Intravenous Induction Agents

Intravenous (IV) induction agents are drugs that produce rapid loss of consciousness (within one arm–brain circulation time, ~15–30 s) when given as a bolus. They are the workhorses of "going to sleep" in modern anaesthesia and a perennial NEET PG favourite — expect questions on the ideal agent, propofol infusion syndrome, ketamine's dissociation, and etomidate's adrenal suppression.

Concept of the ideal IV induction agent

The "ideal" agent is a theoretical construct used to compare real drugs. Knowing the list helps you answer "which agent is closest to ideal" questions.

High-yield: No single agent is ideal. Propofol is closest to the ideal because of rapid smooth induction and rapid clear-headed recovery (good for day-care surgery), but it lacks analgesia and causes cardiovascular depression.

Properties desired: water-soluble, stable in solution, painless on injection, rapid onset (one arm–brain circulation), rapid metabolism to inactive products, cardiovascular & respiratory stability, no histamine release/anaphylaxis, no emetic effect, anti-convulsant, no adrenal suppression, no pain on injection, cheap.

Classification

IV induction agents are grouped by chemistry:

Class	Drug	Key feature
Barbiturate	Thiopentone (thiopental)	Classic, anti-convulsant, no analgesia
Alkylphenol	Propofol	Antiemetic, fastest recovery, infusion syndrome
Phencyclidine derivative	Ketamine	Dissociative, only one with analgesia + CVS stimulation
Imidazole	Etomidate	Most CVS-stable, adrenal suppression
Benzodiazepine	Midazolam	Slow onset, amnesia, has reversal (flumazenil)

A useful mechanistic split:

• GABA-A potentiators (most agents): thiopentone, propofol, etomidate, midazolam → enhance inhibitory chloride currents.
• NMDA antagonist: ketamine → blocks excitatory glutamate at the NMDA receptor (dissociative).

Thiopentone (Thiopental sodium)

A thiobarbiturate; the historical gold standard.

• Presentation/dose: 2.5% solution (yellow), pH ~10.5 (strongly alkaline → tissue necrosis if extravasated). Induction dose 3–5 mg/kg IV.
• Onset/duration: Onset 15–30 s; awakening in 5–10 min due to redistribution from brain (vessel-rich group) to muscle, not metabolism. Slow hepatic metabolism → cumulative with repeat dosing.
• CNS: Potent anti-convulsant; reduces CMRO₂, cerebral blood flow and ICP → useful in neuroanaesthesia and status epilepticus. Causes hyperalgesia (anti-analgesic) at sub-anaesthetic doses.
• CVS: Dose-dependent myocardial depression and venodilation → hypotension.
• Respiratory: Apnoea, laryngospasm risk if airway stimulated under light planes.

High-yield: Intra-arterial thiopentone → intense vasospasm, crystal formation, thrombosis and gangrene. Management: leave the cannula in, dilute with saline, inject intra-arterial papaverine / lignocaine, sympathetic block (stellate ganglion/brachial plexus), heparinise.

Absolute contraindications — mnemonic "PASS":

• Porphyria (acute intermittent) — thiobarbiturates induce ALA synthase, precipitating an attack. Most classic NEET fact.
• Airway obstruction / unstable airway
• Status asthmaticus (bronchospasm)
• Severe hypovolaemia / fixed cardiac output (e.g. tight mitral stenosis, constrictive pericarditis)

Propofol (2,6-di-isopropylphenol)

The most widely used induction agent today; a milky-white lipid emulsion (soybean oil, egg lecithin, glycerol).

• Dose: Induction 1.5–2.5 mg/kg; maintenance infusion 100–200 µg/kg/min (TIVA — total intravenous anaesthesia, often via target-controlled infusion).
• Onset/recovery: Onset ~30 s; recovery fastest of all agents (rapid metabolism + redistribution) → ideal for day-care/ambulatory surgery.
• CNS: Reduces CMRO₂, CBF, ICP. Anti-convulsant overall but may cause excitatory movements/myoclonus on induction.
• CVS: Marked fall in BP (vasodilation + myocardial depression) with no compensatory tachycardia (resets baroreflex) — drops BP more than thiopentone.
• Antiemetic: Strongly antiemetic (sub-hypnotic doses treat PONV) — a favourite distinguishing fact.
• Other: Pain on injection (reduce with lignocaine, large vein); supports bacterial growth (discard after ~6 h); contains egg/soy → caution, though egg allergy to egg white rarely matters as lecithin is from yolk.

High-yield: Propofol Infusion Syndrome (PRIS) — high-dose (>4 mg/kg/h) prolonged (>48 h) infusion, classically in critically ill/paediatric ICU patients. Features: metabolic acidosis, rhabdomyolysis, hyperkalaemia, lipaemia, cardiac failure/arrhythmia, acute renal failure. Mechanism: impaired mitochondrial fatty-acid oxidation. Management: stop propofol, supportive care, haemodialysis/ECMO.

High-yield: Propofol is the agent of choice for induction in day-care surgery and for sedation in the ICU (short context-sensitive half-time).

Ketamine

A phencyclidine derivative and the unique outlier — an NMDA receptor antagonist producing dissociative anaesthesia (functional dissociation between thalamo-neocortical and limbic systems; patient appears awake — eyes open, nystagmus — but is dissociated and analgesic).

• Dose: IV 1–2 mg/kg; IM 5–10 mg/kg (the only induction agent reliably given IM → useful in children, field/disaster, uncooperative patients).
• Analgesia: The only IV induction agent with potent analgesia (especially somatic pain).
• CVS: Sympathomimetic — raises BP, HR, cardiac output (inhibits catecholamine reuptake) → agent of choice in hypovolaemic shock / trauma / haemorrhage and in cardiac tamponade/constrictive pericarditis where preserving preload and rate matters.
• Respiratory: Bronchodilator (good for asthma/status asthmaticus); preserves airway reflexes and respiratory drive relatively well; increases secretions (give an antisialagogue — glycopyrrolate).
• Adverse: Emergence delirium / hallucinations / vivid dreams (reduced by benzodiazepines, esp. midazolam, and dim quiet recovery). Raises ICP, IOP and CBF/CMRO₂.

High-yield: Ketamine is the induction agent of choice in shock/hypovolaemia and acute severe asthma (bronchospasm). It is contraindicated in raised ICP (head injury — debated/relative now), raised IOP (open eye injury, glaucoma), uncontrolled hypertension, ischaemic heart disease, aortic dissection, and psychiatric disease.

High-yield: Ketamine is levorotatory S(+) isomer = more potent, fewer psychomimetic effects than the racemate. Esketamine (S-ketamine) nasal spray is approved for treatment-resistant depression.

Etomidate

A carboxylated imidazole — the most cardiovascularly stable induction agent.

• Dose: 0.2–0.3 mg/kg IV.
• CVS: Minimal effect on BP, HR, CO → agent of choice in haemodynamically unstable / cardiac-compromised patients (ischaemic heart disease, cardiomyopathy, aortic/mitral stenosis, shock when CVS depression must be avoided).
• CNS: Reduces CMRO₂, CBF, ICP while maintaining cerebral perfusion pressure (CVS stability).
• Adverse: Pain on injection, myoclonus, high PONV, thrombophlebitis.

High-yield: Adrenocortical suppression — etomidate inhibits 11-β-hydroxylase (and 17-α-hydroxylase), blocking cortisol/aldosterone synthesis. Even a single induction dose suppresses the adrenal axis for ~24 h. Continuous infusion is contraindicated and it is avoided in sepsis/critically ill patients (increased mortality). Classic NEET enzyme = 11-β-hydroxylase.

Midazolam

A water-soluble benzodiazepine (imidazole ring opens at physiological pH).

• Dose: Sedation 0.02–0.1 mg/kg; induction 0.1–0.3 mg/kg (slow, unreliable for induction → rarely used alone).
• Effects: Anxiolysis, anterograde amnesia (most-tested premedication benefit), sedation, anticonvulsant; minimal CVS effect; respiratory depression synergistic with opioids.
• Onset: Slow for an "induction" agent; best used for premedication, co-induction and procedural sedation.
• Reversal: Flumazenil (competitive antagonist) — the only induction-class agent with a specific antagonist; short-acting, so re-sedation can occur.

High-yield: Midazolam causes profound anterograde amnesia and is the benzodiazepine of choice for premedication and conscious sedation. Flumazenil reverses it.

Comparative pharmacology (the master table)

Agent	Dose (IV)	CVS	Respiratory	Analgesia	ICP/IOP	Signature adverse effect
Thiopentone	3–5 mg/kg	↓ BP	Apnoea, ↑laryngospasm	None (anti-analgesic)	↓	Porphyria, intra-arterial gangrene
Propofol	1.5–2.5 mg/kg	↓↓ BP (no reflex tachy)	Apnoea	None	↓	PRIS, pain on injection
Ketamine	1–2 mg/kg	↑ BP/HR/CO	Bronchodilation, preserved	Strong	↑	Emergence delirium, secretions
Etomidate	0.2–0.3 mg/kg	Stable	Mild	None	↓	Adrenal suppression, myoclonus
Midazolam	0.1–0.3 mg/kg	Minimal ↓	Mild (synergy w/ opioids)	None	↓	Slow onset; reversible (flumazenil)

Effect on cerebral and ocular parameters

Parameter	Most agents (barbiturate/propofol/etomidate)	Ketamine
Cerebral blood flow	↓	↑
CMRO₂	↓	↑ (variable)
ICP	↓	↑
IOP	↓	↑
BP	↓ (etomidate stable)	↑

Stepwise approach — choosing an agent (flow)

Identify the dominant clinical problem, then match:

1. Haemodynamically unstable / cardiac disease → Etomidate (most CVS stable). If trauma with hypovolaemia → Ketamine.
2. Hypovolaemic shock / trauma / acute asthma → Ketamine (sympathomimetic + bronchodilator).
3. Day-care / ambulatory / PONV-prone → Propofol (rapid clear recovery + antiemetic).
4. Status epilepticus / neuroprotection where BP tolerable → Thiopentone or propofol (anticonvulsant, ↓ICP).
5. Acute intermittent porphyria → avoid thiopentone; propofol is safe.
6. Sepsis / critically ill needing infusion sedation → avoid etomidate (adrenal); avoid prolonged high-dose propofol (PRIS).

Clinical scenario → drug: Trauma + hypotension + bronchospasm → Ketamine. Stable patient for short day-care procedure → Propofol. Patient with severe aortic stenosis → Etomidate. Known porphyria → Propofol (never thiopentone).

Complications & important pitfalls

• Anaphylaxis/histamine release: thiopentone and (rarely) propofol; ketamine and etomidate cause least histamine release.
• Pain on injection: propofol and etomidate (mitigate with lignocaine, antecubital vein).
• Tissue injury: thiopentone extravasation/intra-arterial → necrosis/gangrene.
• Emergence phenomena: ketamine (pre-treat with midazolam).
• Adrenal crisis risk: etomidate, especially infusion/sepsis.
• Metabolic catastrophe: propofol → PRIS.
• Respiratory depression/apnoea: all (least with ketamine).

Key differentials & "spot the difference" facts

• Analgesia present? → only ketamine.
• Antiemetic? → propofol (others neutral/pro-emetic; etomidate ↑PONV).
• Raises ICP/IOP/BP? → ketamine (opposite of the rest).
• Specific antagonist exists? → midazolam (flumazenil).
• Inhibits 11-β-hydroxylase? → etomidate.
• Precipitates porphyria? → thiopentone.
• Recovery by redistribution, not metabolism? → thiopentone (classic single-dose pharmacokinetics).
• Fastest recovery, best for TIVA? → propofol.

Recently asked / exam angle

• Mechanism MCQs: "Which IV agent acts on NMDA receptor?" → Ketamine. "GABA-A potentiator that suppresses adrenal cortex?" → Etomidate.
• Enzyme inhibited by etomidate → 11-β-hydroxylase (image/one-liner).
• Propofol infusion syndrome features (metabolic acidosis + rhabdomyolysis + cardiac failure) — recurring stem in critical-care questions.
• Drug of choice in bronchial asthma / acute severe asthma induction → Ketamine.
• Induction agent in shock/trauma → Ketamine; in cardiac patient with stable haemodynamics needed → Etomidate.
• Contraindicated in porphyria → Thiopentone.
• Most CVS-stable induction agent → Etomidate.
• Agent causing intense pain on injection + antiemetic → Propofol.
• Agent with anterograde amnesia used as premedication → Midazolam; reversal → Flumazenil.
• pH of thiopentone solution (~10.5, alkaline) and 2.5% concentration — value-based MCQ.
• Dissociative anaesthesia / eyes-open with nystagmus → Ketamine.
• Assertion–Reason on ketamine raising ICP (avoid in head injury) vs newer evidence it may be acceptable with controlled ventilation — read stems carefully.

Mnemonics

• Thiopentone contraindications "PASS": Porphyria, Airway obstruction, Status asthmaticus, Severe hypovolaemia/fixed CO.
• Etomidate = "E for Endocrine" (adrenal suppression) and "E for ECG-stable" (CVS stability).
• Ketamine "BPS" — Bronchodilation, Pressor (↑BP/HR), Secretions↑ (give glycopyrrolate), plus Psychomimetic emergence.
• Propofol "P's": Painful injection, PONV-prevention (antiemetic), Profound hypotension, PRIS.

Rapid revision

1. Propofol is closest to the ideal induction agent; fastest recovery; agent of choice for day-care surgery and TIVA.
2. Thiopentone awakening is due to redistribution, not metabolism; dose 3–5 mg/kg, 2.5% solution.
3. Thiopentone is contraindicated in acute intermittent porphyria (induces ALA synthase).
4. Intra-arterial thiopentone → vasospasm/gangrene; treat with papaverine/lignocaine + sympathetic block.
5. Propofol is antiemetic and causes pain on injection with no reflex tachycardia during hypotension.
6. Propofol infusion syndrome = metabolic acidosis + rhabdomyolysis + cardiac failure + hyperkalaemia (high-dose, prolonged).
7. Ketamine is the only IV induction agent with analgesia; an NMDA antagonist causing dissociative anaesthesia.
8. Ketamine is sympathomimetic (↑BP/HR) → DOC in shock/trauma; bronchodilator → DOC in asthma; can be given IM.
9. Ketamine raises ICP, IOP and BP and causes emergence delirium (prevent with midazolam).
10. Etomidate is the most CVS-stable agent but inhibits 11-β-hydroxylase → adrenal suppression; avoid infusion and in sepsis.
11. Midazolam → anterograde amnesia, premedication of choice; reversed by flumazenil.
12. CVS depression order roughly: Propofol > Thiopentone > Etomidate ≈ stable; Ketamine raises BP.