What Gases Are Used for Anesthesia During Surgery?

Modern surgical anesthesia relies on a small family of inhaled gases and vapors, most of them halogenated ethers, delivered alongside oxygen or an oxygen-air blend. The three volatile agents you are most likely to encounter in an operating room today are sevoflurane, isoflurane, and desflurane. Nitrous oxide, one of the oldest anesthetic gases still in clinical use, sometimes joins the mix as an adjunct. Each agent has a distinct clinical personality, and the choice between them depends on the patient, the surgery, and even the hospital’s budget.

The Three Volatile Agents in Routine Use

Sevoflurane, isoflurane, and desflurane are all fluorinated ethers. They exist as liquids at room temperature and are converted to vapor by a specialized device called a vaporizer mounted on the anesthesia machine. Once inhaled, they cross from the lungs into the bloodstream and reach the brain within minutes, producing unconsciousness.

Sevoflurane is probably the most widely used of the three worldwide. It has a mild, non-irritating smell and causes very little coughing or breath-holding when a patient breathes it in, which makes it the go-to choice for mask inductions, especially in children who cannot have an IV placed while awake.¹ Its low blood solubility means the anesthesiologist can dial the depth of anesthesia up or down relatively quickly, and patients tend to wake up smoothly once the vapor is turned off.

Isoflurane has been around since the early 1980s and remains in widespread use largely because it is inexpensive. It works well for maintaining anesthesia during longer procedures, though induction and recovery are somewhat slower than with sevoflurane or desflurane. In veterinary medicine, isoflurane is still the workhorse agent for the same economic reason.² One trade-off is that isoflurane can lower blood pressure more noticeably during prolonged cases, something the anesthesia team monitors closely.

Desflurane has the lowest blood solubility of the three, which translates to the fastest wake-up. A meta-analysis comparing emergence times found that patients given desflurane opened their eyes roughly two and a half minutes sooner and responded to commands about two minutes sooner than those given other agents.³ That matters in ambulatory surgery, where a faster recovery can mean earlier discharge. The downside is that desflurane is pungent and irritating to the airway, so it is almost never used for a mask induction. It also requires a heated, pressurized vaporizer, which adds equipment cost. A randomized trial comparing desflurane and sevoflurane under low-flow conditions confirmed that desflurane costs more per hour of anesthesia, though the faster turnover may partially offset that expense.⁴

Carrier Gases and Nitrous Oxide

No patient breathes pure anesthetic vapor. The volatile agent is always mixed into a carrier gas that supplies oxygen to the lungs. The simplest carrier is pure oxygen, which maximizes the oxygen reservoir in the lungs and makes it impossible for the patient to receive a dangerously low oxygen mixture.⁵ Many anesthesiologists use a blend of medical air and oxygen instead, because very high oxygen concentrations over long periods can cause their own problems, including absorption of lung segments that stay collapsed during surgery.

Nitrous oxide occupies a strange middle ground: it is both a carrier gas and a weak anesthetic in its own right. When delivered at high concentrations alongside a volatile agent, nitrous oxide speeds the uptake of the second agent into the blood through a phenomenon called the second gas effect. High inspired concentrations of nitrous oxide create a concentration gradient that pulls other gases along, and this effect is most pronounced with the low-solubility agents now in common use.⁶ Despite this useful property, nitrous oxide has been falling out of favor. It contributes to postoperative nausea, expands air-filled body cavities (a problem in bowel or ear surgery), and is a potent greenhouse gas. Many institutions now omit it routinely and rely on air-oxygen mixtures instead.⁵

How Inhaled Anesthetics Produce Unconsciousness

The honest answer is that scientists still do not fully understand the mechanism. What is clear is that volatile agents interact with multiple targets in the brain rather than flipping a single switch. One important effect involves GABA, the brain’s main inhibitory chemical messenger. Sevoflurane and halothane, for instance, increase the amount of GABA available outside nerve cells, apparently by causing GABA transporters to work in reverse and release GABA rather than mop it up.⁷ More GABA means more inhibition of nerve signaling, which contributes to the loss of consciousness and the suppression of movement.

Anesthesiologists gauge the strength of an inhaled agent using a measure called the minimum alveolar concentration, or MAC. It is defined as the concentration in the lungs at which half of patients will not move in response to a painful stimulus.⁸ Each agent has its own MAC value, and those values shift with the patient’s age, body temperature, and concurrent medications. An older patient, for example, needs a lower concentration than a younger one. MAC gives the anesthesia team a standardized way to compare potency across agents and adjust doses in real time.

Effects on the Heart and Blood Vessels

All volatile agents depress the cardiovascular system to some degree. They relax blood vessel walls, which can lower blood pressure, and they reduce the force of heart muscle contraction. Research in animal models has shown that isoflurane reduces mean arterial pressure in a dose-dependent way, with higher concentrations producing more pronounced drops.⁹ Both isoflurane and halothane directly weaken heart muscle contractility as their concentrations rise.¹⁰ In healthy patients, these effects are manageable and predictable. In someone with severe heart disease, the anesthesia team may adjust the agent, reduce the concentration, or lean more heavily on intravenous drugs to maintain stable blood pressure throughout surgery.

Volatile agents also relax smooth muscle elsewhere in the body, including the uterus. This property matters during cesarean delivery. A Japanese database study found evidence that volatile anesthesia may increase bleeding at delivery compared with total intravenous anesthesia, consistent with the known uterine-relaxing effect of these gases.¹¹ For that reason, when volatile agents are used during a cesarean, the anesthesiologist typically keeps the concentration low and switches promptly after delivery.

Malignant Hyperthermia

One of the most serious risks unique to inhaled anesthetics is malignant hyperthermia, a rare but life-threatening reaction in which the patient’s muscles go into an uncontrolled state of sustained contraction, generating massive amounts of heat. Body temperature can spike dangerously within minutes. The condition is a pharmacogenetic disorder, meaning it requires both a genetic predisposition and a triggering drug.¹² All of the potent volatile agents in current use, including sevoflurane, desflurane, and isoflurane, can trigger it. The depolarizing muscle relaxant succinylcholine is the other common trigger.

The genetic culprit in most cases is a mutation in the gene encoding the ryanodine receptor, a calcium-release channel in skeletal muscle. Over 400 variants have been identified in this gene, and at least 34 are known to cause malignant hyperthermia.¹² In susceptible individuals, exposure to a triggering agent causes the channel to open uncontrollably, flooding the muscle cell with calcium and setting off a metabolic crisis.¹³ Treatment with the drug dantrolene, which blocks the ryanodine receptor, has dramatically improved survival. If you or a blood relative have ever been told you are susceptible, your anesthesiologist needs to know before surgery so that a completely non-triggering anesthetic plan can be arranged.

Gases That Have Come and Gone

The lineup of inhaled anesthetics has changed considerably over time. Diethyl ether and chloroform were the original agents of the 19th century. Both worked but came with serious drawbacks: ether was explosive, and chloroform caused fatal heart arrhythmias with alarming frequency. Beginning in the 1950s, halogenated agents replaced them.¹⁴ Halothane became the dominant agent for decades. It was non-flammable, potent, and pleasant to breathe. But halothane occasionally caused a severe form of liver damage, and once sevoflurane and desflurane arrived in the 1990s, halothane was gradually phased out in high-income countries. Enflurane, another agent from the 1970s, fell out of favor for similar reasons of safety margin.

One artifact of halothane’s long reign is that much of the foundational research on anesthetic mechanisms was done with halothane. You will still see it referenced in pharmacology textbooks and laboratory studies, even though you are very unlikely to encounter it in a modern operating room.

The Environmental Cost of Anesthetic Gases

Less than five percent of a delivered halogenated anesthetic is metabolized by the patient’s body. The other 95 percent or more is exhaled unchanged and vented into the atmosphere through the operating room’s scavenging system.¹⁵ These gases are potent greenhouse agents. The global warming potential of a halogenated anesthetic can be up to 2,000 times that of carbon dioxide on a per-molecule basis.¹⁵ Desflurane is the worst offender among the current agents, with a substantially higher warming potential than sevoflurane, which is one reason some hospitals have begun restricting or eliminating desflurane from their formularies.

Nitrous oxide compounds the problem. It is both a greenhouse gas and an ozone-depleting substance, and because it is often used at high flow rates, the absolute volume released can be large. Reducing fresh gas flow rates is one of the most effective strategies for cutting emissions. Running the anesthesia machine at a flow of one liter per minute instead of several liters can reduce agent consumption by 75 percent or more while also saving money and warming and humidifying the inspired gas.¹⁶ Low-flow techniques also cut costs, since less vapor is wasted into the scavenging system.¹⁷

One technical wrinkle with low-flow anesthesia is that the carbon dioxide absorbent inside the breathing circuit gets more use. Older absorbents containing strong bases like sodium hydroxide could break down volatile agents into unwanted byproducts, including compound A from sevoflurane and carbon monoxide from desflurane and isoflurane.¹⁸ Newer absorbent formulations avoid strong bases and largely eliminate this issue, making low-flow anesthesia both safer and greener.

Protecting Operating Room Staff

Even with scavenging, trace amounts of anesthetic gas escape into the operating room air. Staff members who spend entire shifts in that environment can accumulate meaningful exposure over time. Under U.S. standards, workplace air must be kept below 25 parts per million for nitrous oxide and below 2 parts per million for halogenated agents, with measurements taken every six months.¹⁹ When those limits are observed, few adverse health effects have been documented, though earlier decades of poorly ventilated operating rooms did produce concerns about reproductive harm and organ toxicity among anesthesiologists and nurses.²⁰

Practical measures to keep exposure low include well-maintained scavenging systems, good room ventilation, tight connections on breathing circuits, and, again, reducing fresh gas flows. Avoiding desflurane and nitrous oxide when they are not clinically necessary also helps, since both contribute disproportionately to ambient gas levels.²⁰

Anesthetic Gases and the Developing Brain

Starting around 2000, a series of animal studies raised alarm that common anesthetic agents, including volatile gases, could damage the developing brain and impair learning and memory. The concern was especially acute for infants and toddlers undergoing surgery. Animal models showed that exposure to volatile agents, propofol, and ketamine disrupted normal brain development, while dexmedetomidine and xenon appeared to be protective.²¹

The reassuring news is that several large human studies have not found measurable harm from brief anesthetic exposure in young children. The MASK, PANDA, and GAS studies, three landmark investigations, all concluded that a single, short exposure to general anesthesia in infancy or early childhood was not associated with adverse cognitive outcomes, regardless of the agent used.²¹ The question of whether repeated or prolonged exposure carries risk is harder to answer and is still being studied. For now, the general consensus is that necessary surgery should not be delayed out of fear of anesthetic neurotoxicity, but elective procedures in very young children should be timed thoughtfully.

Xenon as a Future Anesthetic

Xenon is a noble gas that sits at the far edge of the periodic table and at the far edge of clinical anesthesia. It produces unconsciousness, has virtually no side effects on the heart or other organs, and appears to protect the brain after oxygen deprivation.²² Unlike halogenated ethers, xenon is not metabolized at all and has zero global warming potential. It checks almost every box for an ideal anesthetic gas.

The catch is cost. Xenon is extracted from the atmosphere by fractional distillation of air, and it exists in vanishingly small concentrations, roughly one part in 11.5 million. Producing enough for a single surgery can cost hundreds of dollars. Closed-circuit anesthesia systems that recycle exhaled xenon can reduce waste, but the economics remain prohibitive for routine use. Research continues into xenon’s neuroprotective applications, particularly in newborns who have suffered birth asphyxia, where its brain-protecting properties could justify the expense for a specific, high-stakes indication.