Acute Pain

Question 1

Acute Pain: Introduction

The following are general points to bear in mind when prescribing pain relief:

Answer 1

1. Regular analgesia is better than PRN, particularly if pain is predictable (e.g. post-op)
2. Employ multi-modal approach e.g. Paracetamol / NSAID with codeine or morphine, to reduce need for high concentration of opiates.
3. Reduce opiate side effects by prescribing small doses at short intervals
4. Generally, **oral medication better than I.V **if patient can swallow.
5. Prescribe analgesia with adjuncts to manage side effects e.g. nausea / constipation from opiods

Question 2

Pain Ladder

What are the different steps on the analgesic ladder?

Note: be additive in all cases except when a strong opioid replaces a weak one → same mechanism. Don’t necessarily start at the bottom, enter at an appropriate level and adjust accordingly

Answer 2

Question 3

Paracetamol

What is the mechanism, pharmacokinetics, dose, indications, adverse effects, contra-indications and important interactions of paracetamol?

Answer 3

1. Mechanism: inhibits PGE synthesis, but precisie mechanism unclear. It improves effect of other analgesics in moderate / severe pain and should always be part of treatment plan.
2. Pharmacokinetics: hepatic metabolism with saturatable mechanism → hepato-toxic in OD
3. Dose: 1g QDS. I.V not more effective than PO → only used if I.V route is clinically justfied. Dose shoud be reduced in cachectic / malnourised patient
4. **Indications: **1st line mild / mod pain. Anti-pyrectic
5. Adverse effects: liver ± renal damage in OD
6. **Contraindications: **hepatic impairment
7. Interactions: may potentiate warfarin

Question 4

NSAIDs

In which settings are NSAIDs used cautiously, and when should they be avoided?

Answer 4

1. Cautions
   
   1. ​Age > 60
   2. Risk of mucosal GI damage (elderly, previous PUD, on steroid treatment) → gastric protection with PPI
   3. Worsens / precipitates HF
   4. Caution with ACEi and diuretics → increased risk kidney injury
   5. May be associated with ↑ risk thrombosis if high doses for a long time
2. Contra-indications
   
   1. Pregnancy
   2. Patients on warfarin, aspirin, steroids
   3. Bleeding, or previous history of UGI ulceration, aspirin sensitive asthma, renal insufficiency, heart failure, hyperkalaemia

Question 5

Opiates: Morphine

What is the mechanism, pharmacokinetics, adverse effects , contra-indications and interactions of morphine?

Answer 5

1. Mechanism: μ-opiod receptor agonist, acting on antinociceptive, respiratory and cough centres, higher centres (euphoria / dysphoria) and vomiting bowel
2. Pharmcokinetics: high first pass metabolism → oral dose ≃ 3x higher than parenteral. Renal excretion of active metabolites → reduce dose in renal failure. Oxycodone alternative in RF.
3. Adverse effects: sedation, respiratory depression, nausea and vomitimg, ileus and constipation, pruritis, hallucinations and dysphoria
4. Contra-indications: acute respiratory depression, raised intracranial pressure (require pupillary responses to assess neurology)
5. Interactions: sedating medications and alcohol are potentiated

Question 6

Patient Controlled Analgesia

The following are principles of PCA

Answer 6

Suitable for those requiring regular doses of IV morphine (e.g after major abdominal surgery → bowel resection, oesophagectomy etc.)

1. Single dose delivered, then patient locked out (usually 5 min) → standard setting of 1mg bolus, followed by 5 min lockout.
2. Monitor for pain, sedation, RR, N&V, BP and dose delivered.
3. If there is a background dose, if must not be higher (in mg/hr) than the bolus dose to maintain safety
4. For patients remaining in pain on PCA:
   
   1. Is pain relieved by pressing button → if no, increase blous (e.g. by 1mg)
   2. Do you have pain when lying still → if yes, add or increase background (start at 1mg/hr)

Question 7

Naloxone

What is the dosing of naloxone, what is the half-life?

Answer 7

1. Dose:
   
   1. dilute 400mcg vial up to 10ml with 0.9% saline → administer in 40mcg increments (1ml of diluted mixture) and repeat as required.
   2. A single large dose may precipitate severe withdrawal
2. Duration: half-life < opioid, repeat doses may be required.

If there is no response to titrated 400mcg consider alternative cause.

Question 8

Codeine and Dihydrocodeine

What is the dose, indication and adverse effects / interactions of codeine / dihydrocodeine?

Answer 8

1. Dose: codeine may be prescribed alone or as weak (500/8mg) or strong (500/30mg) co-codamol. May prescribe 30-60mg QDS PO
2. Indications: mild / moderate pain, diarrhoea, cough
3. Adverse effects and interactions: as morphine

Dihydrocodeine has similar analgesic efficacy to codeine, usually given at 30mg / 4h. Doubling dose may increase analgesia, but causes more nausea and vomiting.

Question 9

Tramadol

What is the mechanism, dose, and adverse effects of tramadol?

Answer 9

1. Mechanism: has both an opioid effect and an enhancement of **serotonergic and adrenergic pathways **→ unlike other opiods, it is only partially reversed by naloxone
2. **Dose: **
   
   1. PO: 50-100mg, not more than 4hrly
   2. IM / IV injection or IVI: 50 - 100 mg 4-6hrly
   3. Post-op pain: 100mg initially, then 50mg every 10-20 minutes if required during first hour to max 250mg, the 50-100 mg 5-6hrly.
3. Adverse effects:
   
   1. Has fewer typical opioid side effects (less respiratory depression, less constipation, less addictive potential).
   2. Not suitable as a substitue in opioid-dependent patients

Question 10

Oxycodone

What is the efficacy and indication for oxycodone?

Answer 10

1. Efficacy and s/e similar to morphine, but approximately 1.5x as potent
2. Used as a substitute in morphine in tolerant patients, and is safer in patients with renal failure