Choosing an Anaesthetic Flashcards
Is nitrous oxide used as an anaesthetic?
Needs to be combined w/ other anaesthetics to produce anaesthesia
What factors influence choice of inhalant?
- MAC50
- Agent density
- Agent cost
- Speed of onset and recovery
How does MAC50 influence anaesthetic choice?
- Measure of potency of inhalational anaesthetics
- Minimum alveolar conc of anaesthetic than prevents withdrawal response in 50% of animals
- NB for NO (too high to use alone)
How does agent density affect anaesthetic choice?
- Affects quantity of anaesthetic used
- Denser than air = put in bottom
How does agent cost affect anaesthetic choice?
- Mainly purchase of agent specific vaporiser and machine, but overall less than injectables
- Should base on whats best suited to protocol
- Desflurane + sevoflurane = 4x isoflurane
How does speed of onset and recovery affect anaesthetic choice?
- Significant = affects protocol
- Many factors influence it but induction may be slow unless chamber filled rapidly
What are the MAC50 of halothane, isoflurane, sevoflurane, desflurane, and NO?
Halothane = 0.8%
Isoflurane = 1.39%
Sevoflurane = 2.6%
Desflurane = 6%
NO = >150%
Can you use a facemask on a mouse?
- Usually induce anaesthesia in chamber, then move to face mask and maintain on 1.5 MAC50 E.g. 5%-2% isoflurane
- Gas flow needs to be enough to prevent drawing in room air and expel expired gas
=> 3x mouse minute vol = 0.2ml x 80/30g mouse = 16ml x 3 =48ml/min
What are the problems with a face mask?
- Actively scavenging mask may remove too much gas = high flow rates needed E.g. 1L/min systems
- Flowmeter might not be graduated to set flow of 50ml/min
- Vaporisers often only precise >500ml-1000ml/min
=> Check if effective by turning flow rate to 50 ml and see if mouse remains anaesthetised - Margin of safety = fresh gas flow of 100-200ml
How do you calculate the volume of anaesthetic needed for larger animals?
Tidal volume = ~7-10ml/kg in most mammals
Use minute vol X 3
E.g. 20kg breathing 20 breaths/minute = 20kg x 10ml/kg = 200ml tidal vol x 20 breaths = 4000ml minute volume x 3 = 12L/min on open face mask
Should use Bains/Lack/Circle system
What does speed of recovery depend on?
- Agent
- Concentration
- Duration of anaesthesia
- Less significant w/ desflurane = after prolonged anaesthesia, still rapid recovery (10-15 mins)
How does equipment affect choice of inhalant?
- Safe delivery needs purpose made vaporiser fit with controllable source of gas (anaesthetic machine)
How does irritancy affect choice of inhalant?
- Volatile anaesthetics (distinct odour) + 2nd round of anaesthetic = aversive
- Rabbits = breath-holding
-Stressful = irritation of respiratory tract
How do inhalants effects on body systems affect choice?
- All depress cardio and resp systems
- Depends on study aims
How does agent metabolism affect inhalant choice?
- All except isoflurane = liver metabolism = microsomal enzymes post-anaesthetic
- Degree of enzymes = duration of anaesthesia
- All affect liver blood flow and metabolism = affect research aims
