Anaesthetics have transformed modern surgery. Before anaesthetics came into general use 150 years ago, surgery was simply carried out as fast 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.
Anaesthetics can be composed of a number of agents
Gases – which should be non-flammable, non-explosive, non-irritant, lipid-soluble, possess low blood gas solubility, should not be metabolised or have end organ (heart, liver, kidney) toxicity.
Intravenous agents – including sedatives, benzodiaz-epines and propofol which cause unconsciousness but not pain relief, etomidate and ketamine which are often used in emergency settings. Only ketamine also produces pain relief.
Analgesic agents – which, although they can cause unconsciousness, are normally combined with other agents for pain relief.
Muscle relaxants – do not render patients unconscious 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.
Before patients undergo general anaesthesia by inhalation, they are sometimes given a ‘pre-med’ that makes them relaxed and can help reduce salivation. This is because a tube is inserted into the throat to assist ventilation after the patient has become unconscious.
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 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 cosmetic surgery, such as liposuction, 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. The cell membrane 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 4 kilometres under the sea.
The cell membrane has channels that admit or release certain chemicals such as chloride, sodium, potassium and so on. 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, then 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 little chemicals then can open or close the channel, leading to changes in electrical charge on the cell membrane producing 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.
The history of anaesthetics
The use of anaesthetics has transformed modern surgery but they have been around for much longer than you might think. Archaeologists have discovered collected opium poppy capsules from 4,200BC in Spain and their recorded use as an anaesthetic in ancient Sumeria in 1,500BC. In India and China the surgeons of antiquity used cannabis and aconitum with wine to perform operations such as abdominal surgery.
Treatments with plant alkaloids remained the mainstay of anaesthesia until the 19th century, although they had one major drawback if they were potent enough to produce the desired effect, they could also kill the patient. The ability to measure the drugs involved and regulate their application is part of the anaesthetist’s craft to this day, when standardised manufacturing techniques and the use of monitors during surgery provide scientific means of controlling dosage and effect.
In 1846 an American dentist named Horace Wells, after witnessing the effects of nitrous oxide or laughing gas at a carnival sideshow, tested it on himself by having someone pull one of his teeth. His partner William Morton advanced the theory by using the more powerful gas ether, acting as anaesthetist at Massachusetts General Hospital while the surgeon removed a tumour from a patient’s neck. After this public and publicised operation, the influential physician Oliver Wendell Holmes Senior wrote to Morton proposing the procedure be named anaesthesia, from the Greek word meaning lack of sensation.
News of this use for ether spread rapidly to Europe, the British Isles and even Australia. Surgery using the procedure took place in Launceston, Tasmania later during 1846, and leading surgeons adopted the practice. Ether, which was highly flammable and had the tendency to induce vomiting, was rapidly replaced by chloroform which gained royal approval from Queen Victoria after her doctor John Snow gave it to her during the birth of Prince Leopold in 1853. Snow helped pioneer the use of anaesthetics in England, involving himself in the production of equipment for inhalation of vapour.
Apart from the application of ice, the first local anaesthetic was cocaine. In South America, shaman chewed coca leaves and spat the saliva mixture into wounds to nullify pain. After importation of the leaves to Europe, the alkaloid was isolated by the German chemist Friedrich Gaedcke in 1855. Cocaine was introduced into clinical use as a local anaesthetic in Germany in 1884 as a result of its numbing properties and is still used in eye, nose and throat surgery today.
Modern anaesthesia uses combinations of chemical agents that produce unconsciousness and nullify pain.
