Maintenance of anaesthesia

Question 1

What is MAC?

Answer 1

• minimum alveolar concentration
  = the concentration of a vapour in the alveoli of the lungs that is needed to prevent movement in 50% of subjects in response to surgical stimulus

Question 2

What are the 2 licenced inhalation agents in veterinary species in the UK?

Answer 2

• isoflurane
• sevoflurane

Question 3

Difference between isoflurane and sevoflurane

Answer 3

• sevoflurane has a slightly higher MAC and a lower blood gas solubility
  – this means that servo diffuses into tissues and therefore the brain more quickly than iso
  – this results in a quicker change in anaesthetic depth
• sevo is more expensive
• because sevo has a higher MAC you may end up using more of it cf iso at an equivalent flow rate
• sevo is more suitable for mask induction as doesn’t smell and is quicker to effect

Question 4

Disadvantages of inhalation anaesthetic agents

Answer 4

• cause profound dose dependent cardiovascular and respiratory depression

Question 5

How is CV depression mediated?

Answer 5

• mediated via myocardial depression resulting in negative isotropy, peripheral vasodilation, a decrease in vascular reactivity, CNS depression and a reduction in autonomic tone
• this causes a reduction in bp and cardiac output

Question 6

What is the respiratory depression caused by volatile agents the result of?

Answer 6

• a depressed ventilatory response to carbon dioxide (high concentrations of CO2 normally stimulate respiration), depressed hypoxic pulmonary vasoconstriction (pulmonary arteries constrict in parts of the lung tissue in the face of hypoxia to direct blood to better ventilated areas), almost complete suppression of the ventilatory response to hypoxia and by causing bronchodilator which increases dead space

Question 7

Use of nitrous oxide

Answer 7

• extremely high Mac, so isn’t very useful as a sole anaesthetic agent
• reduces the amount of volatile agents needed and provides good analgesia via NMDA receptor antagonism and endogenous opioid production
• CV and respiratory stable so can be used to spare CV effects of iso and sevo
• usually used at 2:1 of NO:O
• ideally want to measure inspired O2 as it’s very poorly soluble in blood and diffusion hypoxia can be life threatening
• leave to breathe 100% O2 for 5-10 mins as poor solubility means that gas will accumulate in the lungs
• rapid effect: increases speed of induction due to low solubility coefficient in blood
• quickly diffuses into gas filled spaces so can’t use in GDV, rabbits/horses, endoscopy as stomach/intestines filled with gas, pneumothorax and ocular procedures where there’s a risk of bubble forming in the eyes

Question 8

What is TIVA?

Answer 8

• total intravenous anaesthesia
• where anaesthesia is both induced and maintained by IV drugs

Question 9

Which drugs in small animals are licenced for TIVA?

Answer 9

• alfaxalone
• propofol

Question 10

What is the risk of using propofol in cats for a long period of time or repeatedly?

Answer 10

• potential for oxidative damage and Heinz body formation

Question 11

Other considerations if using propofol or alfaxalone

Answer 11

• they’re not analgesic so need to provide analgesia by other means e.g. opioids

Question 12

Triple drip TIVA in horses

Answer 12

• guaifenesin
  – centrally acting muscle relaxant
  – not an anaesthetic on its own
  – provides no analgesia
• ketamine
  – dissociative anaesthetic
• detomidine (or xylazine)
  – alpha-2 agonist
• can be used to maintain anaesthesia for up to 1.5h and is given as a CRI to effect
  – time limit is because drugs will accumulate

Question 13

What is PIVA?

Answer 13

• partial intravenous anaesthesia
• combination of IV and inhalational anaesthetic