General Anaesthetics Flashcards
What are the three aims of general anaesthesia
Loss of consciousness
Analgesia
Muscle Relaxation (immobility)
What are the three main drug classes of general anaesthetics
Gases- Nitrous Oxide
Halogenated Hydrocarbons- Isoflurane
Barbiturates- Thiopental
Define minimum alveolar concentration (MAC)
The concentration of vapour in the lungs that is needed to predict movement in 50% of subjects in response to pain stimulus
What does a lower MAC value mean
More potent volatile anaesthetic
Describe how general anaesthetics inhibit the central nervous system at cellular level and give an example
- Inhibits synaptic transmission through excitatory ion channels such as glutamate and acetylcholine inotropic and nicotinic receptors
- Activate inhibitory ion channels such as GABAa and glycine inotropic receptor and two pore potassium channels
Example: general anaesthetics act to reduce excitation or increase inhibition
Where do general anaesthetics inhibiting the central nervous system normally act in the brain and spine
Brain:
Midbrain reticular formation- causes unconsciousness
Thalamic sensory relay nuclei (analgesia)
Spinal:
Loss of reflexes
Contributes to control of motor function and muscle relaxation
Where does flunitrazepam (Rohypnol) act on
Hippocampus to affect short term memory (amnesia)
What are the three things general anaesthetics act on in the peripheral nervous system and what they do to them
Cardiovascular- can cause circulatory depression via decreasing cardiac contractility
- Normally works on sympathatic nervous system, vascular smooth muscle to affect cardiac output or blood pressure
Respiratory- can cause respiratory depression
Muscular- acts on NMJ to cause muscle relaxation
What are the 5 stages of general anaesthesia (PIMPM)
Premedication
Induction
Maintenance
Pain relief
Muscle Relaxation
What is the premedication stage of general anaesthesia and the purpose of it
Combined drug treatment typically 1-3 hours before anaesthesia
Purpose:
Reduction of anxiety and pain
Promotion of amnesia
Reduction of secretions
Enhancing hypnotic effects of general anaesthesia
Reduction of vagal reflexes to intubation- slows heart
Reduction of post-operative nausea or vomiting centre
What are the drug classes used in premeditation, its role and give an example of a drug for each of them
Benzodiazepine:
Diazepam
Increases inhibitory GABA release for sedative and anxiolytic effect
mAChR antagonist
Atropine
Prevents secretion and vagal reflexes
Opioid receptor agonist
Morphine
Reduces pain and sedation
Proton Pump Inhibitors
Metoclopramide
Limits nausea and vomiting
What is the induction stage of the general anaesthetic cycle
IV bolus agents induces loss of consciousness and amnesia
What are the drugs used in the induction stage and the purpose of each
Propofol or barbiturate (sodium thiopental)
Activate GABAa receptors and promote inhibition (system shutdown)
Thiopental
Accumulate in fat which leads to hangover effect
Associated with profound respiratory
depression
Etomidate
Favoured due to high therapeutic index and therefore lack of respiratory and cardiovascular side effects
Ketamine
NMDA glutamate receptor antagonist
Produces conscious anaesthesia
What are the properties that make the induction drugs good for inducing consciousness
Small highly lipophilic compounds with rapid distribution
Very fast onset to brain circulation
Action terminated when distribution into large compartments (muscle)
Why are inhalation agents not used in the induction phase
They act slower and are prone to dangerous induction
What does the stage of maintenance involve
Volatile
Inhalation agents used to maintain loss of consciousness and amnesia
What are the drugs used in the maintenance stages of general anaesthesia and describe them
Isoflurane- halogenated ether
Nitrous Oxide- low potency- used in combination for reducing child birth pain
Halothane- can lead to adverse cardiovascular and respiratory effects
What are the properties that make the maintenance drugs good for maintaining loss of consciousness
Inhaled gases with fast onset and offset of action- able to control anaesthesia depth
Can freely cross the blood brain barrier so kinetics are not determined by rate of transfer from inspired air into blood
Has rapid elimination
Fewer side effects
What is appropriate pain relief in the general anaesthetic cycle
IV opioid agents used to provide analgesia
What are the drugs used in pain relief in the general anaesthetic life cycle
Opioids- morphine and fentanyl
What is muscle relaxation in the general anaesthetic cycle
Agents that act on the nicotinic acetylcholine receptors at the neuromuscular junctions to cause immobility during surgery
What is the mechanism of action of general anaesthetics that cause muscle relaxation via neuromuscular blocking agents
- Competitive antagonists of nAchRs
- Agonist that cause a depolarising block of the muscle end plate (depolarising blocker)
Examples: Atracurium, pancuronium, vercuronium
How do competitive nAchR blockers work via Na+ entry and give some examples
- Depolarises the post synaptic membrane
- Cause: excitatory postsynaptic potential
- Generates action potential (threshold reached)
- Drugs widely used as muscle relaxants alongside anaesthesia
How do depolarising nAChR blockers work and give some examples
They continually stimulate the neuromuscular junction by agonists and cause muscle paralysis
Muscle contracts due to maintained depolarisation but cannot repolarise and relax
Causes loss of excitability (Na+ cannot inactivate)
Example: Suxamethonium (succinylcholine)
Causes paralysis during anaesthesia for 10 minutes
