Maintenance of Anaesthesia Flashcards
When can you intubate?
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)
Describe the process of intubation
- Induction
- Check for sufficient depth of anaesthesia
- Pull tongue out
- Laryngoscope on tongue
+ Don’t touch epiglottis or larynx - Visualise laryngeal opening
- (Local anaesthesia)
- (Lubrication)
- Place endotracheal tube
How can you tell the correct length of endotracheal tube for the patient?
Measure from nose to shoulder point.
Problems associated with endotracheal tubes
Blockage of Murphey’s eye
Placement down one bronchi
Blockage with secretion (particularly cats on ketamine)
Overinflation of cuff
What anaesthetic agents can be used for maintenance?
Injectable
Propofol
Alfaxalone
Ketamine
Inhalational Isoflurane Sevoflurane (Nitrous oxide) (Xenon)
Why don’t we use thiopental for maintenance of anaesthesia?
Very lipid soluble so accumulates and gives long recovery period
Suppresses adrenal gland
Is analgesia required during maintenance?
YES
Most GA agents provide little analgesia (except ketamine)
What methods can we use to maintain anaesthesia?
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
Describe the difference between intermittent and continuous anaesthesia
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
Injectable agents for maintenance (TIVA&PIVA)
TIVA = propofol, alfaxalone
PIVA = ketamine (used at low doses with another agent)
Describe how inhalational agents are used for maintenance
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
Compare the speed of induction using nitrous oxide compared to halothane
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
What is the blood/gas partition coefficient? What does a higher value mean?
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
What is the MAC?
Minimum alveolar concentration required to prevent movement in response to painful stimulus in 50% animals.
What MAC should clinical anaesthesia aim for?
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)
MAC values for isoflurane and sevoflurane (%)
Dog
Cat
Horse
DOG: I = 1.3 S = 2.3
CAT: I = 1.6 S = 2.6
HORSE: I = 1.3 S = 2.3
What is the MAC not affected by?
Length of anaesthesia
Gender
Blood pH
What is the MAC affected by?
Hypothermia/hyperthermia Age Severe hypoxia/hypercapnia (elevated CO2) Severe anaemia Severe hypotension CNS depressant dogs Excitation Pregnancy
How does cardio and respiratory depression and metabolism vary with inhaled agents?
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)
Describe metabolism of sevoflurane
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
Describe the use of nitrous oxide
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
Cons of using nitrous oxide
Diffusion hypoxia at end of anaesthetic
+ Diffuses rapidly into lungs reducing ppO2 in lungs
Health risk with long term exposure/pregnany
Atmospheric pollution
Describe extubation
When swallowing reflex returns
Cats earlier (ear flick) to prevent laryngospasm
Late if concerned about airway protection
+ Brachycephalics
+ Vomiting risk
+ Ruminants (regurgitation)
What should you monitor during recovery?
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
