Key Facts

General anaesthesia is a drug-induced, reversible state comprising unconsciousness, analgesia, amnesia, immobility, and attenuation of autonomic responses. It is delivered by trained anaesthetists who maintain physiological homeostasis throughout the perioperative period.

Epidemiology

Approximately 3 million general anaesthetics are administered annually in the UK. Mortality directly attributable to anaesthesia has fallen from 1 in 10,000 (1980s) to approximately 1 in 100,000-200,000 currently. Serious complications (cardiac arrest, death, awareness) are rare but remain important.

Aetiology

The mechanisms of general anaesthesia are incompletely understood. Key theories include:

• Meyer-Overton hypothesis: Lipid solubility correlates with anaesthetic potency
• Protein receptor theory: Agents act on specific ion channels — GABA-A receptors (propofol, volatile agents), NMDA receptors (ketamine), glycine receptors
• Neural circuit disruption: GA disrupts cortical integration and thalamocortical connectivity

Pathophysiology

Effects on organ systems:

• CVS: Most agents cause myocardial depression and vasodilation → hypotension
• Respiratory: Respiratory depression, loss of airway reflexes, bronchodilation (volatiles)
• CNS: Dose-dependent depression, reduced CMRO2 and CBF, anticonvulsant properties (except enflurane/sevoflurane at high MAC)
• Renal: Reduced RBF and GFR (reversible)
• Hepatic: Reduced hepatic blood flow; halothane hepatitis (historical — now rare)
• Thermoregulation: Impaired — patients become hypothermic

Stages of Anaesthesia (Guedel's Classification — Historical)

• Stage 1 (Analgesia): Conscious but reduced pain perception
• Stage 2 (Excitement): Loss of consciousness, irregular breathing, risk of laryngospasm — pass through quickly
• Stage 3 (Surgical anaesthesia): Target depth — regular respiration, muscle relaxation
• Stage 4 (Overdose): Cardiovascular and respiratory collapse

Induction

• IV induction (propofol, thiopental) — most common in adults
• Inhalational induction (sevoflurane) — preferred in children and difficult IV access

Maintenance

• Inhalational (sevoflurane, desflurane, isoflurane) ± nitrous oxide
• Total intravenous anaesthesia (TIVA) — propofol ± remifentanil infusion

Emergence

• Cessation of anaesthetic agents, reversal of neuromuscular blockade
• Return of consciousness, protective airway reflexes, spontaneous ventilation

Red Flags

• Failed intubation — follow Difficult Airway Society (DAS) guidelines
• Malignant hyperthermia — rising ETCO2, tachycardia, rigidity, fever (dantrolene 2.5mg/kg IV)
• Anaphylaxis — cardiovascular collapse, bronchospasm, rash (adrenaline 50mcg IV boluses)
• Awareness under GA — patient recall of intraoperative events (NAP5 incidence ~1:19,600)

Bedside

• Pre-operative assessment: Airway (Mallampati, thyromental distance, mouth opening), cardiorespiratory examination
• WHO checklist: Sign in, time out, sign out
• AAGBI monitoring standards: ECG, SpO2, NIBP, ETCO2, temperature, peripheral nerve stimulator

Bloods

• As per NICE NG45: Targeted pre-operative testing based on ASA grade and procedure
• FBC: If anaemia or blood loss expected
• U&Es: If renal disease, diuretics, ACEi
• Coagulation: If on anticoagulants or hepatic disease
• HbA1c: Diabetic patients

Imaging

• CXR: Not routine; only if clinically indicated (respiratory disease, cardiac failure)
• Echocardiography: If significant cardiac disease

Special Tests

• Cardiopulmonary exercise testing (CPET): For high-risk patients before major surgery — AT <11 mL/kg/min indicates high risk
• Lung function tests: If severe respiratory disease
• Sickle cell screen: In at-risk populations

Non-pharmacological

• Pre-operative fasting: 6 hours solids, 2 hours clear fluids (1 hour for breast milk in infants)
• Patient positioning: Appropriate positioning, padding of pressure points and nerves
• Temperature management: Active warming (forced-air warmer), warm IV fluids
• Enhanced recovery principles: Minimise fasting, carbohydrate loading

Pharmacological

• IV induction: Propofol 1-2.5mg/kg (standard) or thiopental 3-5mg/kg
• Inhalational induction: Sevoflurane in O2 (MAC 2.0%) — smooth induction
• Maintenance: Sevoflurane/desflurane/isoflurane in O2 ± air ± N2O; or TIVA (propofol TCI 3-6 mcg/mL + remifentanil 0.1-0.5 mcg/kg/min)
• Opioid analgesia: Fentanyl 1-2mcg/kg, morphine 0.1-0.2mg/kg, or remifentanil infusion
• Neuromuscular blockade: Atracurium 0.5mg/kg or rocuronium 0.6mg/kg
• Reversal: Neostigmine 2.5mg + glycopyrrolate 0.5mg (reversal of non-depolarising block); sugammadex 2-4mg/kg (specific rocuronium reversal)
• Antiemetic prophylaxis: Ondansetron 4mg IV + dexamethasone 6.6mg IV

Surgical/Interventional

• Not directly applicable — GA facilitates surgical procedures

Referral Criteria

• Pre-operative anaesthetic review for all patients listed for GA
• High-risk patients — consultant anaesthetist involvement, HDU/ICU bed planning
• Known/suspected difficult airway — advance planning, senior anaesthetist

• Mortality directly attributable to anaesthesia: ~1 in 100,000-200,000
• Perioperative mortality (all causes): ~1 in 1,000 for elective surgery; higher for emergency surgery
• Awareness under GA: ~1 in 19,600 (NAP5) — PTSD can result in ~50% of cases
• Post-operative cognitive dysfunction (POCD): Affects 10-25% of elderly patients at 1 week; mostly resolves
• Perioperative dental damage: Most common GA-related complaint — 1 in 4,500 anaesthetics

ASA Physical Status Classification

AAGBI Minimum Monitoring Standards