Week 10: General anaesthesia Flashcards
What is general anaesthesia?
Total loss of sensation
Balanced anaesthesia requires 3 things. What are they?
- Amnesia
Unconsciousness –> lack of response and recall to noxious stimuli - Analgesia
Pain relief - Akinesis
Muscle relaxation –> immobilisation/ paralysis
Explain the 7 steps carried out from arrival into anaesthetic room to recovery
- Anaesthetic pre-assessment which involves discussion about patients health and obtaining verbal consent
- Monitoring
- Intravenous access: to give anaesthetic agents
- Induction of anaesthesia: induction agents
- Start the analgesia and muscle relation
- Maintain the process: maintenance agents for amnesia/ analgesia/ muscle relaxation
- Reverse the process: reverse muscle relaxation. but maintain post operative analgesia
What is one to two arm-brain circulation time?
- time taken for the anaesthesia to go from site injected into the brain
- is usually 10-20 seconds
What is Cp50?
the concentration of the agent in the blood that can prevent movement after a skin incision in 50% of patient
How do GAs work?
- by hyperpolarising the axon and therefore preventing conduction along the neurones
- they modulate the activity of transmitter-gated ion channels
- GA stimulate the inhibitory receptors and inhibit the excitatory receptors
Which 2 channels to GAs stimulate?
The inhibitory receptors:
- GABAa
- Strychnine-sensitive glycine
Which 3 channels to GAs inhibit?
The excitatory receptors:
- 5HT3
- Neuronal nicotinic
- Glutamate NMDA/ AMPA
What compounds are used for indection?
propofol and sodium thiopentone
What are the ideal properties of an intravenous induction agent?
- simple preparation
- compatibility with other agents and iV fluids
- painless on administration
- high potency and efficacy
- predictable action within one circulation time
- minimal cardiovascular effects
- depression of airway reflexes for intubation
- rapid and predictable offset of effect
- rapid metabolism for minimal hangover
What are the four most commonly used GAs?
- Propofol
- Thiopental
- Etomidate
- Ketamine
What are two advantages of propofol?
- excellent suppression of airway reflexes
- decreases incidence of PONV (post operative nausea and vomiting)
What are the unwanted effects of propofol?
- marked drop in HR and BP
- pain on infection
- involuntary movements
What are the advantages of thiopentone?
- barbiturate
- faster than propofol
- used mainly for rapid sequence induction
- anti epileptic properties and protects brain
What are the unwanted effects of thiopentine?
- drop in BP
- increase in HR
- rash/bronchospasm
- intra-arterial injection: thrombosis and gangrene
- contraindicated in porphyria
Ketamine is a dissociative anaesthesia. What does this mean?
anterograde amnesia and profound analgesia
WHat are the unwanted effects of ketamine?
- rise in HR/BP
- N&V
- patients go into ‘different worlds’
What are the advantages of etomidate?
- rapid onset
- haemodynamic stability
- lowest incidence of hypersensitvity
What are the unwanted effects of etomidate?
- pain on injection
- spontaneous movements
- adreno-cortical suppression
- high incidence post operative N&V
What are the ideal physical properties of inhalation agents?
- non-flammable
- stable with materials, long shelf life
- environmentally friendly
- cheap and easy to manufacture
What are the ideal biological properties of inhalation agents?
- pleasant to inhale, non-irritant
- fast onset
- high potency
- minimal effects other systems
- non toxic to theatre personaell
Explain the Meyer Overton theory on how anaesthetics produce unconsciousness
- related to lipid solubility
- perturbation of lipid membranes
- dissolve in plasma membrane affecting its fluidity, volume, surface tension
- this effects the plasma proteins, causing the patient to stay asleep
- agent with highest solubility = highest potency
Why is the Meyer Overton theory shown to be false?
- some predicted anesthetics ineffective
- lipid membrane effect reproduced by small changes in temp
What theory of how general anaesthesia works is accepted?
A combination of all of these theories:
- Critical volume theory
- Mean excess volume theory
- Multisite expansion theory
- Protein theory of anaesthesia
- Effect on channel
