Anesthesia, or anaesthesia (see spelling differences; from Greek αν-, an-, "without"; and αἲσθησις, aisthēsis, "sensation"), has traditionally meant the condition of having sensation (including the feeling of pain) blocked or temporarily taken away. This allows patients to undergo surgery and other procedures without the distress and pain they would otherwise experience. The word was coined by Oliver Wendell Holmes, Sr. in 1846. Another definition is a "reversible lack of awareness", whether this is a total lack of awareness (e.g. a general anaesthestic) or a lack of awareness of a part of the body such as a spinal anaesthetic or another nerve block would cause. Anesthesia differs from analgesia in blocking all sensation, not only pain.

Today, the term general anesthesia in its most general form can include:

Patients undergoing anesthesia usually undergo preoperative evaluation. It includes gathering history of previous anesthetics, and any other medical problems, physical examination, ordering required blood work and consultations prior to surgery.

There are several forms of anesthesia. The following forms refer to states achieved by anesthetics working on the brain:

  • General anesthesia: "Drug-induced loss of consciousness during which patients are not arousable, even by painful stimulation." Patients undergoing general anesthesia can often neither maintain their own airway nor breathe on their own. While usually administered with inhalational agents, general anesthesia can be achieved with intravenous agents, such as propofol.
  • Deep sedation/analgesia: "Drug-induced depression of consciousness during which patients cannot be easily aroused but respond purposefully following repeated or painful stimulation." Patients may sometimes be unable to maintain their airway and breathe on their own.
  • Moderate sedation/analgesia or conscious sedation: "Drug-induced depression of consciousness during which patients respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation." In this state, patients can breathe on their own and need no help maintaining an airway.
  • Minimal sedation or anxiolysis: "Drug-induced state during which patients respond normally to verbal commands." Though concentration, memory, and coordination may be impaired, patients need no help breathing or maintaining an airway.

The level of anesthesia achieved ranges on a continuum of depth of consciousness from minimal sedation to general anesthesia. The depth of consciousness of a patient may change from one minute to the next.

The following refer to the states achieved by anesthetics working outside of the brain:

  • Regional anesthesia: Loss of pain sensation, with varying degrees of muscle relaxation, in certain regions of the body. Administered with local anesthesia to peripheral nerve bundles, such as the brachial plexus in the neck. Examples include the interscalene block for shoulder surgery, axillary block for wrist surgery, and femoral nerve block for leg surgery. While traditionally administered as a single injection, newer techniques involve placement of indwelling catheters for continuous or intermittent administration of local anesthetics.
    • Spinal anesthesia: also known as subarachnoid block. Refers to a Regional block resulting from a small volume of local anesthetics being injected into the spinal canal. The spinal canal is covered by the dura mater, through which the spinal needle enters. The spinal canal contains cerebrospinal fluid and the spinal cord. The sub arachnoid block is usually injected between the 4th and 5th lumbar vertebrae, because the spinal cord usually stops at the 1st lumbar vertebra, while the canal continues to the sacral vertebrae. It results in a loss of pain sensation and muscle strength, usually up to the level of the chest (nipple line or 4th thoracic dermatome).
    • Epidural anesthesia: Regional block resulting from an injection of a large volume of local anesthetic into the epidural space. The epidural space is a potential space that lies underneath the ligamenta flava, and outside the dura mater (outside layer of the spinal canal). This is basically an injection around the spinal canal.
  • Local anesthesia is similar to regional anesthesia, but exerts its effect on a smaller area of the body.

Anesthesiologists/Anaesthetists (medically-trained physicians)

In the US, medical doctors who specialize in anesthesiology are called anesthesiologists, and dentists who specialize in anesthesiology are called dental anesthesiologists. Such physicians in the UK, Canada and Australia are called anaesthetists or anaesthesiologists.

In the US, a physician specializing in anesthesiology completes 4 years of college, 4 years of medical school, 1 year of internship, and 3 years of residency. According to the American Society of Anesthesiologists, anesthesiologists provide or participate in more than 90 percent of the 40 million anesthetics delivered annually.

In the UK, this training lasts a minimum of seven years after the awarding of a medical degree and two years of basic residency, and takes place under the supervision of the Royal College of Anaesthetists. In Australia and New Zealand, it lasts five years after the awarding of a medical degree and two years of basic residency, under the supervision of the Australian and New Zealand College of Anaesthetists. Other countries have similar systems, including Ireland (the Faculty of Anaesthetists of the Royal College of Surgeons in Ireland), Canada and South Africa (the College of Anaesthetists of South Africa).

In the UK, Fellowship of the Royal College of Anaesthetists (FRCA), is conferred upon medical doctors following completion of the written and oral parts of the Royal College's examination. In the US, completion of the written and oral Board examinations by a physician anesthesiologist allows one to be called "Board Certified" or a "Diplomate" of the American Board of Anesthesiology (or of the American Osteopathic Board of Anesthesiology, for osteopathic physicians).

Other specialties within medicine are closely affiliated to anaesthetics. These include intensive care medicine and pain medicine. Specialists in these disciplines have usually done some training in anaesthetics. The role of the anaesthetist is changing. It is no longer limited to the operation itself. Many anaesthetists perform well as peri-operative physicians, and will involve themselves in optimizing the patient's health before surgery (colloquially called "work-up"), performing the anaesthetic,including specialized intraoperative monitoring (liketransesophageal echocardiography), following up the patient in the post anesthesia care unit and post-operative wards, and ensuring optimal analgesia throughout.

It is important to note that the term anesthetist in the United States usually refers to registered nurses who have completed specialized education and training in nurse anesthesia to become certified registered nurse anesthetists (CRNAs). As noted above, the term anaesthetist in the UK and Canada refers to medical doctors who specialize in anesthesiology.


Nurse anesthetists

In the United States, advance practice nurses specializing in the provision of anesthesia care are known as Certified Registered Nurse Anesthetists (CRNAs). According to the American Association of Nurse Anesthetists, the 36,000 CRNAs in the US administer approximately 27 million anesthetics each year, roughly two thirds of the US total.Thirty-four percent of nurse anesthetists practice in communities of less than 50,000. CRNAs start school with a bachelors degree and at least 1 year of acute care nursing experience,and gain a masters degree in nurse anesthesia before passing the mandatory Certification Exam. Masters-level CRNA training programs range in length from 27 to 36 months.

CRNAs may work with podiatrists, dentists, anesthesiologists, surgeons, obstetricians and other professionals requiring their services. CRNAs administer anesthesia in all types of surgical cases, and are able to apply all the accepted anesthetic techniques – general, regional, local, or sedation. CRNAs do not require Anesthesiologist supervision in any state and only require surgeon/dentist/podiatrists to sign the chart for medicare billing in all but 16 states.


Anaesthesia Assistants

In the US, anesthesiologist assistants (AAs) are graduate-level trained specialists who have undertaken specialized education and training to provide anesthesia care under the direction of an Anesthesiologist. AAs typically hold a masters degree and practice under Anesthesiologist supervision in 18 states through licensing, certification or physician delegation.

In the UK, a similar group of assistants are currently being evaluated. They are named Physician's Assistant (Anaesthesia) (PAAs). Their background can be nursing, Operating Department Practice or another profession allied to medicine or a science graduate. Training is in the form of a post-graduate diploma and takes 27 months to complete. Once finished, a masters degree can be undertaken.


Anesthesia technicians

Anesthesia technicians are specially trained biomedical technicians who assist anesthesiologists, nurse anesthetists, and anesthesiologist assistants with monitoring equipment, supplies, and patient care procedures in the operating room.

In New Zealand, anaesthetic technicians complete a course of study recognized by the New Zealand Association of Anaesthetic Technicians and Nurses.

In the United Kingdom, personnel known as ODPs (Operating Department Practitioners) or anaesthetic nurses provide support to the physician anaesthetist (anaesthesiologist). They can also assist with Surgical procedures alongside the Surgeon and provide Post-Operative Care to patients emerging from Anaesthesia. ODPs can be found in the Operating Department, Accident and Emergency (providing advanced airway assistance), Intensive Care Unit, High Dependancy Unit and for specialist MRI scanners which require Anaesthetic cover.