Principles of Anaesthesia

Question 1

3 Components of General Anaesthesia

Answer 1

1. Loss of consciousness
2. Analgesia
3. Muscle relaxation

Question 2

Monitoring patients receiving Local Anaesthetic:

Answer 2

1. ECG
2. Pulse oximetry
3. BP

Question 3

Safe Maximum Doses of Commonly-used Local Anaesthetics:

1. Lidocaine
   a) With epinephrine
   b) Without epinephine
2. Bupivacaine
   a) With adrenaline
   b) Without adrenaline
3. Prilocaine

Answer 3

1. a) 6 mg/kg
   b) 2 mg/kg
2. a) 2 mg/kg
   b) 2 mg/kg
3. Max 600 mg

Question 4

Signs of Local Anaesthetic Toxicity:

1. Early
2. Late

Answer 4

1. • Numbness/tingling of tongue
     - Perioral tingling
     - Anxiety
     - Lightheadedness
     - Tinnitus
2. • Loss of consciousness
     - Convulsions
     - Cardiovascular Collapse
     - Apnoea

Question 5

Spinal Anaesthesia

Answer 5

• Local anaesthetic ie lidocaine/bupivacaine into subarachnoid space
• adminsitered below L2, usually L3/4 or L4/5
• Addition of 6-8% glucose increases density so that easier to control level of block with gravity

Question 6

Epidural Anaesthesia

Answer 6

• Administered into epidural space
• Larger volumes required compared to spinal anaesthetic since nerve rots are fully covered and myelinated
• Needle aspiration to ensure no dural tap
• Catheter left in epidural space to provide access for ongoing analgesia

Question 7

Complications of Epidural anaesthesia and analgesia

Answer 7

1. Epidural abcess
   - Avoid in skin/systemic sepsis
2. Epidural haematoma
   - Correct coagulopathy and reverse anticoagulation
   - Avoided in patients receiving heparin
3. Respiratory depression
   - Avoid high epidural block
4. Cardiac depression
   - Avoid mid-thoracic epidural

Question 8

Topical Anaesthesia

Answer 8

• Used in procedures involving oral cavity, pharynx, larynx, urethra and conjunctive
• also in children/needle-phobic adults before cannulation/venepuncture
• Eg: Tetracaine(Ametop), prilocaine/lidocaine(Emla), Lignocaine(most common)

Question 9

Advantages of Postoperative Analgesia

Answer 9

1. Minimises physical and psychological morbidity
2. Early mobilisation
3. Optimises respiratory function

Question 10

Postoperative Analgesic Strategy:
- Multimodal analgesia is preferred

1. Epidural analgesia
2. Patient-controlled analgesia(PCA)
3. Parenteral and oral opioids
4. a) Paracetamol
   b) NSAIDs
   c) Selective COX-2 inhibitors

Answer 10

1. • Typical regimen: 0.1% bupivacaine with 2 mg/ml fentanyl up to 16 ml/h
     - used for thoracic, abdominal and major lower limb surgery
     - catheter can remain up to 5 days
     - Advantages: Superior pain relief
     - Disadvantages: Requires monitoring by trained staff, permanent neurological damage(0.005-0.05%), respiratory depression
2. • Typical regmen: 1 mg morphine at 5 min intervals
     - Expensive, requires manual dexterity of patient to control pump, respiratory depression( up to 11.5% patients)
3. a) Strong: buprenorphine, fentanyl, oxycodone, pethidine, morphine
   - Uses: Minor surgery, stepping down from epidural/PCA
   - SE: Respiratory depression, constipation, urinary retention, dysphoria, nausea and vomiting, pruritis, depressed conscious lvel

b) Weak: Codeine(also in cocodamol), Dihydrocodeine(also in codydramol), tramadol
- Uses: mild pain
- Tramadol also useful in neuropathic pain

4. a) Paracetamol
   - All postoperative patients unless contraindicated
   - Reduces opioid requirements by 20-30%
   - When combined with NSAIDs, more effective than NSAIDs alone

b) NSAIDs
- When combined with opioids, increases analgesia and reduces opioid requirement
- SE: Renal impairment, impaired platelet function, increased postoperative bleeding, peptic ulceration, bronchospasm

c) Selective COX-2 inhibitors
- Eg: Celecoxib, Parecoxib, Etoricoxib
- As effective as NSAIDs and better side-effect profile
- Increased risk of thrombotic events
- CI: Ischaemic heart disease, Cerebrovascular and peripheral arterial disease, moderate to severe cardiac failure
- Used when NSAIDs contraindicated but only after assessing cardiovascular risk

Question 11

Neuropathic Pain

Answer 11

• Postoperative Acute neuropathic pain is a risk factor for chronic neuropathic pain
• Tx: Intravenous lidocaine, gabapentin, Tricyclic antidepressants(efficacy in only chronic neuropathic pain confirmed)

Question 12

Risk Factors of Postoperative Nausea and Vomiting:

*Tx: Ondansetron and dexamethasone

Answer 12

1. Female
2. Type of surgery ie gynaecological, laparoscopic
3. Non-smoker
4. History of previous postoperative nausea and vomiting, motion sickness, opioid use
5. Inhalational anaesthetic agents ie nitrous oxide