step is signed.
Where Indian exams catch candidates
Paper questions repeatedly test Dilling's rule (used widely in Indian textbooks) against Young's and Clark's — knowing which formula divides by 20 separates prepared candidates from guessing ones. The 4-2-1 rule appears both as a sum and as a trap (the second 10 kg runs at 2 mL, not 4). Error-pattern questions quote a medication incident and ask what failed — pounds read as kilograms, "per day" given as a single dose, a decimal shift — expecting "independent double-check and unit discipline". Vivas probe drops per minute with a microset (60 drops per mL) and converting mg per kg per hour orders into pump settings. The 4-2-1 logic underpins WHO/IMNCI rehydration planning, and Indian protocols mandate double-checks for high-alert paediatric infusions — say "double-check" early and often.
Frequently asked questions
Which age-based formulas estimate a child's dose from the adult dose?
Dilling's (age ÷ 20), Young's (age ÷ age + 12) and Clark's (weight in pounds ÷ 150) — Dilling's is the commonest in Indian textbooks.
How is the 4-2-1 rule used for maintenance fluids?
4 mL per kg per hour for the first 10 kg, 2 for the next 10, then 1 above 20 kg — summed for the hourly rate, with half given as dextrose-saline per protocol.
What size endotracheal tube fits a 4-year-old?
Uncuffed (4 ÷ 4) + 4 = 5.0 mm; cuffed 4.5 mm — confirmed by leak.
What are the paediatric defibrillation and fluid bolus doses?
Defibrillate at 2 joules per kg, then 4 for subsequent shocks; give crystalloid boluses of 20 mL per kg, reassessing after each.
Why is an independent double-check mandatory for paediatric high-alert drugs?
Because mg-per-kg arithmetic, decimal shifts and dilutions multiply error opportunities, and a second nurse checking the calculation, vial and pump independently intercepts most of them.