General Anaesthesia

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General Anaesthesia

General anaesthesia is the absence of sensation and consciousness and is induced by inhalation or injection of anaesthetic drug(s).

There are four continuous stages (or levels) of depth of general anaesthesia (there is variability depending on the individual, type of drug, speed of induction and skill of anaesthetist):
Stage 1 – analgesia – consciousness and touch sense are still present and analgesia is partial – pain and smell sensation is lost; muscles are moderately relaxed; speech is difficult
Stage 2 – delirium/excitement – unconscious, but automatic movements can still occur – reflex muscle activity is present; irregular respiration; may speak/shout incoherently - delirious and become violent; blood pressure increases; may vomit; may have urinary and/or faecal incontinence;  anaesthetists try to hasten progress through this stage (this stage shows the most variability among individuals).
Stage 3 – surgical anaesthesia – deep unconsciousness; respiration is regular and muscles are relaxed - consists of 4 planes (determined by changes in respiration, pupils, eyeball movements, reflexes, muscle tone)
Stage 4 – medullary paralysis (toxic stage) – overdose – respiration is severely depressed/lost and medullary function is very low; vasomotor collapse; if not reversed death can occur rapidly from circulatory collapse

Stages of general anaesthetic administration:
1) Induction – cover period from administration until stage 3. Speed of induction depends on patient factors and agent used.
2) Maintenance – the period during which patient remains at stage 3
3) Recovery – period from cessation of administration to recovery of consciousness

Balanced anaesthesia is the use of a combination of drugs to achieve the goal of a smooth and rapid induction of the anaesthesia as well as adequate analgesia and muscle relaxation. The drugs used often include a short acting agent for induction, a neuromuscular blocking agent for muscle relaxation and agents for analgesia.

General anaesthetic drugs:
1) Inhalation anaesthetics:
i) Halothane
ii) Enflurane
iii) Ether
iv) Isoflurane
v) Desflurane
vi) Sevoflurane
vii) Nitrous Oxide (gas – others are volatile liquids)

2) Intravenous anaesthetics:
i) Thiopentone
ii) Propofol
iii) Ketamine

Halothane:
• introduced in 1956 – standard against which newer ones are compared
• induction is smooth and relatively rapid; usually awake 1 hour after cessation of inhalation
• weakly analgesic ( used with other agents)
• has muscle relaxant properties, but not usually sufficient for surgery
• declined in use in adults due to risks of liver failure
• adverse effects – hypotension; respiratory depression; liver toxicity

Isoflurane:
• widely used
• induction is smooth and rapid
• quick recovery

Nitrous oxide:
• ‘laughing gas’
• very low anaesthetic potency and high analgesic potency compared to other inhalation anaesthetics
• can not produce surgical anaesthetic without other agent, but widely used to supplement analgesia with other anaesthetics
• can be used alone in dentistry and during delivery

Intravenous anaesthetics
• thiobarbiturates (eg thiopental sodium)– short acting (onset in 10-30secs) used for induction of anaesthesia to produce unconsciousness (needs to be followed by inhalation anaesthesia within 10 minutes)
• benzodiazepines (eg IV diazepam) – used for conscious sedation and perioperative amnesia
• propofol – used for induction and maintenance of anaesthesia; rapid onset and short acting

Adjuncts to general anaesthesia
i) Pre-anaesthetic medications
• used to reduce anxiety, produce a perioperative amnesia, relief of pain and to suppress adverse responses to some anaesthetics
• eg benzodiazepines (for anxiety and amnesia); barbiturates (anxiety and sedation); opioids (pain); alpha2-aderenergic agonist (for anxiety, sedation, permits reduction in anaesthetic dose); anticholinergic drugs (prevent bradycardia)

ii) Neuromuscular blocking agents
• muscles need to be relaxed for surgery to be performed
• anaesthesia does not need to be as deep if muscle relaxant used

iii) Post-anaesthetic medications
• analgesics  for postoperative pain
• antiemetics  for suppression of nausea and vomiting
• muscarnic agonists  for urinary retention

Other drugs:
eg Reglan (reduces gastric motility)

Complications of general anaesthesia:
Malignant hyperthermia (tachycardia is first symptom)

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