🎯 Quiz Mode: Anaesthetics
MCQs will appear as you scroll through the content. Test your knowledge!
title: "Anaesthetics"
Anaesthetics
1. Overview
General Anaesthesia (GA): Three components of a balanced GA:
- Unconsciousness
- Analgesia
- Lack of movement (paralysis)
GA vs Regional Anaesthesia
| General Anaesthetic | Local/Regional Anaesthetics |
|---|---|
| Systemic drugs producing unconsciousness | Block voltage-gated Na channels |
| Requires airway management | Patient awake or sedated |
| Systemic side effects | Localised effects |
2. Stages of General Anaesthesia
| Stage | Key Actions |
|---|---|
| Induction | IV access, positioning, preoxygenation, monitoring, IV anaesthetic agent |
| Maintenance | Propofol infusion OR volatile gas, opioids for analgesia, antibiotics, antiemetics |
| Emergence | Turn off agent, reverse paralysis, remove airway device |
| Recovery | 30-60 min close monitoring, check consciousness, analgesia, PONV |
Preoxygenation: Fill functional residual capacity (FRC) with 100% O2. This provides oxygen reserve during apnoea period of intubation.
Source: CC Bible; local anaesthetic airway teaching.
2.5 Preoperative Assessment
Purpose: Minimize perioperative morbidity/mortality and provide informed consent with realistic risk assessment.
Components of Assessment
Key factors determining workup:
- Operation complexity (minor/intermediate/major)
- Urgency (emergency/urgent/elective)
- Patient comorbidities (ASA classification)
ASA Classification
American Society of Anesthesiologists physical status classification:
| ASA | Status | Example | Periop Mortality Risk |
|---|---|---|---|
| ASA 1 | Healthy | No systemic disease | <0.1% |
| ASA 2 | Mild systemic disease | Well-controlled DM, mild asthma | ~0.2% |
| ASA 3 | Severe systemic disease | Previous MI (stable), morbid obesity | ~1-2% |
| ASA 4 | Severe disease (constant threat to life) | Recent MI with unstable angina | ~8% |
| ASA 5 | Moribund (unlikely to survive without surgery) | Ruptured AAA | ~30-50% |
| ASA 6 | Brain dead (organ donor) | - | - |
Add "E" suffix for emergency surgery (e.g., ASA 1E) - significantly increases risk.
ASA classification predicts perioperative mortality - higher ASA = higher risk, amplified by emergency surgery.
Source: Dr. Kao Lim lecture on preoperative assessment.
History & Examination
Focused assessment:
- Acute surgical condition and severity
- Cardiovascular and respiratory systems (evidence of failure?)
- Exercise capacity (flights of stairs, METs, NYHA classification)
- Specific questions:
- Pregnancy (avoid first trimester if possible)
- Reflux (aspiration risk → consider intubation over LMA)
- Smoking (increased wound/chest infections)
- Previous anaesthetics (PONV, difficult airway)
- Family history (malignant hyperthermia, pseudocholinesterase deficiency)
Exercise capacity = surrogate for cardiorespiratory reserve. Poor exercise capacity indicates higher perioperative risk.
Airway Assessment
Must assess even for regional anaesthetics (may need conversion to GA).
Difficult mask ventilation predictors:
- Obesity
- Obstructive sleep apnoea/snoring
- Edentulous (floppy cheeks, poor seal)
- Beard (poor mask seal)
Difficult intubation predictors:
- Mallampati score III or IV (see Figure below)
- Thyromental distance <6 cm (limit to 7 cm ideal)
- Mouth opening <4 cm (<3 cm for LMA)
- Reduced neck extension (ankylosing spondylitis, C-spine injury)
- Previous difficult airway
Difficult LMA predictors:
- Limited mouth opening
- Neck/airway asymmetry (tumour, radiotherapy, haematoma)
Difficult front-of-neck access:
- Obesity (cannot palpate airway structures)
Previous difficult airway is the strongest predictor of future difficulty. Always check old anaesthetic charts.
Perioperative Medication Management
Continue most medications on day of surgery, except:
| Medication | Management |
|---|---|
| ACE-I/ARBs | May withhold (can cause severe intraoperative hypotension) |
| PPIs/H2 blockers | Continue (reduces aspiration risk) |
| Inhalers | Continue (even on day of surgery) |
| Opioids/analgesics | Continue (baseline pain requirement) |
| Oral hypoglycemics | Withhold (fasting → hypoglycemia risk) |
| SGLT2 inhibitors | Withhold 2+ days pre-major surgery (acid-base disturbance risk) |
| Long-acting insulin | Continue (may reduce dose) |
| Short-acting insulin | Withhold (meal-related) |
| Anticoagulants | Surgeon decision based on bleeding risk |
Aspirin can usually continue for most surgeries. Complex anticoagulation (e.g., mechanical valve) may need bridging therapy and MDT discussion.
Investigation Selection
Tailor to surgical complexity and patient factors - avoid over-ordering.
| Investigation | Indications |
|---|---|
| FBC | Hx haematological disease, major surgery with bleeding risk |
| UEC | Assess renal function, especially if on relevant medications |
| CXR | Specific lung pathology concern (e.g., pneumothorax risk) |
| ECG | Age ≥50, arrhythmia, palpitations, known IHD |
| Coags | Liver failure, on anticoagulants |
| Echo | Valvular disease/murmur (especially AS), poor exercise capacity, pulmonary hypertension |
| Cardiac workup | Ischemic symptoms, depends on surgical urgency |
Minor surgery + healthy patient = minimal investigations. Major surgery = more investigations even if healthy. Balance patient complexity with surgical complexity.
Source: Dr. Kao Lim lecture; UK guidelines on preoperative investigations.
3. Induction Agents
| Drug | Mechanism | Dose (mg/kg) | Onset | Duration | Key Features |
|---|---|---|---|---|---|
| Propofol | Enhances GABA | 1.5-2.5 | 15-45 sec | 5-10 min | Smooth induction, hypotension, pain on injection |
| Thiopentone | Barbiturate, GABA | 3-5 | 15-30 sec | 5-10 min | Less hypotension, groggy recovery |
| Ketamine | NMDA antagonist | 1.5-2 | 30-60 sec | 20-40 min | CVS stable, bronchodilator, emergence delirium |
| Etomidate | GABA | 0.2-0.3 | 15-45 sec | 5-10 min | Most haemodynamically stable |
Propofol provides hypnosis via GABA-A potentiation but no analgesia, and commonly causes hypotension.
Induction dosing: propofol 1.5-2.5 mg/kg, thiopentone 3-5 mg/kg, ketamine 1.5-2 mg/kg, etomidate 0.2-0.3 mg/kg.
Ketamine is an NMDA antagonist that maintains BP (Blood Pressure) via sympathetic stimulation and is a bronchodilator.
Etomidate is the most haemodynamically stable induction agent.
Source: CC Bible; local anaesthetic airway teaching.
Cardiovascularly Stable Induction Agents:
- Ketamine
- Etomidate
- Thiopentone
Use in haemodynamically unstable patients or trauma.
Ketamine advantages:
- Maintains BP (sympathetic stimulation)
- Bronchodilator (good for asthmatics)
- Potent analgesic
- Preserves airway reflexes
Disadvantages: Emergence phenomena (hallucinations, delirium)
4. Neuromuscular Blocking Agents
Classification
| Type | Mechanism | Example |
|---|---|---|
| Depolarising | ACh agonist → persistent depolarisation → refractory state | Suxamethonium |
| Non-depolarising | Competitive ACh antagonist at NMJ | Rocuronium, vecuronium, atracurium |
Commonly Used Agents
| Drug | Onset | Duration | Notes |
|---|---|---|---|
| Suxamethonium | 30-60 sec | 5-10 min | Fastest onset/offset, used for RSI |
| Rocuronium | 60-90 sec | 30-60 min | Can be reversed with sugammadex |
| Vecuronium | 2-3 min | 30-40 min | Minimal cardiovascular effects |
| Atracurium | 2-3 min | 20-35 min | Hofmann degradation (good in renal failure) |
RSI dosing: suxamethonium 1-1.5 mg/kg (onset 45-60 sec, duration 6-8 min) or rocuronium 0.6-1.2 mg/kg (onset 60-90 sec, duration 40-60 min).
Source: eTG; local RSI protocol.
Suxamethonium adverse effects: hyperkalaemia, malignant hyperthermia trigger, myalgias, and anaphylaxis; fasciculations at onset.
Rocuronium has a longer duration (40-60 min) and can be used at higher dose for RSI; rare anaphylaxis.
Non-depolarising agents: vecuronium 0.1 mg/kg (onset 3-4 min) and cisatracurium 0.2 mg/kg (onset 4-6 min), with cisatracurium cleared via Hofmann elimination.
Source: eTG; local ICU/anaesthesia protocols.
Suxamethonium Contraindications:
- Hyperkalaemia or risk factors
- Burns >24 hours old
- Denervation injuries (spinal cord, stroke)
- Prolonged immobilisation
- Malignant hyperthermia susceptibility
- Myopathies
Risk: Massive potassium release due to upregulated ACh receptors
Reversal Agents
| Agent | Target | Dose | Mechanism |
|---|---|---|---|
| Sugammadex | Rocuronium, Vecuronium | 2-16 mg/kg | Encapsulates aminosteroid NMBAs |
| Neostigmine | Non-depolarising NMBAs | 0.05 mg/kg + glycopyrrolate | Anticholinesterase |
Sugammadex advantage: Can reverse even deep neuromuscular blockade within minutes. Far more reliable than neostigmine.
Neostigmine onset is slow (7-10 min) and requires glycopyrrolate to blunt muscarinic effects (e.g. bradycardia).
Sugammadex reverses rocuronium/vecuronium rapidly (about 60-90 sec) and can reverse deep blockade.
Source: Critical Care Specialty Block – Intubation Drugs v3 (1/2026).
Adjuncts for Intubation
Midazolam adjunct dosing: 0.015-0.07 mg/kg (typical adult dose 1-5 mg).
Midazolam is a benzodiazepine (GABA-A) providing anxiolysis and amnesia; reversal is flumazenil.
Fentanyl is a mu-opioid agonist used for analgesic blunting of the intubation response; reversal is naloxone.
Source: Critical Care Specialty Block – Intubation Drugs v3 (1/2026).
5. Volatile Anaesthetics
| Agent | Notes |
|---|---|
| Sevoflurane | Most commonly used, good for gas induction (paediatrics) |
| Desflurane | Fast offset, pungent (not for induction) |
| Isoflurane | Cheaper, more cardiovascular depression |
Gas induction indications:
- No IV access
- Difficult IV access (paediatrics)
- Needle phobia
- Patient preference
Volatile anaesthetic risks:
- Post-operative nausea/vomiting (worst of all agents)
- Malignant hyperthermia trigger
- Environmental impact
6. Airway Management
Airway Devices
| Device | Procedures | Key Features |
|---|---|---|
| Facemask | Very short procedures | Least invasive, requires jaw thrust |
| LMA | Most procedures | Easy insertion, no need for paralysis |
| ETT | Aspiration risk, long procedures | Best protection, requires laryngoscopy |
ETT Sizing
| Patient | ETT Size |
|---|---|
| Adult female | 7.0 mm ID |
| Adult male | 8.0 mm ID |
| Paediatric | (Age/4) + 3.5 mm (cuffed) |
Cormack-Lehane Laryngeal View Grading
| Grade | View | Interpretation |
|---|---|---|
| I | Full glottis visible | Easy intubation |
| II | Partial glottis visible | Usually straightforward |
| III | Only epiglottis visible | Difficult - use bougie |
| IV | Neither glottis nor epiglottis visible | Very difficult - consider surgical airway |
Preoperative Airway Assessment
- L - Look externally (obesity, facial hair, trauma)
- E - Evaluate 3-3-2 rule
- M - Mallampati score (III-IV = difficult)
- O - Obstruction (tumour, abscess, oedema)
- N - Neck mobility (cervical spine injury)
Mallampati Classification
| Class | Visibility |
|---|---|
| I | Tonsillar pillars visible |
| II | Uvula partially obscured |
| III | Only soft palate visible |
| IV | Hard palate only |
Difficult airway predictors:
- Thyromental distance <6 cm
- Mouth opening <3 cm (LMA), <4 cm (laryngoscopy)
- Mallampati III or IV
- Reduced neck extension
7. Neuraxial Anaesthesia
Spinal vs Epidural
| Feature | Spinal | Epidural |
|---|---|---|
| Space | Subarachnoid (CSF) | Epidural |
| Location | Below L1 only | Anywhere |
| Endpoint | CSF flow (reliable) | Loss of resistance |
| Onset | Seconds-minutes | 15-30 minutes |
| Duration | Single shot | Catheter for repeat dosing |
| Hypotension | More | Less |
Local Anaesthetics
| Drug | Onset | Duration | Use |
|---|---|---|---|
| Lignocaine | Fast (5-10 min) | Short (1-3 hr) | Infiltration, nerve blocks |
| Bupivacaine | Slow (10-20 min) | Long (2-12 hr) | Epidural, spinal |
| Ropivacaine | Slow (10-20 min) | Long (2-12 hr) | Epidural, nerve blocks |
Lignocaine Dosing
Q: Lignocaine max dose (plain vs with adrenaline)? A: 3 mg/kg (plain); 7 mg/kg (with adrenaline)
Local Anaesthetic Toxicity
LAST (Local Anaesthetic Systemic Toxicity):
Symptoms (progressive):
- Perioral tingling, tinnitus
- Confusion, seizures
- Arrhythmias, cardiovascular collapse
- Cardiac arrest
Treatment:
- Stop injection
- Call for help
- ABCDE
- Benzodiazepines for seizures
- Intralipid 20%
- Bolus 1.5 mL/kg over 1 min
- Infusion 15 mL/kg/hr
- Repeat bolus up to twice at 5-minute intervals if circulation not restored
- After 5 minutes, double infusion rate if still unstable
Source: CC Bible extract (local anaesthetic systemic toxicity section).
Anticoagulant Cessation
| Drug | Time Before Neuraxial |
|---|---|
| Clopidogrel | 7 days |
| Dabigatran | 5 days |
| Apixaban/Rivaroxaban | 3 days (72 hours) |
| Therapeutic LMWH | 24 hours |
| Prophylactic enoxaparin | 12 hours |
8. Preoperative Assessment
ASA Classification
| Class | Description |
|---|---|
| ASA 1 | Healthy patient |
| ASA 2 | Mild systemic disease (well-controlled HTN, obesity) |
| ASA 3 | Severe systemic disease (poorly controlled DM, COPD) |
| ASA 4 | Life-threatening disease (recent MI, severe valve disease) |
| ASA 5 | Moribund (not expected to survive without surgery) |
| ASA 6 | Brain-dead organ donor |
| E suffix | Emergency surgery |
Fasting Guidelines
Preoperative fasting reduces the risk of aspiration of gastric contents.
Clear fluids: 2 hours fasting.
Q: Breast milk fasting time? A: 4 hours
Infant formula or light meal: 6 hours fasting.
Heavy or fatty meal: 8 hours fasting.
Medications Perioperatively
| Continue | Potentially Withhold |
|---|---|
| Cardiac medications | ARBs (hypotension risk) |
| Antihypertensives (except ARBs) | Anticoagulants |
| Anti-reflux medication | Oral diabetic agents |
| Opioids, chronic pain meds | SGLT2i (3 days - euglycaemic DKA) |
| Inhalers |
9. Monitoring
4 Standard Monitors
Minimum monitoring in GA:
- ECG
- Blood pressure (NIBP)
- Pulse oximetry (SpO2)
- End-tidal CO2 (capnography)
10. Adverse Events
Common (Not Life-Threatening)
| Event | Risk | Prevention |
|---|---|---|
| PONV | 20% | Prophylactic antiemetics |
| Sore throat | 20% | Careful airway management |
| Dental damage | Rare | Careful laryngoscopy |
Rare, Serious
| Event | Risk |
|---|---|
| Awareness under GA | 1:8200 (with muscle relaxant) |
| Major adverse cardiac/cerebrovascular event | Varies |
| Death | 1:57,000 |
11. Practice Questions
Source: Critical Care Specialty Block – Intubation Drugs v3 (1/2026).
What is the MOST important treatment?
What is the diagnosis?
Quick Reference
Induction Agents
| Situation | Agent |
|---|---|
| Standard induction | Propofol |
| Haemodynamically unstable | Ketamine or Etomidate |
| Asthma | Ketamine |
| No IV access | Sevoflurane (gas) |
Muscle Relaxants
| Situation | Agent |
|---|---|
| RSI (fastest) | Suxamethonium |
| RSI (contraindication to sux) | Rocuronium (high dose) |
| Routine intubation | Rocuronium |
Local Anaesthetic Doses
| Drug | Max Dose |
|---|---|
| Lignocaine (plain) | 3 mg/kg |
| Lignocaine + adrenaline | 7 mg/kg |
| Bupivacaine | 2 mg/kg |
High-Risk Medications in Critical Care
- A - Anti-infectives (Aminoglycosides, Amphotericin - nephro/ototoxicity)
- P - Potassium & electrolytes (K+, Mg2+, Ca2+, hypertonic saline)
- I - Insulin (hypoglycaemia risk)
- N - Narcotics & sedatives (respiratory depression)
- C - Chemotherapeutics (immunosuppression, organ toxicity)
- H - Heparin & anticoagulants (bleeding)
High-Risk Medication Considerations
| Drug Class | Main Risks | High-Risk Patients |
|---|---|---|
| Aminoglycosides | Nephrotoxicity, Ototoxicity | Renal dysfunction, elderly, obese |
| Electrolytes (K+) | Cardiac arrest | Incorrect dosing is the biggest issue |
| Insulin | Hypoglycaemia | Variable nutrition, liver disease, sepsis |
| Opioids | Respiratory depression | Respiratory disease, multiple sedatives, elderly |
| Heparins | Haemorrhage | Renal impairment (use UFH not LMWH) |
PK/PD Alterations in Critical Illness
Critically ill patients are MORE SUSCEPTIBLE to drug toxicities due to organ dysfunction. Pharmacokinetic changes are UNPREDICTABLE.
Pharmacokinetic Changes
| Process | Changes in Critical Illness |
|---|---|
| Absorption | Unpredictable - delayed (ileus) or decreased (altered pH, diarrhoea), ↓IM absorption with poor perfusion |
| Distribution | ↑Vd (capillary leak) = under-dosing water-soluble drugs; ↓protein binding = ↑free drug |
| Metabolism | ↑with fever, ↓with hepatic injury/hypothermia |
| Elimination | ↓renal clearance (↓GFR, AKI); ↓biliary clearance |
Loading doses may need to be increased due to increased volume of distribution (capillary leak), but maintenance doses often need reduction due to impaired elimination.
Anaesthetics Study Checklist
Click to expand or view deep dives