An intravenous induction agent has to abolish consciousness in the time it takes blood to travel from an arm to a brain. Everything that makes that possible — the lipid solubility, the receptor density, the bolus delivery — also guarantees that the drug leaves the brain again within minutes, and by moving into muscle rather than by being destroyed. Almost every distinctive feature of these six drugs follows from that one arrangement: why the patient wakes with the whole dose still inside them, why a second bolus lasts longer, why one agent can be infused for eight hours and another cannot, and why a shocked patient needs half of what the textbook says.
Orientation
What this module covers, and how to work through it
This module covers one line of the MMed syllabus — intravenous anaesthetic agents — which is one line of syllabus and four sittings of reading. It runs from the molecular target outwards: what these drugs bind and what decides how fast they act, then each agent in turn, then the argument that puts all six into one clinical decision.
Lesson 1 settles the material every other lesson applies: three molecular targets in mechanistic detail, what each ring and substitution buys, why the formulation is pharmacology rather than packaging, and the front-end kinetics — effect-site equilibration, redistribution and context-sensitive half-time — that actually decide an induction. Lesson 2 takes propofol on its own, because it is the agent every other one is now compared against and because its clearance exceeds hepatic blood flow, which is the fact from which total intravenous anaesthesia follows. Lesson 3 takes thiopentone and etomidate together, because each is one physicochemical property carried all the way to the bedside. Lesson 4 covers ketamine, which produces a different state of unconsciousness altogether, and the two agents given alongside an induction agent rather than instead of one — and then reasons a choice of agent from mechanism across ten clinical situations.
Propofol comes second rather than third, which is not where every textbook puts it. It is placed there because the kinetics lesson 1 has just finished teaching are exactly the ones that make propofol work, and because every subsequent agent in the module is described by what it does differently from propofol.
Prerequisites for the module as a whole
These carry material this module builds on directly and does not repeat. Work through them first if they are not already secure:
- Compartment models: the one-, two- and three-compartment models and the BET infusion scheme. This module applies them to six specific drugs and does not re-derive them.
- The effect site and k(e0): hysteresis, the effect-site compartment and the rate constant that describes it. Lesson 1 uses all three from the first section.
- Stereoselectivity: why a single enantiomer can behave differently from its mirror image. Named here for ketamine and etomidate, explained there.
- Protein binding and volume of distribution: the free fraction, and what happens to it when albumin falls. Lesson 3 turns that into arithmetic for thiopentone.
Syllabus
MMed syllabus mapping
| Syllabus area | Lessons | What is covered |
|---|---|---|
| Anaesthetic drugs — intravenous anaesthetic agents: principles | 1 | The definition and the ideal agent; classification by chemistry and by molecular target; the GABA-A receptor, the NMDA channel and the α2A adrenoceptor; structure–activity relationships; formulation, solvent and incompatibility; arm–brain circulation time, k(e0) and the plasma–effect-site lag; redistribution; context-sensitive half-time; the determinants of induction dose |
| Anaesthetic drugs — intravenous anaesthetic agents: the agents | 2–4 | Propofol in full, with total intravenous anaesthesia and target-controlled infusion; thiopentone's tautomerism, ionisation, saturable metabolism, intra-arterial injection and porphyria; etomidate's haemodynamic profile and adrenocortical suppression; ketamine, dissociative anaesthesia and its isomers; midazolam in co-induction; dexmedetomidine and the locus coeruleus |
| Anaesthetic drugs — choice of agent | 4 | A choice of induction agent reasoned from mechanism in the shocked, fixed-cardiac-output, asthmatic, raised-intracranial-pressure, obstetric, day-case, epileptic, porphyric, malignant-hyperthermia-susceptible and difficult-airway patient |
The sedatives and premedication objective is not settled by this module. Midazolam and dexmedetomidine appear here as intravenous agents; the indication-based grouping of oral premedication, the oral route, the antisialagogue and ward sedation stay with that objective and are not taught here. Remifentanil, and the opioid half of a total intravenous technique, likewise belong to the analgesics objective — lesson 2 states the synergy and gives the maintenance target pair, because a section on the technique without the opioid is not a technique, and stops there.
The pathway
All 4 lessons, in order
Part I
What decides an induction
Three molecular targets, what the chemistry and the bottle contribute, and the front-end kinetics that decide how fast and how deep.
- What they act on, and what decides how fast
The three molecular targets in mechanistic detail, what each ring and substitution buys, why formulation is pharmacology rather than trivia, and the front-end kinetics that actually decide an induction.
Part II
The agents
Propofol on its own, then the two kept for specific indications, then the dissociative agent and the two given alongside — ending with the choice between all six.
- Propofol, TIVA and target-controlled infusion
The agent every other one is now compared against: the emulsion and what it costs, a clearance that exceeds hepatic blood flow, the mechanism behind the pressure fall, and the two things that make total intravenous anaesthesia possible and set its ceiling.
- The barbiturate and the imidazole
Two agents kept for specific indications, and two physicochemical properties carried all the way to the bedside: tautomerism, pKa and protein binding for thiopentone, and inhibition of a single adrenal enzyme for etomidate.
- Ketamine, midazolam, dexmedetomidine, and choosing
The agent that produces a different state of unconsciousness altogether, the two that are given alongside rather than instead, and the argument that puts all six drugs into one clinical decision.
Objectives
The full objective wording
- Principles and mechanismsIntravenous induction agents: targets, structure and front-end kinetics
- PropofolPropofol, total intravenous anaesthesia and target-controlled infusion
- Thiopentone and etomidateThiopentone and etomidate
- Ketamine and the sedativesKetamine, midazolam, dexmedetomidine, and choice of agent