Maintenance of Anaesthesia Flashcards
What are potential problems with ET tubes? how can they be prevented?
- occlusion of end of ET tube (prevented by Murphy’s eye)
- endobronchial intubation - mucus in the tube may cause occlusion and infection, ^ resistance and only ventilating one lung (don’t push too far!)
- compression of inside of tube or stretching of tracheal wall (listen for gas escaping until none escapes)
When can you intubate?
> suficient depth of anaesthesia
- eyes rotated ventrally
- minimal, sluggish palpebral reflex
- loose jaw tone
- no swallowing reflex
What side effects do anaesthetic agents have?
DOse dependant
- CV depression(v CO, vasodialtion, v BP)
- resp depression (v RR, v TV, v MV)
Do most GA provide anaesthesia?
No
except: KETAMINE
Why is analgesia required even if animal is unconcsious?
- prevent unconcious upregulation of pain processing pathways
What ROA are available for anaesthetic maintainence?
> IV
- TIVA
- Intermittent boluses
- CRI
> Inhalational
> Both ("balanced technique)
- partial intravenous anaesthesia (PIVA)
> occasionally (rarely) single IM injection sufficient (eg darting wild animals)Why must you be careful when intubating cts?
Laryngeospasm
- spray lidocaine “intubeaze”
- CAVE: easy to overdose -> local anaesthetic toxicity
What are the 6 aspects of balanced anaesthesia?
- minimise stress
- analgesia
- mm relaxation
- v amount of drugs (so use more types)
- v autonomic reflex activity
- unconciousness
Which GA drugs can be used for injectable maintainence of anaesthesia?
> best for CRI - propofol - alfaxolone > best for intermittent boluses - ketamine - thiopental
Which GA drugs can be used for inhalational anaesthesia?
- isoflurane and sevoflurane [licensed smallies]
- halothane [old fashioned]
- desflurane [new but expensive]
- N2O
- xenon
What 2 ways can TIVA be carried out?
> intermittent boluses
+ simpler, require less equipment
- swinging plane of anaesthesia so ^ risk toxicity
CRI
- need pump/cri infuser
+ TCI : calculate amount of drug to deliver by measuring plasma concentration, ie. minimum infusion rate (=MAC??)
How are most inhalational agents administered and removed?
Lungs
excpetion: halothane metabolised in liver
How may redistribution of inhalational agents affect anaesthesia?
- fat soluability -> slow recovery from long anaesthetic
- vessel-rich (brain, kidneys) v vessel-poor tissues (skin)
Which factors affect inhalational agent uptake?
Pressure gradient from vaporizer to brain
- vaporiser setting
- anaesthetic circuit volume
- alveoli (eg. pulmonary oedema)
- blood
- brain
> Brain concentration approximates alveolar concentration (equililbrium) so expirational concentration (End tidal concentration)
Which factors affect speed of induction?
- ^ partial pressure in lungs = high partial pressure in brain
- if agents v. soluable in blood will remain there -> v partial pressure in brain
- slower induction and recovery for more soluble agents
- CF. injectable agents which SHOULD be dissolvable in blood
What is the blood/gas partition coefficient?
- no. parts gas in blood: alveolus
- high number means gas v. soluble and slower induction and recovery
- more soluble agents also slower change of depth of anaesthesia during maintainence
What is the potency of the inhalational agents? How is this related to blood/gas coefficient?
- how much inhalational agent is needed
- inversely proportional to solubility in blood!!
Define MAC. What does it depend on and how is it used clinically ?
- minimum alveolar concentration required to prevent movement in 50% of animals in response to painful stimulus
- clinical anaesthesia aim for ~1.25-1.5x MAC
- depends on other desative/anaesthetic agents (MAC sparing)
- differs between species
Which factors can change MAC?
- hypothermia v/hyperthermia ^
- age: very young/old v, young fit^
- severe hypoxaemia or hypercapnia v
- severe hypotension v
- CNS depressant drugs v
- exctiation ^
- pregnancy v (eg. ceasar)
Which factors do NOT affect MAC?
- length of anaesthesia
- gender
- blood pH (7.35-7.45 normal, unless severe)
MAC values of isoflurane and sevoflurane in dog, cat horse and human?
Look at table
Is sevoflurane licensed in all animals?
Not cat and horses
Which volatile anaesthetic agent reduces CO most severely?
Halothane
Which inhalational agent has greatest metabolism in the liver?
halothane
Potential side effects of sevoflurane?
- theoretically toxic to kidney
- reacts with hot and dry carbon dioxide absorber (nephrotoxic too) less of an issue with high flow systmes but with low flow systmes bad
Pros and cons of isoflurane?
+ cheap
- irritant and stronger smelling
- vasodilation and CV depression
Pros and cons of sevoflurane
\+ v CV effects \+ maintains better cerebral perfusion better (Always use for MRI) \+ better tolerated, less irritant - compound A reaction with soda lime - expensive
Pros and cons of N2O?
- MAC 200%!! (must be used with other agents)
- less important now that insoluble agents are routinely used
- diffusion hypoxia at end of anaesthesia (diffuses rapidly into lungs v partial pressure of o2. Advise switch off N2O 15 mins before end of surger yand give pure O2)
+ mild analgesic (=ketmine mechanism of action)
+ very insoluble (quick)
+ can speed onset of other agents (2nd gas effect)
Health risk assocaited with N2O?
- teratogen
- vitamin B12 deficiency (> sensory neuropathy, myelopathy, encepalopathy)
- atmospheric pollution (proper scavenging v important)
When is recovery considered to be over?
- 48hrs smallies
- 5-7d equine
When can extubation be carried out?
- when swallowing reflex returns
- cats slightly earlier to prevent laryngospasm
- later if concerned about airway (brachycephalic dogs, vomiting risk, ruminants active regurgitation - leave tube in until they chew it)
What is involved in recovery?
- monitor HR< RR, temp
+- oxygen and fluid therapy - temperature - active/passive warming esp. SA
- post-op analgesia
- nursing care (bladder empty [place catheter in horses as standard, bandages comfortable)
