GENERAL ANESTHETICS

General anesthesia involves the physiological changes: Reversible loss of response to painful
stimuli, loss of consciousness and loss of motor and autonomic reflexes. Loss of consciousness
is associated with inhibition of the activity of reticular formation.
General anesthetics are administered by inhalation or by intravenous routes. They are classified
into two on the basis of their route of administration as inhalation and intravenous anesthetics.
Inhalation anesthetics
The main agents are: Halothane, nitrous oxide, enflurane and ether.

1. Halothane: Is the most widely used agent, highly lipid soluble, potent. It causes arrhythmia,
   hangover and the risk of liver damage is high if used repeatedly.
2. Nitrous oxide: Oderless and colourless gas. It is rapid in action and also an effective
   analgesic agent. Its potency is low, hence must be combined with other agents. It is a relatively
   free of serious unwanted effects.
3. Enflurane: Halogenated ether (similar to halothane). Poorly metabolized in the liver, thus
   less toxic than halothane. It is faster in its action, less liable to accumulate in the body fat
   compared to halothane. It causes seizure during induction and following recovery from
   anaesthesia.
4. Ether: Has analgesic and muscle relaxant properties. It is highly explosive, causes
   respiratory tract irritation, postoperative nausea and vomiting. It is not widely used currently.
   INTRAVENOUS ANESTHETICS
   Intravenous anesthetics act much more rapidly, producing unconsciousness in about 20
   seconds, as soon as the drug reaches the brain from the site of its injection. These agents used
   for induction of anaesthesia followed by inhalation agent. The main induction agent in current
   use is: thiopentone, etomidate, propofol, ketamine and short acting benzodiazepine
   (midazolam).
   Thiopentone: Thiopentone is a barbiturate with very high lipid solubility. After intravenous
   administration the drug enters to tissues with a large blood flow (liver, kidneys, brain, etc) and
   more slowly to muscle. Uptake into body fat occurs slowly because of the low blood flow to this
   tissue, which may cause prolonged effect if given repeatedly. It causes cardiovascular
   depression.
   
   Etomidate: It is more quickly metabolized and the risk of cardiovascular depression is less
   compared to thiopentone. Etomidate suppresses the adrenal cortex, which has been associated
   with an increase in mortality in severely ill patients.
   Ketamine: acts more slowly than thiopentone and produces a different effect, known as
   dissociative anaesthesia in which there is a marked sensory loss and analgesia, as well as
   amnesia and paralysis of movement, without actual loss of consciousness. Ketamine causes
   dysphoria, hallucinations during recovery.
   Benzodiazepines including diazepam, lorazepam, and midazolam are used in general
   anesthetic procedures. Compared with intravenous barbiturates, benzodiazepines produce a
   slower onset of central nervous system effects. Benzodiazepines prolong the postanesthetic
   recovery period but also cause a high incidence of amnesia for events occurring after the drug
   is administered.The benzodiazepines are useful in anesthesia as premedication and
   intraoperative sedation.
   Opioid analgesic anesthesia: Opioid analgesics can be used for general anesthesia, in
   patients undergoing cardiac surgery and fentanyl and its derivates are commonly used for these
   purposes.
   Preanesthetic medication: It is the use of drugs prior to the administration of anaesthetic
   agent with the important objective of making anaesthesia safer and more agreable to the
   patient. The drugs commonly used are, opioid analgesics, barbiturates, anticholinergics, anti
   emetics and glucocorticoids.