Before anaesthetics came into general use around 150 years ago, surgery was simply carried out as quickly as possible. The record for an amputation was 15 seconds, by Napoleon's military surgeon Dominique Laffrey, who used snow to reduce pain.

Of course, many patients just passed out. In fact, this was an advantage because they didn't have to be held down while they were unconscious. Many also died of shock.

Today, highly evolved techniques and state-of-the-art equipment mean modern anaesthetics have significantly reduced the trauma associated with surgical procedures. An anaesthetic is a drug or agent that produces a complete or partial loss of feeling. There are several kinds of anaesthetic: general, regional, local and tumescent.

When a patient undergoes a general anaesthetic, they lose sensation and become unconscious, sometimes requiring assisted breathing, while other types remove sensation from specific areas of the body.

Anaesthetic compositions

Anaesthetics can be composed of a number of agents, such as:

Gases

These should be non-flammable, non-explosive, non-irritant, lipid-soluble, possess low blood gas solubility and should not be metabolised or have end organ (heart, liver, kidney) toxicity.

Intravenous agents

These include sedatives, benzodiazepines and propofol which cause unconsciousness but not pain relief, as well as etomidate and ketamine which are often used in emergency settings. Only ketamine also produces pain relief.

Analgesic agents

Although they can cause unconsciousness, analgesics are normally combined with other anaesthetic agents for pain relief.

Muscle relaxants

These do not render patients unconsciousness or relieve pain but assist with intubation during general anaesthesia.

General anaesthesia

General anaesthetics can be administered by injection or as a gas by inhalation through a mask. Patients are sometimes given a ‘pre-med' that induces relaxation before they undergo general anaesthesia by inhalation. This can also help reduce salivation, particularly beneficial when a tube is inserted into the throat to assist ventilation after the patient has become unconscious and loses sensation.

An anaesthetist is a medical doctor with specialist training in anaesthetics. The anaesthetist controls the length of time the patient is unconscious and constantly monitors their pulse, breathing and blood pressure. If necessary, the anaesthetist administers intravenous fluids before, during and after surgery.

Once the surgery is completed, drugs that reverse the effect of the anaesthetic and any other drugs used during the operation (such as a muscle relaxant) are sometimes administered by injection When the anaesthetist is satisfied the patient's breathing and blood circulation have normalised, the patient is taken to a recovery area.

Local anaesthesia

Local anaesthesia is administered by injection to a small area and is commonly used in dentistry or for minor surgery such as stitching a wound. Local anaesthetic agents are also used in the regional and tumescent approaches.

Regional anaesthesia

This type of anaesthesia is also referred to as a nerve block, because the anaesthetic agent is injected into nerve bundles central to the area to be operated on. Traditionally administered as a single injection, recent developments include the use of an inserted catheter to allow for serial doses of local anaesthetic during surgery.

Common areas where regional anaesthesia is used include the shoulders, arms and legs, spine and lower body during childbirth (epidural).

Tumescent anaesthesia

Tumescent anaesthesia, which originated in liposuction surgery is now widely used in many procedures on subcutaneous tissues, such as the breast or abdominal wall.

The procedure uses a mixture of infiltrate containing local anaesthetic and adrenaline to help numb the area and prevent blood loss. Large volumes of infiltrate are steadily injected into the subcutaneous tissues until the area is swollen. A typical formula uses 25ml of 2 percent lidocaine and 1ml of 1:1000 adrenaline for each litre of sodium lactate intravenous infusion.

It is possible to use large volumes of fluid with higher doses of local anaesthetics, and a commonly recommended dose is up to 35mg per kilogram, although the dose depends on the site and the indication. This type of anaesthesia is combined with twilight sedation.

Twilight sedation

Twilight sedation is the state between wakefulness and complete unconsciousness, where patients are less aware of their surroundings. This is fairly easily accomplished with modern anaesthetic agents such propofol, which allow for rapid adjustment of sedation level and a quick recovery.

The anaesthetic is administered intravenously, intubation is not necessary and the risk of nausea during recovery is minimised.

How anaesthetics work

While modern anaesthetists are experts in understanding how many milligrams of which particular drug to administer depending on the patient's body weight and physical responses, at a cellular level the way these drugs work remains something of a mystery.

In broad terms, a general anaesthetic is carried in by the blood to the nerves in the brain. The nerve cells stop receiving and sending signals so the patient doesn't feel pain and remains immobile during surgery.

The general understanding is that the chemical agent acts on the cell membrane of the nerve cell. All cells are encased by a cell membrane which sandwiches layers of water absorbent and resistant molecules.

The pressure in between the different layers of the cell membrane is around 400 atmospheres, or roughly the pressure 4km under the sea.

The cell membrane has channels that admit or release certain chemicals such as chloride, sodium, potassium. Each channel passes through the sandwich of the cell membrane, with the pressure acting against it, but the channels are held open by cholesterol and other fats that are arranged into fairly rigid liquid crystals. As various chemicals flow in or out of channels, they change the electrical charge on the cell membrane that switches the nerve cell on or off.

There are a number of theories on how anaesthetics achieve this. One of the older theories postulates that anaesthetics can penetrate the cell membrane and interfere with the rigid liquid crystals that hold the channels open. If the shape of the channel changes, so will the flow rate of various chemicals to and from the cell.

A more modern theory is that anaesthetics adhere to little chemicals around the open mouth of the channel. These chemicals then can open or close the channel, leading to changes in electrical charge on the cell membrane that produces insensibility. Another theory says that anaesthetics alter the pressure inside the layers of the cell membrane, changing the shape of the channels and affecting the electrical charge on the cell membrane.