The inhalational anesthetics in clinical use share a common functional purpose, namely maintenance of a reversible state of unconsciousness, analgesia, and immobility during surgical procedures, but they differ substantially in their physical properties, pharmacokinetic behavior, organ system effects, and toxicity profiles. Understanding these differences is not merely of academic interest; agent selection, dosing strategy, and anticipation of adverse effects all depend on a working familiarity with the pharmacology of each individual agent. This module provides a detailed pharmacological account of the six principal inhalational agents in contemporary or recent clinical use: halothane, nitrous oxide, desflurane, isoflurane, enflurane, and sevoflurane. A comparative summary of key pharmacological parameters closes the module and serves as a clinical reference framework.12
MOLECULAR MECHANISMS OF ANESTHETIC ACTION. The precise molecular mechanism by which inhalational anesthetics produce unconsciousness remains incompletely characterized, but the dominant contemporary theory centers on potentiation of inhibitory ion channels and inhibition of excitatory ion channels in the central nervous system.14 The gamma-aminobutyric acid type A (GABA-A) receptor, a ligand-gated chloride channel, is the best-established target: all volatile halogenated agents enhance its activity at clinical concentrations, increasing chloride conductance and hyperpolarizing neurons in cortical, thalamic, and brainstem circuits. Glycine receptors, another inhibitory ligand-gated chloride channel class prominent in brainstem and spinal cord, are similarly potentiated. On the excitatory side, N-methyl-D-aspartate (NMDA) receptors are inhibited by several agents; nitrous oxide is the most prominent example, with xenon and the volatile agents producing lesser degrees of inhibition. Two-pore domain potassium (K2P) channels, particularly TASK-1 two-pore domain potassium channel (TASK-1) and TASK-3 two-pore domain potassium channel (TASK-3), are activated by volatile agents and contribute additional hyperpolarizing current in neurons of the arousal network.
These receptor-level effects translate to circuit-level changes: disruption of thalamocortical connectivity and corticocortical information integration, suppression of the ascending reticular activating system, and dose-dependent EEG changes ranging from slowing and increased amplitude at light planes to burst suppression and electrocerebral silence at deep anesthesia. The observation that multiple structurally unrelated agents (halothane, nitrous oxide, xenon, cyclopropane) all produce anesthesia despite diverse molecular structures led to the older Meyer-Overton hypothesis, which proposed that anesthetic potency correlated with lipid solubility (reflected in the oil:gas partition coefficient). While this correlation holds empirically, it is now understood to reflect indirect modulation of membrane proteins rather than bulk membrane fluidization. Specific protein binding sites on GABA-A and other channel subunits have been identified as direct anesthetic targets, refining the understanding beyond the lipid hypothesis.14
Halothane was introduced into clinical practice in 1956 and represented a major advance over the flammable agents ether and cyclopropane that preceded it. It is a halogenated alkane (2-bromo-2-chloro-1,1,1-trifluoroethane) and is volatile at room temperature, requiring a precision vaporizer for delivery. Although now largely displaced in high-resource settings by newer agents, halothane remains in use in many low-income countries because of its low cost and wide availability, and its pharmacology remains clinically and historically important.1
Physical and Pharmacokinetic Properties. Halothane has a blood:gas partition coefficient of approximately 2.4, making it the most blood-soluble of the volatile halogenated agents in common use. This high solubility translates to a relatively slow rate of rise of alveolar partial pressure and a corresponding slow induction. The oil:gas partition coefficient is high (approximately 224), reflecting substantial lipid solubility and correlating with its high potency (minimum alveolar concentration (MAC) approximately 0.75% in oxygen). Halothane is metabolized to a significant extent (approximately 20% of absorbed dose undergoes hepatic metabolism), predominantly via cytochrome P450 2E1 (CYP2E1), yielding trifluoroacetyl chloride (an oxidative metabolite) and bromide ion (a reductive metabolite). Trifluoroacetylation of hepatic proteins is the mechanistic basis for halothane-associated immune-mediated hepatotoxicity (discussed in Part 4).12
Cardiovascular Effects. Halothane is a potent myocardial depressant. It reduces cardiac output, heart rate, and mean arterial pressure in a dose-dependent fashion. Myocardial depression results from direct inhibition of calcium entry into cardiomyocytes and impaired intracellular calcium handling. Unlike isoflurane and desflurane, halothane does not reflexively increase heart rate in response to vasodilation, so bradycardia is a consistent finding. Halothane sensitizes the myocardium to the arrhythmogenic effects of catecholamines, including exogenous epinephrine, in a clinically important way: ventricular arrhythmias may occur at epinephrine doses as low as 1.5–2 mcg/kg when infiltrated with halothane anesthesia, compared to much higher thresholds with isoflurane. This catecholamine sensitization limits the use of local anesthetics with epinephrine during halothane anesthesia and was a significant clinical disadvantage.3 Systemic vascular resistance is moderately reduced.
CNS Effects. Halothane produces dose-dependent electroencephalographic slowing and reduces cerebral metabolic rate, but it also causes cerebral vasodilation and increases cerebral blood flow (CBF) in a dose-dependent manner. The net effect at clinical doses is an increase in intracranial pressure (ICP), which limits its utility in neurosurgical procedures. Halothane does not have significant epileptogenic potential.1
Respiratory Effects. Halothane produces dose-dependent respiratory depression, reducing tidal volume more than respiratory rate. It is a potent bronchodilator and was historically used in patients with status asthmaticus, though this application is now largely supplanted by sevoflurane. Halothane inhibits hypoxic pulmonary vasoconstriction (HPV), potentially worsening ventilation-perfusion mismatch during one-lung ventilation.
Uterine Effects. Halothane produces potent uterine relaxation in a dose-dependent manner, which facilitates uterine manipulation in obstetric procedures requiring a relaxed uterus (e.g., retained placenta extraction) but carries the risk of uterine atony and postpartum hemorrhage.
Nitrous oxide is the oldest inhalational anesthetic in continuous clinical use, having been employed since the 19th century. It is unique among the inhalational agents in that it is a gas at room temperature, stored as a liquid under pressure, and must be administered as a gas mixture. It is the only inhalational anesthetic that lacks significant halogenation, a structural fact with profound pharmacological consequences.1
Physical and Pharmacokinetic Properties. Nitrous oxide has a blood:gas partition coefficient of approximately 0.47, making it the least blood-soluble of the commonly used inhalational agents and predicting extremely rapid equilibration between alveolar, blood, and brain partial pressures. Its minimum alveolar concentration (MAC), however, is approximately 104%, meaning that at atmospheric pressure it cannot produce surgical anesthesia as a sole agent; even 100% N2O is insufficient. In hyperbaric conditions (e.g., 2 atmospheres), nitrous oxide can produce unconsciousness, but this is impractical clinically. It is therefore always used as a component of a combined anesthetic, exploiting its favorable kinetics and anesthetic-sparing effect on volatile agents.14 Nitrous oxide is essentially not metabolized in mammalian tissues; trace amounts are reduced by gut bacteria to nitrogen, but this is pharmacologically trivial. It is eliminated entirely through the lungs.
Analgesic Properties. Nitrous oxide possesses significant analgesic properties that distinguish it from the volatile halogenated agents. Its analgesia is mediated at least in part through endogenous opioid mechanisms and through N-methyl-D-aspartate (NMDA) receptor antagonism, the latter mechanism shared with ketamine.4 At 50% inspired concentration, nitrous oxide produces analgesia roughly equivalent to 10–15 mg of intramuscular morphine in some studies, accounting for its use in procedural analgesia (e.g., labor analgesia, wound care, dental procedures in some countries).
Cardiovascular Effects. Nitrous oxide has a sympathomimetic effect mediated through stimulation of the sympathetic nervous system, which tends to maintain or slightly increase heart rate, blood pressure, and cardiac output. This makes it a useful adjunct in patients with marginal cardiovascular reserve, in whom the myocardial depression of volatile agents is undesirable. In patients with severe underlying myocardial disease or those who are depleted of endogenous catecholamines, however, the direct myocardial depressant effect of nitrous oxide may become unmasked, as the compensatory sympathetic response is attenuated. The overall cardiovascular profile is substantially more benign than that of the volatile halogenated agents at equivalent analgesic doses.
Expansion of Air-Filled Spaces. Nitrous oxide is highly diffusible into air-filled spaces and equilibrates approximately 34 times faster than nitrogen (which it displaces). Administration of nitrous oxide in the presence of a closed gas-containing body cavity can therefore result in dangerous expansion of that space. Pneumothorax, pneumocephalus (air in the cranial vault following dural tear or craniotomy), bowel obstruction with distended gas-filled loops, middle ear dysfunction (particularly following tympanoplasty), gas embolism, and intraocular gas bubbles (following vitreoretinal surgery using SF6 or C3F8 tamponade gases) are all potential hazards. Nitrous oxide is contraindicated in patients with pneumothorax, following vitreoretinal surgery using intraocular gas, in patients with bowel obstruction, and in procedures where air embolism is a significant risk (e.g., sitting craniotomy, posterior fossa surgery).15
Vitamin B12 and Methionine Synthase Inhibition. Nitrous oxide irreversibly oxidizes the cobalt ion of vitamin B12, inactivating methionine synthase, the enzyme required for conversion of homocysteine to methionine and for thymidylate synthesis. A single prolonged exposure can transiently impair DNA synthesis, and repeated or prolonged exposures (as occurred in occupational settings prior to scavenging systems and in patients receiving repeated N2O anesthetics for dressing changes) can produce megaloblastic anemia and subacute combined degeneration of the spinal cord. This toxicity is of particular concern in patients with pre-existing vitamin B12 deficiency, those receiving methotrexate (which also inhibits folate metabolism), and patients undergoing ICU sedation with prolonged N2O exposure.15
Diffusional Hypoxia and the Second Gas Effect. Both phenomena (described in Part 1) are most clinically relevant with nitrous oxide because of the large volumes at which it is administered (50–70% of inspired gas mixture). These concepts are foundational to safe nitrous oxide administration and emergence.
PONV Contribution. Nitrous oxide increases the incidence of PONV in a duration-dependent manner. A meta-analysis of trials examining nitrous oxide exposure demonstrated that PONV incidence rises with increasing duration of nitrous oxide use, with the overall risk ratio for PONV approximately 1.2 times higher with nitrous oxide than without it, and that the number needed to treat to prevent one PONV case by avoiding nitrous oxide falls from more than 100 for procedures under one hour to approximately 9 for procedures exceeding two hours.13 The mechanism likely involves stimulation of the chemoreceptor trigger zone through dopaminergic and opioid pathways, as well as direct gastrointestinal effects. In high-risk PONV patients (Apfel score 3 or 4), nitrous oxide avoidance is a component of multimodal PONV prevention strategy, alongside TIVA with propofol, multimodal antiemetic prophylaxis, and opioid minimization.
Desflurane is a fluorinated methyl ethyl ether introduced into clinical practice in 1992. Its principal pharmacokinetic advantage over other volatile agents is its extremely low blood:gas partition coefficient, and its principal clinical limitation is its requirement for a heated, pressurized vaporizer due to its near-room-temperature boiling point of 22.8°C.1
Physical and Pharmacokinetic Properties. The blood:gas partition coefficient of desflurane is approximately 0.42, comparable to nitrous oxide and the lowest of the halogenated volatile agents. This extremely low blood solubility produces the fastest alveolar partial pressure equilibration of any halogenated agent, allowing rapid titration of anesthetic depth in both directions and extremely fast emergence, a property highly valued in ambulatory surgery, neuroanesthesia (where rapid neurological assessment after craniotomy is desired), and bariatric surgery (where prolonged emergence from lipid-soluble agents is a concern). The minimum alveolar concentration (MAC) of desflurane is approximately 6–7% in oxygen, making it the least potent of the volatile agents on a percentage basis; however, its low solubility in both blood and tissue compartments means that emergence is rapid even after prolonged administration, without the accumulation seen with more soluble agents.12 Desflurane undergoes minimal hepatic metabolism (less than 0.02% of absorbed dose), the lowest of any volatile agent, producing negligible quantities of inorganic fluoride or trifluoroacetylated protein. This near-zero metabolism accounts for its freedom from the hepatotoxic and nephrotoxic metabolite concerns that characterize halothane and, to a lesser extent, enflurane.
Airway Irritation. Desflurane is a significant airway irritant. At concentrations sufficient for induction (greater than 1 MAC), it commonly provokes coughing, breath-holding, laryngospasm, increased secretions, and occasionally bronchospasm. For this reason, desflurane is not suitable for inhalational induction and is universally administered for maintenance only after induction with an intravenous agent. In patients with reactive airways disease or active upper respiratory infection, desflurane should be used with caution.1
Cardiovascular Effects. At clinical maintenance concentrations, desflurane reduces systemic vascular resistance and mean arterial pressure in a dose-dependent fashion similar to other volatile agents. A unique and clinically important property of desflurane is its tendency to produce transient but marked sympathetic activation, manifesting as tachycardia and hypertension, when inspired concentration is increased rapidly. This response is mediated by stimulation of pulmonary irritant receptors and is most prominent at concentrations above 1 MAC delivered over rapid ramp-up. The practical consequence is that desflurane concentration should be increased gradually, particularly in patients with coronary artery disease or hypertension, in whom this sympathetic surge may provoke ischemia or arrhythmia.6 Despite this concern during rapid increases, desflurane at stable maintenance concentrations does not appear to increase cardiovascular risk relative to other volatile agents.
Environmental Considerations. Desflurane has a global warming potential approximately 3,500 times that of CO2 over a 100-year horizon, the highest of any currently used anesthetic agent. This has prompted calls from anesthesiology societies in several countries to reduce or eliminate desflurane use, and it has been withdrawn from clinical use or substantially restricted in the United Kingdom and several other European countries on environmental grounds.
Isoflurane is a halogenated methyl ethyl ether introduced into clinical practice in 1981. It is the most widely used volatile anesthetic globally and for decades served as the reference standard against which new agents were compared. Its enduring utility reflects a well-characterized safety profile, favorable pharmacokinetics relative to older agents, and low cost.1
Physical and Pharmacokinetic Properties. The blood:gas partition coefficient of isoflurane is approximately 1.4, intermediate between halothane (2.4) and sevoflurane (0.65). This results in a moderately rapid induction and emergence. The minimum alveolar concentration (MAC) is approximately 1.17% in oxygen, falling to approximately 0.5% when 60–70% nitrous oxide is added. Isoflurane undergoes approximately 0.2% hepatic metabolism (cytochrome P450 2E1, CYP2E1), producing inorganic fluoride ions and trifluoroacetic acid, but at levels insufficient to cause clinically significant hepatic or renal toxicity. This metabolic near-inertness contributed to its safety advantage over halothane.12
Cardiovascular Effects. Isoflurane is a peripheral vasodilator. It reduces systemic vascular resistance and mean arterial pressure in a dose-dependent manner. Unlike halothane, which produces myocardial depression with bradycardia, isoflurane's vasodilation reflexively increases heart rate through baroreceptor-mediated sympathetic activation, so cardiac output is relatively maintained despite reduced afterload. At doses up to 1 MAC, myocardial contractility is only minimally depressed, making isoflurane substantially less cardiotoxic than halothane at equivalent depths of anesthesia.
Isoflurane is a coronary vasodilator, and its ability to dilate coronary collateral vessels has raised theoretical concerns about coronary steal syndrome in patients with multivessel coronary artery disease. The steal mechanism postulates that isoflurane-induced vasodilation in patent coronary vessels diverts blood away from territories supplied by fixed stenotic vessels that cannot vasodilate further, potentially reducing perfusion in vulnerable zones of myocardium. This concern was prominent in the 1980s and was the subject of substantial clinical investigation.3 Multiple subsequent clinical trials in patients undergoing coronary artery bypass grafting, including direct comparison studies, failed to demonstrate a meaningful increase in myocardial ischemia or adverse cardiac outcomes attributable to isoflurane at clinical doses. Current consensus is that isoflurane at concentrations of 1 MAC or less does not produce clinically significant coronary steal in the vast majority of patients with coronary artery disease, and it remains widely used in cardiac anesthesia. Caution is reasonable in patients with documented collateral-dependent myocardium or known severe steal-prone anatomy, but isoflurane is not contraindicated in coronary artery disease as a class.37
Respiratory Effects. Isoflurane produces dose-dependent respiratory depression, reducing both tidal volume and respiratory rate, with a resulting increase in PaCO2 under spontaneous ventilation. It is a bronchodilator, though less potent in this regard than halothane or sevoflurane. Inhibition of HPV contributes to ventilation-perfusion mismatch during one-lung ventilation, similar to other volatile agents.
CNS Effects. Isoflurane reduces cerebral metabolic rate for oxygen (CMRO2) in a dose-dependent manner and at doses of 1.5–2 MAC produces an isoelectric EEG, a property that has been exploited for cerebral protection during neurosurgical procedures requiring temporary vessel occlusion. It causes cerebral vasodilation, increasing CBF at doses above approximately 0.5 MAC; however, the increase in CBF is less pronounced than with halothane or enflurane, and the rise in intracranial pressure (ICP) is more modest and more readily blunted by hyperventilation. For these reasons, isoflurane has a more favorable neurosurgical profile than halothane.
Enflurane is a halogenated ether introduced in the 1970s and now largely withdrawn from clinical practice in most high-income countries, having been superseded by isoflurane, sevoflurane, and desflurane. It is included here because its pharmacology, particularly its epileptogenic potential and renal fluoride toxicity, remains of educational and occasionally clinical relevance.1
Physical and Pharmacokinetic Properties. The blood:gas partition coefficient of enflurane is approximately 1.9, close to that of halothane and reflecting moderately high blood solubility with the correspondingly slower induction. The minimum alveolar concentration (MAC) is approximately 1.68% in oxygen. Approximately 2–5% of absorbed dose undergoes hepatic metabolism (cytochrome P450 2E1, CYP2E1), producing inorganic fluoride ions at levels that, while lower than those generated by methoxyflurane, may be sufficient to cause a mild, transient reduction in renal concentrating ability, particularly after prolonged administration or in patients with pre-existing renal impairment.1
Epileptogenic Potential. Enflurane is the only volatile anesthetic with clinically significant epileptogenic potential. At high concentrations (greater than approximately 2 MAC) or in the presence of hypocapnia (which lowers seizure threshold), enflurane produces EEG patterns of high-amplitude spike-and-wave complexes and can induce generalized tonic-clonic seizure activity intraoperatively. This property is dose-dependent and largely concentration-related; at standard clinical doses with normocapnia, clinically significant seizures are uncommon. Nevertheless, enflurane is contraindicated in patients with a known seizure disorder, and its use in neurosurgical procedures requiring cortical mapping or in patients with a history of epilepsy is inadvisable.8 This epileptogenic profile is a primary reason for its displacement by isoflurane and sevoflurane, which do not share this property.
Cardiovascular and Respiratory Effects. Enflurane's cardiovascular profile is broadly similar to isoflurane: dose-dependent reduction in mean arterial pressure, peripheral vasodilation with compensatory heart rate increase, and only modest myocardial depression. It does not sensitize the myocardium to catecholamine-induced arrhythmias to the same extent as halothane. Respiratory depression is dose-dependent and follows the pattern common to all volatile agents.
Sevoflurane is a fluorinated methyl isopropyl ether that entered clinical practice in Japan in 1990 and in the United States in 1995. It has rapidly become the inhalational agent of choice for many clinical scenarios, particularly inhalational induction in pediatric patients and ambulatory surgery, owing to its combination of low blood solubility (fast onset and offset), non-pungent smell (making mask induction well tolerated), and a broadly favorable safety profile.1
Physical and Pharmacokinetic Properties. The blood:gas partition coefficient of sevoflurane is approximately 0.65, lower than isoflurane but higher than desflurane or nitrous oxide. This produces faster equilibration than isoflurane and faster emergence, though not as rapid as desflurane. The minimum alveolar concentration (MAC) is approximately 2.0% in oxygen (decreasing to approximately 1.4% when combined with 60% nitrous oxide). Sevoflurane is metabolized to a greater extent than isoflurane or desflurane (approximately 3–5% of absorbed dose undergoes CYP2E1 (cytochrome P450 2E1)-mediated hepatic metabolism), generating inorganic fluoride ions and hexafluoroisopropanol (HFIP). Serum fluoride levels after sevoflurane can transiently rise above 50 μmol/L (the threshold historically associated with methoxyflurane-induced nephrotoxicity), but clinical nephrotoxicity has not been convincingly demonstrated with sevoflurane, likely because sevoflurane metabolism in the kidney itself is limited and because HFIP, unlike fluoride from inorganic sources, does not appear to be directly nephrotoxic at these concentrations.9
Compound A. A clinically important consideration unique to sevoflurane is its degradation by carbon dioxide absorbents (soda lime, baralyme) at low fresh gas flow rates to produce compound A (fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether), a vinyl ether that causes dose-dependent nephrotoxicity in rats. The relevance of compound A nephrotoxicity to humans remains controversial; multiple clinical studies have failed to demonstrate clinically significant renal dysfunction in patients anesthetized with sevoflurane at low flow rates, and regulatory agencies in most countries permit its use at low fresh gas flows.9 Nevertheless, labeling precautions recommend fresh gas flows of at least 2 L/min with sevoflurane in some jurisdictions, and caution is advised in patients with pre-existing renal impairment undergoing prolonged procedures.
Cardiovascular Effects. Sevoflurane causes dose-dependent reductions in mean arterial pressure and systemic vascular resistance, similar to isoflurane. Heart rate is relatively well maintained or may be slightly decreased. Myocardial depression is modest at clinical doses and intermediate between halothane and isoflurane. Sevoflurane does not sensitize the myocardium to catecholamine-induced arrhythmias. It is not associated with the sympathetic activation on rapid concentration increase seen with desflurane, making it more suitable for inhalational induction.1 Sevoflurane has been shown in several studies to exhibit myocardial preconditioning effects, specifically reduction of ischemia-reperfusion injury through mechanisms involving mitochondrial ATP-sensitive potassium channel (KATP) channels, though the clinical magnitude of this benefit in routine anesthetic practice continues to be investigated.
Respiratory Effects. Sevoflurane is the preferred agent for inhalational induction because of its non-irritating, somewhat sweet odor, which is well tolerated by children and adults during mask induction. It produces dose-dependent respiratory depression and bronchodilation. The bronchodilator effect is clinically significant and makes sevoflurane the preferred volatile agent in patients with asthma or reactive airways disease, largely replacing halothane for this indication in high-resource settings.
CNS Effects. Sevoflurane reduces CMRO2 and has only modest effects on CBF compared to halothane. At doses less than 1.5 MAC, increases in CBF and intracranial pressure (ICP) are limited. Sevoflurane does not have the epileptogenic potential of enflurane; however, there are isolated reports of EEG spike activity with sevoflurane, and it should be used cautiously in patients with known seizure disorders, though it is not contraindicated in the way enflurane is.8 It is the agent of choice for pediatric inhalational induction, where IV access may not be established before induction.
Emergence Agitation. A clinically important adverse effect of sevoflurane, particularly in pediatric patients, is emergence agitation (also called emergence delirium). It occurs in 20 to 80% of children following sevoflurane anesthesia in various reports, presenting as inconsolable crying, thrashing, disorientation, and failure to recognize or respond to caregivers, beginning within minutes of awakening and typically resolving within 15 to 30 minutes.8 The mechanism is incompletely understood but is thought to relate to the rapid offset of sevoflurane sedation, which produces a dysphoric transitional state before full cortical reintegration occurs. Risk factors include young age (peak incidence in preschool children aged 2 to 5), ear, nose, and throat (ENT) procedures (particularly tonsillectomy and adenoidectomy), pre-existing anxiety, and pain. Preventive strategies with established efficacy include midazolam premedication, a small dose of propofol (1 mg/kg IV) at the end of anesthesia to smooth emergence, fentanyl 1 to 2 mcg/kg administered before emergence, and adequate multimodal analgesia to minimize pain as a contributing trigger. Dexmedetomidine (0.3 to 0.5 mcg/kg IV) administered near the end of the procedure is also effective. Emergence agitation is distinct from postoperative delirium in adults, which is a separate and more prolonged syndrome occurring in elderly patients and involving different pathophysiology.
The following summary integrates the key pharmacological parameters across the agents discussed above, providing a clinical reference framework. Approximate values are provided; specific values vary by source and experimental conditions.
PHYSICAL AND KINETIC PROPERTIES Halothane: blood:gas partition coefficient 2.4; oil:gas coefficient approximately 224; minimum alveolar concentration (MAC) (O2) 0.75%; boiling point 50.2°C; metabolism approximately 20% (CYP2E1 (cytochrome P450 2E1)); molecular formula C2HBrClF3.12 Nitrous Oxide: blood:gas partition coefficient 0.47; oil:gas coefficient approximately 1.4; MAC (O2) 104%; gas at room temperature; metabolism negligible (<0.004%); molecular formula N2O.1 Desflurane: blood:gas partition coefficient 0.42; oil:gas coefficient approximately 19; MAC (O2) 6–7%; boiling point 22.8°C; metabolism <0.02% (CYP2E1); molecular formula C3H2F6O.12 Isoflurane: blood:gas partition coefficient 1.4; oil:gas coefficient approximately 99; MAC (O2) 1.17%; boiling point 48.5°C; metabolism approximately 0.2% (CYP2E1); molecular formula C3H2ClF5O.1 Enflurane: blood:gas partition coefficient 1.9; oil:gas coefficient approximately 98; MAC (O2) 1.68%; boiling point 56.5°C; metabolism approximately 2–5% (CYP2E1); molecular formula C3H2ClF5O.1 Sevoflurane: blood:gas partition coefficient 0.65; oil:gas coefficient approximately 47; MAC (O2) 2.0%; boiling point 58.6°C; metabolism approximately 3–5% (CYP2E1); molecular formula C4H3F7O.1
COMPARATIVE ORGAN SYSTEM EFFECTS Speed of induction/emergence (fastest to slowest): desflurane ≈ nitrous oxide > sevoflurane > isoflurane > enflurane > halothane. This order directly reflects blood:gas partition coefficient ranking. Cardiovascular depression: Halothane > enflurane > isoflurane ≈ sevoflurane ≈ desflurane (at stable maintenance); nitrous oxide is mildly stimulatory via sympathomimetic mechanisms. Catecholamine sensitization is a unique property of halothane not shared by modern agents.3 CNS: All volatile agents cause cerebral vasodilation and increase CBF at doses above 0.5–1 MAC; this effect is greatest with halothane and least with desflurane and sevoflurane. CMRO2 reduction is seen with all agents; isoflurane produces burst suppression at 1.5–2 MAC. Epileptogenic potential is unique to enflurane.8 Respiratory: All agents are dose-dependent respiratory depressants. Bronchodilation is a shared property; sevoflurane and halothane are most potent bronchodilators. Desflurane is a significant airway irritant; sevoflurane and halothane are non-pungent and suitable for inhalational induction.
Hepatotoxicity: Halothane carries the highest risk (Type I and Type II hepatotoxicity; immune-mediated hepatitis with re-exposure). Enflurane and isoflurane carry minimal but non-zero risk via similar trifluoroacetylation mechanism at much lower metabolic rates. Desflurane and sevoflurane have a very low hepatotoxic potential; sevoflurane hepatitis is exceedingly rare.10 Renal: Fluoride-dependent nephrotoxicity is a theoretical concern with enflurane (historically) and sevoflurane (compound A); clinically significant renal dysfunction attributable to sevoflurane at standard fresh gas flows has not been consistently demonstrated. Halothane, isoflurane, and desflurane do not pose significant renal metabolite risk.9 Uterine relaxation: All volatile agents produce dose-dependent uterine relaxation, potentially contributing to uterine atony and postpartum hemorrhage at concentrations above 0.5 MAC. Nitrous oxide has minimal uterine effects at clinically used concentrations. Malignant hyperthermia: All volatile halogenated agents are triggering agents for malignant hyperthermia (MH). Nitrous oxide is not an MH trigger. This distinction is critical in MH-susceptible patients.11
CLINICAL SELECTION GUIDE Pediatric inhalational induction: sevoflurane is the agent of choice. Non-pungent, fast equilibration, bronchodilator, hemodynamically stable at induction doses. Ambulatory surgery requiring rapid emergence: desflurane or sevoflurane; desflurane provides fastest emergence for long cases due to lowest solubility. Neurosurgery / elevated intracranial pressure (ICP): isoflurane or sevoflurane preferred over halothane; minimize inspired concentration, combine with propofol TIVA where possible; avoid enflurane. Reactive airways / asthma: sevoflurane or halothane; sevoflurane preferred in high-resource settings given halothane's cardiovascular and hepatic concerns. Cardiac surgery with compromised contractility: isoflurane or sevoflurane at low concentrations; consider total IV anesthesia (TIVA) with propofol; avoid halothane. Prolonged surgery in obese patients: desflurane or sevoflurane, both of which have low tissue solubility and do not accumulate significantly in adipose tissue relative to isoflurane. MH-susceptible patient: nitrous oxide or total IV anesthesia; no halogenated volatile agent of any kind. Resource-limited settings: halothane and nitrous oxide remain available and affordable where modern agents are not; knowledge of halothane pharmacology and its limitations is essential for clinicians in these settings.
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