polypharmacy Flashcards
common side effects of polypharmacy
cognition + consciousness - increased risk of delirium
bowel function - constipation esp
dizziness + balance - postural hypotension + parkinsonism - risk of falls/immobility
anticholinergic side effects
Short term
o Confusion + hallucinations
o Tachycardia
o Blurred vision
o Urinary retention
o Constipation
o Dizziness
Long term
o Increased risk of developing dementia
gabapentin for chronic low back pain?
no!
no evidence for MSK pain (only neuropathic)
clear evidence of dizziness, fatigue, visual disturbances esp in old
risk benefit balnce in prescribing anticholinergics for urinary incontinence
Reduction in symptoms vs severe side effects
- Independent active – might benefit
- Residential home with mild dementia – more likely to harm
pharmacokinetics vs pharmacodynamics
pharmacokinetics = movement of drug through body, ADME (absorption, distribution, metabolism, excretion)
pharmacodynamics = drug action on body, biological response to drug (+ADRs)
2 basic principles of absorption
- acidic drugs require an acidic environment for absorption - phenytoin, aspirin, ppenicillins
- basic drugs require a basic environment for absorption - diazepam, morphine, pethidine
changes in elderly affecting absorption of a drug
increased gastric pH (less acidic), decrease small bowel surface area
usually not a problem except if -
- previous GI surgery
- NJ tube or PEG feed composition
- transdermal patches + oedema
- proportionally more basic drugs absorbed
distribution
largely transported by protein binding - free vs bound proportion of drugs
- albumin (basic) binds to acidic drugs
- alpha-1 acid glycoprotein (acid) binds to basic drugs
–> elderly often low albumin but higher A-1 AG
differences in distribution in elderly
lipid binding
- increased fat - proportional to muscle mass
- increase the Vd of lipophilic drugs (diazepam, anaesthetics), longer half life
decreased body water (10-15%)
- most important change
- lower Vd of hydrophilic drugs - lithium, digoxin
- lower Vd AND CrCl = t1/2 usually unchanged significantly in elderly
volume of distribution
theoretical volume into which all of drug is fully dissolved in plasma
Example –
100mg drug X given to patient
Steady state conc = 0.1mg/L
0.1mg = 1L
100mg = 1000L
-> volume of distribution = 1000L
half life calculation
t1/2 = In2 (0.69) x Vd / CL
(CL= CL(R(kidney)) + CL (H (liver)))
biological half-life = elimination half-life
hepatic metabolism in elderly
reduced liver function due to decreased liver size (30%), blood flow, and disease (CHF)
first pass metabolism reduced (propanolol)
in general, bio transforming enzymes are reduced in elderly
3 phases that facilatate metabolism + excretion
mainly affect phase 1 pathways (oxidation, recution or hydrolysis), functional “polar groups” to facilitate metabolism or excretion
older patients rely on phase II metabolism but slow acetylators at risk of prolonged exposure/toxicity
renal metabolism in elderly
lower GFR - decreased size, tubular secretion, renal blood flow
serum creatinine not a reliable measure
pharmacodynamics in elderly
general principle = lower doses achieve the same effect in elderly (eg alcohol)
BUT some effects are decreased - beta blockers + HR
