General Anaesthetics
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Introduction
Anaesthesia is a reversible state of CNS depression to unconsciousness.
General anaesthesia has four primary objectives:
- unconsciousness
- amnesia
- analgesia
- muscle relaxation
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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
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
- 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
- rapid onset and offset
- analgesia
- minimal metabolism
- minimal hemodynalic changes
- low cost
Halothane: most potent, but slowest; least potentiation of muscle relaxation
Isoflurane: most common
Sevoflurane:
Desflurane
Resources and References
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