Clinical Use of Analgesics II- Opioids and other Medicines

Question 1

Which analgesics are naturally occuring?

Answer 1

Morphine, codeine, thebaine, papaverine (from the poppy)

Question 2

Which analgesics are semi-synthetic?

Answer 2

Oxycodone, diamorphine, buprenorphine, hydromorphone (core features of alkaloid modified)

Question 3

Which analgesics are synthetic?

Answer 3

Tramadol, pethidine, fentanyl and alfentanil (no chemical relationship but produce the same clinical effect)

Question 4

What is the opioid mechanism of action?

Answer 4

Interact with opioid receptors, primarily found in brain and spinal cord

• spinal level; inhibits transmission of nociceptor signals through dorsal horn and spinal cord
• supraspinal level; inhibit descending signals to brainsten and spinal cord

Question 5

Which opioids are used for moderate pain and what are their features?

Answer 5

Codeine, dihydrocodeine and dextropropoxyphene

Ceiling effect, as increase the dose analgesia doesn’t increase but side effects do

Question 6

Which opioids are used for severe pain and what properties do they possess?

Answer 6

Morphine, diamorphine, oxycodone, fentanyl, tramadol

Linear dose response curve, side effects normally limiting factor in increasing the dose

Question 7

What are common opioid side effects?

Answer 7

Nausea, vomiting, constipation, sedation, respiratory depression, hypotension, urinary retention

Question 8

What are the less common opioid side effects?

Answer 8

Narcotic bowel syndrome
Immunosupression
Endocrine dysfunction
Addiction

Question 9

How can we treat opioid induced constipation?

Answer 9

Diet and Fluids
Laxatives, stimulant and a stool softener
Peripherally resticted opioid antagonist (naloxonePR)

Question 10

How do you initiate morphine administration?

Answer 10

Determine opioid requirement by using a short acting preparation plus PRN doses
Calculate 24 hour requirements
Convert to modified release formulation
In addition, 1/10th-1/6th of the total daily dose can be prescribed for breakthrough pain every 4 hours

Question 11

What are the advantages of PCA (patient controlled analgesia)?

Answer 11

Rapid analgesic once pain at steady state 
Readily prepared 
Patient satisfaction 
No dose delay 
Patient acceptability 
No peaks and troughs

Question 12

What are the disadvantages of PCA (patient controlled analgesia)?

Answer 12

Expensive
Need IV access
Training
Monitoring

Question 13

What are epidural opioids?

Answer 13

An alternative to PCA, lower limb, abdo or spine surgery
Mixture of local anaethestic and opioid normally
Act synergistically
Commonly fentanyl and bupivacaine
Respiratory depression uncommon due to lipophilicity of fentanyl

Question 14

Why is fentanyl less likely to cause respiratory depression?

Answer 14

More lipophillic

Question 15

What are the adverse effects of epidural?

Answer 15

Wrong route, hypotension, infection, localised haematoma / bleeding on administration

Question 16

What do you need to monitor in opioid use?

Answer 16

BP, pulse, respiration rate, pain, o2 saturation, sedation score, opioid usage, opioid side effects

Question 17

What is naloxone?

Answer 17

Opioid antagonist
Higher affinity for the receptor than agonist
Short half life when given IV (may need repeated doses)
May induce pain
Titrate gradually until effect is achieved

Question 18

What is tramadol?

Answer 18

MOR agonist (30% of analgesic effect)
Inhibits noradrenaline and serotonin reuptake
70% absorbed PO (peak at 2 hours)
Less pronounced opioid side effects (constipation)
More pronounced opioid side effects (nausea and halluncinations)

Question 19

What are tricyclic antidepressants?

Answer 19

Inhibit neuronal reuptake of noradrenaline and serotonin
Precise effect on neuropathic pain unclear
Use limited by adverse effects
Adverse effects minimised by starting with low dose and small incremental changes
50-70mg od

Question 20

What are antiepileptic drugs?

Answer 20

Carbamezapine, phenytoin, sodium valporate
Block voltage gated Na+channels in peripheral neurones
Lots of adverse events
Effective in certain neuropathic pain syndromes
Limited licence for neuropathic pain

Question 21

What are anticonvulsant drugs?

Answer 21

Gabapentin and pregabalin
Prevents voltage dependent Ca2+ channel activation in dorsal horn neurones
Does not affect Na+ channels
Fewer adverse effects and drug interactions
Licensed for neuropathic pain

Question 22

What is capsaicin? Topical

Answer 22

TRPV1 receptor agonist
8% patch, neuralgia
0.075% diabetic neuropathy
0.025% oa

Causes a burning sensation upon application
Counselling on application required

Question 23

What are the uses of lidocaine 5% medicated plaster?

Answer 23

PHN but used for other neuropathic pain
Requires application to the painful site and can be cut to size
Apply up to 3 plasters for 12 hours each day