Techniques of calculation and prescribing drug dosage in

pediatric practice

Dr. Paul, Shubhra PrakashAssistant Registrar

Rajshahi Shishu(children) Hospital

Dose of Commonly used drugs (parenteral)

Drugs Indication with Dose

Frequency Preparation

Aminophylline 5mg/kg/dose stat2.5mg/kg/dose for maintenance therapy

125mg/5mL

Meropenem Other than meningitis 20-30 mg/kg/dose

8 hourly 500 mg/vial

Meningitis60mg/kg/dose

8 hourly

Azithromycin 10mg/kg/dose Once daily for 3 days

500mg/vial

Vancomycin 10-15mg/kg/dose 8 hourly 500mg/vial

Dopamine 5gm/kg/min followed by gradual increase up to 20gm/kg/min

200mg/5mL vial

Dose of Commonly used drugs (parenteral)

Drugs Indication with Dose

Frequency

Preparation

Netilmycin 3mg/kg/dose 12 hourly 50mg/2mL

Phenobarbitone 20mg/kg/dose as loading dose

8 hourly 200 mg/ml

Maintenance5-20mg/kg/day in divided dose

Clarithromycin 15mg/kg/day in2 divided dose

12 hourly 500mg/vial

Phenytoin 20mg/kg/dose as loading dose

125 mg/5ml

Maintenance5-20mg/kg/day in divided dose

Relation of Dosage frequency with weight and age

Once daily

Twice daily

Thrice daily

Neonate

Children

Adult

Dosage frequency of Gentamycin for an example

The lesser is the weight the more is the dilution

The younger the child the more is the dilution

Weig

ht

Age

Example

Example I with Meropenem

Example II with Azithromycin

Example I

Case I:

• A 4 months old child weighing 5kg requiring Meropenem for an indication of meningitis.

How will you calculate the dose and infusion rate?

Basic Calculation

As the indication is meningitis the baby will

receive Injection Meropenem 60mg/kg/dose

8 hourly.So the dose for this baby is

300mg/kg/dose

Example I with meropenemAs Meropenem is to be infused not less than 30 minutes, it should be infused with IV fluid. So for the example case IV fluid of choice either Normal saline or Dextrose in aqua or Dexstrose saline is chosen. A measured volume , suppose 30 ml may be taken as carrier for meropenem infusion

Example I with meropenem

A vial of Meropenem contains 500mg of active drug in powder format. So it should be reconstituted with distilled water. Here comes the measurement of Dilution. If smaller dose is required the dilution should be with more amount of diluents and vice versa.

Example I with meropenemSo if dilution is made with 10 ml of D/W then each ml will contain 50 mg of the drug, similarly if dilution is made with 5 ml of D/W then each ml will 100 mg of meropenem. From this diluted drug the exact dose is taken and poured into measured infusion fluid and to be infused.

Example I with meropenem

In our example case I the dose is required 6 ml (if diluted with 10 ml D/W ) or 3 ml (if diluted with 5 ml D/W). Now this 3 or 6 ml is poured in to desired amount of fluid to be infused cat calculated time.

Calculation of infusion rate

e.g. 30 ml fluid should be infused over 1 hour.

So infusion rate is 30ml/hr which stands for 30ml/60min i.e. 0.5ml/min is the infusion rate.

Then 0.5ml/min means

0.5 X 15 drops/min

=7.5 drops/min (if used with infusion set)

=7.5X 4 drops/min (if used with micro burette )

= 30 drops/min

Calculation of infusion rate

e.g. 30 ml fluid should be infused over 1/2 hour.

So infusion rate is 30ml/30min which stands for 30ml/60min i.e. 1ml/min is the infusion rate.

Then 1ml/min means

1X 15 drops/min

=15 drops/min (if used with infusion set)

=15X 4 drops/min (if used with micro burette )

= 60 drops/min

Calculation of infusion rate

2 hours

• 15

1hour

• 30

So in case of infusing 30 ml fluid the rate will be as follows

(drops/min)

Example II

Case II:

• A 7 months old child weighing 8 kg requiring Azithromycin for an indication of Acute watery Diarrhea

How will you calculate the dose and infusion rate?

Basic Calculation

As the indication is acute watery Diarrhoeathe baby

will receive Injection Azithromycin @ 10mg/kg/day.

So the dose for this baby is 80mg/kg/dose

Example II with Azithromycin As Azithromycin is to be infused over 30 minutes, it should be infused with IV fluid. So for the example case IV fluid of choice either Normal saline (as it is provided with the drug as combipack) or Dextrose in aqua or Dexstrose saline is chosen. A measured volume , suppose 30 ml may be taken as carrier for Azithromycin infusion.

Example II with Azithromycin

A vial of Azithromycin contains 500mg of active drug in powder format. So it should be reconstituted with distilled water. Here comes the measurement of Dilution. If smaller dose is required the dilution should be with more amount of diluents and vice versa.

Example II with Azithromycin So if dilution is made with 10 ml of D/W then each ml will contain 50 mg of the drug, similarly if dilution is made with 5 ml of D/W then each ml will 100 mg of Azithromycin From this diluted drug the exact dose is taken and poured into measured infusion fluid and to be infused.

Example II with Azithromycin

In our example case II the dose is required 1.6 ml (if diluted with 10 ml D/W ) or 0.8 ml (if diluted with 5 ml D/W). Now this 0.8 or 1.6 ml is poured in to desired amount of fluid to be infused cat calculated time.

Example III

Case II:

• A 2 months old child weighing 4 kg requiring Netilmycin for an indication of sepsis.

How will you calculate the dose?

Example III

Recommended dose of netilmycin in sepsis is 3mg/kg/dose 12 hourly.•The baby requires 4.5mg/dose of Netilmycin only.

A vial of Netilmycin contains 50mg/2ml.

•So the baby requires only 0.18 ml of netilmycin

Example III

Here an insulin syringe of 100 U will be helpful for the required dose.

As in 100 unit insulin syringe 1ml=100 unit, 0.18 ml refers to 18 unit.

So, the dose becomes 18 unit/dose.

Thank you all