General anaesthetic

# Overview
A general anaesthetic (or anesthetic) drug is an anaesthetic drug that brings about a reversible loss of consciousness. These drugs are generally administered by an anesthesia provider in order to induce or maintain general anaesthesia to facilitate surgery.
Drugs given to induce or maintain general anaesthesia are either given as:
- Gases or vapors (inhalational anaesthetics)
- Injections (intravenous anaesthetics)
Most commonly these two forms are combined, with an injection given to induce anaesthesia and a gas used to maintain it, although it is possible to deliver anaesthesia solely by inhalation or injection.
Inhalational anaesthetic substances are either volatile liquids or gases and are usually delivered using an anaesthesia machine. An anaesthesia machine allows composing a mixture of oxygen, anaesthetics and ambient air, delivering it to the patient and monitoring patient and machine parameters. Liquid anaesthetics are vaporized in the machine.
Many compounds have been used for inhalation anaesthesia, but only a few are still in widespread use. Desflurane, isoflurane and sevoflurane are the most widely used volatile anaesthetics today.  They are often combined with nitrous oxide. Older, less popular, volatile anesthetics, include halothane, enflurane, and methoxyflurane. Researchers are also actively exploring the use of xenon as an anaesthetic.
Injection anaesthetics are used for induction and maintenance of a state of unconsciousness. Anaesthetists prefer to use intravenous injections as they are faster, generally less painful and more reliable than  intramuscular or subcutaneous injections. Among the most widely used drugs are:
- Propofol
- Etomidate
- Barbiturates such as methohexital and thiopentone/thiopental
- Benzodiazepines such as midazolam and diazepam (commonly known as Valium)
- Ketamine is used in the UK as "field anaesthesia", for instance at a road traffic incident, and is more frequently used in the operative setting in the US.
The volatile anaesthetics are a class of general anaesthetic drugs. They share the property of being liquid at room temperature, but evaporating easily for administration by inhalation (some experts make a distinction between volatile and gas anesthetics on this basis, but both are treated in this article, since they probably do not differ in mechanm of action). All of these agents share the property of being quite hydrophobic (i.e., as liquids, they are not freely miscible with in water, and as gases they dissolve in oils better than in water
Pediatric Anaesthesia:
Anatomic and physiologic differences among neonates, children and adults require differences in the administration of anesthesia compared to adults.
Preoperative Preparation:
The anesthesiologist first assesses the medical condition of the child and psychological status of the child and family while considering the surgical procedure. The anesthesia induction plan is made and explained to the family in order to alleviate fears the child and family may have.  Chart review, physical examination is performed. Any medications to be used and equipment monitoring should be explained to patient. Informed consent should be obtained. Care should be taken to alleviate the anxiety of a child and parents, as anxiety felt by the parents my be transferred to the child. For the extremely anxious child that cannot be consoled by these methods heavy premedication is often used. Premedication combination is often midazolam, Ketamine, and atropine.
Fasting: Infants and children have a higher metabolic rate and a larger body surface area:weight than adults. As such they become dehydrated more easily than adults.  Studies have found not difference in gastric residual volume and pH in children permitted unlimited clear liquids 2 to 3 hours prior to anesthesia induction.
Current guidelines American Association of Anesthesiologists:
Ingested Material Minimum Fasting Period
Clear liquids 2 h
Breast milk 4 h
Infant formula 6 h
Nonhuman milk 6 h
Light meal 6 h
Premedication: Premedication is not often necessary for children 6 months and younger. Premedication is often used in the extremely fearful child or 10-12 month olds as this is the peak time a child has severe separation anxiety from parents. Oral midazolam is most commonly used .25-.33 mg/Kg in the US. Premedications may be administered orally, intravenously, intramuscularly, nasally, rectally and sublingually. Premedication combination is often midazolam, Ketamine, and atropine.
Except for atropine, anticholinergics are not commonly used in infants and children as they often do not reduce laryngeal reflexes during anesthesia induction. Atropine in infants less than 6 months of age less than 45 minutes before induction reduces hypotension during in induction with inhaled anesthetics.
Induction of Anesthesia: