Induction and maintenance Flashcards
There are 4 stages of anaesthesia. Describe stage 1.
Voluntary excitement
Lasts until unconsciousness is present
Patient may resist induction and show fear and apprehension, then disorientation
Heart rate increased due to sympathetic stimulation
There are 4 stages of anaesthesia. Describe stage 2.
Involuntary excitement
From unconsciousness until rhythmic breathing is present
All cranial nerve reflexes present, hyperactive
Heart rate increased
Struggling movements and howling may be seen though the animal is not really aware of what it is doing
There are 4 stages of anaesthesia. Describe stage 3 (plane I)
Reflexes less prominent
Regular deep respiratory movements
Heart rate slightly reduced and regular
Eye central but beginning to rotate ventrally
Muscle tone (eg jaw) still present and responsive
There are 4 stages of anaesthesia. Describe stage 3 (plane II)(surgical anaesthesia)
Palpebral, pedal reflexes absent
Slightly reduced respiratory movements, will still increase with painful stimuli
Heart rate slightly reduced and regular
Eye rotated ventrally, pupil may be constricted
Muscles relaxed
There are 4 stages of anaesthesia. Describe stage 3 (plane III)(heading to overdose)
All reflexes absent but corneal
Shallow respiration and rate decreased
Reduced HR and BP
No muscle tone
There are 4 stages of anaesthesia. Describe stage 4.
Reduced respiration rate, irregular/jerky progressing to agonal Heart rate low, pulses weak and slow Reflexes absent Eye central, pupil fixed and dilated Muscle tone flacid CARDIAC ARREST IMMINANT!!!!
Describe the ideal induction agent
Takes the patient as smoothly and rapidly as possible from a conscious state to being anaesthetised (stage III)
Is short acting- but allows enough time for an inhalation agent to build up to effective levels
Has minimal effects on the cardiovascular and respiratory systems
What drugs can be used as an induction agent?
Barbiturates No longer licensed except for euthanasia Propofol Alfaxalone Ketamine (as part of a combination)
Describe propofol
Hindered Phenol
May be used on its own. Best after a pre-med
Given slowly (but not too slowly!) over 30-40 seconds
Duration is only 5 minutes
Top up doses can be given but may prolong recovery (especially cats)
Usually patient is intubated and inhalational agent used for maintenance
What are the advantages of propofol?
Non irritant on injection
Can be given slowly to effect (intravenous)
Top up doses licenced to be given
Smooth, rapid induction and recovery
Rapidly metabolised in liver
Can be used for caesarean- but allow time for mother to metabolise drug (15 mins) before removing pups
What are the disadvantages of propofol?
Pain occasionally occurs on injection
Cardiovascular and respiratory depression occur
Post injection apnoea if injected too quickly
No analgesic properties
Two types-
-Emulsion containing benzyl alcohol (Propoflo plus) risk of side effects if top ups given, can keep 28d after opened
-emulsion containing soya and egg with no bacteriostat. Cannot be stored once open (refrigeration will NOT help!)
Describe alfaxalone
Steroid anaesthesia
Is NOT Saffan. Can be used in dogs and cats.
Given slowly IV over ~60 seconds
Duration 5-10 mins
Can use on its own but better after premed.
Usually patient intubated and maintained with inhalational agent
Can use to maintain anaesthesia but recovery prolonged.
What are the advantages of alfaxalone?
Less cardiopulmonary suppression than propofol.
Very rapidly metabolised by the liver
High safety margin
No pain on injection
What are the disadvantages of alfaxalone?
Post injection apnoea if injected too quickly
Been licenced for 10 years but anecdotally less use than propofol – vets reluctant to change
Prolonged recovery if ‘top-up’
No preservative so unused product should be discarded
Limited analgesia
Can get paddling/twitching on recovery if animal moved
Describe ketamine
Dissociative anaesthetic
Needs to be used in combination with other drugs
-Double/Triple combination (cats/dogs)
-Benzodiazepine (cats)
Excreted unchanged in urine of cats (hepatic metabolism in dogs)
What are the advantages of ketamine?
Good somatic analgesia
Can be given IM or IV
In combination with a benzodiazepine relatively safe in sick animals
Cardiovascular system stimulated (via sympathetic nerves)
Respiratory function not compromised (though apneustic breathing with slow inspiration, a pause then rapid expiration does occur)
Rapid onset, especially when used IV
Also absorbed over mucous membranes
What are the disadvantages of ketamine?
Pain on IM injection
Reclassified as Schedule 2 Controlled drug – formerly Schedule 4
No muscle relaxation
Eyes remain open – must lubricate corneas
Cannot be used on its own without risk of extreme excitement or seizures
What inject-able agents can be used for maintenance?
Propofol or alfaxalone – Total Intra-Venous Anaesthesia
Ketamine
- Combined with α2 and opioid (triple comb)
- Combined with benzodiazepine
Describe triple (dom/torb/ket)
Complimentary actions
good supporting literature/dose charts
Wide range of species
Large range of data available
Mixed in same syringe? If used immediately
Reversible- atipamezole can be given at any time after iv injection in the cat, but only after 40 minutes if given im. Not at all in the dog!
name some inhalation agents used for maintenance
Isoflurane
Sevoflurane
Nitrous oxide
Describe how inhalation agents are used
Must be taken from the alveolus into the bloodstream and then circulated to the brain.
They require the use of an anaesthetic circuit
With the exception of nitrous oxide (a gas) these agents are volatile liquids
Can be used for induction but more commonly used for maintenance
How is the strength of an inhalation agent calculated?
Minimum alveolar concentration = MAC
The minimum concentration of anaesthetic agent in the alveoli required to keep 50% patients asleep
Expressed as a percentage
Relates to the numbers on the vaporiser dial
What is blood:gas solubility?
the portion of agent that will dissolve in the blood compared to the portion that remains in a gaseous state
The greater the MAC,…
the higher the required vaporiser setting to maintain anaesthesia.
The lower the blood-gas coefficient,…
the more rapid induction and recovery will be
Describe isoflurane
Higher vaporiser setting than halothane
Quicker induction/recovery than halothane
Good muscle relaxation
Some analgesia
Irritant to respiratory tract
Animals don’t like the smell (consider when using as induction agent)
A struggling animal will have high levels of adrenaline. However, unlike halothane the heart is not sensitised to adrenaline, so there is less likelihood of an arrhythmia developing.
Describe sevoflurane
More rapid induction/recovery than isoflurane
No smell and non-irritant to respiratory tract
Now licenced for dogs and cats
Higher vaporiser setting (↑cost).
Reacts with soda-lime in rebreathing circuits
Depth of anaesthetic can change quickly- care when monitoring
In skilled hands this is a very useful anaesthetic and we are likely to see its use increasing.
Describe health ans safety surrounding anaesthetic agents
Each anaesthetic will have an SPC
Drugs must be stored correctly, in a locked cabinet if controlled
Local rules should be drawn up to indicate how a spill should be dealt with
There are published maximum exposure limits for the volatile agents, and these must be recorded annually by personal dosimetry
Describe nitrous oxide
Weak anaesthetic, so cannot be used alone, given with O2 and isoflurane or sevoflurane
Given at ~50%
Very good analgesia
Will reduce volatile requirement
Minimal cardiopulmonary depression
Will diffuse into gas filled structures (GDV, pneumothorax)
What is the second gas effect of nitrous oxide?
Nitrous oxide will increase the rate of uptake of other inhalational agents into the blood stream
Therefore an appropriate level of anaesthesia will be reached more quickly
Describe diffusion hypoxia from nitrous oxide
Due to rapid movement of gas
When recovering animal and nitrous switched off
Must maintain on O2 for 10-15 minutes
If not oxygen can reach dangerously low levels in alveoli as nitrous diffuses out of the blood stream
What three pieces of equipment are required to administer inhalation agents?
Anaesthetic machine
Breathing system
Patient connector
Describe a scavenging system
Purpose – prevents contamination of the theatre environment with volatile agent
Active scavenging
-(this removes the need for personal dosimetry)
Passive scavenging
Fluosorbers
How should you dispose of anaesthetic agents?
Sharps contaminated with medicines
Bottles/vials contaminated with medicines
Controlled drugs
