[5] Various drugs are used to achieve unconsciousness, amnesia, analgesia, loss of reflexes of the autonomic nervous system, and in some cases paralysis of skeletal muscles.

[6] Attempts at producing general anaesthesia can be traced throughout recorded history in the writings of the ancient Sumerians, Babylonians, Assyrians, Egyptians, Greeks, Romans, Indians, and Chinese.

[9] In the 20th century, general anaesthesia's safety and efficacy improved with routine tracheal intubation and other advanced airway management techniques.

Advances in monitoring and new anaesthetic agents with improved pharmacokinetic and pharmacodynamic characteristics also contributed to this trend, and standardized training programs for anaesthesiologists and nurse anaesthetists emerged.

General anaesthesia interrupts or changes the functions of CNS components including the cerebral cortex, thalamus, reticular activating system, and spinal cord.

Theories of anaesthesia identify target sites in the CNS, neural networks and arousal circuits linked with unconsciousness, and some anaesthetics can potentially activate specific sleep-active regions.

A study from 2020 showed that inhaled anaesthetics (chloroform and isoflurane) could displace phospholipase D2 from ordered lipid domains in the plasma membrane, which led to the production of the signaling molecule phosphatidic acid (PA).

For example, a patient who consumes significant quantities of alcohol or illicit drugs could be undermedicated during the procedure if they fail to disclose this fact, and this could lead to anaesthesia awareness or intraoperative hypertension.

[6] An important aspect of pre-anaesthetic evaluation is an assessment of the patient's airway, involving inspection of the mouth opening and visualisation of the soft tissues of the pharynx.

[6] However, a randomized controlled trial from 2021 demonstrated that clonidine is less effective at providing anxiolysis and more sedative in children of preschool age.

[6] Melatonin has been found to be effective as an anaesthetic premedication in both adults and children because of its hypnotic, anxiolytic, sedative, analgesic, and anticonvulsant properties.

[30] Another example of anaesthetic premedication is the preoperative administration of beta adrenergic antagonists, which reduce the burden of arrhythmias after cardiac surgery.

Despite newer anaesthetic agents and delivery techniques, which have led to more rapid onset of—and recovery from—anaesthesia (in some cases bypassing some of the stages entirely), the principles remain.

Inhalational anaesthesia may be chosen when intravenous access is difficult to obtain (e.g., children), when difficulty maintaining the airway is anticipated, or when the patient prefers it.

General anaesthesia can also be induced with the patient spontaneously breathing and therefore maintaining their own oxygenation which can be beneficial in certain scenarios (e.g. difficult airway or tubeless surgery).

Intravenous anaesthesia to maintain spontaneous respiration has certain advantages over inhalational agents (i.e. suppressed laryngeal reflexes) but requires careful titration.

[39] General anaesthesia reduces the tonic contraction of the orbicularis oculi muscle, causing lagophthalmos (incomplete eye closure) in 59% of people.

[42] Muscle relaxation allows surgery within major body cavities, such as the abdomen and thorax, without the need for very deep anaesthesia, and also facilitates endotracheal intubation.

This is achieved by allowing the patient to breathe a carefully controlled mixture of oxygen and a volatile anaesthetic agent, or by administering intravenous medication (usually propofol).

Advantages include faster recovery from anaesthesia, reduced incidence of postoperative nausea and vomiting, and absence of a trigger for malignant hyperthermia.

Shivering is also fairly common and can be clinically significant because it causes an increase in oxygen consumption, carbon dioxide production, cardiac output, heart rate, and systemic blood pressure.

[49] Cardiovascular events such as increased or decreased blood pressure, rapid heart rate, or other cardiac dysrhythmias are also common during emergence from general anaesthesia, as are respiratory symptoms such as dyspnoea.

[6] Postoperative pain is managed in the anaesthesia recovery unit (PACU) with regional analgesia or oral, transdermal, or parenteral medication.

[52][53] In the recovery unit, many vital signs are monitored, including oxygen saturation,[54][55] heart rhythm and respiration,[54][56] blood pressure,[54] and core body temperature.

Apart from causing discomfort and exacerbating pain, shivering has been shown to increase oxygen consumption, catecholamine release, risk for hypothermia, and induce lactic acidosis.

[60][61] If the shivering cannot be managed with external warming devices, drugs such as dexmedetomidine,[62][63] or other α2-agonists, anticholinergics, central nervous system stimulants, or corticosteroids may be used.

Administration of a μ-opioid antagonist such as alvimopan immediately after surgery can help accelerate the timing of hospital discharge, but does not reduce the development of paralytic ileus.

Adherence to the pathway and guidelines has been shown to associate with improved post-operative outcomes and lower costs to the health care system.

Interventions found to reduce mortality include pharmacotherapy, ventilation, transfusion, nutrition, glucose control, dialysis and medical device.

[69] Mortality directly related to anaesthetic management is very uncommon but may be caused by pulmonary aspiration of gastric contents,[70] asphyxiation,[71] or anaphylaxis.