Maintenance of anaesthesia Flashcards
When can you intubate?
Sufficient depth of anaesthesia (eyes rotate ventrally, minimal/sluggish palpebral reflex, loose jaw tone, no swallowing reflex on stimulation) –> Pull tongue out –> laryngoscope on tongue (don’t touch epiglottis or larynx) –> visualise laryngeal opening –> LA and lubricaton
How do you measure an ETT?
measure to point of shoulder
What are problems with ETTs? 5
- Occlusions at end - prevented by murphy’s eye
- Endobronchial intubation
- Mucus in tube (occulsion and infection)
- Compression of inside of tube
- Stretching of tracheal wall
Special considerations - cats and ETTs
spray larynx with local anaesthetic (desensitises and reduces laryngospasm). Use intubeaze (lidocaine spray)
List 2 alternatives to ETT in cats
V-gel
Laryngeal mask
T/F- mist anaesthetics don’t provide analgesia
True (one exception = ketamine). (Analgesia is still required even though patient is unconscious to prevent upregulation of pain processing pathways_.
How can anaesthesia be maintained? 4
Inhalational
Intravenous
Combination (injectional and inhalational)
Single IM injection (occasionally)
Define PIVA
Partial IntraVenous Anaesthesia
List 4 injectable maintenance anaesthetics
Best for TIVA:
Propofol
Alfaxalone
Accumulate so not good for CRI, use for intermittent:
Ketamine
Thiopental
List 6 inhalational maintenance anaesthetics
Isoflurane Sevoflurane Halothane Desflurane NO2 Xenon
Outline intravenous maintenance
TIVA = total intravenous anaesthesia
Intermittent boluses or continuous rate infusion (CRI)
Advantages of intermittent bolus/CRI for maintaining anaesthesia
INTERMITTENT: simpler, less equipment, swinging plane of anaesthesia
CRI: target controlled infusion (TCI) possible, minimum infusion rate (MIR)
How are inhalational agents metabolised?
Administered and removed from body via lungs (except halothane - liver.
Redistributed to brain and other tissues
How does fat solubility affect inhalational agents?
Fat solubility may slow recovery from a long anaesthetic (think vessel rich vs. vessel poor tissues)
What factors affect inhalational agent uptake? 2
- Pressure gradient from vaporiser to brain
- Brain concentration approximates alveolar concentration
What factors affect speed of induction?
- High pp in lungs = high pp in brain
BUT agents that are very soluble in blood will have lower pp in lungs –> lower pp in brain therefore speed of induction/recovery for soluble agents is slower.
What is the blood/gas partition coefficient?
Number of parts of gas in blood versus alveolus
High number = gas very soluble in blood = slower induction and slower to change depth of anaesthesia during maintenance
Define MAC
Minimum Alveolar Concentration that is required to prevent movement in response to painful stimulus in 50% animals. For clinical anaesthesia aim for 1.25-1.5 times MAC. Depends on other sedatives/anaesthetics (may cause a MAC sparing effect). MAC values vary between species.
List factors that influence MAC and how.
Hypothermia (decrease), hyperthermia (increase)
Age - young/old (decrease), young/fit (increase)
Severe hypoxia/hypercapnia (decrease)
Severe hypotension (decrease)
CNS depressant drugs (decrease)
Excitation (increase)
Pregnancy (decrease)
Name 3 factors that don’t influence MAC
length of anaesthesia
gender
normal blood pH
What is the range for normal blood pH?
7.35-7.45
What is the MAC value in dogs for isoflurane and sevoflurane?
ISOFLURANE = 1.3 SEVOFLURANE = 2.3
What is the MAC value in cats for isoflurane and sevoflurane?
ISOFLURANE = 1.6 SEVOFLURANE = 2.6
What is the MAC value in horses for isoflurane and sevoflurane?
ISOFLURANE = 1.3 SEVOFLURANE = 2.3
Is sevoflurane licensed in dogs, cats and horses?
Yes - dogs
No - cats and horses
Which anaesthetic reduces CO the most?
Halothane - mainly
Also some reduction with iso and sevo
T/F: respiratory depression is similar for all anaesthetics
True
How much metabolism is performed by the liver for differentinhalational anaesthetics?
Isolfurane - 0.2%
Sevoflurane - 2%
(Halothane - 20% - the exception)
Desflurane - 0.02%
Predominantly the lung
How is sevoflurane metabolised?
- Theoretically free fluoride ions are released (nephrotoxic but no clinically reported problems)
- Compound A formed during reaction with hot and dry CO2 absorber (nephrotoxic, newer absorbers prevent this)
- Low flow anaesthesia potentiate these processes
T/F: sevoflurane is the best anaesthetic choice for renal disease
False (nephrotoxtic free radicals and compound A produced). Isoflurane is a much better option.
Compare and contrast isoflurane and sevoflurane
ISOFLURANE: vasodilation, CV depression, cheap, stronger smell, patient less compliant, more irritant
SEVOFLURANE: less CVS side effects that iso, maintains better cerebral perfusion than iso, more expensive, better tolerated, less irritant, compound A produced (upon reaction with CO2 absorber)
Outline NO2 as an anaesthetic agent
- MAC in animals is 200% –> mist be used with other anaesthetics otherwise risk of hypoxic mistures.
- Mild analgesic properties
- very insoluble in blood –> very fast onset
- can speed onset of another agent (second gas effect)
- less important now insoluble agents are routinely used
What is ‘diffusion hypoxia’ as seen with NO2?
at end of anaesthetic, NO2 diffuses rapidly into lungs reducing pp of O2 in the lungs –> therefore to avoid this switch of NO2 15 mins before switching off O2 at end of surgery to prevent this.
How does NO2 cause a health risk with chronic exposure or pregnancy?
Causes vitamin B12 deficiency (sensory neuropathy, myelopathy and encephalopathy). Also adds to atmospheric greenhouse effect.
When should you extubate? What affects this?
When swallowing reflex returns (earlier in cats to prevent laryngospams) or later if concerned about airway protection (brachycephalics, vomiting risk and ruminants with risk of regurgitation).
What should happen during the recovery period?
Monitoring continues (HR, RR, temp) \+/- oxygen administration \+/- fluids Temperature (active/passive warming) Post-op analgesia Nursing care and TLC - check bladder is empty and bandages are comfortable.
