Introduction
Intravenous anaesthetic agents are used for induction and maintenance of general anaesthesia, as well as procedural sedation. Understanding their comparative pharmacology is essential for appropriate drug selection and safe patient management. The major classes include barbiturates, propofol, etomidate, ketamine, benzodiazepines, and alpha-2 agonists.
Mechanisms of Action
GABA-A Receptor Agonists
Most IV anaesthetics potentiate GABA-A receptors, the primary inhibitory neurotransmitter system in the CNS. GABA-A receptors are ligand-gated chloride channels composed of pentameric subunits (typically 2α, 2β, 1γ).
Propofol acts primarily at the β subunit, enhancing chloride conductance and producing hyperpolarisation. It also exhibits direct activation at higher concentrations.
Barbiturates (thiopentone) bind at distinct sites on the α and β subunits, prolonging channel opening time. At high concentrations, they can directly activate GABA-A receptors even without GABA present.
Etomidate binds between α and β subunits, enhancing GABA-mediated chloride influx with minimal direct activation.
Benzodiazepines (midazolam) bind at the α-γ interface, potentiating GABA effects but requiring GABA to be present (positive allosteric modulation).
NMDA Receptor Antagonists
Ketamine is a non-competitive antagonist at the NMDA receptor's phencyclidine binding site within the ion channel. This blocks glutamate-mediated excitatory neurotransmission. Ketamine also interacts with opioid receptors, monoaminergic systems, and muscarinic receptors, contributing to its unique clinical profile.
Alpha-2 Adrenoceptor Agonists
Dexmedetomidine is a highly selective α2-adrenoceptor agonist (α2:α1 selectivity ratio 1600:1). It acts primarily on α2A receptors in the locus coeruleus, producing sedation via reduced noradrenergic output. α2B receptors in the dorsal horn mediate analgesic effects.
Central Nervous System Effects
Consciousness and Depth of Anaesthesia
Propofol produces rapid loss of consciousness (one arm-brain circulation time, approximately 15-30 seconds) with smooth, reliable hypnosis. It acts on cortical and subcortical structures, causing dose-dependent unconsciousness. Recovery is rapid (5-10 minutes) due to redistribution and rapid clearance (30-60 minutes).
Thiopentone similarly produces rapid unconsciousness (10-20 seconds) but has a longer context-sensitive half-time due to saturable hepatic metabolism and extensive redistribution. Recovery after a single dose occurs within 5-15 minutes but is slower than propofol.
Etomidate causes loss of consciousness in one arm-brain circulation time with minimal cumulative effects due to rapid ester hydrolysis. Duration after single bolus is 3-12 minutes.
Ketamine produces "dissociative anaesthesia" characterised by catalepsy, amnesia, and analgesia while maintaining some protective reflexes. Onset is slower (60-90 seconds IV) with duration of 10-20 minutes for single bolus.
Midazolam produces anxiolysis and sedation in a dose-dependent manner but is less reliable for inducing unconsciousness, requiring doses of 0.2-0.3 mg/kg for anaesthesia induction. Onset is 1-2 minutes with duration of 15-30 minutes.
Dexmedetomidine produces cooperative sedation where patients are easily arousable. Onset is gradual (5-10 minutes) with peak effect at 15-25 minutes. It does not reliably produce unconsciousness even at high doses.
Cerebral Metabolic Rate and Blood Flow
Propofol reduces cerebral metabolic rate of oxygen consumption (CMRO₂) by up to 50-60%, coupled with decreased cerebral blood flow (CBF) and intracranial pressure (ICP). The reduction in CBF is proportional to reduced metabolic demand. Cerebral autoregulation is preserved but shifted to lower pressures.
Thiopentone similarly decreases CMRO₂ (up to 55%), CBF, and ICP. It provides maximal cerebral protection at burst suppression on EEG. The cerebrovascular response to CO₂ is maintained.
Etomidate reduces CMRO₂ by 35-45% with corresponding decreases in CBF and ICP. Of the GABA agonists, it causes the least cardiovascular compromise while still providing cerebral protection.
Ketamine uniquely increases CMRO₂, CBF, and potentially ICP by 25-60%. However, this effect may be attenuated by concurrent use of GABA-ergic agents or controlled ventilation. The increase in CBF exceeds the increase in metabolic demand.
Benzodiazepines modestly reduce CMRO₂ (approximately 25%), CBF, and ICP with preservation of autoregulation.
Dexmedetomidine causes minimal changes in CMRO₂ with modest reductions in CBF through central sympatholysis.
Seizure Activity
Propofol is anticonvulsant at anaesthetic doses but may cause excitatory phenomena (myoclonus, opisthotonos) during induction, which are not true seizures. It is used to treat status epilepticus.
Thiopentone is profoundly anticonvulsant, used historically for refractory status epilepticus at doses inducing burst suppression.
Etomidate may activate epileptiform activity on EEG, even in non-epileptic patients. Myoclonus occurs in 30-60% of patients but represents subcortical disinhibition rather than seizure activity.
Ketamine generally has anticonvulsant properties despite theoretical concerns about limbic excitation.
Benzodiazepines are potent anticonvulsants used for acute seizure management.
Dexmedetomidine has no significant effect on seizure threshold.
Nausea and Vomiting
Propofol has antiemetic properties at subhypnotic doses (10-20 mg), acting on area postrema and chemoreceptor trigger zone.
Thiopentone and etomidate have no inherent antiemetic properties.
Ketamine is highly emetogenic, particularly in the recovery phase, with incidence of 15-40% in adults (reduced by benzodiazepines).
Respiratory System Effects
Respiratory Drive and Pattern
Propofol causes dose-dependent respiratory depression through reduced hypoxic and hypercapnic ventilatory responses. Induction doses typically cause apnoea (30-90 seconds). The CO₂ response curve is shifted rightward and has reduced slope. Infusion doses for sedation (25-100 mcg/kg/min) still significantly depress respiratory drive.
Thiopentone causes similar respiratory depression with apnoea following induction doses. The duration of apnoea (30-90 seconds) is often longer than with propofol.
Etomidate causes less respiratory depression than other GABA agonists but still produces apnoea in 10-20% of patients after induction doses. The duration is typically brief.
Ketamine uniquely preserves respiratory drive at subaesthetic doses, with minimal depression of hypoxic and hypercapnic responses. Apnoea is rare even with induction doses, though respiratory depression can occur with rapid boluses or very high doses.
Benzodiazepines cause dose-dependent respiratory depression, particularly affecting hypoxic ventilatory response. Synergistic depression occurs when combined with opioids, which is clinically significant for conscious sedation.
Dexmedetomidine has minimal effect on respiratory drive, even at high doses, making it attractive for sedation without airway protection. The hypoxic ventilatory response is largely preserved.
Airway Effects
Propofol reduces upper airway tone, potentially worsening obstruction. It also suppresses laryngeal and cough reflexes, facilitating laryngeal mask airway insertion and fiberoptic intubation.
Thiopentone similarly reduces airway reflexes and tone.
Etomidate preserves airway reflexes better than other GABA agonists but can still cause laryngospasm, particularly in lightly anaesthetised patients.
Ketamine maintains airway reflexes and tone, though hypersalivation may pose an aspiration risk. Laryngospasm can occur, especially in children.
Benzodiazepines reduce upper airway muscle tone, potentially worsening obstructive sleep apnoea.
Dexmedetomidine preserves upper airway patency better than other sedatives through differential effects on airway dilator muscles versus diaphragm.
Bronchomotor Tone
Propofol causes bronchodilation through multiple mechanisms including direct smooth muscle relaxation, reduced vagal tone, and reduced sympathetic outflow. It attenuates bronchospasm associated with intubation.
Thiopentone may precipitate bronchospasm through histamine release (5-10% incidence), particularly with rapid bolus administration.
Etomidate has no significant bronchomotor effects but may cause histamine release.
Ketamine produces bronchodilation through direct smooth muscle relaxation and increased catecholamine levels. It is beneficial in asthmatic patients requiring intubation.
Benzodiazepines and dexmedetomidine have minimal direct bronchomotor effects.
Cardiovascular System Effects
Myocardial Contractility and Cardiac Output
Propofol causes significant myocardial depression through reduced calcium influx into myocytes and sensitisation to calcium. Cardiac output decreases by 15-30% at induction doses, proportional to plasma concentration. This effect is more pronounced in elderly patients and those with impaired cardiac function.
Thiopentone has negative inotropic effects but these are typically masked by compensatory tachycardia in healthy patients. Direct myocardial depression occurs at high concentrations. In patients with poor cardiac reserve, significant decreases in cardiac output may occur.
Etomidate has minimal direct myocardial depressant effects, making it the most cardiovascularly stable induction agent. Cardiac output is typically maintained or minimally reduced.
Ketamine maintains or increases cardiac output through central sympathetic stimulation, increasing heart rate, contractility, and systemic vascular resistance. However, direct myocardial depression occurs when sympathetic reserves are depleted (critical illness, beta-blockade), potentially unmasking shock.
Benzodiazepines cause minimal direct myocardial depression. Midazolam may reduce cardiac output by 10-20% through vasodilation and reduced sympathetic tone.
Dexmedetomidine reduces cardiac output by 10-30% through decreased heart rate (baroreceptor-mediated bradycardia and reduced sympathetic outflow) despite maintained or slightly increased stroke volume.
Heart Rate
Propofol typically causes bradycardia through reduced sympathetic tone and possibly vagal predominance, particularly when combined with opioids. Heart rate decreases by 10-25%.
Thiopentone increases heart rate by 10-20% through baroreceptor-mediated reflex tachycardia responding to decreased blood pressure.
Etomidate typically maintains heart rate with minimal change.
Ketamine increases heart rate by 15-30% through central sympathetic stimulation and inhibition of noradrenaline reuptake.
Benzodiazepines have minimal effect on heart rate.
Dexmedetomidine causes bradycardia (20-30% reduction) through central sympatholysis and baroreceptor activation. Profound bradycardia and sinus arrest can occur, particularly with rapid administration or high doses.
Systemic Vascular Resistance and Blood Pressure
Propofol reduces systemic vascular resistance (SVR) by 15-25% through direct vasodilation (sympathetic inhibition and endothelium-mediated mechanisms). Mean arterial pressure typically decreases by 25-40% after induction, combining reduced SVR and cardiac output.
Thiopentone reduces SVR through direct vasodilation and central sympatholysis. Blood pressure decreases by 10-30%, partially offset by compensatory tachycardia.
Etomidate maintains SVR with minimal blood pressure changes (<10% reduction typically), making it suitable for haemodynamically unstable patients.
Ketamine increases SVR by 20-40% through sympathetic stimulation. Blood pressure increases by 20-30%, though this may not occur in catecholamine-depleted states.
Benzodiazepines cause modest decreases in SVR (10-20%) through reduced sympathetic tone. Blood pressure changes are typically minor in healthy patients.
Dexmedetomidine initially may increase SVR through peripheral α2B receptor activation, causing transient hypertension, followed by decreased SVR through central sympatholysis. Overall, blood pressure may decrease by 10-30%.
Coronary Circulation
Propofol reduces myocardial oxygen consumption proportionally more than it reduces blood pressure, potentially improving myocardial oxygen balance. Coronary perfusion pressure may decrease with significant hypotension.
Thiopentone similarly reduces myocardial oxygen demand but hypotension may compromise coronary perfusion in coronary artery disease.
Etomidate maintains coronary perfusion pressure through cardiovascular stability while reducing oxygen consumption.
Ketamine increases myocardial oxygen consumption through increased heart rate, contractility, and blood pressure. This may be detrimental in patients with coronary artery disease, though evidence is mixed.
Dexmedetomidine reduces myocardial oxygen consumption through bradycardia and reduced sympathetic tone, potentially beneficial in coronary disease.
Special Considerations
Pain on Injection
Propofol causes pain on injection in 30-70% of patients due to venous irritation. This is reduced by prior lignocaine, larger veins, or lipid formulation modifications.
Etomidate causes pain in up to 50% of patients, worse with aqueous formulations.
Thiopentone as an alkaline solution (pH 10-11) causes severe pain and tissue necrosis if extravasated. Intra-arterial injection causes severe arterial spasm and potential limb loss.
Ketamine, benzodiazepines, and dexmedetomidine typically do not cause injection pain.
Adrenocortical Function
Etomidate inhibits 11β-hydroxylase enzyme in the adrenal cortex, blocking cortisol and aldosterone synthesis. Even a single induction dose reduces cortisol production for 4-8 hours. Continuous infusions cause prolonged adrenal suppression and are contraindicated.
Other agents do not significantly affect adrenocortical function.
Clinical Relevance
Agent Selection
Propofol is the most commonly used induction agent due to rapid onset, predictable recovery, antiemetic properties, and smooth emergence. It is ideal for day surgery and total intravenous anaesthesia (TIVA). Caution is required in haemodynamically unstable patients, elderly, and those with cardiovascular disease due to significant hypotension.
Thiopentone is now rarely used due to propofol's superiority but remains relevant for rapid sequence induction in haemodynamically stable patients and neuroprotection. Contraindications include porphyria, severe cardiovascular disease, and airway obstruction.
Etomidate is reserved for haemodynamically unstable patients requiring general anaesthesia (trauma, cardiac disease, shock). Single doses for induction are acceptable, but concerns about adrenal suppression limit repeated dosing.
Ketamine is invaluable for patients requiring haemodynamic stability (shock, tamponade, constrictive pericarditis) and procedures requiring analgesia. It is the agent of choice in resource-limited settings. Cautions include elevated ICP, ischaemic heart disease, and psychiatric disorders. Combining with benzodiazepines reduces emergence phenomena.
Midazolam is used primarily for anxiolysis and conscious sedation rather than induction. Its reversibility with flumazenam (though rarely used clinically) and cooperative sedation properties are useful for awake procedures.
Dexmedetomidine excels for sedation requiring preserved respiratory drive (awake fiberoptic intubation, ICU sedation, regional anaesthesia supplementation). The major limitation is bradycardia and slow onset. Loading doses should be given slowly (>10 minutes) to avoid initial hypertension.
Drug Combinations
Combining agents exploits synergistic effects while reducing doses and side effects. Propofol-remifentanil combinations for TIVA provide stable depth, though both cause profound respiratory depression and hypotension. Ketamine-propofol combinations ("ketofol") balance cardiovascular stability with smooth hypnosis. Benzodiazepine premedication reduces ketamine emergence reactions and overall anaesthetic requirements.
Context-Sensitive Half-Time
Understanding duration of action after infusions is crucial. Propofol maintains rapid recovery even after prolonged infusions due to extensive extravascular redistribution and high clearance. Thiopentone's context-sensitive half-time increases dramatically with duration, making it unsuitable for maintenance. Dexmedetomidine has a relatively short context-sensitive half-time (approximately 250 minutes after 8-hour infusion), allowing predictable emergence.
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