PAEDIATRIC DOSING

1. Pediatrics pharmacology 2
Dr.Azad Abduljabar Haleem Al Mezori
azad82d@gmail.com

2. Pediatrics prescribing
• Legal responsibility lies with the doctor who
signs the prescriptions??

4. PAEDIATRIC DOSING
• Pharmacokinetic and pharmacodynamic data are
seldom available for the paediatric population,
• this is because most medications are only
licensed for adult use and have not undergone
specific pre-marketing clinical studies in children.
• Data on therapeutic dosing for children are often
anecdotal and based on case reports or very
small population studies.

5. • The surface area and the weight are the only
common methods currently available to
predict paediatric therapeutic doses from
those used for adults.

6. Surface area
• The surface-area or percentage method for estimating
doses is calculated as follows:
• (Surface area of child (m2)/1.76m2)*100
• where 1.76 m2 is the average adult surface area.
• Children are often said to tolerate or require larger doses of
drugs than adults based on a weight basis, and the
percentage method helps to explain this phenomenon.
• Body water (total and extracellular) is known to equate
better with surface area than body weight.
• It thus seems appropriate to prescribe drugs by surface
area if they are distributed through the extracellular fluid
volume in particular.

7. Weight
• Adult dose (mg)/70kg=mg/kg dose
• where 70 kg is the average adult weight.
• This method will give lower doses than the
surface-area method.
• It is far less accurate in clinical terms and is
usually inappropriate for accurate therapeutic
dosing.
• However, as it gives lower and thus safer
estimates of what the toxic dose may be, it is a
more practical and reasonably cautious method
for extrapolating toxic doses.

8. • Most paediatric doses given in textbooks are
described in small age or weight groups on a
mg/kg basis.
• However, these will often have been originally
obtained from surface-area data and thus are
larger than the adult dose divided by 70.
• There are, however, many medications which can
be used in children and accuracy of prescribing is
essential because of the vast physical size
differences in children (i.e. from 0.5 kg to 120 kg)
let alone their changes in kinetics.

9. • Children should be regularly weighed so that up-
to-date weights can be used for prescribing.
• However, remember the practicalities of what is
to be given to the child.
• If a child is 6.5 kg and requires 2 mg/kg of
ranitidine, it would seem wiser to prescribe 15
mg, which is 1 ml of the oral liquid, rather than
13 mg, which equates to 0.867 ml.
• Some knowledge of the therapeutic range of the
drug is required to know when and by how much
it is suitable to round doses, however, and
sometimes awkward quantities are required.

10. DRUG MONITORING
• Most drugs have wide therapeutic windows
and thus toxicity is unlikely at 'normal doses'.
• It is usually easy to see a medication effect,
e.g. analgesics take away pain.
• There are, however, certain circumstances
when it is important to measure drug levels
to ensure that there are adequate levels for
effect and/or that the levels are unlikely to
cause toxicity.

11. • A drug with a narrow therapeutic window has
a narrow range between the drug
concentration exhibiting maximum efficacy
and that producing minimum toxicity.
• Medications that have narrow therapeutic
windows are often monitored, as it is hard to
predict whether a dose for an individual
patient will be clinically effective or will cause
toxicity.

12. • Common drugs for therapeutic monitoring:
• Phenytoin
• Phenobarbital
• Warfarin
• Carbamazepine
• Gentamicin
• Vancomycin
• Digoxin
• Theophylline

13. Indications for monitoring:
• • To confirm levels are not toxic and are at a level
that is normally effective (usually checked once
steady-state has been reached), e.g. gentamicin,
vancomycin
• • If toxicity is expected
• • If external factors may have changed a level
(change in renal/hepatic function)
• • To check compliance It is important to know
that a drug was at a steady state when a level was
measured and whether trough levels or peak
levels are important.

14. INTERACTIONS
• Many situations arise where interactions
between different medications are important.
• Drugs may either inhibit or induce the liver
enzyme systems as follows:

15. Drugs and the liver
• Liver induction
• • Will lead to treatment failure of:
• • Warfarin, phenytoin, theophylline.
• • Caused by:
• • Phenytoin, carbamazepine, barbiturates,
rifampicin.

16. • liver inhibition
• • Will lead to potentiation of:
• • Warfarin, phenytoin, carbamazepine,
theophylline, cyclosporin
• • Caused by:
• • Omeprazole, erythromycin, valproate,
isoniazid, cimetidine.

17. Compatibility
• When medications are administered, always
be aware of their interactions before they
enter the body.
• This is particularly important with parenteral
medication.
• Many medications interact to produce non-
effective products, toxic products or
precipitates.

18. • Examples of incompatible injections:
• Gentamicin: is partly inactivated by penicillin,
so lines must be flushed between
administrations.

19. Drugs in breast milk
• There is now a great deal of evidence
establishing the benefits of breast-feeding for
both the mother and the baby.
• The ability of a drug to pass into the breast
milk will depend on its partition coefficient,
pKa, its plasma and milk protein binding and
selective transport mechanisms.

20. • Unfortunately, lack of knowledge about drug
safety in breast-feeding often causes women
to:
• stop breast-feeding or never start in the first
place because of their own anxiety or that of
the health-care professionals involved in
their care.

21. • It must always be remembered that even if
drugs do enter the breast milk they must still
go through the infant's gut to be absorbed
and this offers another barrier to many drugs
such as omeprazole.
• The infant's short gastric emptying time also
reduces exposure to some drugs.

22. • The frequency and volume of the feeding
must also be considered as there will be less
exposure to the drug if the infant is receiving
supplementary feeds and/or other liquids.
• Further reassurance can be given to a parent if
the medication that is being given has a long
term history of safety in children at
therapeutic levels.

23. • Taking all this into consideration there are
very few drugs that are contraindicated in
breastfeeding women.
• The benefits of breast-feeding usually
outweigh the small theoretical risk of harm to
the neonate.
• This said we should never become
complacent and should look for the evidence
and approach newer drugs with more caution.

24. Drugs which are contraindicated in
breast feeding
• Cancer chemotherapy
• Radiopharmaceuticals
• Antithyroid drugs, Iodine, Iodides
• Amiodarone
• Androgens
• Indomethacin
• Nalidixic acid
• Nitofurantoin & Sulphonamide (in G6PD-deficient infants)
• Chloramphenicol
• Tetracyclines
• Vitamins A and D (high dose)

25. Thanks