General Anaesthetics

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Introduction

Anaesthesia is a reversible state of CNS depression to unconsciousness.

General anaesthesia has four primary objectives:

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Hypnotics

rapid inducers of unconsciousness

  • propofol
  • etomidate
  • thiopental
  • ketamine

propofol

  • extremely short acting, with rapid onset and offset
  • not water soluble: painful, risk of infection due to emulsion
  • hypotension
  • expensive
  • used in the ICU during intubations

Etomidate

 

thiopental

  • mimics GABA action
  • ultra-short acting, but long half life - due to redistribution of drug from brain to tissues
  • garlicy taste, skin rash and erythema, painful, hypotension

ketamine

  • potent analgesic effect
  • rapid onset, short duration
  • risk of psychosis and nightmare
  • increased increased intracranial pressure
  • increased hypertension and tachycardia

 

 

 

 

Amnesics

  • diazepan
  • midazolam

Diazepam

  • enhances GABA
  • potent amnesic and anticonvulsant
  • rapid onset but long elimination half life
  • highly lipophilic, so painful injection

Midazolam

  • water soluble
  • slightly slower onset than diazepam

 

 

 

Analgesics

  • morphine
  • fentanyl
  • sufentanil
  • remifentanil
  • meperidine

Morphine

  • slower onset (10-15 min) but long acting (3-4 hours)
  • used post-operatively

Fentanyl

  • 100x more potent than morphine
  • rapid onset (5 minutes) but shorter duration (30-60 minutes)

Sufentanil

  • similar pharmacokinetics as fentanyl, but 10x more potent

Remifentanil

  • ultra short acting
  • useful for short procedures where you want the pt awake right away, ie risk of stroke
  • metabolized by plasma esterases

Meperidine

  • semisynthetic, similar profile as morphine
  • higher incidence of nausea and vomiting, but lower risk of biliary spasm

 

 

 

 

 

 

 

 

 

Muscle Relaxants

To ensure the patient doesn't move during intubation and surgery

  • succinylcholine
  • pancuronium
  • rocuronium

Succinylcholine

  • neuromuscular junction blocker
  • rapid onset and short duration due to removal by plasma pseudocholinesterase
  • intense myalgia afterwards
  • risk of malignant hyperthermia

pancuronium

  • slow onset, long acting, non-depolarizing relaxant
  • can be antagonised by anticholinesterases
  • primarily removed by kidneys; duriation will be prolonged in patients with renal insufficiency

rocuronium

  • rapid onset, shorter acting, non-depolarizing muscle relaxant

 

 

 

 

Inhalational Agents

Most inhalational agents (except nitrous oxide) provide hypnosis, analgesia, amnesia, and muscular relaxation.

Potency is measured by minimal alveolar concentration (MAC) - the concentration in oxygen that will prevent 50% of patients from making a purposeful movement in response to a painful stimulus such as surgical incision.

 

Nitrous Oxide is good because of

Halothane: most potent, but slowest; least potentiation of muscle relaxation

Isoflurane: most common

Sevoflurane:

Desflurane

 

 

 

 

Resources and References

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