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Prescribing in special populations - Coggle Diagram
Prescribing in special populations
Children
Problem drugs
Antihistamines - paradoxical excitation
Aspirin - associated with Reyes syndrome (rash, vomiting, liver damage)
Phenothiazine anti-emetics (e.g. prochlorperazine) - extrapyramidal side effects
Sulphonamides - risk of kernicterus if under 2mths
Chloramphenicol - grey baby syndrome
Tetracyclines - teeth discoloration, impaired bone growth if <8yrs
Dosing
By age
By weight
By body surface area - most accurate
PK Changes
Absorption
Oral - neutral to acidic pH at 2yrs, gastric emptying delay in infants under 6-8mths, immature intestinal mucosa, frequent feeding
Intramuscular - variable blood flow to muscle in neonates
Percutaneous - absopriotn faster & more extensive
Distribution
Increased body water:fat ratio highest in neonates, plasma protein binding lower in neonates, greater membrane permeability (BBB)
Metabolism
Reduced hepatic metabolism in neonates till 6-12mths, then exceed adult levels until 4yrs
Liver volume, blood flow & biliary function correlate with body surface area (BSA)
Excretion
Clearance increases non-linearly with weight so maintenance dose per kg is highest around 2yrs
Renal function reaches maturity at 1-2yrs when GFR correlates with BSA
Top tips
Use non-drug measures where appropriate
Start low & go slow with careful monitoring
Regularly review dose & need for ongoing therapy, discontinue if no longer indicated
Simplify regimens (lower frequency better)
Counsel on adherence if applicable
Give relevant instructions and warning (caregiver)
Beware of adverse drug reactions
Renal impairment
Prescribing tips
Avoid nephrotoxic drugs when possible
Reduce dose &/or interval (except in loop diuretics - increase dose)
Always check if drug is contra-indicated or if dose adjustment is required
Caution with drugs that increase K+
Consider TDM if appropriate
Many drugs cleared by dialysis so dose timing is important
PK Changes
Kidney is by far commonest route of elimination of drugs and/or metabolites
Drug/metabolite may accumulate in renal failure causing toxicity
Insulin - metabolised by kidneys, dose decrease in diabetics with renal failure
Vitamin D - activated by kidneys, active form must be given in renal failure
Kidney produces erythropoetin - anaemia of renal failreu managed est with exogenous erythropoeitin administration
Pregnancy
Risks to mother
Underlying illness or condition, treatment, delayed or suboptimal treatment, adherence
Risks to fetus
Maternal illness (and failure of treatment of mother) & treatment
Pharmacodynamic effects
Teratogenicity, miscarriage, spontanous preterm birth, low birth weight, postnatal effects (toxicity, withdrawal, premature closure of ductus arteriosus, adverse developmental effects)
PK Changes
Distribution
Increased plasma volume in later stage leading to increased Vd of some drugs & changes in drug binding
Metabolism
Variable changes in hepatic metabolism/elimination
Absorption
Rate & extent of absorption may be affected
Elimination
Increase in GFR, renally eliminated drug concentrations may be reduced
Challenges
Poor understanding of background risk
Spontaneous abortions, genetic congenital malformations, risk from disease, other exposures (smoking & ETOH)
Many pregnancies are unplanned
Exposure to known teratogens in 1st trimester
Limited info on majority of drugs
Absence of evidence of harm does not mean that the drug is safe
General principles
Choose drugs with relatively more safety data
TDM recommended where appropriate
Risk benefit assessment
Counsel on adherence when essential drugs prescribed
Valid indication for drug therapy needed
Warnings about teratogenic drugs/substances
Prescribing in lactation
Essential drugs only
Careful timing of drugs ingestion & breastfeeding
Avoided or monitored drugs
Think about ADRs in the nursing infant
Absolute contraindications
Drugs that affect milk production
Liver failure
PD: drug (substance)-induced hepatotoxicity
No easy measure of impaired function (Child-Pugh score used rather than LFTs)
PK: liver has a central role in drug metabolism
Top tips
Avoid hepatotoxins when possible
Drugs metabolised by the liver may have prolongation of half-lives/elimination times resulting in toxicity
Reduce dose of drugs that undergo extensive first pass elimination as portal hypertension reduces pre-systemic elimination by bypassing the liver with vascular shunting
Consider TDM
Use drug information sources for advice on prescribing in liver disease
Elderly
PK Changes
A: relatively unchanges
M: oxidative metabolism reduced, administer drugs with high hepatic metabolism with caution
E: decline in GFR with advanced age
D: decrease in total body water & lean body mass, with increase in fat mass
Top tips
Regularly review need for ongoing therapy
Start low & go slow with careful monitoring
Give vaccines when indicated
Use non-drug measures where appropriate
Simplify regimens (lower frequency better)
Counsel on adherence
Give relevant instructions & warnings, and check understanding
Beware adverse drug reactions & interactions
PD & suitability issues
Decrease in receptor numbers / activity
Adverse drug reactions - NSAIDs, hypnotics, anticholinergics, warfarin
Polypharmacy
Adherence
Obesity
Lipophilicity of the drug - increased Vd of fat-soluable drugs
TBW vs IBW (ideal body weight) vs adjusted body weight
TDM (therapeutic drug monitoring) if appropriate
Also consider PD effects of meds
Palliative care
Goal: Best possible quality of life
Drug therapy is part of holistic care - needs regular review due to progressive nature
Common reversible symptoms: pain, N&V, constipation, seep disturbances
Competitive sports
Check what is banned during competitions & in general (some are sport specific)
Rational Prescribing
1: Define the patient's problem
2: Specify therapeutic objectives
3: Non-drug measures +/- patient drug
4: Write prescription
5: Give info, instructions & warnings
6: Monitor (change/stop) the treatment