General anaesthesia
General anaesthesia (UK) or general anesthesia (US) is a method of medically inducing loss of consciousness that renders a patient unarousable even with painful stimuli.[5] This effect is achieved by administering either intravenous or inhalational general anaesthetic medications, which often act in combination with an analgesic and neuromuscular blocking agent. Spontaneous ventilation is often inadequate during the procedure and intervention is often necessary to protect the airway.[5] General anaesthesia is generally performed in an operating theater to allow surgical procedures that would otherwise be intolerably painful for a patient, or in an intensive care unit or emergency department to facilitate endotracheal intubation and mechanical ventilation in critically ill patients. Depending on the procedure, general anaesthesia may be optional or required. Regardless of whether a patient may prefer to be unconscious or not, certain pain stimuli could result in involuntary responses from the patient (such as movement or muscle contractions) that may make an operation extremely difficult. Thus, for many procedures, general anaesthesia is required from a practical perspective.
General anaesthesia
Facilitating surgery, terminal sedation[1]
Anaesthesia awareness,[2] overdose,[3] death[4]
A variety of drugs may be administered, with the overall goal of achieving unconsciousness, amnesia, analgesia, loss of reflexes of the autonomic nervous system, and in some cases paralysis of skeletal muscles. The optimal combination of anesthetics for any given patient and procedure is typically selected by an anaesthetist, or another provider such as a nurse anaesthetist (depending on local practice and law), in consultation with the patient and the surgeon, dentist, or other practitioner performing the operative procedure.[6]
Purpose and indications[edit]
Purpose of general anesthesia
General anesthesia serves as a critical tool in surgical practice, facilitating procedures by inducing a state of reversible unconsciousness in patients. Its primary objectives encompass ensuring patient safety, comfort, and pain relief throughout the surgical process.
Induction of unconsciousness
An essential aspect of general anesthesia is the induction of complete unconsciousness, rendering patients oblivious to sensory stimuli and surgical events. This profound state of unawareness is achieved through the administration of pharmacological agents targeting the central nervous system, effectively suppressing consciousness and perception.
Analgesia and pain control
In addition to inducing unconsciousness, general anesthesia provides effective analgesia to eliminate intraoperative pain. By interrupting the transmission of nociceptive signals within the nervous system, specialized medications mitigate surgical discomfort, enhancing patient comfort and expediting postoperative recovery.
Muscle relaxation and facilitation of surgical procedures
General anesthesia induces muscle relaxation and abolishes reflex responses, optimizing surgical conditions for precise intervention. This relaxation of skeletal muscles assists surgeons in executing procedures with meticulous precision, ensuring optimal outcomes and minimizing the risk of intraoperative complications.
Overall management of physiological responses
General anesthesia plays a pivotal role in maintaining physiological stability during surgery, attenuating stress responses and preserving hemodynamic equilibrium. Anesthesiologists vigilantly monitor patients' vital signs and administer medications as necessary to mitigate adverse physiological reactions, promoting procedural safety and minimizing perioperative risks.
Psychosocial considerations and anxiety management in surgery
Addressing psychosocial concerns and managing anxiety are integral components of perioperative care, particularly in patients facing challenges with stress tolerance or immobility. General anesthesia may be warranted for individuals with movement disorders, while elective use can alleviate anxiety in patients with learning disabilities or severe apprehension. Implementing a patient-centered approach, interdisciplinary collaboration, and comprehensive support are essential strategies for optimizing patient experience and surgical outcomes.[10][11][12]
Indications for general anesthesia
General anesthesia is employed in a variety of medical situations to ensure patient comfort, safety, and successful procedural outcomes. Understanding the indications for general anesthesia is essential for healthcare providers to make informed decisions and optimize patient care.
Surgical procedures: One of the most common indications for general anesthesia is surgical intervention. General anesthesia is utilized across a wide range of surgical specialties, from, on occasion, minor procedures such as dental extractions to major surgeries like cardiac bypass surgery. It allows surgeons to operate on patients without them feeling pain or discomfort, ensuring a smooth and successful procedure.
Complex non-surgical medical procedures: Certain medical procedures, such as endoscopies, colonoscopies, and imaging studies, may occasionally require general anesthesia to ensure patient cooperation and comfort. General anesthesia is particularly beneficial in cases where patients need to remain still for an extended period or if the procedure is invasive and potentially uncomfortable.
Emergency situations: In emergencies, where immediate intervention is necessary, general anesthesia may be indicated to facilitate life-saving procedures. This could include surgeries to treat traumatic injuries, control bleeding, or relieve acute medical conditions. General anesthesia helps ensure patient stability and safety during critical interventions.
Pediatric care: Children often require general anesthesia for various medical procedures, ranging from surgeries to diagnostic tests. Due to their unique physiological and psychological needs, general anesthesia is often preferred to ensure that pediatric patients remain still, pain-free, and cooperative during procedures.
Obstetric care: While regional anesthesia techniques like epidurals are more common in obstetrics, there are situations where general anesthesia may be indicated, such as emergency cesarean sections or certain fetal interventions. General anesthesia ensures that the mother remains unconscious and pain-free during these procedures, prioritizing both maternal and fetal well-being.
Special populations: Certain patient populations, such as those with intellectual disabilities, severe anxiety, or medical conditions that preclude other anesthesia options, may benefit from general anesthesia. Tailoring anesthesia management to the individual needs of these patients ensures optimal safety, comfort, and procedural success.[10][11][12]
Biochemical mechanism of action[edit]
The biochemical mechanism of action of general anaesthetics is still controversial.[13] Theories need to explain the function of anaesthesia in animals and plants.[14] To induce unconsciousness, anaesthetics have myriad sites of action and affect the central nervous system (CNS) at multiple levels. General anaesthesia commonly interrupts or changes the functions of CNS components including the cerebral cortex, thalamus, reticular activating system, and spinal cord. Current theories on the anaesthetized state identify not only target sites in the CNS but also neural networks and arousal circuits linked with unconsciousness, and some anesthetics potentially able to activate specific sleep-active regions.[15]
Two non-exclusionary mechanisms include membrane-mediated and direct protein-mediated anesthesia. Potential protein-mediated molecular targets are GABAA,and NMDA glutamate receptors. General anesthesia was hypothesized to either enhance the inhibitory transmission or reduce the excitatory transmission of neuro signaling.[16] Most volatile anesthetics have been found to be a GABAA agonist, although the site of action on the receptor remains unknown.[17] Ketamine is a non-competitive NMDA receptor antagonist.[18]
The chemical structure and properties of anesthetics, as first noted by Meyer and Overton, suggest they could target the plasma membrane. A membrane-mediated mechanism that could account for the activation of an ion channel remained elusive until recently. A study from 2020 demonstrated that inhaled anesthetics (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). The signaling molecule activated TWIK-related K+ channels (TREK-1), a channel involved in anesthesia. PLDnull fruit flies were shown to resist anesthesia, the results established a membrane mediated target for inhaled anesthetics.[19]
Preoperative evaluation[edit]
Prior to a planned procedure, the anesthesiologist reviews medical records, interviews the patient, and conducts a physical examination to obtain information regarding their medical history and current physical state, and to determine an appropriate anesthetic plan, including what combination of drugs and dosages will likely be needed for the patient's comfort and safety during the procedure. A variety of non-invasive and invasive monitoring devices may be necessary to ensure a safe and effective procedure. Key factors in this evaluation are the patient's age, gender, body mass index, medical and surgical history, current medications, exercise capacity, and fasting time.[20][21] Thorough and accurate preoperative evaluation is crucial for the effective safety of the anesthetic plan. 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.[2][22] Commonly used medications can also interact with anaesthetics, and failure to disclose such usage can increase the risk during the operation. Inaccurate timing of last meal can also increase the risk for aspiration of food, and lead to serious complications.[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.[23] The condition of teeth and location of dental crowns are checked, and neck flexibility and head extension are observed.[24][25] The most commonly performed airway assessment is the Mallampati classification, which evaluates the airway base on the ability to view airway structures with the mouth open and the tongue protruding. Mallampati tests alone have limited accuracy, and other evaluations are routinely performed addition to the Mallampati test including mouth opening, thyromental distance, neck range of motion, and mandibular protrusion. In a patient with suspected distorted airway anatomy, endoscopy or ultrasound is sometimes used to evaluate the airway before planning for the airway management.[26]
Premedication[edit]
Prior to administration of a general anaesthetic, the anaesthetist may administer one or more drugs that complement or improve the quality or safety of the anaesthetic or simply provide anxiolysis. Premedication also often has mild sedative effects and may reduce the amount of anaesthetic agent required during the case.[6]
One commonly used premedication is clonidine, an alpha-2 adrenergic agonist.[27][28] It reduces postoperative shivering, postoperative nausea and vomiting, and emergence delirium.[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. Oral clonidine can take up to 45 minutes to take full effect,[29] The drawbacks of clonidine include hypotension and bradycardia, but these can be advantageous in patients with hypertension and tachycardia.[30] Another commonly used alpha-2 adrenergic agonist is dexmedetomidine, which is commonly used to provide a short term sedative effect (<24 hours). Dexmedetomidine and certain atypical antipsychotic agents may be also used in uncooperative children.[31]
Benzodiazepines are the most commonly used class of drugs for premedication. The most commonly utilized benzodiazepine is Midazolam, which is characterized by a rapid onset and short duration. Midazolam is effective in reducing preoperative anxiety, including separation anxiety in children.[32] It also provides mild sedation, sympathicolysis, and anterograde amnesia.[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. Recovery is more rapid after premedication with melatonin than with midazolam, and there is also a reduced incidence of post-operative agitation and delirium.[33] Melatonin has been shown to have a similar effect in reducing perioperative anxiety in adult patients compared to benzodiazepine.[34]
Another example of anaesthetic premedication is the preoperative administration of beta adrenergic antagonists, which reduce the burden of arrhythmias after cardiac surgery. However, evidence also has shown an association of increased adverse events with beta-blockers in non-cardiac surgery.[35] Anaesthesiologists may administer one or more antiemetic agents such as ondansetron, droperidol, or dexamethasone to prevent postoperative nausea and vomiting.[6] NSAIDs are commonly used analgesic premedication agent, and often reduce need for opioids such as fentanyl or sufentanil. Also gastrokinetic agents such as metoclopramide, and histamine antagonists such as famotidine.[6]
Non-pharmacologic preanaesthetic interventions include playing cognitive behavioral therapy, music therapy, aromatherapy, hypnosis massage, pre-operative preparation video, and guided imagery relaxation therapy, etc.[36] These techniques are particularly useful for children and patients with intellectual disabilities. Minimizing sensory stimulation or distraction by video games may help to reduce anxiety prior to or during induction of general anaesthesia. Larger high-quality studies are needed to confirm the most effective non-pharmacological approaches for reducing this type of anxiety.[37] Parental presence during premedication and induction of anaesthesia has not been shown to reduce anxiety in children.[37] It is suggested that parents who wish to attend should not be actively discouraged, and parents who prefer not to be present should not be actively encouraged to attend.[37]
Anesthesia and the brain[edit]
Anesthesia has little to no effect on brain function, unless there is an existing brain disruption. Barbiturates, or the drugs used to administer anesthesia do not affect auditory brain stem response.[38] An example of a brain disruption would be a concussion.[39] It can be risky and lead to further brain injury if anesthesia is used on a concussed person. Concussions create ionic shifts in the brain that adjust the neuronal transmembrane potential. In order to restore this potential more glucose has to be made to equal the potential that is lost. This can be very dangerous and lead to cell death. This makes the brain very vulnerable in surgery. There are also changes to cerebral blood flow. The injury complicates the oxygen blood flow and supply to the brain.
Maintenance[edit]
The duration of action of intravenous induction agents is generally 5 to 10 minutes, after which spontaneous recovery of consciousness will occur.[48] 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.[49] 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.[50][51]
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).[6]
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.[52]
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.[6]
Emergence[edit]
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.[53] 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.[6]