Anesthesia or anaesthesia from Greek "without sensation" is a state of controlled, temporary loss of sensation or awareness that is induced for medical purposes. It may include analgesia relief from or prevention of pain , paralysis muscle relaxation , amnesia loss of memory , or unconsciousness.
A patient under the effects of anesthetic drugs is referred to as being anesthetized. Anesthesia enables the painless performance of medical procedure that would otherwise cause severe or intolerable pain to an unanesthetized patient, or would otherwise be technically unfeasible. Three broad categories of anesthesia exist:. In preparing for a medical procedure, the clinician chooses one or more drugs to achieve the types and degree of anesthesia characteristics appropriate for the type of procedure and the particular patient.
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The types of drugs used include general anesthetics , local anesthetics , hypnotics , sedatives , neuromuscular-blocking drugs , narcotics , and analgesics. The risks of complications during or after anesthesia are often difficult to separate from those of the procedure for which anesthesia is being given, but in the main they are related to three factors: the health of the patient, the complexity and stress of the procedure itself, and the anaesthetic technique.
Of these factors, the health of the patient has the greatest impact. Major perioperative risks can include death, heart attack , and pulmonary embolism whereas minor risks can include postoperative nausea and vomiting and hospital readmission. Some conditions, like local anesthetic toxicity, airway trauma or malignant hyperthermia , can be more directly attributed to specific anesthetic drugs and techniques.
The purpose of anesthesia can be distilled down to three basic goals or endpoints:  : Different types of anesthesia affect the endpoints differently.
Regional anesthesia , for instance, affects analgesia; benzodiazepine -type sedatives used for sedation, or " twilight anesthesia " favor amnesia ; and general anesthetics can affect all of the endpoints. The goal of anesthesia is to achieve the endpoints required for the given surgical procedure with the least risk to the patient.
To achieve the goals of anesthesia, drugs act on different but interconnected parts of the nervous system. Hypnosis, for instance, is generated through actions on the nuclei in the brain and is similar to the activation of sleep. The effect is to make people less aware and less reactive to noxious stimuli. Loss of memory amnesia is created by action of drugs on multiple but specific regions of the brain.
Memories are created as either declarative or non-declarative memories in several stages short-term , long-term , long-lasting the strength of which is determined by the strength of connections between neurons termed synaptic plasticity. Inhalational anesthetics will reliably produce amnesia through general suppression of the nuclei at doses below those required for loss of consciousness.
Drugs like midazolam produce amnesia through different pathways by blocking the formation of long-term memories. Tied closely to the concepts of amnesia and hypnosis is the concept of consciousness.
Consciousness is the higher order process that synthesizes information. Likewise, a person can have dreams a state of subjective consciousness during anesthetic or have consciousness of the procedure despite having no indication of it under anesthetic. Anesthesia is unique in that it is not a direct means of treatment; rather, it allows others to do things that may treat, diagnose, or cure an ailment which would otherwise be painful or complicated.
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The best anesthetic, therefore, is the one with the lowest risk to the patient that still achieves the endpoints required to complete the procedure.
The first stage in anesthesia is the pre-operative risk assessment consisting of the medical history , physical examination and lab tests. Diagnosing a person's pre-operative physical status allows the clinician to minimize anesthetic risks.
Safe anesthesia care depends greatly on well-functioning teams of highly trained healthcare workers. The medical specialty centred around anesthesia is called anesthesiology , and medical doctors who practise it are termed anesthesiologists. Ancillary healthcare workers involved in anesthesia provision have varying titles and roles depending on the jurisdiction, and include nurse anesthetists , anesthetic nurses , anesthesiologist assistants , anaesthetic technicians , physicians' assistants anaesthesia , operating department practitioners and anesthesia technologists.
International standards for the safe practice of anesthesia, jointly endorsed by the World Health Organization and the World Federation of Societies of Anaesthesiologists , highly recommend that anesthesia should be provided, overseen or led by anesthesiologists, with the exception of minimal sedation or superficial procedures performed under local anesthesia.
One part of the risk assessment is based on the patients' health.
The American Society of Anesthesiologists has developed a six-tier scale that stratifies the patient's pre-operative physical state. It is called the ASA physical status.
The scale assesses risk as the patient's general health relates to an anesthetic. The more detailed pre-operative medical history aims to discover genetic disorders such as malignant hyperthermia or pseudocholinesterase deficiency , habits tobacco , drug and alcohol use , physical attributes such as obesity or a difficult airway and any coexisting diseases especially cardiac and respiratory diseases that might impact the anesthetic.
The physical examination helps quantify the impact of anything found in the medical history in addition to lab tests. Aside from the generalities of the patients health assessment, an evaluation of specific factors as they relate to the surgery also need to be considered for anesthesia. For instance, anesthesia during childbirth must consider not only the mother but the baby.
Cancers and tumors that occupy the lungs or throat create special challenges to general anesthesia. After determining the health of the person undergoing anesthetic and the endpoints that are required to complete the procedure, the type of anesthetic can be selected.
Choice of surgical method and anesthetic technique aims to reduce risk of complications, shorten time needed for recovery and minimise the surgical stress response. Anesthesia is a combination of the endpoints discussed above that are reached by drugs acting on different but overlapping sites in the central nervous system. General anesthesia as opposed to sedation or regional anesthesia has three main goals: lack of movement paralysis , unconsciousness , and blunting of the stress response.
In the early days of anesthesia, anesthetics could reliably achieve the first two, allowing surgeons to perform necessary procedures, but many patients died because the extremes of blood pressure and pulse caused by the surgical insult were ultimately harmful.
Eventually, the need for blunting of the surgical stress response was identified by Harvey Cushing , who injected local anesthetic prior to hernia repairs. The most common approach to reach the endpoints of general anesthesia is through the use of inhaled general anesthetics. Each anesthetic has its own potency which is correlated to its solubility in oil.
This relationship exists because the drugs bind directly to cavities in proteins of the central nervous system, although several theories of general anesthetic action have been described. Inhalational anesthetics are thought to exact their effects on different parts of the central nervous system.
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For instance, the immobilizing effect of inhaled anesthetics results from an effect on the spinal cord whereas sedation, hypnosis and amnesia involve sites in the brain. The higher the MAC, generally, the less potent the anesthetic.
The ideal anesthetic drug would provide hypnosis, amnesia, analgesia, and muscle relaxation without undesirable changes in blood pressure, pulse or breathing. In the s, physicians started to augment inhaled general anesthetics with intravenous general anesthetics.
The drugs used in combination offered a better risk profile to the person under anesthesia and a quicker recovery.
A combination of drugs was later shown to result in lower odds of dying in the first 7 days after anesthetic. For instance, propofol injection might be used to start the anesthetic, fentanyl injection used to blunt the stress response, midazolam injection given to ensure amnesia and sevoflurane inhaled during the procedure to maintain the effects.
More recently, several intravenous drugs have been developed which, if desired, allow inhaled general anesthetics to be avoided completely.
The core instrument in an inhalational anesthetic delivery system is an anesthetic machine. It has vaporizers , ventilators , an anesthetic breathing circuit, waste gas scavenging system and pressure gauges.
The purpose of the anesthetic machine is to provide anesthetic gas at a constant pressure, oxygen for breathing and to remove carbon dioxide or other waste anesthetic gases. Since inhalational anesthetics are flammable, various checklists have been developed to confirm that the machine is ready for use, that the safety features are active and the electrical hazards are removed.
There are also many smaller instruments used in airway management and monitoring the patient. The common thread to modern machinery in this field is the use of fail-safe systems that decrease the odds of catastrophic misuse of the machine.
Patients under general anesthesia must undergo continuous physiological monitoring to ensure safety. In the US, the American Society of Anesthesiologists ASA has established minimum monitoring guidelines for patients receiving general anesthesia, regional anesthesia, or sedation. These include electrocardiography ECG , heart rate, blood pressure, inspired and expired gases, oxygen saturation of the blood pulse oximetry , and temperature.
For minor surgery, this generally includes monitoring of heart rate , oxygen saturation , blood pressure , and inspired and expired concentrations for oxygen , carbon dioxide , and inhalational anesthetic agents.
For more invasive surgery, monitoring may also include temperature, urine output, blood pressure, central venous pressure , pulmonary artery pressure and pulmonary artery occlusion pressure , cardiac output , cerebral activity , and neuromuscular function. In addition, the operating room environment must be monitored for ambient temperature and humidity, as well as for accumulation of exhaled inhalational anesthetic agents, which might be deleterious to the health of operating room personnel.
Sedation also referred to as dissociative anesthesia or twilight anesthesia creates hypnotic , sedative , anxiolytic , amnesic , anticonvulsant , and centrally produced muscle-relaxing properties. From the perspective of the person giving the sedation, the patient appears sleepy, relaxed and forgetful, allowing unpleasant procedures to be more easily completed.
Sedatives such as benzodiazepines are usually given with pain relievers such as narcotics , or local anesthetics or both because they do not, by themselves, provide significant pain relief. From the perspective of the person receiving a sedative, the effect is a feeling of general relaxation, amnesia loss of memory and time passing quickly.
Many drugs can produce a sedative effect including benzodiazepines , propofol , thiopental , ketamine and inhaled general anesthetics. The advantage of sedation over a general anesthetic is that it generally does not require support of the airway or breathing no tracheal intubation or mechanical ventilation and can have less of an effect on the cardiovascular system which may add to a greater margin of safety in some patients.
When pain is blocked from a part of the body using local anesthetics , it is generally referred to as regional anesthesia.
There are many types of regional anesthesia either by injecting into the tissue itself, a vein that feeds the area or around a nerve trunk that supplies sensation to the area. The latter are called nerve blocks and are divided into peripheral or central nerve blocks. The following are the types of regional anesthesia:  : — When local anesthetic is injected around a larger diameter nerve that transmits sensation from an entire region it is referred to as a nerve block or regional nerve blockade.
Nerve blocks are commonly used in dentistry, when the mandibular nerve is blocked for procedures on the lower teeth. With larger diameter nerves such as the interscalene block for upper limbs or psoas compartment block for lower limbs the nerve and position of the needle is localized with ultrasound or electrical stimulation. The use of ultrasound may reduce complication rates and improve quality, performance time, and time to onset of blocks.
Nerve blocks are also used as a continuous infusion, following major surgery such as knee, hip and shoulder replacement surgery, and may be associated with lower complications. Central neuraxial anesthesia is the injection of local anesthetic around the spinal cord to provide analgesia in the abdomen , pelvis or lower extremities.
Pharmacology - GENERAL & LOCAL ANESTHETICS (MADE EASY)
It is divided into either spinal injection into the subarachnoid space , epidural injection outside of the subarachnoid space into the epidural space and caudal injection into the cauda equina or tail end of the spinal cord.
Spinal and epidural are the most commonly used forms of central neuraxial blockade. Spinal anesthesia is a "one-shot" injection that provides rapid onset and profound sensory anesthesia with lower doses of anesthetic, and is usually associated with neuromuscular blockade loss of muscle control.
Epidural anesthesia uses larger doses of anesthetic infused through an indwelling catheter which allows the anesthetic to be augmented should the effects begin to dissipate. Epidural anesthesia does not typically affect muscle control.
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Because central neuraxial blockade causes arterial and vasodilation , a drop in blood pressure is common. The physiologic effects are much greater when the block is placed above the 5th thoracic vertebra.
An ineffective block is most often due to inadequate anxiolysis or sedation rather than a failure of the block itself.
Pain that is well managed during and immediately after surgery improves the health of patients by decreasing physiologic stress and the potential for chronic pain. Instead, it is a dynamic process wherein persistent painful stimuli can sensitize the system and either make pain management difficult or promote the development of chronic pain. Pain management is classified into either pre-emptive or on-demand.