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Anesthesia Fact Sheet

What is anesthesia?

Photo of a woman recieving anesthesia.

Anesthesia is a medical treatment that prevents patients from feeling pain during surgery. Anesthesia has made possible countless procedures that improve human health, longevity and quality of life. Every year, millions of Americans safely undergo surgery with anesthesia, although some risks exist.

Anesthesia consists of several components, including sedation, unconsciousness, immobility, analgesia (lack of pain) and amnesia (lack of memory). Scientists have developed drugs called anesthetics that target each of these elements.

Most anesthetics fall into one of two broad categories: general or local/regional.

What is general anesthesia?

General anesthesia affects the entire body. It is used when it is important for a patient to be unconscious. Many major, life-saving procedures like open-heart surgery, brain surgery or organ transplantation would be impossible without general anesthesia.

General anesthetics are either delivered intravenously or breathed in as a gas. Intravenous anesthetics act quickly and disappear rapidly from the bloodstream, so patients can go home sooner after surgery. Inhaled anesthetics may take longer to wear off.

Although general anesthetics are usually considered quite safe for most patients, they can pose risks, particularly for elderly patients, those with certain genetic variations, and those with some chronic, systemic diseases, such as diabetes. In addition, some patients, especially the elderly and children, may have lingering effects for several days after general anesthesia. Fortunately, serious side effects—such as dangerously low blood pressure—are much less common than they once were.

What are local and regional anesthesia?

Photo of a woman recieving local anesthesia to her mouth.
Dental injections deliver local anesthetics that numb part of the mouth.

Doctors use local and regional anesthetics to block pain in a part of the body. Local anesthetics affect a small part of the body, such as a single tooth. Regional anesthetics affect larger areas, such as everything below the waist.

With local and regional anesthetics, patients can remain conscious and comfortable during surgery. But, like all drugs, these anesthetics can have side effects, and delivering them to the right spot is sometimes difficult.

How does anesthesia work?

For many decades after anesthetics became a routine part of surgery, practically nothing was known about how they work. Virtually all scientists believed that anesthetics blocked nerve cell signaling by disrupting fatty molecules in the membranes that envelop cells. This theory, first put forward in the early 1900s, dominated research on anesthetics for much of the 20th century. Anesthetics are difficult to work with in the laboratory, and the lack of tools to study them at the molecular level contributed to this period of slow scientific progress.

Today, advances in cell biology, genetics and molecular biology have transformed anesthesiology into an active area of research. Scientists no longer think that anesthetics work by acting on fatty molecules in cell membranes. The bulk of the evidence now supports the idea that the drugs interfere with nerve signals by targeting specific protein molecules embedded in nerve cell membranes. Researchers also believe that inhaled and intravenous anesthetics each act on a different set of molecules to bring about their characteristic effects.

How was surgery performed before anesthesia was available?

Prior to the 1840s, doctors and dentists did not routinely use anesthesia when operating on patients. Most doctors attempted surgery only when it was absolutely necessary to save a person's life, and operations were largely limited to amputations and removal of external growths. Although alcohol, opium or other botanicals sometimes helped alleviate the agony, most surgical patients remained conscious and endured excruciating pain.

When did doctors and dentists begin using anesthesia?

Replica of the inhaler used by William T. G. Morton in 1846 in the first public demonstration of surgery using ether.
Replica of the inhaler used by William T.G. Morton in 1846 in the first public demonstration of surgery using ether.
Wood Library/Museum, Park Ridge, IL.

In 1846, a dentist publicly demonstrated that ether, a colorless liquid that vaporizes quickly, would put patients to sleep during surgery. The practice began to spread. Doctors and dentists soaked a sponge or a cloth with ether and had patients breathe in the fumes through an inhaler.

The fumes knocked the person out, but there was no way to control the amount inhaled. If patients inhaled too little, they could wake up during surgery and flail about in pain; if they inhaled too much, they might never wake. To make matters worse, ether is highly flammable, and a spark in the operating room could cause a dangerous explosion. Despite the problems with ether, its use enabled surgeons to perform internal procedures that would have been too painful or complicated to conduct on conscious patients.

The introduction of less flammable anesthetic gases made operating rooms safer, and the discovery of intravenous anesthetic agents such as sodium thiopental made it possible for surgeons to control the dose. But well into the 1950s, doctors still usually sedated their patients using some type of anesthetic gas and monitored them with nothing more sophisticated than a stethoscope. Dangerous side effects were common and included heart rhythm and breathing problems, lowered blood pressure, nausea and vomiting.

Anesthesiology students training with a patient simulator.
Anesthesiology students training with a computer-controlled mannequin called a patient simulator.
Jeffrey Taekman.

How are anesthesiologists trained?

Like all medical doctors, anesthesiologists earn an undergraduate degree (often in a life sciences field) and a medical degree (M.D. or D.O.).Then they must complete a 4-year residency program in anesthesiology. Many choose to complete an additional 1-year fellowship in a specialty such as pain management, pediatric anesthesiology or critical care medicine.

What do anesthesiologists do during surgery?

Anesthesiologists carefully monitor patients throughout surgery using electronic devices that continually display vital signs. Major advances in monitoring include the continuous measurement of blood pressure, blood oxygen levels, heart function and respiratory patterns. These advances have dramatically improved the safety of general anesthesia and make it possible to operate on many patients who were previously considered too sick to undergo surgery.

What do anesthesiologists do outside of the operating room?

Photo of an anesthesiologist monitoring a patient's vital signs.
Digital devices enable anesthesiologists to monitor patient vital signs throughout surgery.

The role of the anesthesiologist has expanded to include caring for patients during postoperative recovery. Anesthesiologists also provide anesthesia for nonsurgical procedures such as endoscopy and various cardiac interventions, as well as during labor and delivery. As experts in pain management, anesthesiologists may manage pain clinics or advise other specialists on how to manage pain.

What new advancements in anesthesia are on the horizon?

As scientists learn more about the molecular mechanisms by which anesthetics cause their various effects, they will be able to design agents that are more targeted, more effective and safer, with fewer side effects.

Observations of the short- and long-term effects of anesthetics on subsets of the population, such as the elderly or cancer survivors, will reveal whether certain anesthetics are better than others for members of those groups. Research on how a person's genetic makeup influences the way he or she responds to anesthetics will enable doctors to further tailor anesthesia to individual patients.

How does anesthesia research contribute to other fields of science and medicine?

Knowledge of how anesthetics affect pain and consciousness helps scientists gain a better understanding of the mechanisms that underlie these physiological states. Understanding these mechanisms could lead to new ways to alleviate pain and to new treatments for conditions associated with a decrease or loss of consciousness, such as epilepsy and coma. Studies of anesthesia may also provide insights into the nature of consciousness itself.

NIGMS is a part of the National Institutes of Health that supports basic research to increase our understanding of biological processes and lay the foundation for advances in disease diagnosis, treatment and prevention. For more information on the Institute's research and training programs, see

Content updated May 2015

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​Learn More

Waking Up to Anesthesia: Learn More Before You Go Under from the April 2011 issue of NIH News in Health

Anesthesiology Research and Training programs supported by the National Institute of General Medical Sciences

Anesthesia from MedlinePlus

Profile of anesthesiologist Daniel Sessler in the NIGMS magazine, Findings

This page last reviewed on October 01, 2015