Anesthesia is a medical state in which pain, sensation, movement responses, or consciousness are temporarily and reversibly suppressed, usually by drugs. The word comes from Greek roots meaning "without sensation." It covers several techniques, from numbing a single tooth to inducing a deep, controlled unconsciousness during major surgery, and it is delivered by specialists who manage the drug effects and the patient's vital functions throughout a procedure. Its work sits at the meeting point of medicine, pharmacology, physiology, and neuroscience, and it draws on how drugs act on the nervous system and on ion channels in nerve membranes.
Anesthesia matters because much of modern surgery, obstetrics, and diagnostic medicine depends on it. Before its development in the 1840s, operations were performed on conscious patients, with intense pain and high risk of shock. Today's practice combines a range of agents, precise monitoring, and team protocols, and it has greatly lowered the risk of death and serious injury attributable to the anesthetic itself. Anesthesia also remains an active research field, with open questions about how general anesthetics switch off consciousness, how to individualize dosing, and how to reduce environmental and access-related costs.
This entry first defines anesthesia and its components and then describes the main types, the drugs used, and how local and general agents work at the molecular and neural levels. It next covers pharmacokinetics, the clinical workflow, monitoring technology, physiological effects, risks, and postoperative pain control, followed by special patient groups, the professions involved, and historical development. The final sections address research frontiers, environmental impact, and the ethical and global-health questions surrounding access to safe anesthesia.
Definition and components of the anesthetic state
The word comes from the Greek anaisthēsia, "want of feeling or perception," built from an- ("without") and aisthēsis ("feeling"). In modern medicine it names a deliberately induced, reversible loss of sensation. That loss may be partial or total, and it may or may not include loss of consciousness. The surgical sense, the prevention of pain during operations, became common after the first public ether demonstration in 1846. Anesthesia is distinct from pain relief alone, which is analgesia. Opioids given without hypnotic drugs can relieve pain while the patient stays awake and aware. It is also distinct from sedation, which lowers alertness or anxiety while the patient keeps responding to commands. Sedation may or may not block pain. The "reversible" qualifier separates anesthesia from numbness caused by nerve injury, disease, or psychological conversion, which is not a pharmacological state a clinician can switch off.
Clinical practice treats the anesthetic state as a set of separable components rather than a single switch. The standard list includes unconsciousness (hypnosis), amnesia, analgesia, immobility in response to painful stimulation, and attenuation of autonomic reflexes, meaning the cardiovascular and hormonal surges triggered by surgical incision. Some texts also list skeletal muscle relaxation. These components can be dissociated. Barbiturates produce unconsciousness but are weak analgesics, so a patient can be asleep and still feel pain. Because each component is mediated by different receptors and neural pathways, no single agent produces all of them reliably. This is why pharmacological practice uses "balanced anesthesia," which combines inhaled or intravenous hypnotics, opioids, and neuromuscular blocking drugs to cover different components with different drugs.
Anesthesiology is a physician specialty built on this understanding, but its scope is wider than the operating room. Anesthesiologists manage the physiological state of patients undergoing surgery and assess their risk before and after it. They also work in intensive care, where they provide ventilation and sedation for critically ill patients. In obstetrics they provide labor analgesia and anesthesia for cesarean delivery. Many also run chronic pain services and contribute to perioperative medicine, which covers the whole period around an operation. The specialty therefore draws on medicine, physiology, and pharmacology together.
Two concepts must be kept apart: the depth of anesthesia and the level of surgical stimulus. Depth refers to the drug-induced state of the central nervous system, judged from responsiveness, reflexes, and sometimes processed electroencephalographic signals from the cerebral cortex. The level of stimulus refers to the intensity of noxious input from the operation, such as skin incision, tissue traction, or manipulation of internal organs. That input changes continually during a procedure. The two do not move together. A patient may be deeply unconscious yet still show sympathetic surges to a strong stimulus if analgesia is inadequate. Another may have adequate analgesia but insufficient hypnosis, which risks awareness. For this reason anesthesiologists titrate both: they adjust hypnotic and sedative agents to maintain unconsciousness and amnesia, and they adjust analgesic and immobilizing agents to match the stimulus. Monitoring and drug titration of both aspects are discussed in later sections.
Types of anesthesia
Clinical practice usually sorts anesthesia into four broad categories: general, regional, local, and sedation. There are four main categories of anesthesia used during surgery and other procedures: general anesthesia, regional anesthesia, sedation (sometimes called monitored anesthesia care), and local anesthesia. The categories differ mainly in how far they reach into consciousness and how much of the body they affect. Within that framework, the choice is usually made for each case rather than by a fixed rule.
General anesthesia
General anesthesia is the most complete form. It produces a drug-induced loss of consciousness from which the patient cannot be roused even by painful stimulation. General anesthesia frequently impairs the ability to independently maintain ventilatory function. For this reason, the anesthesia team usually secures the airway, most often with an endotracheal tube, and takes over breathing with a mechanical ventilator. Drugs may be inhaled, given intravenously, or both, and the team monitors vital signs continuously. General anesthesia is generally reserved for long operations, invasive procedures in the chest or abdomen, and cases in which the surgeon needs the patient to be completely still.
Regional anesthesia
Regional anesthesia blocks sensation in a larger part of the body by placing local anesthetic close to nerves or the spinal cord. Neuraxial techniques act on the central nerves in the lower trunk. Two of the most frequently used are spinal anesthesia and epidural anesthesia, which are produced by injections made with great exactness in the appropriate areas of the back. Spinal anesthesia delivers a single dose into the cerebrospinal fluid, whereas an epidural places the drug in the epidural space outside the dura, which allows catheter-based dosing. A caudal block uses the same epidural principle through the sacral hiatus at the base of the spine and is most often seen in pediatric surgery.
Peripheral nerve blocks target the bundles of nerves that supply a limb or part of the trunk. Two of the most frequently used are femoral nerve block, which is produced by injection in the leg region, and brachial plexus block, which is produced by injection in the arm and shoulder region. Truncal blocks, which numb the chest or abdominal wall, form another group. Regional techniques can be used alone, or combined with sedation or general anesthesia, depending on the surgery and patient's needs. Spinal and epidural techniques are frequently chosen for childbirth and prostate surgery.
Local anesthesia
Local anesthesia numbs a small, defined area while the patient stays awake. You're awake and can hear and see what's going on around you. There are two main approaches. Topical anesthesia applies a gel, cream, or spray to the surface of the skin or mucosa, while infiltration anesthesia injects the drug into the tissue. In dentistry, topical agents can numb the gum surface before a needle is used, and infiltration or intraligamentary injections support fillings, crowns, root canal treatment, and extractions. Podiatric procedures such as nail removal and bunion correction are other common examples. Local anesthesia is when a medication — often lidocaine — is injected into your skin. Because its effects are confined to one area, recovery is typically brief. Some anesthetists and surgeons argue that major operations can be safer under local anesthesia than under general anesthesia, although this view is not universal.
Sedation and monitored anesthesia care
Sedation is best understood as a continuum of depth rather than a single procedure. In the widely used framework of the American Society of Anesthesiologists, minimal sedation (anxiolysis) leaves patients responding normally to verbal commands. In moderate sedation, formerly called "conscious sedation," patients respond purposefully to verbal commands or light touch, and the airway is maintained without intervention. In deep sedation, patients cannot be easily aroused but respond purposely to repeated or painful stimulation. General anesthesia lies at the far end of the scale. The boundaries are not sharp, so practitioners who give a particular level of sedation must be able to rescue a patient who becomes deeper than intended.
Monitored anesthesia care (MAC) is easily confused with this continuum, but it is a different concept. MAC refers to a service provided by a qualified anesthesia provider for a diagnostic or therapeutic procedure, regardless of level of sedation. In practice, the provider assesses the patient before the procedure, adjusts medication during it, and directs recovery afterward. Sedation of this kind is common in endoscopy, imaging, dental clinics, and emergency departments.
Factors guiding the choice
The type of anesthesia is selected for each patient and operation. The location and expected duration of the procedure matter first: a brief skin procedure may require only local anesthesia, while a lengthy abdominal operation may require general anesthesia. The patient's comorbidities are equally important. Severe lung disease, airway abnormalities, heart conditions, and bleeding disorders can each change which option is safest. The best kind of anesthesia for you is based on your health and the procedure or surgery you are having. Patient preference also plays a role, and sometimes patients may choose which type of anesthesia will be used. Decisions of this kind are normally reached in discussion between the patient and the anesthesia team, which connects them to the broader principles of informed consent and of medical ethics.
General anesthetic agents
Inhaled (volatile) agents and nitrous oxide
Volatile agents are liquids at body temperature that are turned into vapor and carried to the lungs in fresh gas. The main halogenated ethers in clinical use are isoflurane, sevoflurane and desflurane. Nitrous oxide is a gas, so it comes from a cylinder through a flowmeter and does not pass through a vaporizer. Desflurane boils near room temperature, so it needs its own electrically heated vaporizer. The fresh gas enters a breathing circuit. In a circle system most exhaled gas is rebreathed after carbon dioxide is absorbed, so the anesthesiologist adjusts the inspired concentration and the fresh-gas flow rather than the vapor delivered alone.
Potency is expressed as minimum alveolar concentration (MAC). Reported MAC values place desflurane near 6%, sevoflurane near 2% and isoflurane near 1.2%, and nitrous oxide has a MAC above 100% at one atmosphere, so it cannot produce surgical anesthesia on its own and is used as an adjunct. Blood:gas partition coefficients and MAC values are tabulated for desflurane, sevoflurane and isoflurane in common teaching references, and blood-gas solubility sets how quickly the alveolar concentration can follow the inspired one. Measured coefficients vary by study. A 2015 study of 120 fasting adults found values about 5% to 12% higher than earlier published means, 5.07%, 12.12%, and 7.55% higher for isoflurane, sevoflurane, and desflurane, respectively, which suggests that uptake and elimination may be slower in some patients than older tables imply.
Intravenous induction agents
Propofol is the most widely used induction drug. It enhances GABA-type inhibition, acts within roughly a minute, and clears quickly after a single bolus because it redistributes from the brain into muscle and fat. Its main drawbacks are dose-dependent hypotension and pain on injection. Barbiturates such as thiopental, which was once the standard induction agent, depress breathing and lower blood pressure; methohexital is still used in some settings. Etomidate gives stable hemodynamics and is favored in patients with cardiac disease, but it suppresses adrenal steroid synthesis and can provoke myoclonus. Ketamine produces dissociative anesthesia by blocking NMDA receptors. Unlike most induction agents it tends to raise blood pressure and heart rate, preserves airway reflexes, and can cause emergence phenomena such as vivid dreams or hallucinations. The rapid onset and short duration of these drugs are exploited for induction and for brief procedures, but repeated doses or prolonged infusion cause accumulation in fat and slow awakening.
Pharmacological differences and choice of agent
Blood solubility mainly governs how fast the anesthetic level in the brain changes, and the low blood:gas partition coefficient of sevoflurane suggests that effective anesthetic concentrations should be achieved rapidly. Desflurane's low solubility allows the fastest emergence, while isoflurane's higher solubility makes adjustments slower. Lipid solubility, by contrast, relates to potency and to tissue storage, and it is the basis of the lipid theory discussed in another section. Sevoflurane is pleasant to inhale and is often used for inhalational induction in children, whereas desflurane is pungent and irritates the airway, so it is generally not used for induction. Nitrous oxide spreads rapidly but can expand gas-filled cavities such as the bowel or a pneumothorax. Its rapid outflow at the end of anesthesia can also dilute alveolar oxygen, a phenomenon known as diffusion hypoxia. Agent choice therefore depends on the patient's body fat, cardiac and hepatic status, age, the expected length of surgery, and the need for a rapid recovery.
Adjunct drugs
Opioids such as fentanyl, sufentanil, morphine and hydromorphone provide analgesia during surgery, while remifentanil, which is cleared by nonspecific esterases in the blood, has a very short offset. High doses can cause respiratory depression and chest wall rigidity. Neuromuscular blocking agents fall into two classes. Succinylcholine is a depolarizing agent with rapid onset, and it can raise serum potassium and trigger malignant hyperthermia in susceptible patients. Nondepolarizing agents such as rocuronium, vecuronium and cisatracurium provide longer relaxation. Their effects are reversed either with neostigmine, which increases acetylcholine by inhibiting cholinesterase and is paired with glycopyrrolate, or with sugammadex, which encapsulates rocuronium and vecuronium. Benzodiazepines, most often midazolam, reduce anxiety, promote amnesia and enhance GABA-type inhibition; flumazenil can reverse them. Anticholinergics such as glycopyrrolate and atropine reduce airway and salivary secretions and block vagal bradycardia, and glycopyrrolate, being quaternary, penetrates the brain poorly.
Local anesthetic chemistry and nerve blockade
Molecular architecture
Nearly every local anesthetic follows the same blueprint: an aromatic lipophilic ring, usually benzene, an amphipathic intermediate chain, and a terminal tertiary amine that acts as a base and accepts a proton. The middle linkage sorts the drugs into two families. Esters include procaine, chloroprocaine and tetracaine, while amides include lidocaine, bupivacaine, ropivacaine and mepivacaine. Ester agents are broken down by plasma enzymes called pseudocholinesterases, whereas amides are processed mainly by liver enzymes. The split also has allergy implications, since cross-reactivity between amide and ester agents is very rare.
The terminal amine works as a molecular switch. In its tertiary form the amine is liposoluble and can penetrate cell membranes, whereas in its quaternary form it is positively charged and water-soluble, which makes it hard to pass through lipid membranes.
Blocking sodium channels
Local anesthetics act as reversible blockers of voltage-gated sodium channels in the cell-membrane of neurons. By interrupting the sodium influx that drives the rising phase of an action-potential, they raise the threshold needed to fire the nerve, so pain signals fail to propagate. Because the drugs are weak bases, the balance between charged and uncharged molecules is governed by the pKa of the drug and the pH of body fluids. Only the neutral base crosses the lipid bilayer. It is the uncharged base that freely diffuses through the lipid bilayer of neurons. Once inside, the more acidic cytoplasm favors the charged cation, which reaches its binding site from the inner side of the channel. The bound drug then locks the channel in its inactive conformation, so sodium influx stays blocked.
Experiments indicate that neither form is inert. Both charged and neutral forms of local anesthetics are able to block channels. The two forms reach the site by different routes: uncharged compounds, including the free base, appear to reach the site through the membrane's lipid interior, bypassing the channel gates. Onset speed follows from pKa. Lidocaine, which has a relatively low pKa, acts quickly because a larger share of it is uncharged at physiological pH, while agents with higher pKa act more slowly.
Differential nerve blockade
Blockade is not uniform across nerve fibers. Fiber diameter is a major determinant, and sensitivity to local anesthetics is inversely proportional to axon diameter, a pattern supported by direct recordings from rat dorsal root axons. The clinical result is a predictable sequence of loss. The sensation of pain disappears first, followed by temperature, touch, proprioception, and skeletal muscle tone. Within a spinal root, the explanation given involves internodal spacing. Blocking three consecutive nodes is rare in larger nerve fibers, which have longer distances between nodes of Ranvier, but blockade is much more likely in smaller fibers with shorter internode distances. This is why dilute epidural solutions can relieve labor pain while leaving leg movement largely intact.
The position of unmyelinated C fibers in this order is less settled. One source reports that the smallest axons, non-myelinated C-fibers, being the least susceptible to block, whereas another notes that with subcutaneous infiltration of local anesthetic, the smallest, unmyelinated fibers are affected first. The discrepancy likely reflects the differing routes of administration and drug concentrations used in the studies.
Potency and duration
Potency is largely governed by fat solubility. This is largely the result of differences in lipid solubility, which enhances diffusion through nerve sheaths and neural membranes. Clinical concentrations typically range from 0.5 to 4%. The fraction of charged molecules in tissue also matters: the cation-base concentration ratio is critical to optimal neural blockade. Inflamed or acidic tissue therefore tends to blunt the effect of a given dose. Clinically, the onset of numbness usually takes just a few minutes, and full sensation returns a few hours later, after the anesthetic wears off. Duration is further extended by protein binding, which keeps drug molecules in the tissue longer, and by vasoconstrictors such as epinephrine, which slow absorption into the bloodstream; these effects are well established but are not detailed in the sources consulted here.
Local anesthetic systemic toxicity
When drug reaches the circulation in excess, the result is local anesthetic systemic toxicity. Local anesthetic toxicity cannot be explained by voltage-gated sodium channel blockade alone, since membrane effects and mitochondrial dysfunction also contribute to injury. Clinically, the drugs produce analgesia and anesthesia, but also seizures, arrhythmias, and cardiac arrest. Central symptoms such as perioral numbness, tinnitus, metallic taste and convulsions typically appear first, followed by cardiovascular depression, which is more pronounced with bupivacaine.
Management focuses on stopping further injection, protecting the airway and ventilation, controlling seizures, and giving intravenous lipid emulsion. Prolonged advanced cardiac life support is needed if arrest occurs. Dose limits are set per kilogram of body weight and are lower when epinephrine is co-administered. Commonly cited ceilings are about 4.5 mg/kg for plain lidocaine and 2.5 mg/kg for plain bupivacaine, but figures differ between guidelines and should be checked against current local protocols.
Mechanisms of general anesthesia
The search for how general anesthetics work began with an empirical regularity. In the 1890s, Hans Meyer and Ernest Overton, working independently, found that the potency of many structurally unrelated agents rose in step with their solubility in oil. This Meyer–Overton correlation suggested that anesthetics act by dissolving into the lipid bilayer of cell membranes and disturbing it. Overton proposed in 1901 that a generic physical mechanism must lie behind the correlation, but he did not specify one, and the lipid theory shaped thinking for most of the twentieth century. Its limits became clear over time. Some compounds with the expected lipid solubility are not anesthetic at all. In homologous series of straight-chain alcohols and alkanes, potency stops increasing beyond a certain molecular length, a "cut-off" that points to a binding site of restricted size. Membrane effects also appear too small to account for anesthesia by themselves, which shifted attention toward specific proteins. The correlation nonetheless still holds for many agents, so any complete explanation has to accommodate it.
Protein-target theories dominate current work. At clinically relevant concentrations, most intravenous and volatile agents enhance the function of GABA-A receptors, ligand-gated chloride channels that make neurons less excitable. Propofol, barbiturates, and volatile agents such as isoflurane all potentiate this receptor, and some studies have linked specific receptor subunits to sedative versus anesthetic effects. Ketamine and nitrous oxide act differently, inhibiting NMDA receptors, which carry excitatory glutamate signals. Volatile agents also modulate two-pore potassium channels that help set the resting membrane potential, and sodium and calcium channels appear to be affected as well. Because so many chemically diverse molecules act on unrelated proteins, researchers have struggled to explain why a single, highly specific binding model should apply. Most authors now consider multiple targets to contribute in different proportions.
Network-level explanations address the question of how such molecular actions produce unconsciousness. Neuroimaging and electrophysiology indicate that anesthesia disrupts communication between the thalamus and the cerebral cortex, as well as among cortical regions. In one human study using dexmedetomidine, metabolism fell in the thalamus and in key cortical networks, and connectivity between the thalamus and the default mode network was impaired, although connectivity within cortical networks was preserved. Other evidence contradicts parts of this picture: cortico-cortical connectivity may persist during unconsciousness, and thalamic connectivity with primary sensory cortex is relatively spared during propofol anesthesia. In rats, propofol produces highly synchronized oscillations between the medial prefrontal cortex and the thalamus, which the authors link to loss of consciousness. The prevailing hypothesis is that consciousness depends on integrated, long-range information flow, and that anesthetics break this integration rather than simply silencing any single region.
Several questions remain open. Chemically different agents, acting on different proteins, produce a remarkably similar state, and it is not settled whether they converge on a common network endpoint or affect the same circuits by different routes. It is also unclear how much of the state depends on the thalamus and how much on the cortex. Immobility is another puzzle. Experimental work suggests that the spinal cord plays a substantial role in the movement response to painful stimulation, and that this response can be suppressed at drug concentrations that differ from those needed to abolish consciousness. This implies that the mechanisms of immobility and unconsciousness are at least partly separable, although the precise spinal targets and their relationship to brain-level effects are still being mapped.
Pharmacokinetics and measurement of dose
Minimum alveolar concentration
Inhaled anesthetics are dosed by concentration, and the minimum alveolar concentration (MAC) is the common yardstick for comparing their potency. It is the concentration of gas at one atmosphere that leaves half of patients unresponsive to a surgical stimulus. Anesthesiologists have accepted MAC as the measure for comparing the relative potencies of anesthetic gases. Because MAC is a population median rather than a value for any one patient, a clinician reads it as a relative target: one MAC is the dose that works for half the population, and individual needs vary around it.
Its most practical feature is additivity. MAC values are additive. For instance, when applying 0.3 MAC of drug X and 1 MAC of drug Y the total MAC achieved is 1.3 MAC. Nitrous oxide exploits this. In one study, 65% nitrous oxide decreased MAC about 50% in each group at any age studied. Since nitrous oxide has a MAC above 100% on its own, it is used as a carrier gas that lowers the volatile agent needed, rather than as a stand-alone anesthetic.
Age is the most important modifier. MAC begins to rise at one month of age with a peak at approximately 6 months of age, and then falls. In adults, for patients older than 1 year, MAC decreases by approximately 6% to 7% with each increasing decade of life. Drugs also shift the figure. Acute use of amphetamines, cocaine, ephedrine, and chronic use of alcohol increase MAC, whereas propofol, etomidate, barbiturates, benzodiazepines, ketamine, opiates, local anesthetics, lithium, verapamil, and alpha 2-agonists decrease MAC. Monitoring has to account for this. Gas analyzers either adjust for age and body temperature or assume a default state of 40 years of age and a normal body temperature. A clinical study also found that the end-tidal MAC fraction in older patients declined by 3.01% per decade, less than the modeled age-related fall, so older patients often receive relatively more agent than the age-adjusted figure implies.
Uptake and distribution of inhaled agents
The effect of an inhaled agent depends on its partial pressure in the brain, not on the total amount inhaled. It is the partial pressure of the inhalation agent in the blood, and therefore in the brain, that induces anesthesia. Transfer from the lungs into blood is governed mainly by the blood–gas partition coefficient, which describes how much agent blood takes up relative to alveolar gas. Reported values differ slightly between sources, but they are consistently low for desflurane and nitrous oxide and high for isoflurane. Partition coefficients at 37°C: desflurane 0.45, nitrous oxide 0.47, sevoflurane 0.65, isoflurane 1.4, enflurane 1.8.
Cardiac output and ventilation set the pace. As cardiac output increases, anesthetic uptake increases, the rise in alveolar partial pressure slows, and induction is delayed. The effect is stronger for soluble agents. With highly soluble agents, a potentially dangerous positive feedback exists in which uptake into blood slows induction and then, as the blood saturates, speeds it. Tissue distribution follows blood flow. Tissues are often grouped by perfusion: the vessel-rich group (brain, heart, lungs, kidney, liver, spinal cord) rapidly reaches equilibrium within minutes, while muscle equilibrates over hours. Fat accumulates slowly but holds a large share of soluble agents, and because fat/blood partition coefficients are greater than 1, blood/gas solubility is increased by postprandial lipidemia and is decreased by anemia.
Metabolism, elimination, and recovery
Volatile agents are cleared mainly by the lungs. Elimination of inhalational anesthetics is primarily accomplished by exhalation, though minimal amount of elimination occurs percutaneously. Recovery is therefore largely a matter of how fast the agent leaves the brain through the blood, and low-solubility agents wake patients fastest. Intravenous agents are different. They are cleared by hepatic metabolism and renal excretion of metabolites and, in many cases, by redistribution from the brain into peripheral tissues. After a short bolus, redistribution dominates and recovery is quick. After prolonged infusion, peripheral tissues become loaded and drug slowly returns to plasma, so the time for concentrations to fall after stopping the infusion grows with duration. This is why a drug with a short bolus profile may take much longer to clear after hours of infusion, and why the choice of agent and total dose matters in long procedures and intensive care sedation.
Target-controlled infusion and PK–PD models
Intravenous dosing is increasingly guided by target-controlled infusion (TCI), in which a pump is programmed with a pharmacokinetic model and the clinician sets a target concentration rather than a rate. Open target-controlled infusion systems can be programmed with any pharmacokinetic model, and allow either plasma- or effect-site targeting. Effect-site targeting is more clinically relevant, because the target is the concentration at the site of action. With effect-site targeting the goal is to achieve a user-defined target effect-site concentration as rapidly as possible, by manipulating the plasma concentration around the target. The effect-site concentration is a model estimate, not a measured value, linked to the plasma compartments by a rate constant. Second-generation open-loop TCI systems incorporate the site of action into a three-compartment model, enabling better control of drug concentration in the brain through the ke₀ parameter.
Models differ, and the choice matters. Currently systems are pre-programmed with the Marsh and Schnider pharmacokinetic models for propofol. The Schnider model integrates individual variants of age, height, weight and gender to calculate the infusion profile to achieve predetermined steady "target effect site concentration". Two models given the same target can produce different plasma curves: one study found that MARSH predicted a higher concentration during the early phase, whereas SCHNIDER was maintained at a higher concentration. Newer models aim to reduce this variability. In a small comparison in sleep endoscopy, the Eleveld model was associated with a shorter time to attain the optimal sedation plane compared to the S group (6.1 ± 1.7 vs. 9.8 ± 2.2 min), though the sample was small and the result has not been established across wider populations.
Clinical process from assessment to recovery
Preoperative evaluation
Anesthetic care starts before the patient reaches the operating room. The anesthesiologist reviews the medical history, examines the airway, checks current medications, and gives fasting instructions. A widely used summary of the patient's baseline health is the ASA physical status classification. Its grades run from I, for a person in good health, to V, for a person with a severe, life-threatening condition, and class VI is reserved for deceased organ donors. Surgery that cannot wait receives an "E" suffix, so a class 3 emergency case is written as 3E. The grade rests on comorbid conditions that threaten life or limit activity, so it is read alongside the type of surgery, frailty, and the level of deconditioning. The grade is a clinical judgment, and one study found that different anesthesiologists may assign different scores to the same patient. Sources also disagree on its history: one reports that the system was first introduced in 1941 and revised in 1961, while another dates the five-category version to 1963.
Fasting rules are designed to lower the risk of aspiration. International guidelines recommend six hours of fasting for solids and two hours for clear liquids. Pediatric practice often follows a shorter schedule: a minimum of 6 hours for solid food or formula, 4 hours for breast milk, and 2 hours for clear fluids. In practice, the rules are often not followed exactly. One audit found that most patients fasted from clear fluids for more than two hours and from solid food for more than six to 12 hours, and over-long fasting can cause dehydration, hypoglycemia, discomfort, and delayed recovery.
Induction and airway management
Induction usually begins with preoxygenation and an intravenous hypnotic, and an opioid or similar drug may be given to blunt the response to laryngoscopy. For many elective cases a neuromuscular blocking agent is also given so that the airway can be secured without a reflex response. Before laryngoscopy, the anesthesiologist normally confirms that the block has taken effect, most often with train-of-four monitoring, which is often used to monitor the degree of neuromuscular block when such drugs are used. Intubation with an endotracheal tube gives direct control of the airway and allows mechanical ventilation. Supraglottic airways, such as laryngeal mask devices, sit above the vocal cords and are often chosen for shorter or lower-risk procedures. Failed or difficult intubation remains one of the main sources of harm during this phase, and aspiration of gastric contents is a leading cause of harm associated with tracheal intubation, which is why fasting and rapid-sequence techniques matter.
Maintenance
During surgery, the anesthesiologist combines agents rather than relying on one drug, as described in the sections on agents and pharmacokinetics. Balanced anesthesia can mean adjusting inhaled or intravenous hypnotics, opioids, and neuromuscular blockade to the stimulus of the operation. Fluid and blood management aims to maintain circulating volume and oxygen delivery while avoiding overload. Ventilator settings are adjusted to keep oxygenation and carbon dioxide within target ranges and to limit lung injury, and the anesthesiologist reassesses drug delivery as the surgical stimulus changes.
Emergence and transfer
At the end of surgery, residual neuromuscular block must be reversed or allowed to wear off. Neostigmine, an older agent, has limitations: it takes at least 8 minutes to reach its maximal effect and works only once recovery from block has begun. In a randomized trial of 80 patients, sugammadex at 1.0 mg/kg reversed block to a train-of-four ratio of 1.0 in about two minutes, while a 0.5 mg/kg dose took longer, up to about eight minutes, and neostigmine took about 8.5 minutes. Sugammadex has its own risks: anaphylaxis is rare but more common with sugammadex than with neostigmine, and after sugammadex lowers free drug levels, rocuronium can move back out of tissues and recurarization can occur, a risk that is greater in obese or elderly patients and in those with renal impairment or hypothermia.
Guidance on extubation centers on neuromuscular recovery. Clinicians are advised to remove the endotracheal tube only once the train-of-four ratio has recovered to at least 90 percent of baseline. Inadequate recovery after extubation has been linked to more postoperative complications. Quantitative monitoring is the most reliable way to confirm this threshold, because it is the only means of determining whether a train-of-four ratio of at least 0.9 has been reached. Other clinical criteria, such as being awake and following commands, adequate spontaneous breathing, intact airway reflexes, and stable hemodynamics, are assessed alongside it.
After extubation, the patient is usually moved to a monitored recovery unit, where pulse oximetry, ventilation, circulation, and temperature are watched closely. In one reported case, pulseless electrical activity developed in the recovery unit as the train-of-four count fell back to 1, which shows why monitoring continues after the operating room. Care is then handed over in a structured verbal report between the anesthesia team and the receiving nurses or physicians, so that the patient's course, medications, and risks are clearly communicated.
Monitoring technology
Monitoring serves two purposes in anesthesia. The first is to confirm that the patient is being oxygenated, ventilated and circulated safely. The second is to estimate how strongly drugs are affecting the brain and the neuromuscular system. A standard set of devices is used in nearly every general anesthetic, and newer tools are mostly added on top of it.
Standard monitoring
Routine monitoring includes pulse oximetry, which estimates the saturation of hemoglobin with oxygen from light absorbed by pulsating arterial blood; electrocardiography of heart rhythm; cuff measurement of blood pressure; and temperature monitoring, which matters because anesthesia impairs thermoregulation. Analysers at the airway also measure the concentrations of inhaled agents and oxygen, so that the delivered and exhaled gas composition is known at each breath.
Capnography is the continuous measurement of carbon dioxide in respiratory gas, and the reading taken at the end of exhalation is called end-tidal CO2. Anesthesiologists have been using capnography for decades to monitor end-tidal carbon dioxide in patients receiving general anesthesia. Because the waveform combines the CO2 level with the breathing rate, it can expose hypoventilation or an obstructed airway before oxygen saturation falls, which is why it is considered more sensitive for detecting ventilatory compromise than pulse oximetry alone. The American Society of Anesthesiologists endorses capnography as a standard of care for general anesthesia as well as moderate and deep procedural sedation.
Invasive monitoring is used when continuous or repeated measurements are needed. An arterial line gives beat-to-beat blood pressure and allows frequent blood-gas sampling. A central venous catheter provides access for vasoactive or high-osmolality drugs and for fluid management, and it can also yield central venous pressure readings.
Depth-of-anesthesia monitoring
Because the anesthetic state cannot be read directly from the patient, anesthesiologists rely on processed signals from the brain. The Bispectral Index (BIS) monitor uses a simplified form of EEG recording via four flat skin electrodes attached to the patient's forehead. The device condenses the electroencephalogram into a single number from 0 to 100. The US Food and Drug Administration cleared BIS monitoring in 1996 for assessing the hypnotic effects of general anesthetics and sedatives. A value in the range of 40 to 60 is generally sought during surgical anesthesia, and a Cochrane review including more than 40,000 patients concluded that BIS reduces awareness compared to clinical signs, but not compared to end-tidal concentration monitoring.
Other processed-EEG indices and evoked-potential methods, which record the brain's response to repeated auditory or somatosensory stimuli, have been developed on similar principles. None of them measures consciousness itself. Their output tracks physiological signals that correlate with the depth of sedation, so readings can be distorted by artefacts such as electromyographic noise, certain drugs, and patient-specific factors. For this reason, depth monitors are generally treated as an adjunct to clinical judgment and to the measured inhaled concentration rather than as a stand-alone endpoint.
Neuromuscular monitoring
Muscle relaxants are often given to allow intubation and surgical access, and their depth is assessed with a peripheral nerve stimulator. The most common pattern is train-of-four (TOF) stimulation, which delivers four electrical impulses to a peripheral nerve at half-second intervals and measures the evoked muscle responses. The TOF ratio compares the fourth response with the first, and in the unblocked muscle, the TOF ratio is 1.0. As nondepolarizing block deepens, successive twitches weaken because each nerve impulse depletes the releasable pool of acetylcholine at the presynaptic terminal. Depolarizing agents such as succinylcholine behave differently, so the pattern also helps identify which drug is acting.
Subjective assessment is unreliable near the threshold of recovery. Fade is not detected qualitatively, by visual or tactile means, until the train-of-four ratio is below 0.4. A 2018 international consensus concluded that subjective evaluation is not predictive of adequate neuromuscular recovery, and is not sensitive to the presence of residual weakness. Current guidance therefore favors quantitative monitoring, and avoiding residual weakness can be accomplished by measuring the patient's train-of-four ratio and ensuring a ratio of at least 0.9 prior to extubation.
Emerging data-driven tools
Research is also moving toward systems that combine many signals. Machine-learning models have been trained on arterial pressure waveforms and other intraoperative data to anticipate episodes of hypotension, so that the anesthesiologist can intervene before blood pressure falls to dangerous levels. Such tools are still being evaluated for accuracy, alarm burden, and whether they improve outcomes, and they are intended to support rather than replace clinical decisions.
Physiological effects on the body
Cardiovascular effects
Most general anesthetic agents lower arterial pressure in a dose-dependent way. Propofol and the volatile agents cause vasodilation, mainly by reducing systemic vascular resistance, and they blunt the baroreflex response that would normally raise heart rate and vasoconstrictor tone when pressure falls. Many agents also depress myocardial contractility to some degree, although the effect varies widely between drugs. Because sympathetic outflow is reduced, hypotension is common, especially in patients who are hypovolemic, elderly, or have chronic hypertension or limited cardiac reserve. Bradycardia can occur through vagal reflexes during airway instrumentation or surgical traction, and opioids can contribute. Ketamine is a notable exception: it tends to maintain or raise heart rate and blood pressure, which is why it is chosen for patients at risk of hypotension.
Respiratory effects
General anesthesia depresses the ventilatory response to carbon dioxide and to low oxygen levels, so breathing becomes shallow or stops altogether without support. Opioids and volatile agents add to this depression. Upper airway muscle tone falls and protective reflexes such as coughing and laryngeal closure are suppressed, so the airway must be secured or supported. Loss of diaphragmatic activity and the supine position reduce functional residual capacity, allowing atelectasis to develop in dependent lung regions. High inspired oxygen fractions can worsen this through absorption atelectasis. Volatile agents also impair hypoxic pulmonary vasoconstriction, which can increase ventilation–perfusion mismatch. Together these effects raise the risk of hypoxemia, which is why controlled ventilation, positive end-expiratory pressure, and continuous pulse oximetry are standard.
Thermoregulation
General anesthetics impair the body's thermoregulatory defenses. They lower the core temperature at which peripheral vasoconstriction and shivering are triggered, so the body loses its usual responses to cold. Core temperature typically falls quickly during the first hour as heat moves from the warm core to cooler peripheral tissues, then continues to decline more slowly because of heat loss from exposed skin, cold operating rooms, and cold intravenous or irrigation fluids. This perioperative hypothermia, usually defined as a core temperature below 36 °C, has several downstream consequences. Clotting enzymes and platelet function work less effectively, increasing blood loss and transfusion needs. Reduced peripheral perfusion lowers tissue oxygen tension and is associated with higher rates of surgical site infection and impaired wound healing. Shivering, when it occurs, raises oxygen consumption and cardiac workload, and slowed drug metabolism can prolong recovery. Forced-air warming and warmed fluids are the usual countermeasures.
Effects on other systems
Surgery and anesthesia reduce renal blood flow and glomerular filtration, particularly when mean arterial pressure falls, which can contribute to acute kidney injury in vulnerable patients. Hepatic blood flow follows cardiac output and portal and arterial supply, so hypotension and some volatile agents can reduce it, which matters for drugs cleared mainly by the liver. Surgical trauma activates the hypothalamic–pituitary–adrenal axis and the sympathetic system, raising cortisol, catecholamines, and glucagon, and promoting insulin resistance and hyperglycemia. Opioids and neuraxial or regional blocks attenuate this stress response, but whether that attenuation improves outcomes remains debated. Opioids and anesthesia slow gastrointestinal motility, and postoperative ileus is common; splanchnic vasoconstriction during hypotension may also compromise mucosal perfusion. Finally, surgery and anesthetic drugs alter immune function, including changes in leukocyte activity and cytokine release. The clinical significance of these changes, including any influence on infection or cancer recurrence, is still under investigation and is not settled.
Risks, complications, and safety
Mortality trends
Deaths directly attributable to anesthesia have become much rarer over the past half-century. A systematic review of studies published between 1954 and 2007 found that anesthesia-only mortality had fallen to under one death per 10,000 anesthetics, and that perioperative mortality in developed countries had dropped below 20 per 10,000 Brazilian and worldwide studies demonstrated a similar decline in anesthesia-related mortality rates, which amounted to fewer than 1 death per 10,000 anesthetics in the past two decades. Perioperative mortality rates also decreased during this period, with fewer than 20 deaths per 10,000 anesthetics in developed countries. The Anesthesia Patient Safety Foundation credits the roughly tenfold fall since the 1970s to better training, monitoring equipment, safer drugs and tighter safety standards Anesthesia-related mortality has decreased tenfold since the 1970s. This decline is due to advances in training, monitoring equipment, safer medications, and improved safety standards.
The trend is not uniform across datasets. A serial study of about 9.4 million anaesthetics in Hubei Province, China, recorded a 57.6% fall in anaesthesia-related deaths between 2017 and 2021 The incidence of anaesthesia-related deaths experienced a significant reduction over the 2017–2021 period, with a percentage change of 57.6%. By contrast, a US analysis of death records from 1999 to 2020 reported a rising age-adjusted rate, from 1.01 to 4.35 Between 1999 and 2020, the AAMR rose 124% going from 1.01 in 1999 to 4.35 in 2020. That figure comes from a conference abstract, so it should be read with caution. Across the literature, the main causes of anesthesia-related death are airway failures and cardiocirculatory events The main causes of anesthesia-related mortality were problems with airway management and cardiocirculatory events related to anesthesia and drug administration.
Airway and cardiovascular complications
Airway management remains the central technical hazard. A failed or difficult intubation can quickly lead to hypoxia. Aspiration of gastric contents is a particular danger during induction, because protective reflexes are suppressed by anesthetic drugs, and it is most serious in emergencies or in patients who have not fasted. Dental injury, such as loosened or broken teeth, is among the most common airway-related injuries, and laryngeal trauma, including sore throat, vocal cord injury and laryngospasm, also occurs. Improvements in monitoring have shifted the pattern of fatal events. Deaths caused by airway management failures seem to have decreased. One analysis suggested that the predominance of cardiovascular events in anesthesia-related cardiac arrest may reflect the frequent use of pulse oximetry and capnography, which may be more effective in preventing respiratory rather than cardiovascular events.
Awareness and postoperative complications
Awareness during general anesthesia, meaning explicit recall of intraoperative events, is uncommon but well documented, and it can result in post-traumatic stress disorder Awareness with recall after general anesthesia is an infrequent, but well described, phenomenon that may result in posttraumatic stress disorder. Reported rates vary by study and population. Estimates of 0.1–0.2% of patients in low-risk procedures are common Recent studies showed the incidence to be 0.1-0.2% in low-risk surgical procedures; however, it can reach 1% for patients at increased risk. Obstetric and cardiac patients have higher rates Incidence rates for obstetric and cardiac patients are higher overall, at 0.4 percent and 1.1-1.5 percent, respectively. Difficult airway management is also a recognized contributor, since it prolongs the induction phase and delays the delivery of maintenance anesthetics and increases the likelihood of awareness. Neuromuscular blocking drugs raise the risk considerably. One dataset found the rate rising from about 1 in 135,000 to about 1 in 8,200 when these drugs were used The incidence of AAGA increased from 1 out of 135,000 general anaesthetics to 1 out of 8200 general anaesthetics when neuromuscular blocking drugs were used. Depth-of-anesthesia monitoring with the bispectral index is intended to reduce this risk, and the US FDA has noted that its use may be associated with fewer cases The US Food and Drug Administration states that the "Use of BIS monitoring to help guide anesthetic administration may be associated with the reduction of the incidence of awareness and recall in adults during general anesthesia and sedation.
Postoperative nausea and vomiting is common and is driven by patient factors such as female sex, non-smoking status, a history of motion sickness or prior nausea, and the use of postoperative opioids; it is usually managed with prophylactic combinations of drugs. Postoperative delirium, an acute disturbance of attention and cognition, is most frequent in older adults, and some patients have lasting cognitive decline. Its causes are multifactorial and only partly understood, which is why it is discussed separately as postoperative delirium.
Rare but serious reactions
Malignant hyperthermia is a hypermetabolic crisis triggered by volatile halogenated anesthetics or succinylcholine. MH episodes are caused by exposure to volatile halogenated anesthetics or succinylcholine. These episodes are characterized by heightened muscle metabolism, increased temperature, rhabdomyolysis, and if untreated, can be fatal. The disorder is linked to mutations in the skeletal muscle ryanodine receptor, a calcium release channel. These can trigger a potentially lethal malignant hyperthermia (MH) episode via intracellular calcium increase mainly from RyR1 channel leakage. Treatment relies on dantrolene, which is poorly water soluble and has a long plasma half-life, and dantrolene is the only known treatment option to prevent death. For that reason, early recognition, such as a rising end-tidal carbon dioxide and muscle rigidity, and stopping the trigger drugs are critical. The condition is discussed in more detail as malignant hyperthermia.
Anaphylaxis is a severe, systemic allergic reaction that can produce circulatory collapse and bronchospasm within minutes. Among the drugs most often implicated are neuromuscular blocking agents and antibiotics, while latex and chlorhexidine are recognized triggers; management follows standard emergency protocols, with intramuscular or intravenous allergic treatment such as adrenaline. Local anesthetic systemic toxicity occurs when too much drug enters the bloodstream. Central effects such as tinnitus, perioral numbness, and seizures typically come first, followed by cardiac effects including arrhythmia and cardiac arrest. Bupivacaine is particularly cardiotoxic. Management includes stopping the injection, securing the airway, controlling seizures, and giving intravenous lipid emulsion, and dose limits based on body weight are the main preventive measure.
Pain control after surgery and persistent pain
Postoperative analgesia has moved away from relying on a single drug class. The current approach, called multimodal analgesia, combines several agents that act at different points in the pain pathway, typically a non-opioid analgesic, a lower-dose opioid, local anesthetic infiltration or a nerve block, and sometimes an intravenous adjunct. Ketamine and lidocaine are the adjuncts studied most often. The evidence for ketamine is inconsistent: the reported results on ketamine are mixed, and some studies have shown a significant reduction in acute and chronic postoperative pain and in opioid requirements, while others have shown little to no benefit. A Cochrane review concluded that postoperative ketamine probably reduces postoperative analgesic consumption and pain intensity, without a significant difference in central nervous system adverse events. Benefit also varies by operation: the effect was minimal in pediatric and thoracic surgeries, while ketamine has a valuable role in multimodal regimens, especially in orthopedic and spine procedures. Dosing is not standardized either, with bolus doses ranging from 0.15 to 1 mg·kg⁻¹ and infusion doses from 2 to 5 µg·kg⁻¹·min⁻¹ across studies. In one randomized double-blind trial after sleeve gastrectomy, an intravenous lidocaine infusion gave better pain control in the early postoperative period compared to dexmedetomidine and ketamine and decreased the hospital length of stay.
Multimodal regimens are usually embedded in enhanced recovery after surgery (ERAS) pathways, which standardize perioperative care so that the hospital course becomes more predictable. Opioid-sparing strategies are a central part of these pathways. In a meta-analysis of cardiac surgery, opioid-sparing approaches were linked to shorter durations of mechanical ventilation and reduced ICU length of stay, without significant differences in postoperative mortality. Outcomes in other operations are reported in the same direction but vary in size. In thoracic outlet decompression, daily opioid use fell from a median of 33 mg to 14 mg with multimodal pain management alone, and to 11 mg with the full ERAS protocol, yet only the full protocol shortened stay (1.44 days versus 3.26 days). After pancreaticoduodenectomy, hospital stay decreased from 8.92 to 5.72 days, and opioids for the first five hospital days fell from 130.13 to 71.2 morphine milligram equivalents. A meta-analysis of esophagectomy trials found lower rates of non-surgical and pulmonary complications in the ERAS groups, along with a significant reduction in length of stay. Reducing opioids does not automatically worsen pain control; in one adolescent scoliosis cohort, pain scores did not differ between the ERAS and traditional groups.
Persistent postsurgical pain (PPSP) is pain that continues beyond the normal healing period. Acute pain after surgery typically resolves within 1–2 weeks with appropriate control therapies, but a minority of patients do not recover. Moderate-to-severe PPSP has been confirmed as common after outpatient surgery. In a systematic review of predictors, preoperative pain, anxiety, age, and surgery type were the four most significant predictors of acute postsurgical pain, and the same pre-operative factors are also linked to the later transition. Acute intensity is a plausible route to chronicity: the severity of acute postoperative pain is often cited as a risk factor, implying that individuals prone to intense early pain may be the most vulnerable, although not every patient with severe postoperative pain goes on to develop PPSP. This is why pain trajectories, which track pain over time, are used to identify abnormal resolution. Psychological factors such as anxiety, pain catastrophizing, expectation of pain, and depression are also associated with the condition. Mechanistically, research points to sensitisation of peripheral sensory neurons, neuroplasticity in the central nervous system, and nociceptive signalling along the neuro-immune axis. Whether aggressive early analgesia actually prevents chronic pain remains debated: the attenuation of chronic pain by early analgesic medication fits with early pain intensity being a risk factor, suggesting that a sensitizing influence of high early pain may drive chronic pain. The prevention strategy also has a cost, since long-term opioid consumption can lead to tolerance, addiction, opioid-induced hyperalgesia, and death.
Acute pain services are multidisciplinary teams, typically led by anesthesiologists with nursing and pharmacy staff, that take responsibility for postoperative analgesia across a hospital. Their work generally includes planning analgesic regimens before surgery, monitoring pain scores and sedation levels on the ward, and adjusting drug doses, nerve-block catheters, and opioid protocols as the patient recovers.
Special populations and clinical settings
Children
Infants and young children differ from adults in body water, organ maturity, airway proportions, and heat loss, so anesthetic dosing and monitoring are adjusted for age and weight. The more contested question is whether anesthesia in early childhood affects later neurodevelopment. Preclinical studies, mainly in rodents and primates, showed neuronal death and behavioral deficits after exposure to common anesthetics during periods of rapid synapse formation, and this work led to regulatory warnings and investigation into the potential long-term cognitive and neurodevelopmental consequences in children who have been exposed to anesthesia before the age of three. Human findings are mixed. A 2019 pooled analysis reported a modest elevation in risk, with a pooled hazard ratio of 1.25 for adverse behavioral or developmental outcomes after a first exposure before age four, but the number of times of exposure (HR = 1.75) rather than the time at exposure before 4-year was the stronger risk factor. Randomized evidence is more reassuring for a single exposure. The GAS trial, along with the PANDA and MASK cohorts, found that a single exposure of <1 h is not associated with long-term neurodevelopmental abnormalities. Interpretation remains difficult, because translating these findings to humans is complex due to confounding factors such as surgical stress, preexisting conditions, and perioperative management. Sedation for imaging, such as MRI or CT in young children, is therefore planned to use the lowest effective exposure, and the neuroscience of this question remains under active study.
Older adults
Older patients usually need less volatile anesthetic to produce the same effect, and their brain is more vulnerable to postoperative complications. The most important complication is postoperative delirium, a fluctuating disturbance of attention and cognition. In a pooled analysis of older patients undergoing elective surgery, the pooled incidence of postoperative delirium was 18.4%. Predictors include history of delirium, frailty, cognitive impairment, and impairment in activities of daily living, and the strongest single factor is advanced age. Some drug-related factors can be modified, including patients premedicated with benzodiazepine, and intraoperative ketamine use. Depth of anesthesia is one such modifiable factor. A prospective cohort reported that those who received a MAC ≥ 0.9 had an RR of 2.22 (95%CI: 1.2-4.2) of presenting POD. A meta-analysis of randomized trials found that compared with standard care, EEG-guided anesthesia demonstrated a beneficial effect in reducing postoperative delirium among elderly patients (RR = 0.76, 95% CI: 0.61–0.96). These findings support individualized dosing guided by electrode-based monitoring, although the evidence base is still being refined.
Obstetric anesthesia
Labor analgesia is most often delivered by epidural catheter, which provides pain relief while keeping the mother awake and allows the dose to be adjusted over many hours. For cesarean delivery, neuraxial techniques, especially spinal anesthesia, are the usual first choice for planned operations, and the main concern is maternal hypotension from sympathetic blockade. As one review puts it, there have been concerns about spinal anesthesia used in cesarean sections about fetal safety because of sympathetic blockade-induced hypotension which can dangerously lower uteroplacental perfusion. For the newborn, the comparison is informative but not uniform. Some observational studies found that GA administration correlated with more frequent resuscitative interventions, increased admission rates to NICU, and a greater incidence of neonatal respiratory distress syndrome. However, general anesthesia is often used in more urgent situations, which may confound the results, as GA is used more frequently in higher-grade emergencies. A small randomized trial of elective repeat cesareans found that anesthesia type does not seem to influence the short-term outcome of the newborn infants. Clinical guidance therefore weighs maternal safety, urgency, and the infant's condition together rather than relying on a single outcome.
Non-surgical and remote settings
Anesthesia is also given outside the main operating theatre. Dental clinics, endoscopy and imaging suites, and interventional radiology rooms often use sedation or short general anesthesia with portable or compact equipment, which places particular demands on monitoring and emergency readiness. Emergency departments and disaster-response teams must provide anesthesia with limited staff, equipment, and drugs, sometimes without the usual preoperative assessment. Veterinary anesthesia follows the same basic principles of induction, airway protection, monitoring, and recovery, but adapts drugs and doses to different species and body sizes, and it draws on many of the same pharmacological tools used in human practice.
Anesthesia professionals and training
Anesthesiologists are physicians who practice anesthesiology as a medical specialty. In most systems they hold overall responsibility for the anesthetic plan, even when much of the hands-on work is carried out by others. In the United States, specialty training typically follows medical school with a residency of about four years, and the anesthesia services themselves are often delegated under the anesthesiologist's direction. Staffing models frequently let one attending anesthesiologist oversee two to four hands-on providers at once, who may be nurse anesthetists or resident physicians.
Nurse anesthetists are a major part of the workforce in some countries. In the United States, CRNAs are advanced practice registered nurses (APRN) certified as a nurse anesthetist. Nurse anesthesia organizations established educational institutions for nurse anesthetists in 1952, and established the CRNA certification in 1957. The rules governing their supervision have shifted over time. Before 2001, Medicare required that physicians supervise CRNAs in the administration of anesthesia. In 2001, Medicare's rules changed, allowing individual states to decide whether CRNAs may administer anesthesia without physician supervision. One policy review reported that twenty-four states have partially or fully opted out of the supervision requirement. Some states go further than others; in Arizona, for example, it is within the scope of practice for a CRNA to provide epidural analgesia without the presence of a physician.
The debate over scope of practice is contested. Nurse anesthesia bodies argue that a nurse anesthetist is qualified independently to perform, without supervision by an anesthesiologist or other physician, all the services that an anesthesiologist can perform. The American Society of Anesthesiologists takes a different view: nurse anesthetists are qualified to perform some, but not all, of these services, and only under the supervision of a physician, preferably an anesthesiologist. Comparative studies have generally found similar results, with one review noting that CRNA and anesthesiologist services have similar clinical outcomes. Still, the policy question is not settled, and the effects of expanded scope on access and cost remain unclear.
The shortage of providers is most acute in low- and middle-income countries. One scoping review estimated that five billion people worldwide face challenges to access anesthesia care, with more severity in low- and middle-income countries. Several sub-Saharan countries have fewer than one anesthesia provider per 100,000 residents. To cover demand, more than 100 countries globally use anesthetic task shifting/sharing, that is, having NPAPs or general practitioners, instead of specialist anesthesiologists, provide anesthesia care. The World Health Organization uses the term task shifting for moving responsibilities to workers with shorter training so that scarce skills are used efficiently. Non-nursing providers appear under various titles, such as clinical officer in Kenya, Uganda and Malawi. In Uganda, the country's anesthesia services rely on about 400 anesthetic officers, while it has only around 60 anesthesiologists. Anesthesia is also unusual in that anesthesia is perhaps the only specialty where formal, nonphysician cadres practice in some settings with full scope of practice.
Task-sharing is contentious. Supporters argue that non-physician providers can provide safe, high-quality anesthesia care without anesthesiologist oversight and in doing so decrease costs and increase access, while critics raise concerns about safety and training, and both sides acknowledge that job security, salary and medicolegal liability also shape the debate. Financing rules can also constrain the model: in some countries, such as Iran, insurance policies reimburse anesthesia services only if delivered by physician anesthesiologists, limiting task-sharing models. Emigration adds further strain, as many anesthesiologists, particularly in low-income countries, migrate for better opportunities, further straining local services.
Teamwork is treated as a safety measure in its own right. Anesthesia crisis resource management was adapted from aviation: a thorough analysis of 60 airline accidents, including data from cockpit voice and flight data recorders, disclosed lethal decision-making errors by individual crew members or inadequate teamwork by crews. Human error is a recurring finding in anesthesia incident studies; in one Australian series, human factors were causal or contributive to 83% of the first 2000 events reported. Training in this area emphasizes communication, managing fatigue and avoiding fixation errors, along with practical habits such as calling for help early, distributing the workload and cross-checking. Checklists are a related tool. Since the initial publication of the World Health Organization (WHO) surgical safety checklist in 2009, the potential for checklists and other cognitive aids to reduce errors during surgical procedures has been recognized worldwide.
Historical development
Pre-modern pain control
Long before chemical anesthesia, surgeons relied on a mix of intoxication, plant preparations and physical methods. Opium, henbane, acupuncture, herb mixtures, alcohol, cannabis, carotid compression, mandrake, and others were used to ease pain or produce "insensibility" in patients. Most of these methods were unreliable, and operations were typically performed quickly on conscious or partly sedated patients.
Nitrous oxide, ether and chloroform (late 18th to mid 19th century)
Nitrous oxide was the first gas studied for surgery. At the beginning of the nineteenth century, Sir Humphry Davy suggested that nitrous oxide might be used with advantage in surgical operations. The idea went unused until the American dentist Horace Wells, after seeing the gas given at a public lecture in December 1844, had a tooth extracted while under its effects and reported little discomfort. A later public attempt in 1845 failed, and nitrous oxide did not become a standard surgical agent.
Ether was the decisive step. Crawford Long gave ether to a patient for a minor operation in Georgia in 1842, but it was not until December, 1849, that Dr. Long saw fit to publish his results. The more influential event came on 16 October 1846, when William T. G. Morton administered ether to a patient undergoing removal of a neck tumour at Massachusetts General Hospital in Boston. Morton is credited with developing a delivery instrument that could control the level of ether given during surgery, which made the practice more controllable, although the priority of his claim remains disputed. Within a year, James Young Simpson introduced chloroform in 1847, and it was widely used in surgery in the following decades, particularly in Britain.
Local anesthesia, regional blocks and synthetic drugs (1880s to early 1900s)
Cocaine, isolated from coca leaves, was the first local anesthetic used in clinical practice; Carl Koller applied it to the eye in 1884. Its addictive properties and toxicity drove the search for alternatives. In 1905 a synthetic substitute, procaine, was developed from cocaine's chemistry. It was developed from cocaine as a local anaesthetic without the euphoric and addictive effects of cocaine. It was better known by its brand name, novocaine, and was widely used in dentistry.
Regional anesthesia advanced in the same period. August Bier performed spinal anesthesia in 1898 and designed a larger-bore needle that did not require an introducer. The Quincke–Babcock needle, designed in 1914, became the standard spinal needle for the future.
The 20th century
The period after 1900 brought the agents and tools of modern practice. Intravenous barbiturates, beginning with thiopental in the 1930s, allowed rapid induction without an inhaled gas. Purified curare-derived neuromuscular blockers entered clinical use in the early 1940s, enabling muscle relaxation for intubation and surgery without deep anesthesia. Lidocaine, synthesized in 1943, became the dominant amide local anesthetic. Volatile agents such as halothane, introduced in the 1950s, replaced flammable ether and chloroform.
Monitoring developed more slowly. Anesthesia researchers were early pioneers of physiological measurement; in 1957 Severinghaus developed the first blood gas analyzer to measure the partial pressure of CO2 and O2 in a sample of arterial blood. Pulse oximetry, which measures oxygen saturation without blood sampling, was first presented in 1974 by Takuo Aoyagi, an engineer at Nihon Kohden. The ratio of red to infrared light absorption of pulsating components was used in the design, which was commercialized by BIOX/Ohmeda in 1981 and by Nellcor in 1983. Its adoption in the operating room was rapid: in 1986, intraoperative pulse oximetry became a standard of care. Together with capnography and standard monitoring guidelines, it marked a shift from reliance on clinical signs to continuous measurement during anesthesia.
Research frontiers
Neuroscience of consciousness
Anesthetics are used as research tools as well as clinical drugs. Because they can switch consciousness off and on in a controlled way, investigators can ask which brain processes disappear when awareness is lost. Machine-learning classifiers trained on short EEG segments from volunteers given propofol have tracked unconsciousness with high accuracy, and they still performed well when applied to surgical patients recorded under different hardware and conditions. The reported patient-cohort AUC values, a measure of how well a model separates the two states, fell between about 0.95 and 0.98 (1-4). Other groups have focused on connectivity rather than power. In a study of 31 surgical patients with high-density EEG, alpha-band coupling between parietal, occipital and subcortical regions appeared essential for maintaining consciousness, and its breakdown marked the point of transition into unconsciousness (2-2, 2-4).
Different drugs can produce different signatures. A volunteer study comparing dexmedetomidine, propofol and natural sleep found that sensory disconnection and true unconsciousness had separable EEG features, and that loss of consciousness was linked to reduced activity in the anterior and posterior cingulate cortices (6-6). A small retrospective study of ketamine induction, by contrast, described a "gamma burst" pattern of alternating slow-delta and gamma oscillations, which the authors linked to NMDA-receptor circuit mechanisms (8-5, 8-8).
A persistent methodological problem is that many EEG markers were long assumed to indicate unconsciousness, even though patients cannot report their experience during anesthesia (3-8). Whether a given signature reflects the loss of experience itself, or only the loss of responsiveness, remains an open question.
Pharmacogenomics and personalized anesthesia
Individual differences in genes encoding drug-metabolizing enzymes, transporters and receptors can change how anesthetic drugs behave. A 2026 scoping review of literature from 2015 to 2025 summarized several examples. Variants in CYP3A4/5 and POR alter midazolam metabolism and can prolong sedation, a variant of CYP2B6 is associated with reduced clearance of propofol and ketamine, and butyrylcholinesterase deficiency markedly prolongs paralysis after succinylcholine (18-3, 18-4, 18-5, 18-6). Propofol itself is cleared mainly through glucuronidation by UGT1A9, and a systematic review suggested that genotype-based adjustment could be combined with target-controlled infusion algorithms (12-1, 12-2). Point-of-care genetic tests that return results within about an hour are also being developed for other drugs, which may eventually be adapted to anesthetic genes (12-3).
Population genetics adds another layer. A whole-exome study of 625 healthy Colombian adults found that allele frequencies in anesthesia-related genes differed from those in other populations, and that most variants were rare (14-7, 14-8). Genotype-guided dosing is therefore promising but still largely at the research stage; the pharmacogenetic disorders already recognized in clinical practice, such as RYR1-linked malignant hyperthermia, are the best-established examples (15-2).
Drug design
Current development aims to produce agents with more selective receptor actions, shorter recovery and fewer side effects. Two examples from recent practice illustrate the direction. Remimazolam is a short-acting benzodiazepine that is broken down by tissue esterases, which allows a fast offset that does not depend on liver clearance. Sugammadex, a modified cyclodextrin, reverses rocuronium-induced muscle blockade by encapsulating the drug, a mechanism distinct from the older cholinesterase inhibitors. Ongoing work seeks to separate the desired sedative or analgesic actions from cardiovascular and respiratory depression, but the most effective compounds may differ from those that are already approved.
Digital and automated systems
Closed-loop systems adjust drug infusion automatically in response to a measured signal, usually a depth-of-anesthesia index derived from EEG. A registry analysis identified about ten closed-loop anesthesia and sedation trials, including closed-loop propofol delivery in pediatric cardiac surgery (26-3). Evidence for these systems is growing. A systematic review of eight randomized controlled trials (568 patients) examined AI-assisted intraoperative management, most often using the Hypotension Prediction Index, a machine-learning tool that forecasts low blood pressure (19-8, 19-9). Reinforcement-learning controllers for propofol hypnosis have been tested in human volunteers, with reported improvements in accuracy and stability relative to published manual performance (20-4).
Validation is the main barrier. A pilot study of closed-loop delivery of propofol and remifentanil guided by EEG feedback was designed to test feasibility before any controlled trial of clinical benefit (21-2, 21-3). An earlier automated sedation system, McSleepy, received FDA approval but was later withdrawn, while the Sedasys system remained a research prototype (24-5). These cases show that regulatory approval, clinical adoption and the demonstration of consistent benefit across populations are separate hurdles that automated anesthesia must still clear.
Environmental impact
Anesthesia has a measurable climate footprint, and most of it comes from a small group of agents. Inhaled anesthetics are potent greenhouse gases. Their heat-trapping effect, measured as global warming potential (GWP), is hundreds to thousands of times that of an equal mass of carbon dioxide. Most of the agent is exhaled unchanged, and in clinical use it is either collected by scavenging systems or eventually released to the air mostly vented through gas scavenging (vacuum) systems to protect against indoor occupational exposure but are ultimately released. Estimates place volatile agents at roughly 0.01–0.10% of total global carbon dioxide equivalent (CO2e) emissions, a small share, but one the specialty can influence directly. On a 100-year horizon, the ASA's figures rank desflurane highest, with a GWP of 2540, followed by isoflurane (539), nitrous oxide (273) and sevoflurane (144) Desflurane is the inhaled anesthetic that is the most potent greenhouse gas. Values depend on the time horizon chosen. A 20-year calculation gives sevoflurane 349, isoflurane 1401 and desflurane 3714 , so the gap between desflurane and the others widens over shorter periods.
Raw GWP ignores how much agent is actually used. Because nitrous oxide is weak, clinical effect requires large masses, and in clinically equivalent doses, ranked climate impacts: desflurane > nitrous oxide > isoflurane > sevoflurane. Desflurane's persistence in the atmosphere adds to its impact. One source gives an atmospheric lifetime of about 10 years, compared with 1.2 years for sevoflurane and 3.6 years for isoflurane , while another gives 14 years for desflurane, so the exact figure depends on the source. Nitrous oxide is a different kind of problem. It is a 'stock' pollutant i.e. has a long lifetime and accumulates overtime. It also depletes the ozone layer . Its footprint grows sharply when it is used as a carrier gas: in one analysis, a 60% nitrous oxide carrier raised the 20-year footprint of sevoflurane 5.9-fold and of isoflurane 2.9-fold, while desflurane's footprint fell . Over 100 years, the same carrier raised sevoflurane's footprint 19-fold and isoflurane's 9-fold, while desflurane was unchanged .
Several strategies reduce these emissions. The most direct is to avoid the highest-impact agents: guidance recommends avoiding desflurane and N_2O, practising 'low-flow' anaesthesia, and utilising non-inhalational methods, such as regional anaesthesia or TIVA. Lowering fresh gas flow reduces the amount of agent consumed, and one modelling study found that using low fresh gas flows or end-tidal control reduces the carbon footprint of inhalational general anaesthesia. Switching to total intravenous anesthesia (TIVA) with propofol is the largest single change. A French prospective life-cycle study of general, spinal and inhalation techniques found median emissions of 10.8 kgCO_2e [9.1-12.6] for inhalation anaesthesia, 6.6 kgCO_2e [6.4-6.9] for TIVA and 5.5 kgCO_2e [5.3-6.2] for spinal anaesthesia. The advantage is not automatic, however. Other work found that spinal being a default low carbon approach does not hold, and that single-use equipment, electricity for the patient air warmer, and pharmaceuticals were major sources of carbon dioxide equivalent emissions across all anesthetics. In a modelling study of adults, TIVA's footprint rose by 0.04–0.60 kgCO_2e.h^-1, compared with 4.01 kgCO_2e.h^-1 for the median consumption sevoflurane scenario for each hour beyond the first 22 minutes, which led the authors to conclude that TIVA suits appropriate cases lasting longer than 22 min. A later exchange of letters showed that the break-even point depends on assumptions, including whether processed EEG monitoring is used .
Anesthesia staff also face direct exposure to waste gases. Leaks come from tank valves, high- and low-pressure machine connections; connections in the breathing circuit, defects in rubber and plastic tubing, hoses, reservoir bags, and ventilator bellows, and the Y-connector. OSHA estimates that more than 200,000 health care professionals are potentially exposed, and reported effects include nausea, dizziness, headaches, fatigue, and irritability, as well as sterility, miscarriages, birth defects, cancer, and liver and kidney disease. A 2023 meta-analysis linked chronic exposure to deoxyribonucleic acid (DNA) damage, although the evidence base is limited and observational. The main control is engineering: engineering controls such as an appropriate anesthetic gas scavenging system are the first line of defense. Scavenging alone is not sufficient, since even when scavenging and venting systems are in place, workers may be exposed to these gases during disconnection or through leaks. A Brazilian study found that all isoflurane and sevoflurane concentrations in unscavenged operating room were higher than the US recommended limit (2 parts per million), and that scavenging reduced levels without bringing them fully within limits . US exposure limits are dated, since NIOSH has not revised its recommended exposure limits since 1977, and the Occupational Safety and Health Administration has voluntary guidelines but no regulatory exposure limit for anesthetic gases.
Choosing an anesthetic therefore involves trade-offs among clinical benefit, cost and environmental footprint. The clinical need comes first: regional and intravenous techniques are not always appropriate, and volatile agents remain useful for some patients. Where the choice is open, the evidence suggests that the environmental and financial costs can point in the same direction. One simulation of health-system procedures estimated that minimal flow sevoflurane produces 26.5 times more CO2e than TIVA, rising to 61.8 times more at 2 l min⁻¹ FGF. Waste from TIVA, such as disposable equipment, and procedure length also matter, so the lowest-carbon option depends on the case. Regulation and practice have already moved in this direction. One analysis of climate impact reported that greenhouse gas impact from anesthetic gases has decreased by 27% between 2014 and 2023, and the same reporting credited restrictions on desflurane. Measures such as patient safety checks, scavenging maintenance and occupational monitoring must be weighed alongside these choices.
Ethics, access, and global health
Informed consent and patient choice
Anesthesia carries ethical weight that differs from many other fields of medicine. Its hazards are built into the care itself. Khan and Merry describe anesthesia as "intrinsically hazardous and not intrinsically therapeutic," which means that its safety, and not only its benefit, must be weighed explicitly.Safety is particularly important in anesthesia because anesthesia is intrinsically hazardous and not intrinsically therapeutic. Consent discussions therefore need to address specific risks that vary by technique, such as aspiration, awareness during general anesthesia, airway injury, and reactions to drugs. Patients also need to understand the realistic alternatives, including regional techniques or sedation, so that their autonomy in choosing among them is meaningful. This process draws on general principles of informed consent and medical ethics.
Global disparities in access
Access to anesthesia is one of the widest gaps in global health care. Around five billion people lack access to safe and affordable surgical and anesthesia care, and the burden falls heavily on low-income countries, where most of this population lives.Despite of global recommendations and efforts, five billion lack access to safe, affordable surgical and anesthesia care with more than 80% from low income countries Surgical capacity is also concentrated: 75% of operations worldwide are performed in the world's wealthiest one-third of countries.
The workforce gap is central. A target of 40 specialists per 100,000 people implied a worldwide shortage of about one million, and the Lancet Commission judged that goal unrealistic and recommended 20 per 100,000 instead. Densities are far lower in some regions. In East Africa, the figure runs from 0.02 per 100,000 in Burundi to 0.39 in Kenya. Afghanistan was reported to have only nine anaesthetists for a population of 32 million.
The shortfall is not only about people. Khan and Merry argue that many countries face a problem of maldistribution rather than an absolute scarcity, since there appears to be no global shortage of equipment or medicines so much as a shortage of funding for them in many regions. They also point to large gaps in infrastructure, including operating rooms, and a substantial global shortage of adequately trained health professionals. Access to care is therefore framed as a basic right, and not a matter of charity, in many analyses of global anesthesia.
Essential medicines and regulation
The World Health Organization's model list of essential medicines defines the core drugs a functioning anesthesia service should be able to obtain. Its 2025 edition, the 24th, includes inhaled agents such as isoflurane, sevoflurane and nitrous oxide, and injectable agents such as ketamine and propofol. Earlier WHO-based guidance for anesthesia and pain management extended the core set to muscle relaxants, reversal agents such as neostigmine and naloxone, and rescue drugs such as epinephrine and atropine.
Inclusion on the list does not mean unrestricted use. Opioids such as morphine, fentanyl and codeine appear on the same list as anesthetic agents, and they are subject to national controlled-substance rules and international narcotic-control treaties, which limit prescribing, storage, and record-keeping. Balancing availability for surgery and pain relief against the risk of misuse is a continuing regulatory task, particularly in settings where supply is thin.
Diversion and substance use among professionals
Anesthesia professionals have unusually direct access to potent drugs. Anesthesia providers use highly addictive medications each day and can easily divert or misuse these medications. Drug diversion, which occurs when controlled or prescription medications are obtained or used illegally, is a particular concern because clinicians often prepare and give intravenous drugs with little direct supervision. Estimates of substance use disorder among anesthesia professionals range from 1 to 2 percent, with a rise over the past four decades. Opioids remain the most commonly misused drugs in this group, but propofol misuse has also become common, and anesthesia providers accounted for 68% of the cases in 88 published reports of propofol diversion between 1990 and 2020.
Diversion also harms patients. Misuse during work can impair judgment and lead to errors, and diverted analgesics can leave patients under-treated for pain. Institutional responses include structured diversion monitoring, unannounced checks of drug-use patterns, and recovery programs. Clinicians returning to practice after treatment are often assigned routine lists under direct supervision, with agreed rules for how they obtain and administer opioids and close monitoring for aberrant behavior. Such programs aim to protect both the clinician and the patients who depend on safe drug handling.
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