General anesthetics bring about a reversible loss of consciousness and loss of pain in order for surgeons to operate on a patient. The use of general anesthetics is common, but how they produce their effects is still not fully understood.

General anesthesia is a medicine that is administered intravenously (IV) or through a tube or mask. It is performed by an anesthesiologist or nurse anesthetist, a specially trained doctor or nurse who will also monitor a patient’s vital signs and rate of breathing during the procedure.

Under general anesthesia, people are unable to feel pain (analgesic) and will be unconscious. It is more commonly used for major operations and during surgery. A person may also experience amnesia temporarily following the anesthetic.

General anesthetics have been widely used in surgery since 1842 when Crawford Long administered diethyl ether to a patient and performed the first painless operation.

In this article, we will cover a number of topics, including the differences between general anesthesia and sedation, potential side effects of general anesthesia, associated risks, and some theories regarding how they work.

Fast facts on general anesthesia

  • An anesthesiologist or anesthetist normally administers the general anesthetic prior to an operation.
  • There are some risks associated with taking general anesthetics, but they are relatively safe when administered correctly.
  • Very rarely, a patient may experience unintended intraoperative awareness.
  • Side effects of general anesthesia can include dizziness and nausea.
  • The mechanisms by which anesthesia works are still only partially understood.

A patient receiving anesthetic which is being administered through a mask.Share on Pinterest
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There are a number of potential side effects of general anesthesia.

Some individuals may experience no side effects, whereas others may experience a few. None of the side effects are particularly long lasting, and they tend to occur right after the anesthesia.

Side effects of general anesthesia can include:

  • temporary confusion and memory loss, although this is more common in older adults
  • dizziness
  • difficulty passing urine
  • bruising or soreness from the IV drip
  • nausea and vomiting
  • shivering and feeling cold
  • sore throat, due to the breathing tube

Overall, general anesthesia is very safe. Even particularly ill patients can be safely anesthetized. The surgical procedure itself offers the most risk.

However, older adults and those undergoing lengthy procedures are most at risk of negative outcomes. These outcomes can include:

Some specific conditions increase the risk to the patient undergoing general anesthetic, such as:

Death as a result of general anesthetic does occur, but only very rarely — roughly 1 in every 100,000.

Unintended intraoperative awareness

This refers to rare cases in which patients report a state of awareness during an operation, after the point at which the anesthetic should have removed all sensation. Some patients are conscious of the procedure itself, and some can even feel pain.

Unintended intraoperative awareness is incredibly rare, affecting an estimated 1 in every 19,000 patients undergoing general anesthesia.

Because of the muscle relaxants given alongside anesthesia, patients are unable to signal to their surgeon or anesthetist that they are still aware of what is happening.

Patients who experience unintended intraoperative awareness can suffer long-term psychological problems. Most often, the awareness is short-lived, and a person is only aware of vague sensory experiences such as sounds, touches, or movements.

As unintended intraoperative awareness is so infrequent, it is not clear exactly why it occurs.

The following are considered to be potential risk factors:

  • heart or lung problems
  • daily alcohol use
  • emergency surgery
  • cesarean delivery
  • anesthesiologist error
  • use of some additional medications
  • depression

Medical teams may administer IV sedation for the following procedures:

General anesthesia is also used for more lifesaving procedures such as heart surgeries or treatments for cancer, although it does carry some risks.

Sedation is administered through an IV.

There are three types of sedation, including:

  • Mild: A person is awake and responsive.
  • Moderate: A person may be sleepy but can wake up.
  • Deep: Similar to general anesthesia, as a patient is in a deep sleep.

Sedation and general anesthesia are both forms of anesthesia, but people will experience different effects depending on the type administered by doctors during a procedure, including the level of consciousness, breathing support, and possible side effects.

Sedation is generally characterized by people feeling drowsy but being in a relaxed state of consciousness. Whereas, when people are under general anesthesia, they have a complete loss of consciousness.

Cardiovascular function is usually maintained throughout sedation, and people are able to breathe independently. However, this does not mean that respiratory assistance may not be used in some instances with sedation.

On the other hand, general anesthesia usually impairs breathing, and monitoring and breathing assistance are required.

The adverse effects that may be associated with general anesthesia are avoided with sedation, as the recovery period from sedation is typically quicker.

Alongside general anesthetic, there are other types, including:

  • Local anesthesia: This type of anesthesia is given before minor surgeries, such as the removal of a toenail. This reduces pain in a small, focused area of the body, but the person receiving the treatment remains conscious.
  • Regional anesthesia: This type numbs an entire portion of the body and prevents pain sensation, such as in the lower half of the body during childbirth.

There are two main forms of regional anesthesia:

  • Spinal anesthetic: This type is used for surgeries of the lower limbs and abdomen. The professional administering the anesthetic injects it into the lower back and numbs the lower body.
  • Epidural anesthesia: This type of anesthesia is often used to reduce the pain of childbirth and lower limb surgery. It is administered to the area around the spinal cord through a small catheter instead of a needle injection.

There are a number of reasons why general anesthesia may be chosen over local anesthesia. In some instances, the patient is asked to choose between general and local anesthetic.

This choice depends on:

  • age
  • overall health
  • personal preference

The main reasons for opting for general anesthetic are:

  • The procedure is likely to take a long time.
  • There is a likelihood of significant blood loss.
  • Breathing may be affected, such as during a chest operation.
  • The procedure may make the patient feel uncomfortable.
  • The patient may be young, and they may have difficulty remaining still.

The purpose of general anesthetic is to induce:

  • analgesia, or removing the natural response to pain
  • amnesia, or memory loss
  • immobility, or the removal of motor reflexes
  • unconsciousness
  • skeletal muscle relaxation

However, using general anesthetic poses a higher risk of complications than local anesthesia. If the surgery is more minor, an individual may choose local anesthesia, especially if they have an underlying condition, such as sleep apnea.

Presurgical evaluation

Before receiving general anesthesia, patients will have a presurgery assessment to determine the most appropriate drugs to use, the quantities of those drugs, and in which combination.

Some factors to explore in a presurgical evaluation include:

  • body mass index
  • medical history
  • age
  • current medications
  • fasting time
  • alcohol or drug intake
  • pharmaceutical drug use
  • mouth, dental, and airway inspection
  • observation of neck flexibility and head extension

It is essential that these questions are answered accurately. For instance, if a history of alcohol or drug use is not mentioned, an inadequate amount of anesthesia might be given, which could lead to dangerously high blood pressure or unintended intraoperative awareness.

Guedel’s classification, designed by Arthur Ernest Guedel in 1937, describes the four stages of anesthesia.

Modern anesthetics and updated delivery methods have improved the speed of onset, general safety, and recovery, but the four stages remain essentially the same:

Stage 1, or induction: This phase occurs between the administration of the drug and the loss of consciousness. The patient moves from analgesia without amnesia to analgesia with amnesia

Stage 2, or excitement stage: The period following a loss of consciousness, characterized by excited and delirious activity. Breathing and heart rate become erratic, and nausea, pupil dilation, and breath-holding might occur.

Because of irregular breathing and the risk of vomiting, there is a danger of choking. Modern, fast-acting drugs aim to limit the time spent in stage 2 of anesthesia.

Stage 3, or surgical anesthesia: Muscles relax, vomiting stops, and breathing is depressed. Eye movements slow and then cease. The patient is ready to be operated on.

Stage 4, or overdose: Too much medication has been administered, leading to the brain stem or medullary suppression. This results in respiratory and cardiovascular collapse.

The anesthetist’s priority is to take the patient to stage 3 of anesthesia as quickly as possible and keep them there for the duration of the surgery.

The exact mechanisms that produce the state of general anesthesia are not well known. The general theory is that their action is induced by altering the activity of membrane proteins in the neuronal membrane, possibly by making certain proteins expand.

Of all the drugs used in medicine, general anesthetics are an unusual case. Rather than a single molecule acting at a single site to produce a response, there is a huge variety of compounds, all of which generate quite similar but widespread effects, including analgesia, amnesia, and immobility.

The chemical makeup of general anesthetic drugs ranges from the simplicity of alcohol’s chemical makeup to the complexity of sevoflurane.

General anesthetics are known to act at a number of sites within the central nervous system. The importance of these sites on the induction of anesthesia is not fully understood. These sites include:

  • Cerebral cortex: This is the brain’s outer layer, which is involved in tasks relating to memory, attention, and perception, among other functions.
  • Thalamus: Its roles include relaying information from the senses to the cerebral cortex and regulating sleep, wakefulness, and consciousness.
  • Reticular activating system: This is important in regulating sleep-wake cycles.
  • Spinal cord: The spinal cord passes information from the brain to the body and vice versa. It also houses circuitry that controls reflexes and other motor patterns.

A number of different neurotransmitters and receptors are also known to be involved in general anesthesia:

  • N-Methyl-D-aspartic acid (NMDA) receptors: Some general anesthetics, including ketamine and nitrous oxide (N2O), bind to NMDA receptors. They are known to be important in controlling synaptic plasticity and memory functions.
  • 5-hydroxytryptamine (5-HT) receptors: Normally activated by the neurotransmitter serotonin, they play a part in controlling the release of a number of other neurotransmitters and hormones.
  • Glycine receptor: Glycine can act as a neurotransmitter and has a number of roles. It has been shown to enhance sleep quality.

Although general anesthetics hold many mysteries, they are hugely important in surgery and the field of medicine at large.

General anesthesia can cause some potential side effects and complications. However, overall, it is a very safe medicine, and it is usually the surgery itself that carries the most risk.

Before any type of anesthesia is administered, patients will have an assessment to determine the most appropriate combination of drugs to use and the quantities, depending on if a person has any risk factors or family history of allergies to an anesthetic.