Pediatric & critical-care dosing: practice problems with step-by-step solutions

1Why these doses need extra care

Children are not small adults. Their doses are worked out from body weight, so a 10 kg toddler and a 40 kg teenager get very different amounts of the same drug.

In critical care, many drugs run as continuous infusions. Heparin and insulin are ordered in units per hour. Drugs that support blood pressure or heart function, such as dopamine, dobutamine and norepinephrine, are ordered in mcg/kg/min or mcg/min and adjusted (titrated) to the patient’s response.

Heparin, insulin and these drips are high-alert medications: a calculation error can cause serious harm. Many hospitals require an independent double check by a second nurse. Always follow local policy.

The calculations themselves are short. They use only multiplying, dividing and unit conversions you already know.

2Weight-based doses divided over the day

A paediatric order often gives a daily dose per kilogram, divided into several doses, for example 30 mg/kg/day every 8 hours.

1. Make sure the weight is in kg. Divide pounds by 2.2.
2. Daily dose = dose per kg per day × weight.
3. Number of doses per day = 24 h / dosing interval. Every 8 hours is 3 doses, every 6 hours is 4, every 12 hours is 2.
4. Dose each time = daily dose / number of doses.
5. For a liquid, turn the mg into mL with desired over have: (D/H) × Q.

Read the order carefully. “mg/kg/day” must be divided; “mg/kg per dose” is already one dose.

3Infusions in units per hour

Heparin and insulin are measured in units, not in mg. Write the word “units” in full: the abbreviation “U” can be misread as a 0 or a 4.

First find the concentration of the bag, the units in every mL:

Then turn the ordered dose per hour into a pump rate:

If the order is per kg, for example 18 units/kg/h, multiply by the weight first to get units/h.

To go the other way, from a running pump to the dose: units/h = mL/h × units/mL.

4Titrated drips in mcg/kg/min

Here the order is per kg and per minute, but the pump runs in mL per hour. The steps are:

1. Concentration in mcg/mL: convert the mg in the bag to mcg (× 1000) and divide by the volume.
2. Dose per minute: mcg/kg/min × weight (kg).
3. Dose per hour: × 60.
4. Pump rate: mcg/h ÷ mcg/mL = mL/h.

In one line:

Example: dopamine 400 mg in 250 mL is 400 000/250 = 1600 mcg/mL. For 5 mcg/kg/min and 80 kg: 5 × 80 × 60 = 24 000 mcg/h, and 24 000/1600 = 15 mL/h.

If the order is in mcg/min without “/kg”, leave the weight out.

5A worked example

Follow every step. Try to predict the next one before you open it. All values are made up for practice.

6Common mistakes

• Forgetting to divide a daily dose by the number of doses, or dividing by the interval in hours instead.
• Using the weight in pounds as kilograms.
• Forgetting the × 60 between per minute and per hour.
• Leaving the bag in mg while the dose is in mcg: an error of 1000 times.
• Using the weight when the order is in mcg/min or units/h without “/kg”.

7How exam questions look

• Easy: paediatric weight-based doses: mg/kg/day divided into doses, sometimes with the weight in pounds or a liquid to measure in mL.
• Medium: heparin and insulin infusions: the pump rate in mL/h for an order in units/h or units/kg/h, the units/h from a running pump, or a heparin bolus in mL.
• Hard: titrated drips: the mL/h for an order in mcg/kg/min or mcg/min, or the mcg/kg/min a patient receives at a given pump rate.

All values in these questions are made up for practice and are not clinical guidance. In real care, always follow the prescription, the product information and local protocols.