General Anaesthesia — Principles & Conduct

📘 1 · Miller's Anesthesia — What General Anaesthesia Is

The Anaesthetic Triad & Balanced Anaesthesia

"General anaesthesia is a drug-induced, reversible state comprising unconsciousness, amnesia, analgesia and immobility, with control of the autonomic and reflex responses to surgical stimulation. No single drug provides all of these ideally, so modern practice uses a 'balanced' technique — small doses of several agents combined to achieve each component while minimising the side-effects of any one."

Synthesised from Miller's Anesthesia, 9th Ed — Basic Principles of Anaesthesia.

💤 Hypnosis

Unconsciousness & amnesia — IV induction agents and volatile agents.

🩹 Analgesia

Blunting the response to pain — opioids, regional blocks, ketamine, α2-agonists.

💪 Relaxation

Immobility & muscle relaxation for surgery & intubation — neuromuscular blockers.

🧩 Why "balanced" anaesthesia

A pure inhalational technique deep enough to prevent movement would cause profound cardiovascular depression. Adding opioids (analgesia) and muscle relaxants (immobility) lets you run a lighter, safer plane of hypnosis.
TIVA (total intravenous anaesthesia) — usually propofol ± remifentanil by target-controlled infusion — is an alternative to volatile-based maintenance (useful in PONV-prone patients, malignant-hyperthermia risk, neuromonitoring, shared/difficult airways).
The four questions during maintenance: Is the patient asleep? Pain-free? Still enough? Physiologically stable?

📗 2 · Barash & Morgan/Mikhail — Agents & MAC

Induction, Maintenance & the Concept of MAC

"The potency of an inhaled anaesthetic is expressed as the Minimum Alveolar Concentration — the alveolar concentration, at one atmosphere, that prevents movement in response to a standard surgical incision in 50% of subjects. MAC is additive between agents, is reduced by age, opioids, hypothermia, pregnancy and other sedatives, and is increased in the young, in chronic alcohol use and in hyperthermia."

Synthesised from Barash — Clinical Anesthesia; Morgan & Mikhail's Clinical Anesthesiology.

Induction agents (intravenous)

Propofol — smooth, antiemetic, rapid recovery; causes hypotension and pain on injection; agent of choice for TIVA.
Ketamine — dissociative; maintains blood pressure & airway reflexes; bronchodilator and analgesic; emergence phenomena.
Etomidate — cardiovascularly stable; transient adrenal suppression; myoclonus.
Thiopentone — rapid barbiturate; anticonvulsant, cerebroprotective; avoid in porphyria; contraindicated intra-arterially.

Maintenance — inhalational agents

Sevoflurane — sweet, non-irritant → the agent of choice for inhalational (gas) induction, especially in children; low blood-gas solubility = fast on/off.
Isoflurane — cheap, stable; pungent (not for gas induction).
Desflurane — lowest solubility → fastest emergence; airway-irritant, needs a heated vaporiser.
Nitrous oxide — analgesic adjunct, "second-gas effect"; avoid in closed air spaces (pneumothorax, bowel obstruction, middle ear) and with prolonged use (B₁₂/megaloblastic effects).

📊 MAC — the numbers & the modifiers

Approx. MAC (in 100% O₂): sevoflurane ~2.0%, isoflurane ~1.15%, desflurane ~6.0%, nitrous oxide ~104%. MAC-awake ≈ 0.3–0.4 MAC; MAC-BAR (blocks autonomic response) ≈ 1.5 MAC.

MAC decreases with: increasing age (~6% per decade after 40), opioids/sedatives, hypothermia, pregnancy, hyponatraemia, acute alcohol. MAC increases with: infancy (peak ~6 months), hyperthermia, chronic alcohol, chronic stimulant use, hypernatraemia.

📙 3 · Morgan/Mikhail — Equipment

The Anaesthetic Machine & Breathing Systems

🛠 The machine — safety by design

Gas supply → flowmeters → vaporisers → common gas outlet → breathing system → scavenging. Multiple safety features prevent a hypoxic mixture: oxygen failure alarm, hypoxic guard (links N₂O to O₂ so FiO₂ can't fall below ~25%), pin-index & colour coding, and non-return valves.
Always perform the machine check (AAGBI checklist) and confirm a self-inflating bag is immediately available in case of machine failure.

🔄 Breathing systems & CO₂

Circle system — the workhorse: soda-lime absorbs CO₂, allowing low fresh-gas flows (economy, warmth, humidity). Watch for exhausted absorbent (rising inspired CO₂, colour change).
Mapleson systems — classified A–F; Mapleson A is most efficient for spontaneous ventilation, Mapleson D/Bain for controlled ventilation, and the Jackson–Rees (F) for paediatrics.
Capnography is the single most important monitor on the breathing system — it confirms ventilation and detects disconnection, obstruction, rebreathing and (as a sudden fall) cardiac output loss or embolism. See the Monitoring page.

📋 4 · Stages, Depth & Awareness

Depth of Anaesthesia

Guedel's stages (classical, ether-era — still examined)

I — Analgesia: induction to loss of consciousness.
II — Excitement/delirium: loss of consciousness to onset of automatic breathing — disconjugate gaze, hyperreactivity, risk of laryngospasm/vomiting; move through it quickly and avoid stimulation.
III — Surgical anaesthesia: regular breathing to respiratory paralysis (four planes; pupils central, reflexes progressively lost).
IV — Overdose: apnoea, cardiovascular collapse — too deep.

Accidental awareness under GA (AAGA) NAP5

Awareness is rare (~1 in 19,000, NAP5) but distressing and medico-legally important. Highest-risk situations: use of neuromuscular blockade (paralysis masks the movement that would warn you), RSI, obstetric & cardiac anaesthesia, TIVA with pump/cannula problems, and the transfer/emergence periods. Prevent it by giving adequate hypnotic doses, monitoring end-tidal agent (or using processed EEG/depth monitoring with TIVA + paralysis), checking TIVA delivery, and being vigilant at induction and emergence.

🇮🇳 Indian Context

Volatile-based anaesthesia with end-tidal agent monitoring remains the most widely available and economical way to guard against awareness where processed-EEG monitors are limited. Where TIVA is used with muscle relaxants, prioritise a depth-of-anaesthesia monitor and meticulous checking of the infusion line/cannula. Draw-over/older machines are still in service in some settings — a disciplined machine check and a self-inflating bag on standby are essential.

💊 5 · Drug & MAC Reference

Core General-Anaesthesia Drugs

Drug	Class / role	Typical adult dose	Key points
Propofol	IV induction / TIVA	Induction 1.5–2.5 mg/kg; TIVA by TCI	Hypotension, pain on injection, antiemetic; propofol-infusion syndrome with prolonged high-dose
Ketamine	IV induction / analgesia	1–2 mg/kg IV (0.25–0.5 mg/kg analgesia)	Preserves BP & airway drive; bronchodilator; emergence phenomena; ↑ secretions
Etomidate	IV induction	0.3 mg/kg	Cardiostable; adrenal suppression, myoclonus
Thiopentone	IV induction (barbiturate)	3–5 mg/kg	Anticonvulsant; avoid in porphyria; never intra-arterial
Sevoflurane	Volatile (induction & maintenance)	MAC ~2.0%	Non-irritant → gas induction; fast on/off
Isoflurane / Desflurane	Volatile (maintenance)	MAC ~1.15% / ~6.0%	Des = fastest emergence but airway-irritant; not for gas induction
Nitrous oxide	Adjunct gas	50–70% (with O₂)	Analgesic; avoid in closed air spaces & prolonged use (B₁₂)
Fentanyl	Opioid (analgesia)	1–3 µg/kg	Blunts stimulation; respiratory depression
Rocuronium / Vecuronium	Non-depolarising relaxant	Roc 0.6 mg/kg (1.2 for RSI)	Reversed by sugammadex (roc/vec) or neostigmine + glycopyrrolate
Atracurium / Cisatracurium	Non-depolarising relaxant	Atr 0.5 mg/kg	Hofmann elimination — good in renal/hepatic failure; atracurium histamine release
Neostigmine + glycopyrrolate	Reversal	Neostigmine 50 µg/kg + glyco 10 µg/kg	Only reverses partial block; confirm TOF ratio >0.9

🗺 6 · Conduct of General Anaesthesia

From Sign-In to Recovery

Prepare & check (before induction)

Confirm PAC, consent, fasting, allergies; WHO checklist "sign-in"
Machine check, suction, drugs drawn & labelled, monitoring attached, difficult-airway kit available
Preoxygenate; IV access; baseline observations

Induction

IV (propofol etc.) or inhalational (sevoflurane) induction; opioid to obtund the pressor response
Secure the airway (SGA or tracheal tube ± relaxant); confirm with capnography
RSI if aspiration risk (see Airway page)

Maintenance

Volatile (end-tidal MAC guided) or TIVA; analgesia; relaxation as needed
Ventilate to normocapnia; titrate to surgical stimulus; maintain physiology (BP, temperature, fluids, glucose)
Continuous monitoring (see Monitoring)

Emergence

Stop agents; ensure full reversal of neuromuscular block (TOF >0.9); analgesia & antiemesis on board
Restore spontaneous ventilation; extubate awake (or deep per plan) with a strategy for the at-risk airway

Recovery / PACU

Hand over to trained recovery staff with monitoring; oxygen, analgesia, PONV control
Watch for airway obstruction, residual block, hypoventilation, hypothermia, delirium; discharge by criteria

⚠️ 7 · Common Mistakes

Common Mistakes in General Anaesthesia

❌ Mistake 1 — Stimulating the patient in stage II

Inserting an airway or moving the patient during the excitement phase provokes laryngospasm, coughing and vomiting. Move through stage II quickly and only instrument the airway at an adequate depth.

❌ Mistake 2 — Paralysis without adequate hypnosis (awareness)

A muscle relaxant hides the movement that would warn you a patient is light. Always ensure sufficient hypnotic (end-tidal agent or checked TIVA ± depth monitoring), especially during RSI, transfer and emergence.

❌ Mistake 3 — Skipping the machine check

Undetected circuit leaks, an empty vaporiser or exhausted soda-lime cause awareness, hypercapnia or hypoxia. The pre-use machine check and an available self-inflating bag are non-negotiable.

❌ Mistake 4 — Nitrous oxide in a closed air space

N₂O diffuses into air-filled cavities faster than nitrogen leaves — expanding a pneumothorax, obstructed bowel, or middle-ear/eye gas. Avoid it in these situations.

❌ Mistake 5 — Incomplete reversal of neuromuscular block

Residual block causes airway obstruction, hypoventilation and aspiration in recovery. Confirm a TOF ratio >0.9 (quantitatively) before extubation; use sugammadex for deep rocuronium block.

❌ Mistake 6 — Ignoring temperature

Inadvertent perioperative hypothermia increases wound infection, bleeding, cardiac events and drug duration. Warm actively and monitor core temperature for any case beyond the briefest.

❌ Mistake 7 — Under-treating the volatile-free MAC gap

During transfers or a vaporiser running dry, patients can become light. Bridge with an IV agent and keep an eye on end-tidal agent concentration continuously.

🎓 8 · Exam Pearls — DrNB / MD / EDAIC

Exam Pearls

Q: What is the anaesthetic triad?
Hypnosis (unconsciousness/amnesia), analgesia, and muscle relaxation — with control of autonomic/reflex responses. Balanced anaesthesia combines agents to achieve each while limiting side-effects.

Q: Define MAC and list what changes it.
The alveolar concentration preventing movement to a standard incision in 50% of subjects. ↓ by age, opioids, hypothermia, pregnancy, sedatives, acute alcohol; ↑ by infancy, hyperthermia, chronic alcohol/stimulants. MAC-awake ≈ 0.3–0.4 MAC; MAC-BAR ≈ 1.5 MAC.

Q: Which volatile for inhalational induction, and why?
Sevoflurane — sweet-smelling and non-irritant with low blood-gas solubility, giving a smooth, rapid gas induction (especially paediatrics).

Q: Describe Guedel's stages.
I analgesia; II excitement (risk of laryngospasm/vomiting — pass through quickly); III surgical anaesthesia (four planes); IV overdose (apnoea, cardiovascular collapse).

Q: What safety features prevent a hypoxic gas mixture?
Oxygen failure alarm, hypoxic guard (N₂O–O₂ ratio link keeping FiO₂ ≥25%), pin-index system, colour coding, non-return valves, and oxygen analyser/capnography monitoring.

Q: When is TIVA preferred over volatile maintenance?
PONV-prone patients, malignant-hyperthermia susceptibility, neurophysiological monitoring, shared/difficult airway or tubeless surgery, and to reduce environmental pollution.

Q: Which Mapleson system is most efficient for spontaneous vs controlled ventilation?
Mapleson A for spontaneous ventilation; Mapleson D (Bain) for controlled ventilation; Jackson–Rees (F) for paediatrics.

Q: What did NAP5 teach about awareness?
Incidence ~1:19,000; strongly associated with neuromuscular blockade, RSI, obstetric/cardiac cases, TIVA delivery failure, and the induction/emergence/transfer periods. Prevent with adequate dosing, end-tidal agent or depth monitoring, and vigilant TIVA checks.

📚 9 · References

References

Gropper MA, et al. (eds). Miller's Anesthesia, 9th Edition. Inhaled & Intravenous Anaesthetics; Basic Principles. Elsevier; 2020.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail's Clinical Anesthesiology, 6th Edition. Inhalation & Intravenous Anaesthetics; Breathing Systems; The Anaesthesia Machine. McGraw-Hill; 2018.
Barash PG, et al. (eds). Clinical Anesthesia, 9th Edition. Wolters Kluwer; 2023.
Pandit JJ, Andrade J, Bogod DG, et al. (NAP5). 5th National Audit Project on accidental awareness during general anaesthesia. Br J Anaesth. 2014;113:549–559.
Klein AA, Meek T, Allcock E, et al. (AAGBI). Recommendations for standards of monitoring during anaesthesia and recovery 2021. Anaesthesia. 2021;76:1212–1223.
Stoelting RK, Hillier SC. Pharmacology & Physiology in Anesthetic Practice, 6th Edition. Wolters Kluwer; 2021.