Maintenance of Anaesthesia

Question 1

When can you intubate?

Answer 1

Sufficient depth of anaesthesia

Eyes rotated ventrally
Minimal sluggish palpebral reflex
Loose jaw tone
No swallowing reflex on stimulation
(Animal that is too light can regurgitate)

Question 2

Describe the process of intubation

Answer 2

1. Induction
2. Check for sufficient depth of anaesthesia
3. Pull tongue out
4. Laryngoscope on tongue
   + Don’t touch epiglottis or larynx
5. Visualise laryngeal opening
6. (Local anaesthesia)
7. (Lubrication)
8. Place endotracheal tube

Question 3

How can you tell the correct length of endotracheal tube for the patient?

Answer 3

Measure from nose to shoulder point.

Question 4

Problems associated with endotracheal tubes

Answer 4

Blockage of Murphey’s eye
Placement down one bronchi
Blockage with secretion (particularly cats on ketamine)
Overinflation of cuff

Question 5

What anaesthetic agents can be used for maintenance?

Answer 5

Injectable
Propofol
Alfaxalone
Ketamine

Inhalational
Isoflurane
Sevoflurane
(Nitrous oxide)
(Xenon)

Question 6

Why don’t we use thiopental for maintenance of anaesthesia?

Answer 6

Very lipid soluble so accumulates and gives long recovery period
Suppresses adrenal gland

Question 7

Is analgesia required during maintenance?

Answer 7

YES

Most GA agents provide little analgesia (except ketamine)

Question 8

What methods can we use to maintain anaesthesia?

Answer 8

Inhalational

Intravenous (total intravenous anaesthesia = TIVA)
+ Intermittent bolus
+ Continuous rate infusion

Combination of injectable and inhalational
+ PIVA (partial intravenous anaesthesia)

Occasionally single IM injection sufficient eg. wild animal

Question 9

Describe the difference between intermittent and continuous anaesthesia

Answer 9

Intermittent bolus = increase concentration very quickly then distribution, metabolism and elimination means conc will drop
+ Give another bolus when animal starts to wake up = not good as want stable plane of anaesthesia

Continuous = stable plane of anaesthesia

Question 10

Injectable agents for maintenance (TIVA&PIVA)

Answer 10

TIVA = propofol, alfaxalone

PIVA = ketamine (used at low doses with another agent)

Question 11

Describe how inhalational agents are used for maintenance

Answer 11

90% agents used inhalational

Administered and removed from body by lungs

From alveoli, agent absorbed into blood to brain

Redistributed into other tissues including fat
+ Fat solubility slow recovery from long anaesthetic
+ Vessel rich vs vessel poor tissues

Question 12

Compare the speed of induction using nitrous oxide compared to halothane

Answer 12

Nitrous oxide = very insoluble in blood, get quick induction and recovery (reaches high conc in brain quickly)

Halothane = highly soluble in blood, slow induction and recovery

Question 13

What is the blood/gas partition coefficient? What does a higher value mean?

Answer 13

Number of parts of has in blood vs alveolus.

Higher number = more soluble, so longer induction and recovery

Halothane = 2.4
Isoflurane = 1.4
Sevoflurane = 0.6
Nitrous oxide = 0.47
Desflurane = 0.42

Question 14

What is the MAC?

Answer 14

Minimum alveolar concentration required to prevent movement in response to painful stimulus in 50% animals.

Question 15

What MAC should clinical anaesthesia aim for?

Answer 15

1.25-1.5xMAC

BUT MAC calculated with no premedication, so we don’t have to work with MAC values this high now (MAC sparing effect)

Question 16

MAC values for isoflurane and sevoflurane (%)

Dog
Cat
Horse

Answer 16

DOG: I = 1.3 S = 2.3
CAT: I = 1.6 S = 2.6
HORSE: I = 1.3 S = 2.3

Question 17

What is the MAC not affected by?

Answer 17

Length of anaesthesia
Gender
Blood pH

Question 18

What is the MAC affected by?

Answer 18

Hypothermia/hyperthermia
Age
Severe hypoxia/hypercapnia (elevated CO2)
Severe anaemia
Severe hypotension
CNS depressant dogs
Excitation
Pregnancy

Question 19

How does cardio and respiratory depression and metabolism vary with inhaled agents?

Answer 19

Some reduced CO with iso/sevo (most with halothane)

Respiration better maintained with sevo

Metabolism:
Iso 0.2%
Sevo 2%
Halothane 20% (no longer used but could cause hepatitis)

Question 20

Describe metabolism of sevoflurane

Answer 20

Theoretical free fluoride ions release
+ Toxic to kidney
+ No problems resported clinically

Compound A formed during reaction with soda lime
+ Nephrotoxic
+ Newer absorbers prevent this
+ Low flow anaesthesia potentiates this process
+ Minimum fresh gas flow 2L/min

Question 21

Describe the use of nitrous oxide

Answer 21

MAC 200% (can’t be used as sole agent)

Mild analgesic

Very insoluble
+ Fast onset
+ Speeds up onset of other agents
+ Less important now as well have insoluble agents

Question 22

Cons of using nitrous oxide

Answer 22

Diffusion hypoxia at end of anaesthetic
+ Diffuses rapidly into lungs reducing ppO2 in lungs

Health risk with long term exposure/pregnany

Atmospheric pollution

Question 23

Describe extubation

Answer 23

When swallowing reflex returns

Cats earlier (ear flick) to prevent laryngospasm

Late if concerned about airway protection
+ Brachycephalics
+ Vomiting risk
+ Ruminants (regurgitation)

Question 24

What should you monitor during recovery?

Answer 24

TPR

Oxygen administration

Fluid therapy

Temp - active/passive warming

Analgesia

Bladder empty

Bandages comfortable

Check for dysphoria with opioids
+ Barking as uncomfortable
+ Check for pain and lower dose