Prescribing in the Elderly Flashcards
what are the 4 principles of pharmacokinetics?
ADME absorption distribution metabolism excretion
what are the 2 principles of absorption?
acidic drugs require an acidic environment for absorption (phenytoin, aspirin, penicillin)
basic drugs require basic environment for absorption (diazepam, morphine, pethidine)
how is drug absorption different in elderly?
less stomach acid so stomach is more basic
basic drugs are therefore more absorbed in elderly than in younger people
decreased small bowel surface area
what are the principles of distribution?
protein binding (free vs bound proportion of drugs)
- albumin (basic) binds to acidic drugs
- alpha 1 acid glycoprotein (acidic) binds to basic drugs
lipid binding
- increases the volume of distribution of lipophilic drugs
how is distribution different in the elderly?
elderly generally have low albumin but a higher A-1 AG, therefore better distribution of basic drugs increased fat (proportional to muscle mass), therefore lipophilic drugs are better distributed
what can the volume of distribution tell you about a drug?
higher volume of distribution = higher lipid solubility = longer half life
how is volume of distribution calculated?
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what is volume of distribution?
the theoretical volume necessary to contain all of the administered drug if it was at the concentration observed in the blood plasma
how can weight loss affect drug action?
loss of fat = Vd reduced = higher concentration of drug in plasma = side effects (delirium etc)
how does clearance affect half life of a drug?
lower clearance rate = longer half life
how is hepatic metabolism different in the elderly?
reduced liver function due to decreased size and blood flow and disease
reduced first pass metabolism
reduced levels of bio-transforming enzymes
reduces clearance and can cause increased half life/action of the drug
how is renal metabolism different in the elderly?
lower GFR (due to decreased size, tubular secretion and renal blood flow)
general principle of pharmacodynamics in the elderly?
lower doses have same effect in the elderly but some effects are decreased (B Blockers don’t have as much of a bradykinic affect)
how is the therapeutic index calculated?
50% lethal dose / 50% effective dose
or
minimum toxic dose / minimum effective dose
if both values are similar will give a small therapeutic index
give examples of drugs with a small therapeutic index?
theophylline warfarin lithium digoxin gentamicin vancomycin
how does therapeutic window change with age?
gets narrower
therapeutic response reduces
toxic response increases
name 4 drugs associated with adverse effects in elderly?
warfarin
digoxin
insulin
benzodiazepines
name 6 drugs with side effects in elderly?
opioids = constipation
- also give lactulose/senna prophylaxis
steroids = osteoporosis, diabetes
levothyroxine
- calcium interferes with levothyroxine absorption - stop calcium tablets
NSAIDs = GI bleed, reduced GFR, decreased effect of diuretics/anti-hypertensives
- swap to paracetamol
benzodiazepines = impaired cognitive function
antibiotics = resistance, C. Diff
what is Beers criteria?
3 categories of drugs
- always avoided
- potentially bad
- used with caution
principles of polypharmacy?
review medications regularly
avoid treating side effects with more drugs - reduce dose or swap drug instead
try and prescribe one drug that helps more than one problem
if stable, use combination preparation
what is the STOPP-START tool?
list of drugs in 2 categories used for prescribing in elderly
STOPP = Screening Tool of Older People’s Potentially inappropriate Prescriptions
START = Screening Toll to Alert Doctors to Right Treatments
STOPP GI drugs?
loperamide/codeine phosphate in certain circumstances
metoclopramide in people with parkinsons
stimulant laxatives in people with bowel obstruction
START GI drugs?
PPI for reflux, stricture or patients >80 on anti-platelets and SSRIs
fibre supplement
STOPP cardio drugs?
beta blockers if taking verapamil
non-selective beta blocker in COPD patients
CCBs in chronic constipation or taking diltiazem/verapamil with heart failure
dipyridamole as monotherapy
statins
START cardio drugs?
antihypertensives
ACE inhibitor (chronic heart failure or post MI)
PPI (with aspirin + warfarin)
warfarin/DOACs (chronic AF)
how is adherence in elderly?
generally the same as younger people
