General Anesthetics — Inhaled and IV

General anesthetics are a diverse class of drugs that produce reversible unconsciousness, amnesia, analgesia, and muscle relaxation sufficient for surgical procedures. These agents work through multiple mechanisms including enhancement of inhibitory GABAergic neurotransmission and/or antagonism of excitatory NMDA receptors, fundamentally altering cerebral electrophysiology to produce the anesthetic state. Both inhaled volatile anesthetics and intravenous agents are essential components of modern anesthesia practice, with selection based on patient factors, operative characteristics, and institutional preferences. Understanding the pharmacodynamics and pharmacokinetics of these agents is critical for safe perioperative management, as improper dosing or monitoring can result in awareness under anesthesia, hemodynamic instability, or severe complications such as malignant hyperthermia. This topic is high-yield for board examinations due to frequent testing of mechanism of action, adverse effects, relative contraindications, and clinical decision-making in anesthetic selection.

The majority of general anesthetics—including all volatile anesthetics and most IV agents—enhance inhibitory signaling through GABA_A receptors, which are pentameric chloride channels distributed throughout the CNS. At clinically relevant concentrations, anesthetics increase the frequency of chloride channel opening (positive allosteric modulation), resulting in hyperpolarization of neuronal membranes and suppression of action potential generation. This occurs through direct binding to allosteric sites on the GABA_A receptor complex, distinct from the benzodiazepine binding site but sharing similar regional specificity. The resulting increase in membrane potential and decreased neuronal excitability produces dose-dependent depression of consciousness, with volatile anesthetics demonstrating greater potency at GABA_A receptors in the thalamus and cortex compared to spinal cord neurons initially, though this selectivity diminishes at higher concentrations.

Ketamine and nitrous oxide (N₂O) produce anesthesia through non-competitive antagonism of the N-methyl-D-aspartate (NMDA) receptor, an ionotropic glutamate receptor channel permeable to calcium and sodium. These agents block the ion channel pore in a voltage-dependent manner, preventing calcium influx and excitatory neurotransmission despite glutamate binding to the receptor. This mechanism differs fundamentally from GABA_A enhancement, accounting for ketamine's unique pharmacodynamic profile: preservation of airway reflexes, spontaneous ventilation, and analgesia despite dissociative side effects. The NMDA antagonism occurs at the phencyclidine binding site within the channel pore; ketamine binds more tightly (lower off-rate) than N₂O, explaining its greater potency and longer duration of action.

General anesthesia is characterized by specific patterns of electroencephalographic (EEG) activity that correlate with anesthetic depth. At induction, the EEG demonstrates progressive slowing and increase in amplitude, with burst suppression occurring at deeper levels of anesthesia. The thalamocortical system, particularly thalamic reticular nuclei projecting to cortical layers, represents a critical site of anesthetic action; disruption of thalamic oscillatory activity impairs sensory relay to the cortex and prevents arousal. Volatile anesthetics and IV agents produce this effect through convergent pathways despite distinct molecular targets, explaining why different anesthetics produce similar endpoint effects despite mechanistic differences.

The blood-gas partition coefficient determines the rate at which an inhaled anesthetic reaches equilibrium between alveolar gas and blood, directly affecting speed of induction and emergence. Agents with low blood-gas solubility (e.g., desflurane, blood-gas coefficient = 0.42) reach brain equilibrium rapidly, producing fast induction (minutes) and rapid emergence allowing quick recovery of consciousness. Conversely, agents with higher solubility (e.g., methoxyflurane, blood-gas coefficient = 0.97) achieve slower equilibration, prolonging both induction and emergence. The brain-blood partition coefficient is similarly low for volatile anesthetics, meaning anesthetic effect closely tracks blood concentration due to rapid CNS equilibration.

MAC represents the alveolar concentration of an anesthetic at 1 atmosphere of pressure at which 50% of patients do not move in response to a standard surgical incision. MAC serves as the primary measure of anesthetic potency and varies inversely with lipophilicity—more lipophilic agents require lower MAC values. MAC is additive between agents; for example, 0.5 MAC of one volatile agent combined with 0.5 MAC of another produces approximately 1.0 MAC, enabling multiagent combinations. MAC increases in hyperthermia, chronic alcohol use, and certain genetic polymorphisms (linked to altered GABA_A receptor function), and decreases with advancing age (approximately 1-2% per decade after age 40), pregnancy, and use of opioids or other CNS depressants.

Inhaled anesthetics are absorbed through alveolar epithelium based on the alveolar-to-venous partial pressure gradient. Uptake is governed by three factors: alveolar ventilation (VA), cardiac output (CO), and the blood-gas partition coefficient. In the initial phase (0-10 minutes), uptake is predominantly limited by cardiac output; as blood becomes saturated, ventilation becomes rate-limiting. The rapid uptake phase explains why volatile anesthetics reach therapeutic concentrations quickly when cardiac output is adequate. Regional blood flow distribution means highly perfused tissues (brain, heart, kidney) achieve equilibrium rapidly, while poorly perfused tissues (fat, bone) reach equilibrium over hours to days.

Volatile anesthetics are eliminated predominantly through exhalation (>95%), with minimal hepatic metabolism except for methoxyflurane (~50% metabolized) and sevoflurane (~2-5% metabolized). The metabolism of sevoflurane produces inorganic fluoride and hexafluoroisopropanol (HFIP); when fluoride concentrations exceed ~50 μM, risk of fluoride-induced nephrotoxicity increases, limiting sevoflurane use in prolonged procedures. Desflurane undergoes negligible metabolism (<0.02%). The context-sensitive half-time (time for blood concentration to decline 50% after infusion cessation) is relevant primarily for volatile anesthetics used in closed-circuit systems or when emergence must be rapid.

Propofol is a lipophilic IV agent producing rapid induction (30-40 seconds) due to fast blood-brain equilibration. Following bolus administration, propofol distributes rapidly to highly perfused tissues (CNS, heart), then redistributes to less perfused tissues (muscle, fat) over 10-15 minutes, resulting in rapid emergence despite continued drug presence in the body. Propofol undergoes rapid hepatic conjugation and extrahepatic metabolism (including lung and kidney), producing inactive metabolites with clearance exceeding hepatic blood flow (suggesting non-hepatic metabolism sites). Continuous infusions produce context-sensitive half-times of 30-60 minutes after 1-2 hours of infusion, but extend significantly with prolonged infusions (>10 hours), a consideration for ICU sedation. Propofol causes dose-dependent cardiovascular depression through multiple mechanisms: direct myocardial depression, peripheral vasodilation via GABA_A potentiation, and sympathetic inhibition.

These agents produce induction similarly to propofol through GABA_A enhancement but with slower emergence due to greater lipophilicity and slower hepatic metabolism. Thiopental's redistribution half-life is 3-8 minutes, prolonged compared to propofol. Metabolism is entirely hepatic through oxidative pathways; in patients with hepatic disease, elimination is significantly prolonged. Barbiturates are rarely used for induction in modern anesthesia due to propofol's more favorable pharmacokinetics and reduced cardiovascular effects, though still utilized in some settings (status epilepticus, increased intracranial pressure management).

Etomidate produces rapid, smooth induction (30-60 seconds) with minimal cardiovascular effects and preserved airway reflexes better than propofol. However, a single bolus dose causes transient (4-8 hours) suppression of 11β-hydroxylase, the enzyme catalyzing the final step of cortisol synthesis, resulting in decreased cortisol production even without adrenal insufficiency symptoms. Prolonged infusions (>12 hours) or repeated boluses may cause clinically significant adrenal suppression. Etomidate is rapidly hydrolyzed by non-specific plasma and tissue esterases to inactive metabolites, producing redistribution kinetics similar to propofol with emergence in 5-10 minutes. The adrenal suppression, even transient, has led many institutions to reserve etomidate for emergency induction when hemodynamic stability is critical, though recent literature questions the clinical significance of brief cortisol suppression.

Ketamine produces dissociative anesthesia through NMDA antagonism rather than GABA_A enhancement, preserving airway reflexes, spontaneous ventilation, and analgesia while producing hallucinations and emergence delirium in significant proportions of patients. IV bolus produces induction in 30-60 seconds with longer CNS residence time than propofol. Ketamine undergoes hepatic N-demethylation to norketamine, an active metabolite with ~1/3 the potency of the parent compound; norketamine can produce prolonged effects in hepatic dysfunction. Redistribution half-life is 10-15 minutes, with total elimination half-life of 2-3 hours. Ketamine uniquely produces sympathomimetic effects (increased heart rate, blood pressure, cardiac output) through inhibition of norepinephrine reuptake and direct sympathomimetic activity, making it valuable in hemodynamically unstable patients. However, in critically ill patients with catecholamine depletion, ketamine may paradoxically cause hypotension.

Midazolam is rarely used as sole induction agent but commonly used for anxiolysis and mild sedation in perioperative settings. It enhances GABA_A receptor function through binding to the classical benzodiazepine site (distinct from anesthetic binding sites), producing sedation at lower doses without complete unconsciousness at clinical concentrations. Midazolam is metabolized hepatically via 1-hydroxylation and 4-oxidation, with elimination half-life of 1.5-2.5 hours; formation of active metabolites (1-hydroxymidazolam, α-hydroxymidazolam) extends effects beyond parent drug half-life. These active metabolites are typically inactive at clinical concentrations but accumulate with repeated dosing or infusions, potentially producing prolonged sedation.

Isoflurane is the prototypical volatile anesthetic still widely used, with MAC of 1.15% and blood-gas solubility of 1.4. It produces dose-dependent GABA_A enhancement with greatest effects on thalamic reticular nuclei. Isoflurane undergoes minimal metabolism (0.2%), producing a small amount of inorganic fluoride and inorganic chloride. Its slower emergence compared to desflurane is offset by intermediate speed and stable anesthetic properties, making it suitable for balanced anesthesia.

Sevoflurane has become the predominant volatile anesthetic in pediatric anesthesia due to non-pungent properties allowing gaseous induction without IV access. MAC is 2.0%, and blood-gas solubility is 0.69 (faster emergence than isoflurane but slower than desflurane). Sevoflurane undergoes 2-5% hepatic metabolism producing inorganic fluoride; under conditions of low fresh gas flow (<2 L/min) and prolonged anesthesia, fluoride concentrations can approach nephrotoxic levels (~50 μM), a consideration for long cases. Sevoflurane also degrades to compound A (an unsaturated ether) when passing through desiccated CO₂ absorbent; compound A is potentially nephrotoxic at high concentrations, though clinical relevance remains debated. Recent evidence suggests sevoflurane may have lower incidence of emergence delirium in children compared to other volatile agents.

Desflurane has the lowest blood-gas solubility (0.42) and lowest MAC (6.0%), providing the most rapid emergence among volatile anesthetics. However, desflurane is pungent, irritating airway mucosa and potentially causing laryngospasm, coughing, or airway obstruction during gaseous induction; it is therefore unsuitable for inhalational induction and must be preceded by IV induction. Desflurane undergoes negligible metabolism (<0.02%), eliminating concerns about fluoride toxicity. Its rapid emergence makes it ideal for short procedures requiring quick postoperative assessment. The pungency limits its use for gaseous induction but not for maintenance anesthesia.

N₂O is an inorganic gas producing anesthesia through NMDA receptor antagonism rather than GABA_A enhancement. It has MAC of 104% (meaning 100% concentration cannot produce anesthesia alone—requires other agents), necessitating combination with volatile anesthetics or IV agents. N₂O is rapidly absorbed and eliminated via exhalation with negligible metabolism. When used with volatile anesthetics, N₂O allows reduction in volatile anesthetic MAC (typically by ~0.5 MAC), decreasing volatile agent exposure and potentially improving emergence profiles. However, N₂O is associated with postoperative nausea and vomiting (PONV), diffusion hypoxia if not carefully managed at emergence, and rare but severe adverse effects including megaloblastic anemia and subacute combined degeneration with chronic exposure (occupational exposure in poorly ventilated operating rooms). These risks have led to reduced N₂O use in recent years, with many modern anesthetics using volatile agents without N₂O.

While general anesthetics are pharmacologic agents rather than disease states, understanding their selection requires recognition of patient factors and clinical contexts that influence anesthetic choice and tolerability:

Patients with severe hypotension, ongoing sepsis, hemorrhagic shock, or cardiogenic shock require anesthetics with minimal cardiovascular depressant effects. Ketamine is specifically indicated in these patients due to sympathomimetic properties (increased HR, BP, and CO despite some direct myocardial depression balanced by sympathomimetic effects). Etomidate is also preferred for induction in hemodynamically compromised patients due to minimal vasodilation and preserved sympathetic tone, though single-dose adrenal suppression requires consideration. Propofol must be used with extreme caution in shock states due to dose-dependent cardiovascular depression; if essential, doses should be markedly reduced and vasopressors prepared. Volatile anesthetics similarly depress cardiac output and systemic vascular resistance in dose-dependent manner and should be minimized in unstable patients.

Patients with traumatic brain injury, intracranial mass, or other conditions increasing ICP benefit from anesthetics that reduce cerebral metabolic rate and cerebral blood flow while maintaining cerebral perfusion pressure. Propofol and thiopental reduce cerebral metabolic rate of oxygen consumption (CMRO₂) maximally among all anesthetics while decreasing ICP and cerebral blood flow; propofol is typically preferred due to superior pharmacokinetics. Isoflurane and sevoflurane also decrease CMRO₂ but may increase ICP through cerebral vasodilation at higher doses; they are used with care. Volatile anesthetics should be used at MAC <1.0 to minimize cerebrovascular effects. Ketamine was traditionally avoided due to concerns about ICP elevation, but recent evidence suggests safe use with controlled ventilation and adequate sedation, as ketamine's preservation of airway reflexes and sympathomimetic effects may be beneficial in certain scenarios.

Patients with personal or family history of malignant hyperthermia (MH) or confirmed MH susceptibility represent an absolute contraindication to all volatile anesthetics and succinylcholine. These patients require "safe" anesthesia using IV in