Acute Pain Management Flashcards
What are implications of inadequate acute pain management?
- CVS → ↑ HR, ↑ BP, ↑ PVR, MI, venous stasis, thrombosis
- Resp → diaphragmatic splinting, atelectasis, sputum retentions, hypoxaemia
- GI → delayed gastric emptying, ↓ intestinal motility
- GU → urinary retention
- Met/End → ↑ protein breakdown, impaired wound healing, sodium/water retention, ↑ metabolic rate
- Chronic Pain
- Psych → anxiety, sleeplessness, fatigue, distress
What is the World Health Organisation Analgesic Ladder?
- Initially → paracetamol / NSAIDs
- Next → weak opioids
- Next → strong opioids
How does paracetamol work?
- Number of central mechanisms
- Effects on PG production, serotonergic, opioid, NO + cannabinoid pathways
- Analgesic and antipyretic without anti-inflammatory activity
- Excreted renally, after flucuronide + sulphate conjugation in liver
- Hepatotoxic metabolite N-acetyl-p-benzoquinoneimine normally inactivated by conjugation with hepatic glutathione
- In paracetamol OD, pathway is overwhelmed → hepatic cell necrosis
How is paracetamol administered?
- PO or PR
- Available as IV prep
- Particularly effective when IV
What is the recommended dose of paracetamol?
- 4 g/d in adults
- Effective when prescribed regularly rather than PRN
- MRHA licensed dose of paracetamol is same for all routes of administration in adults 50kg+
- Dose in children weighing _<_10kg is now 7.5mg/kg (>10kg: 15 mg/kg)
NSAIDs are, in general, more useful for superficial pain arising from the skin, buccal mucosa, joint surfaces and bone. What is the mechanism of action of NSAIDs?
- Analgesic, anti-inflammatory, antiplatelet and antipyretic
- Due to inhibition of COX → ↓PGs, ↓prostacyclins, ↓Tx A2
- COX-1: kidney, GI mucosa, platelets (PGs contribute to normal fxn)
- COX-2: inflammatory mediators following tissue damage
- NSAID therapeutic effects (COX-2) and adverse effects (COX-1)
- Selective COX-2 inhibitors developed in attempt to reduce adverse effects of NSAIDs
What are adverse effects of NSAIDs?
- GI toxicity
- Renal impairment (all)
- Increased risk of CV events (eg MI / stroke)
- Lowest GI effects → iboprufen
- Lowest CV events → naproxen and low-dose iboprufen
- COX-2 inhibitors have higher CV risk but lower GI effects
- Other adverse effects: hypersensitivity rxn, fluid retention
What are differences between NSAIDs and COX-2 inhibitors?
What are contraindications to NSAIDs?
Avoid in severe renal impairment, heart failure, liver failure and known NSAID hypersensitivity
What is the treatment for neuropathic pain?
NICE Guidelines:
- First line → amitriptyline (or imipramine if untolerated) or pregabalin
- Second line → amitriptyline AND pregabalin
- Third line → refer to pain specialist, tramadol in interim + avoid morphine
- Diabetic neuropathic pain → duloxetine
What are examples of inhaled analgesia?
- Entonox (50/50 N2O, O2) → quick-acting, potent, short duration, self-admin
- Isonox (isofluorane in entonox) → lower concs of isofluorane produce less drowsiness
What are side-effects of inhaled analgesia?
- Ideal for short procedures (dressings, drain removal, catheter, labour pain, traction)
- Entonox:
- Side-effects → drowsiness, nausea, excitability
- Diffuses rapidly into + increases gas-containing cavities
- CIs → pneumothorax, decompression sickness, intoxication, bowel obstruction, bullous emphysema + head injury
Opioid drugs act as agonists at opioid receptors, found mainly in brain + spinal cord, but also peripherally.
What are the 3 principal classes of opioid receptor?
- μ (mu) → analgesia, n+v, bradycardia, resp depression, miosis, inhibition of gut motility, pruritus, endogenous agonists are b-endorphins
- κ (kappa) → analgesia, sedation, dysphoria, diuresis, endogenous agonists are dynorphins
- δ (delta) → analgesia, endogenous agonists are enkephalins
What are features of morphine?
- Gold standard against which all new analgesics compared
- Least lipid-soluble opioid in common use
- Metabolised in liver, excreted by kidney
- Metabolite morphine-6-flucuronide is more potent than morphine
- Accumulation can occur after prolonged use in pts w/ impaired renal fxn
- Dose ranges and dose intervals vary according to route of admin
What are features of diamorphine?
- A prodrug (diacetylmorphine)
- Rapidly hydrolysed to 6-monoacetylmorphine and then morphine
- Much more lipid-soluble than morphine
- More rapid onset of action than morphine when given by epidural or IV
What are features of fentanyl?
- Highly lipid-soluble synthetic opioid
- Short-acting because of rapid tissue uptake
- Suitable for transdermal administration
- Commonly administered IV, epidurally, intrathecally, buccally, or via nasal spray
What are features of pethidine?
- Analgesic w/ anticholinergic + some LA activity
- Metabolised in liver, excreted in kidney
- Main metabolite is norpethidine (a potent analgesic)
- High blood conc → CNS excitation
- Pts with impaired renal fxn are at risk
- Can be used to treatpost-op shivering associated w/ volatile anaesthetic agents, and epidural + spinal anaesthesia
What are features of codeine?
- Prodrug for morphine
- For mild-mod pain
- About 10% of dose is converted to morphine
- Metabolism ⇒ morphine requires CYP2D6 enzyme (part of cytochrome p450 system)
- 8-10% of caucasians lack this enzyme, obtaining little or no benefit
What are features of tramadol?
- Synthetic, centrally acting opioid-like drug
- < 50% analgesic activity is at μ-receptor
- Inhibits noradrenaline + serotonin uptake
- Compared to other opioids → ↓tolerance + abuse, ↓resp depression, ↓constipation
- Main metabolite more potent, also depends on cytochrome p450 enzyme (same as codeine)
All opioids are equianalgesic if adjustments are made for the dose + route of admin. Allowance should be made for long-term opioid therapy, incomplete cross-tolerance between opioids, differing half-lives and interpatient variability.
What are the equianalgesic dosages?
Opioids have a similar spectrum of side-effects. There is considerable interpatient variability, and some patients may suffer from more side-effects with one particular drug compared to another.
What are side-effects of opioids?
- Resp depression → reduced RR, VT and irregular rhythm
- Sedation
- Euphoria + Dysphoria
- Nausea + Vomiting
- Muscle rigidity
- Miosis
- Bradycardia
- Myocardiac depression
- Vasodilatation
- Delayed gastric emptying
- Constipation
- Pruritus
What do opioid antagonists do?
- Act at all opioid receptors
- Naloxone most commonly used
- Possible to reverse side-effects such as respiratory depression, N+V, sedation
- Without antagonising the analgesic effects
- Naloxone effective for ~60 min
What are routes of administration for opioids?
- Oral → limited due to first-pass metabolism, immediate-release oral opioids (moprhine syrup, oxycodone) for early management of acute pain, oral fentanyl should be restrictred to treating breakthrough pain in pts receiving opioid therapy for chronic cancer pain
- SC or IM (intermittent) → 4hrly PRN, titrate for each pt
- IV (intermittent) → adequate pain relief w/out XS drowsiness + resp depression, suitable for recovery wards. Eg. 1-3mg morphine or 20-60mcg of fentanyl every 5 min, until pt comfortable
- IV (continuus) → close obs + monitoring essential
- Intrathecal → good for day-case arthroscopic surgeries, or post-elective C section
- Intransal → v effective in children (>1yr), for breakthrough pain
- Transmucosal → fentanyl lollipops for children
- Transdermal → for lipid-soluble opioids
What are recommended doses of transdermal fentanyl, based on morphine doses?
What is patient-controlled analgesia?
- Pts administer own IV analgesia + titrate dose to own end-point of pan relief using small microprocessor-controlled pump
- Morphine most common (more pruritus w/ morphine)
- Others: fentanyl, pethidine, tramadol
- Equipment malfunction rare
- Operator error more common
How do you manage nausea + vomiting with opioids?
- Add an antiemetic → ondansetron 4mg, cyclizine 50-100mg, haloperidol 2mg
- Prescribe on a regular basis
- Change the opioid
How do you manage breakthrough pain?
- Add regular NSAID + paracetamol (if not contraindicated)
- Increase bolus dose
- Consider background infusion if severe
How do you manage respiratory depression, with opioids?
- All opioids have same potential for resp depression
- Relatively uncommon
- Early indicator → increasing sedation
- Opioid doses adjusted so sedation score remains <2
- Resp depression (RR < 8) is reversed with IV naloxone (100-400 micrograms)
For epidural analgesia, an indwelling epidural catheter is inserted used to provide continuous infusion of analgesic agents. Provide excellent analgesia.
What are benefits of epidural analgesia?
- Gold standard for major surgery
- Prevent postoperative respiratory compromise resulting from pain
- Post-operative complications reduced → MI, atelectasis, pulmonary infection, DVT, ileus, blood transfusion requirements
- Improved intestinal motility → permitted earlier feeding
What are the disadvantages and contraindications for epidural analgesia?
- No survival benefit in high-risk pts
- Confine pts to bed (esp if motor block present)
- Indwelling urinary catheter → infection risk
- Epidural haematoma
- Contraindicated with patient refusal and untrained staff
- Other contraindications → sepsis, hypovolaemia, coagulation disorders, anticoagulants
What are complications of epidural analgesia?
Extreme vigilance is needed for all pts who have epidural analgesia, because of the risk of spinal infection.
What are the clinical features of spinal infection?
- Back pain
- Fever
- Leucocytosis
- ↑ ESR
Epidural catheter should be removed immediately + sent to lab for cultures, Staph aureus most common (90%). MRI w/ gadolinium is investigation of choice - scan early! Once onset of muscle weakness, only 20% of pts regain full function even after surgery.
Quality of analgesia is better w/ peripheral nerve blocks, compared with opioids. Incidence of post-operative side-effects is reduced.
What are examples of continuous peripheral nerve blocks?
- Interscalene
- Axillary
- Femoral / fascia illiaca
- Sciatic
Complications → bruising, haematoma, LA toxicity, peripheral nerve damage, infection, catheter kinking, catheter migration
Physical opioid dependence is a physiological phenomenon characterised by a withdrawal rxn when the drug is withdrawn or antagonist administered.
What are symptoms of opioid withdrawal?
- Yawning
- Sweating + Anxiety
- Rhinorrhoea + Lacrimation
- Tachycardia, hypertension
- Diarrhoea, nausea + vomiting
- Abdo pain + cramps
Symptoms peak 26-72hr after last dose
What is buprenorphine?
- Partial opioid agonist
- Used in treatment of opioid addiction
- Dose → 8-32mg
What are features of ketamine?
- NMDA receptor antagonist in CNS + peripheral NS
- Major adjuvant analgesic in variety of settings
- Major side effects uncommon
- Insufficient evidence to recommend ketamine as routine perioperative analgesic
