Purpose
General anaesthesia has many purposes including:
- Analgesia — loss of response to pain,
- Amnesia — loss of memory,
- Immobility — loss of motor reflexes,
- Hypnosis — loss of consciousness,
- Skeletal muscle relaxation.
Biochemical mechanism of action
The biochemical mechanism of action of general anaesthetics is not yet well understood. To induce unconsciousness, anaesthetics affect the GABA and NMDAsystems. For example, halothane is a GABA agonist,and ketamine is an NMDAreceptor antagonist.
Preanaesthetic evaluation
Prior to planned operation or procedure, the anaesthetist reviews the medical record and/or interviews the patient to determine the best combination of drugs and dosages and the degree to which monitoring will be required to ensure a safe and effective procedure. Key factors of this evaluation are the patient's age, body mass index, medical and surgical history, current medications, and fasting time. Thorough and accurate answering of the questions is important so that the anaesthetist can select the proper anaesthetic drugs and procedures. For example, a patient who consumes significant quantities of alcohol or illicit drugs could be undermedicated if s/he fails to disclose this fact. This in turn could then lead to anaesthesia awarenessor dangerously high blood pressure.[citation needed] Commonly used medications (e.g., sildenafil) can interact with anaesthesia drugs; failure to disclose such usage can also increase the risk to the patient.[citation needed] There are some situations (where patients are on a certain medication and must undergo a given procedure) in which local anaesthesia or regional anaesthesia can be given, but instead general anaesthesia is chosen, though this is not extremely common due to the fact that general anaesthesia is by nature more dangerous and the agents used react with many more medications.
An important aspect of the preanaesthetic evaluation is that of the patient's airway, involving inspection of the mouth opening and visualisation of the soft tissues of thepharynx. The condition of teeth and location of dental crowns and caps are checked, neck flexibility and head extension observed. If a tracheal tube is indicated and airway management is deemed difficult, then alternative methods of tracheal intubation, such as fibreoptic intubation, may be required as part of the anaesthetic management.
Premedication
Anaesthesiologists may prescribe or administer a premedication prior to administration of a general anaesthetic. Anaesthetic premedication consists of a drug or combination of drugs that serve to complement or otherwise improve the quality of the anaesthetic.
One example of this is the preoperative administration of clonidine, an alpha-2 adrenergic agonist. Clonidine premedication reduces the need for anaesthetic induction agents[citation needed], as well as the need for volatile anaesthetic agents during maintenance of general anaesthesia,[citation needed] and the need for postoperative analgesics.[citation needed] Clonidine premedication also reduces postoperative shivering[citation needed], postoperative nausea and vomiting and emergence delirium.[citation needed] In children, clonidine premedication is at least as effective as benzodiazepines, in addition to having a more favourable side effect profile.[citation needed] It also reduces the incidence of post-operative delirium associated with sevoflurane anaesthesia.[citation needed] As a result clonidine has become a popular agent for anaesthetic premedication. Drawbacks of oral clonidine include the fact that it can take up to 45 minutes to take full effect,hypotension and bradycardia.[citation needed]
Midazolam, a benzodiazepine characterized by a rapid onset and short duration relative to other benzodiazepines, is effective in reducing preoperative anxiety, including separation anxiety associated with separation of children from their parents and induction of anaesthesia. Dexmedetomidine and certain atypical antipsychotic agents are other drugs that are used in particular in very uncooperative children.
Melatonin has been found to be effective as an anaesthetic premedication in both adults and children due to its hypnotic, anxiolytic, sedative, antinociceptive, andanticonvulsant properties. Unlike midazolam, melatonin does not impairpsychomotor skills or adversely affect the quality of recovery. Recovery is more rapid after melatonin premedication than with midazolam, and there is also a reduced incidence of post-operative agitation and delirium. Melatonin premedication also reduces the required induction dose of propofol andthiopental.
Another example of anaesthetic premedication is the preoperative administration ofbeta adrenergic antagonists to reduce the incidence of postoperative hypertension,cardiac dysrhythmia or myocardial infarction.[citation needed] One may choose to administer an antiemetic agent such as droperidol or dexamethasone to reduce the incidence of postoperative nausea and vomiting,[citation needed] or subcutaneousheparin or enoxaparin to reduce the incidence of deep vein thrombosis.[citation needed] Other commonly used premedication agents includeopioids such as fentanyl or sufentanil, gastrokinetic agents such asmetoclopramide, and histamine antagonists such as famotidine.
Non-pharmacologic preanaesthetic interventions include playing relaxing music,massage, and reducing ambient light and noise levels in order to maintain the sleep-wake cycle. These techniques are particularly useful for paediatric and mentally retarded patients. Other options for children who refuse or cannot tolerate pharmacologic premedication include interventions by clowns and child life specialists. Minimizing sensory stimulation or distraction by video games may also help to reduce anxiety prior to or during induction of general anaesthesia.
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