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
What are the neurobiological effects of anaesthetics?
- amnesia
- hypnosis
- immobility
What is MAC?
Minimum alveolar concentration = concentration of the vapour that prevents the reaction to a standard surgical stimulus in 50% of subjects
1 MAC = all patients are asleep
What are some commonly used inhalation agents?
- sevoflurane
- desflurane
- isoflurane
What are the adverse effects of GA?
- vasodilation
- decrease cardiac contractility
- can potentially affect organ perfusion
- malignant hyperthermia
- hepatotoxicity
Which inhalation agent is sweet smelling and used when we cannot establish I.V access?
sevoflurane
Which inhalation agent has rapid onset and offset, lowest lipid solubility and can be used for long operations?
desflurane
Which inhalation agent has the least effect on organ blood flow?
Isoflurane
The next step after anaesthesia is analgesia. Why is this required?
- insertion of airway
- laryngeal mask airway
- intubation
- intra operative pain relief
- post operative pain relief
What is the next step after analgesia and what is this required for?
muscle relaxation
required for intubation and surgery
Name some opoids
- fentanyl (commonest, short acting)
- morphine
- oxycodon
- paracetmol
- NSAIDS
- tramadol
- dihydrocodeine
Name some muscle relaxants for akinesis?
- suxamethonium
- atracurium
- vecuronium
- pancuronium
How do we reverse muscle relaxants?
with neostigmine and glycopyrrolate
How do we assess conciousness?
- clinical signs
- measure level MAC
- BIS (bispectral index) monitor
- isolated forearm
- evoked potentials
