Prescribing for children

  • Created by: z
  • Created on: 22-02-16 19:39

Pharmacokinetics in children: absorption

  • absorption
    • oral 
      • developmental changes in surfaces of gut, GI motility and intraluminal pH alter rate and extent of absortion
      • slower gastric emptying for first 6-8 months
      • first pass metabolism increased for some drugs
    • IM
      • absorption erratic due to reduced muscle mass and variabilty of blood flow to/from site
    • percutaneous
      • increases the younger the pt because of thinner stratum corneum and increased skin hydration
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Pharmacokinetics in children: distribution

  • water soluble drugs
    • higher volume of distribution thus need higher dose/kg compared to adults for some drugs (e.g. gentamicin)
      • younger children have greater % of body weight as water/ less as fat (prem baby= 85%:1% whilst adult (60%:20%)
  • protein bound drugs
    • reduced plasma proteins in neonates thus get reduced plasma-protein binding and a  get greater free conc of certain drugs (e.g. morphine, phenytoin, diazepam)
    • sulphonamides displace bilirubin from albumin resulting in worse neonatal jaundice (possible kernicterus)
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Pharmacokinetics in children: metabolism

  • presence and activity of heaptic enzyme pathways vary in neonates and infants
    • most phase 1 enzymes appear AFTER birth but mature rapidly
      • e.g. in a preterm baby half-life of caffeine is 20-36 hours (use for apnea of prematurity)
      • by 1 month caffeine is of little use due to rapid incr in metabolism
    • phase 2 enzyme development is less well understood
      • morphine shows some conjugation at 24 weeks gestation but quadruples by 27-40weeks (thus need greater doses for pain relief)

NB hepatic metabolism phases:

  • Phase 1: oxidation/reduction/hydrolisis of drug (oxidation catalysed by cytochrome P450 enzymes)
  • Phase 2: conjugation of metabolite 
    • an ion group (glutathione, methyl or acetyl) is added- increases water solubility for excretion and decr pharmacological activity
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Pharmacokinetics in children: excretion

  • GFR and renal function are immature- reach adult values at 6 months
    • GFR in neonate is 30-40% of that of an adult
    • renally excreted drugs (e.g. gentamicin) therefore accumulated so need longer dosing intervals and monitoring
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Age categorisation of children

  • Prem baby
    • <36 weeks gestation
  • Neonate/newborn
    • 0 to 27 days
  • Infant
    • 28 days to 23 months
  • Child
    • 2 to 11 years
  • Adolescent
    • 12 to 16/18 years
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Dosing in children

  • approximate through formulas (e.g. (child body SA/adult body SA) x adults dose)
    • NOT reliable for pre-termneonates and infants
  • base on:
    • age banding
      • usually for low therapeutic index drugs 
      • e.g. paractemaol, amoxacillin
    • weight (mg/kg)
      • NB childhood obesity- don't exceed adult dose when going by weight
    • body surface area (mg/m^2)
      • for narrow therapeutic index drugs
      • e.g. chemotherapy, immunosuppression
  • over 12 years:
    • often clasify as adults for dosing
    • but need to acknowledge may be prepubescent (not at adult height/weight)
  • in emergency:
    • Broselow tape
    • reference documents
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Drug administration routes

  • oral
    • liquids (syrups, suspensions) better tolerated than tablets
    • NB suspensions should be shaken before use
    • NB don't add to feed- may interact or not be fully taken
  • IV better tolerated than IM
    • IM discouraged in neonates and young children due lack of suitable muscle and unpredictable absorption
  • rectal
    • if oral not tolerated, esp in emergency (e.g. rectal diazepam)
  • inhalation
  • special routes:
    • intraosseus (IO)
      • via bone marrow b/c highly vascularised
      • esp in emergency
    • buccal
      • non-invasive
      • e.g. buccal midazolam
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Drugs to avoid in children

  • IV chloramphenicol
    • grey baby syndrome
    • a/e in neonates
    • cyanosis, grey skin colour, decr BP, CV collapse
  • aspirin
    • Reye's syndrome
    • acute encephalopathy
    • mitochondrial damage leading to rash, vomiting, liver damage
  • tetracycline
    • affects growing teeth and bone
    • discolouration of teeth and growth stunting

NB try to avoid school time dosing (compliance and practicality issues) 

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