severe brainstem or medullary depression, resulting in a cessation of respiration and potential cardiovascular collapse. this stage is lethal without cardiovascular and respiratory support. induction general anaesthesia is usually induced in an operating theatre or in a dedicated anaesthetic room adjacent to the theatre. general anaesthesia may also be conducted in other locations, such as an endoscopy suite, intensive care unit, radiology or cardiology department, emergency department, ambulance, or at the site of a disaster where extrication of the patient may be impossible or impractical. anaesthetic agents may be administered by various routes, including inhalation, injection (intravenous, intramuscular, or subcutaneous), oral, and rectal. once they enter the circulatory system, the agents are transported to their biochemical sites of action in the central and autonomic nervous systems. most general anaesthetics are induced either intravenously or by inhalation. commonly used intravenous induction agents include propofol, sodium thiopental, etomidate, methohexital, and ketamine. 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. sevoflurane is the most commonly used agent for inhalational induction, because it is less irritating to the tracheobronchial tree than other agents. as an example sequence of induction drugs: pre-oxygenation or denitrogenation to fill lungs with 100% oxygen to permit a longer period of apnea during intubation without affecting blood oxygen levels fentanyl for systemic analgesia during intubation propofol for sedation for intubation switching from oxygen to a mixture of oxygen and inhalational anesthetic once intubation is complete laryngoscopy and intubation are both very stimulating. the process of induction blunts the response to these maneuvers while simultaneously inducing a near-coma state to prevent awareness. physiologic monitoring several monitoring technologies allow for a controlled induction of, maintenance of, and emergence from general anaesthesia. standard for basic anesthetic monitoring is a guideline published by the asa, which describes that the patient's oxygenation, ventilation, circulation and temperature should be continually evaluated during anesthetic. continuous electrocardiography (ecg or ekg): electrodes are placed on the patient's skin to monitor heart rate and rhythm. this may also help the anaesthesiologist to identify early signs of heart ischaemia. typically lead ii and v5 are monitored for arrhythmias and ischemia, respectively. continuous pulse oximetry (spo2): a device is placed, usually on a finger, to allow for early detection of a fall in a patient's hemoglobin saturation with oxygen (hypoxaemia). blood pressure monitoring: there are two methods of measuring the patient's blood pressure. the first, and most common, is non-invasive blood pressure (nibp) monitoring. this involves placing a blood pressure cuff around the patient's arm, forearm, or leg. a machine takes blood pressure readings at regular, preset intervals throughout the surgery. the second method is invasive blood pressure (ibp) monitoring, which allows beat to beat monitoring of blood pressure. this method is reserved for patients with significant heart or lung disease, the critically ill, and those undergoing major procedures such as cardiac or transplant surgery, or when large blood loss is expected. it involves placing a special type of plastic cannula in an artery, usually in the wrist (radial artery) or groin (femoral artery). agent concentration measurement: anaesthetic machines typically have monitors to measure the percentage of inhalational anaesthetic agents used as well as exhalation concentrations. these monitors include measuring oxygen, carbon dioxide, and inhalational anaesthetics (e.g., nitrous oxide, isoflurane). oxygen measurement: almost all circuits have an alarm in case oxygen delivery to the patient is compromised. the alarm goes off if the fraction of inspired oxygen drops below a set threshold. a circuit disconnect alarm or low pressure alarm indicates failure of the circuit to achieve a given pressure during mechanical ventilation. capnography measures the amount of carbon dioxide exhaled by the patient in percent or mmhg, allowing the anaesthesiologist to assess the adequacy of ventilation. mmhg is usually used to allow the provider to see more subtle changes. temperature measurement to discern hypothermia or fever, and to allow early detection of malignant hyperthermia. electroencephalography, entropy monitoring, or other systems may be used to verify the depth of anaesthesia. this reduces the likelihood of anaesthesia awareness and of overdose. airway management main article: airway management anaesthetized patients lose protective airway reflexes (such as coughing), airway patency, and sometimes a regular breathing pattern due to the effects of anaesthetics, opioids, or muscle relaxants. to maintain an open airway and regulate breathing, some form of breathing tube is inserted after the patient is unconscious. to enable mechanical ventilation, an endotracheal tube is often used, although there are alternative devices that can assist respiration, such as face masks or laryngeal mask airways. generally, full mechanical ventilation is only used if a very deep state of general anaesthesia is to be induced for a major procedure, and/or with a profoundly ill or injured patient. that said, induction of general anaesthesia usually results in apnea and requires ventilation until the drugs wear off and spontaneous breathing starts. in other words, ventilation may be required for both induction and maintenance of general anaesthesia or just during the induction. however, mechanical ventilation can provide ventilatory support during spontaneous breathing to ensure adequate gas exchange. 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). spontaneous ventilation has been traditionally maintained with inhalational agents (i.e. halothane or sevoflurane) which is called a gas or inhalational induction. spontaneous ventilation can also be maintained using intravenous anaesthesia (e.g. propofol). intravenous anaesthesia to maintain spontaneous respiration has certain advantages over inhalational agents (i.e. suppressed laryngeal reflexes) however it requires careful titration. spontaneous respiration using intravenous anaesthesia and high-flow nasal oxygen (strive hi) is a technique that has been used in difficult and obstructed airways. eye management main article: eye injuries during general anaesthesia general anaesthesia reduces the tonic contraction of the orbicularis oculi muscle, causing lagophthalmos (incomplete eye closure) in 59% of peoples. in addition, tear production and tear-film stability are reduced, resulting in corneal epithelial drying and reduced lysosomal protection. the protection afforded by bell's phenomenon (in which the eyeball turns upward during sleep, protecting the cornea) is also lost. careful management is required to reduce the likelihood of eye injuries during general anaesthesia. some of the methods to prevent eye injury during general anesthesia includes, taping eyelid shut, use of eye ointment, specialty designed eye protective goggle. neuromuscular blockade syringes prepared with medications that are expected to be used during an operation under general anaesthesia maintained by sevoflurane gas: - propofol, a hypnotic - ephedrine, in case of hypotension - fentanyl, for analgesia - atracurium, for neuromuscular block - glycopyrronium bromide (here under trade name robinul), reducing secretions paralysis, or temporary muscle relaxation with a neuromuscular blocker, is an integral part of modern anaesthesia. the first drug used for this purpose was curare, introduced in the 1940s, which has now been superseded by drugs with fewer side effects and, generally, shorter duration of action. 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. acetylcholine, a natural neurotransmitter found at the neuromuscular junction, causes muscles to contract when it is released from nerve endings. muscle paralytic drugs work by preventing acetylcholine from attaching to its receptor. paralysis of the muscles of respiration—the diaphragm and intercostal muscles of the chest—requires that some form of artificial respiration be implemented. because the muscles of the larynx are also paralysed, the airway usually needs to be protected by means of an endotracheal tube. paralysis is most easily monitored by means of a peripheral nerve stimulator. this device intermittently sends short electrical pulses through the skin over a peripheral nerve while the contraction of a muscle supplied by that nerve is observed. the effects of muscle relaxants are commonly reversed at the end of surgery by anticholinesterase drugs, which are administered in combination with muscarinic anticholinergic drugs to minimize side effects. examples of skeletal muscle relaxants in use today are pancuronium, rocuronium, vecuronium, cisatracurium, atracurium, mivacurium, and succinylcholine.novel neuromuscular blockade reversal agents such as sugammadex may also be used; it works by directly binding muscle relaxants and removing it from the neuromuscular junction. sugammadex was approved for use in the united states in 2015, and rapidly gained popularity. a study from 2022 has shown that sugammadex and neostigmine are likely similarly safe in the reversal of neuromuscular blockade. maintenance the duration of action of intravenous induction agents is generally 5 to 10 minutes, after which spontaneous recovery of consciousness will occur. in order to prolong unconsciousness for the duration of surgery, anaesthesia must be maintained. 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). inhaled anaesthetic agents are also frequently supplemented by intravenous analgesic agents, such as opioids (usually fentanyl or a fentanyl derivative) and sedatives (usually propofol or midazolam). propofol can be used for total intravenous anaesthetia (tiva), therefore supplementation by inhalation agents is not required. general anesthesia is usually considered safe; however, there are reported cases of patients with distortion of taste and/or smell due to local anesthetics, stroke, nerve damage, or as a side effect of general anesthesia. at the end of surgery, administration of anaesthetic agents is discontinued. recovery of consciousness occurs when the concentration of anaesthetic in the brain drops below a certain level (this occurs usually within 1 to 30 minutes, mostly depending on the duration of surgery). in the 1990s, a novel method of maintaining anaesthesia was developed in glasgow, scotland. called target controlled infusion (tci), it involves using a computer-controlled syringe driver (pump) to infuse propofol throughout the duration of surgery, removing the need for a volatile anaesthetic and allowing pharmacologic principles to more precisely guide the amount of the drug used by setting the desired drug concentration. advantages include faster recovery from anaesthesia, reduced incidence of postoperative nausea and vomiting, and absence of a trigger for malignant hyperthermia. at present, tci is not permitted in the united states, but a syringe pump delivering a specific rate of medication is commonly used instead. other medications are occasionally used to treat side effects or prevent complications. they include antihypertensives to treat high blood pressure; ephedrine or phenylephrine to treat low blood pressure; salbutamol to treat asthma, laryngospasm, or bronchospasm; and epinephrine or diphenhydramine to treat allergic reactions. glucocorticoids or antibiotics are sometimes given to prevent inflammation and infection, respectively. emergence emergence is the return to baseline physiologic function of all organ systems after the cessation of general anaesthetics. this stage may be accompanied by temporary neurologic phenomena, such as agitated emergence (acute mental confusion), aphasia (impaired production or comprehension of speech), or focal impairment in sensory or motor function. 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. the proposed mechanism is based on the observation that the spinal cord recovers at a faster rate than the brain. this results in uninhibited spinal reflexes manifested as clonic activity (shivering). this theory is supported by the fact that doxapram, a cns stimulant, is somewhat effective in abolishing postoperative shivering. 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. responding and following verbal command, is a criterion commonly utilized to assess the patient's readiness for tracheal extubation. postoperative care anaesthetized patient in postoperative recovery. postoperative pain is managed in the anaesthesia recovery unit (pacu) with regional analgesia or oral, transdermal, or parenteral medication. patients may be given opioids, as well as other medications like non steroidal anti-inflammatory drugs and acetaminophen. sometimes, opioid medication is administered by the patient themselves using a system called a patient controlled analgesic. the patient presses a button to activate a syringe device and receive a preset dose or "bolus" of the drug, usually a strong opioid such as morphine, fentanyl, or oxycodone (e.g., one milligram of morphine). the pca device then "locks out" for a preset period to allow the drug to take effect, and also prevent the