Induction and maintenance

Question 1

There are 4 stages of anaesthesia. Describe stage 1.

Answer 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

Question 2

There are 4 stages of anaesthesia. Describe stage 2.

Answer 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

Question 3

There are 4 stages of anaesthesia. Describe stage 3 (plane I)

Answer 3

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

Question 4

There are 4 stages of anaesthesia. Describe stage 3 (plane II)(surgical anaesthesia)

Answer 4

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

Question 5

There are 4 stages of anaesthesia. Describe stage 3 (plane III)(heading to overdose)

Answer 5

All reflexes absent but corneal
Shallow respiration and rate decreased
Reduced HR and BP
No muscle tone

Question 6

There are 4 stages of anaesthesia. Describe stage 4.

Answer 6

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!!!!

Question 7

Describe the ideal induction agent

Answer 7

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

Question 8

What drugs can be used as an induction agent?

Answer 8

Barbiturates 
No longer licensed except for euthanasia
Propofol 
Alfaxalone 
Ketamine (as part of a combination)

Question 9

Describe propofol

Answer 9

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

Question 10

What are the advantages of propofol?

Answer 10

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

Question 11

What are the disadvantages of propofol?

Answer 11

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!)

Question 12

Describe alfaxalone

Answer 12

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.

Question 13

What are the advantages of alfaxalone?

Answer 13

Less cardiopulmonary suppression than propofol.
Very rapidly metabolised by the liver
High safety margin
No pain on injection

Question 14

What are the disadvantages of alfaxalone?

Answer 14

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

Question 15

Describe ketamine

Answer 15

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)

Question 16

What are the advantages of ketamine?

Answer 16

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

Question 17

What are the disadvantages of ketamine?

Answer 17

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

Question 18

What inject-able agents can be used for maintenance?

Answer 18

Propofol or alfaxalone – Total Intra-Venous Anaesthesia

Ketamine

• Combined with α2 and opioid (triple comb)
• Combined with benzodiazepine

Question 19

Describe triple (dom/torb/ket)

Answer 19

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!

Question 20

name some inhalation agents used for maintenance

Answer 20

Isoflurane
Sevoflurane
Nitrous oxide

Question 21

Describe how inhalation agents are used

Answer 21

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

Question 22

How is the strength of an inhalation agent calculated?

Answer 22

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

Question 23

What is blood:gas solubility?

Answer 23

the portion of agent that will dissolve in the blood compared to the portion that remains in a gaseous state

Question 24

The greater the MAC,…

Answer 24

the higher the required vaporiser setting to maintain anaesthesia.

Question 25

The lower the blood-gas coefficient,…

Answer 25

the more rapid induction and recovery will be

Question 26

Describe isoflurane

Answer 26

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.

Question 27

Describe sevoflurane

Answer 27

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.

Question 28

Describe health ans safety surrounding anaesthetic agents

Answer 28

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

Question 29

Describe nitrous oxide

Answer 29

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)

Question 30

What is the second gas effect of nitrous oxide?

Answer 30

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

Question 31

Describe diffusion hypoxia from nitrous oxide

Answer 31

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

Question 32

What three pieces of equipment are required to administer inhalation agents?

Answer 32

Anaesthetic machine
Breathing system
Patient connector

Question 33

Describe a scavenging system

Answer 33

Purpose – prevents contamination of the theatre environment with volatile agent
Active scavenging
-(this removes the need for personal dosimetry)
Passive scavenging
Fluosorbers

Question 34

How should you dispose of anaesthetic agents?

Answer 34

Sharps contaminated with medicines
Bottles/vials contaminated with medicines
Controlled drugs