General Anaesthetics: Putting the Brain to Sleep
"Count backwards from a hundred." By ninety-seven, the patient is unconscious, and a surgeon can open their body without them feeling a thing. General anaesthesia is one of medicine's everyday miracles — and one of its deepest mysteries, since we still don't fully know how it erases consciousness. But we know the drugs cold: what puts you under in seconds, what keeps you there, and the rare, lethal reaction the anaesthetist is always watching for.
In the operating theatre, the anaesthetist injects a milky-white drug into the vein and asks the patient to count. "One hundred… ninety-nine… ninety…" and they're gone — unconscious in a single arm-to-brain circulation. For the next two hours a controlled cocktail keeps them asleep, pain-free, and still, while a machine breathes for them. Then the drugs are switched off and they wake in recovery, the surgery a blank. That whole journey is built from a handful of drugs, each with a specific job — and understanding them means understanding what 'anaesthesia' actually is.
What anaesthesia actually requires
General anaesthesia is really several goals at once. 'Being put under' bundles four separate needs: unconsciousness, amnesia (no memory of the operation), analgesia (no pain), and immobility (no movement or reflex response). No single drug does all four well, so modern anaesthesia is 'balanced' — a combination: a fast intravenous drug to induce sleep, an inhaled agent to maintain it, an opioid for pain, and often a neuromuscular blocker for immobility (the paralytics from the cholinergic article — which is why 'paralysis is not anaesthesia' matters so much). Broadly, the anaesthetics themselves fall into two families: intravenous and inhaled.
Intravenous anaesthetics: the induction drugs
IV drugs put you to sleep fast — usually within one circulation. Propofol is the workhorse: a rapid, smooth induction with an anti-nausea bonus, but it lowers blood pressure, stings on injection, and provides no pain relief. Thiopental, an ultra-short barbiturate, is the classic example of redistribution from the pharmacokinetics series — it works in seconds and wears off as it moves from brain to fat. Etomidate is gentle on the heart (good for unstable patients) but can suppress the adrenal glands. And ketamine is the odd one out: it blocks NMDA glutamate receptors to produce 'dissociative' anaesthesia — it actually PROVIDES pain relief, keeps blood pressure up, and preserves breathing and airway reflexes, making it valuable in shock, in asthma (it dilates airways), and in children — though it can cause vivid emergence hallucinations.
Inhaled anaesthetics and MAC
Once asleep, patients are usually kept under with inhaled anaesthetics — volatile liquids like sevoflurane, isoflurane, and desflurane, and the gas nitrous oxide. They work by broadly enhancing inhibition (GABA and glycine) and blunting excitation across the brain. Their potency is measured by MAC (minimum alveolar concentration) — the amount needed to keep half of patients still to a surgical stimulus; a LOW MAC means a POTENT drug. A great advantage is control: because they're breathed in and out through the lungs (recall the lungs as a route of excretion), the anaesthetist can deepen or lighten anaesthesia moment to moment and wake the patient by simply letting them breathe the drug off. Nitrous oxide is a weak anaesthetic but a good analgesic, often used to supplement the others.
- Anaesthesia needs 4 things: unconsciousness, amnesia, analgesia, immobility — hence 'balanced' drug combos.
- IV drugs induce fast: propofol (common), thiopental (redistribution), etomidate (cardiac-stable), ketamine (dissociative).
- Ketamine (NMDA blocker) uniquely provides analgesia and preserves blood pressure & airway.
- Inhaled agents maintain anaesthesia; potency = MAC (low MAC = potent); easy to titrate via the lungs.
The dreaded reaction is malignant hyperthermia. In genetically susceptible people, volatile anaesthetics (and the paralytic succinylcholine) can trigger a runaway rise in body temperature with muscle rigidity, a racing heart, and dangerous metabolic chaos — potentially fatal within minutes. The anaesthetist watches for a rising end-tidal CO2 and temperature as early warnings, stops the trigger, cools the patient, and gives dantrolene (which stops the muscle from releasing calcium). It's the single most important emergency to recognize in the anaesthetic room.
- Assuming one drug does everything. Anaesthesia is a balanced combination of several drugs.
- Thinking a low MAC means a weak drug. Low MAC = MORE potent.
- Forgetting propofol gives no analgesia — pain relief needs a separate drug (an opioid).
- Missing early malignant hyperthermia (rising CO2 & temperature). Give dantrolene fast.
A drug has a low MAC. What does that tell you?
- Anaesthesia = unconsciousness + amnesia + analgesia + immobility, achieved by a balanced drug combo.
- IV: propofol (common), thiopental (redistribution), etomidate, ketamine (dissociative, keeps BP/airway).
- Inhaled agents maintain anaesthesia; potency = MAC (low = potent), easily titrated via the lungs.
- Malignant hyperthermia (volatiles + succinylcholine) is the lethal emergency — treat with dantrolene.
- Katzung BG. Basic & Clinical Pharmacology — General Anesthetics.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — General anesthetics.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — General anaesthetic agents.
- Miller RD. Miller's Anesthesia — Intravenous & inhaled anaesthetics; MAC; malignant hyperthermia.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Anesthetics.
