Induction of anaesthetics

Question 1

What are the 2 main methods of administering GAs? Examples of each?

Answer 1

INJECTABLE (usually, IV mainly, or IM): propofol, alfaxalone, ketamine (thiopentone/thiopental, etomidate)
INHALATIONAL: isoflurane, sevoflurance (halothane, desflurane, N2O, xenon)

Question 2

What affects choice of anaesthetic agent?

Answer 2

• species, temperament
• underlying medical conditions
• nature of procedure
• available drugs, equipment and conditions
• personal preference and experience

Question 3

Outline basic anaesthesia versus sedation

Answer 3

Dose dependent CNS depression
Continuum: sedation –> anaesthesia
Low dose of sedative = sedation
High dose of sedative = anaesthesia

Question 4

What are the stages of induction?

Answer 4

Conscious patient –> premeds/sedation –> anaesthetic administration –> (excitement and inadequate anaesthesia) –> deep sedation –> unconsciouness

Question 5

What are the CVS effects of GAs? 3

Answer 5

OVERALL = CVS DEPRESSION
Drop in CO
Vasodilation
Reduced BP

Question 6

Respiratory effects of GAs?

Answer 6

OVERALL = RESPIRATORY DEPRESSION
decreased RR
Decreased tidal volume
Reduced MV

Question 7

Outline pharmacokinetics of GAs - 4

Answer 7

UPTAKE - time to reach adequate blood and brain levels, varies with ROA
DISTRIBUTION INTO TISSUES: depends on BF, disease influence, lowers blood levels
METABOLISM: species and disease influences
ELIMINATION

Question 8

Define ROA

Answer 8

Route of administration

Question 9

What is propofol classified as?

Answer 9

a Phenol

Question 10

Administration - propofol

Answer 10

IV (essential). Give to animal very slowly particularly if v sedated.

Question 11

Use - propofol

Answer 11

Induction and maintenance of anaesthesia

Question 12

Appearance - propofol

Answer 12

White emulsion - contains lipid (preparations are now available that contain a lipid so shelf life extended to 28 days)

Question 13

Pharmacokinetics - propofol

Answer 13

Rapid onset after injection, blood levels decrease by redistribution,
METABOLISM - rapid, hepatic and extra-hepatic (lungs)
EFFECTS: CVS and respiratory depression

Question 14

Things to bear in mind when using it in cats?

Answer 14

Cats lack the enzymes to conjugate glucoronides (slow propofol metabolis) also problems metabolising triglycerides (metabolism of lipid carrier) –> slower recovery (vs. dogs), also cumulative (increases toxicity risk).
REPEATED USE: oxidative injury and Heinz Body production (–> anaemia and death)

Question 15

What are Heinz Bodies (HBs)?

Answer 15

Inclusions in RBCs of denatured Hb.

Question 16

What is Propoclear?

Answer 16

Propofol in a nano-droplet micro emulsion (clear, colourless). 28 shelf life (preservatives). Rapidly withdrawn due to pain on administration.

Question 17

What is PropofolPlus?

Answer 17

a UK propofol. No pain on injection. Dogs and cats have smooth, rapid inductions and recoveries. 28 day shelf life, Familiar milky white appearance. Rarely causes local tissue reaction with inadvertant perivascular administration.

Question 18

What benzyl alcohol?

Answer 18

a preservative. Toxicity may lead to prolonged recovery and hyperkinesia in cats and neurological signs in dogs and fatalities in both species.

Question 19

Alfaxalone:
classification
administration

Answer 19

CLASSIFICATION: steroid, poorly soluble in water so marketed with a solubilising agent (cyclodextrin)
ADMINISTRTION: IV or IM
USE: induction and maintenance of anaesthesia
EFFECTS: rapid onset, short duration of action, less CVRS depression than propfol

Question 20

KETAMINE
type
actions
administration
use

Answer 20

TYPE: dissociative anaesthetic
ACTIONS: analgesic actions at sub-anaesthetic doses (NMDA-Rs), excitation if given alone
ADMINISTRATION: IM or IV
USE: induction and maintenance

Question 21

What are common ways to use ketamine?

Answer 21

TRIPLE COMBINATION - aggressive cats, ketamine, medetomidine and opioid, IM (onset of action 5 mins), single injection alone suitable for short procedures
HORSES: after profound sedation with alpha2 agonist

Question 22

Ketamine - effects

Answer 22

CNS - reflexes preserved
CVS - stimulation of SNS (mild hypertension and tachycardia, some direct CV depression)
RESP - some mild depression
ANALGESIA

Question 23

Thiopentone/thiopental 
USE
ACTION
METABOLISM
RECOVERY 
FORM

Answer 23

Used to be a very common induction agent - no longer licensed. Rapid onset of action. Relatively slow metabolism. Cumulative (fat, esp repeated injections), slow recovery, reconstitute with water as it is powdered form .

Question 24

Effects - Thiopentone/thiopental

Answer 24

Irritant - if administered extravascularly (alkaline)
CV and Resp depression - similar to propofol
Occasionally used in horses to top up during maintenance

Question 25

Effects etomidate

Answer 25

Minimal CV and Resp. depression
Good for sick/compromised patients
Depresses adrenal gland function (BAD)
unlicensed

Question 26

What is a fentanyl/midazolam combination? Use?

Answer 26

Opioid + benzodiazepine

For sick/compromised patients

Question 27

Why use inhalational anaesthesia?

Answer 27

aggressive patients where IV injection impossible

Question 28

Risks of inhalational anaesthesia?

Answer 28

Increased risk of pollution and personnel exposure
Excitement/stress during induction
Isoflurane - more irritant to respiratory passages (–> breath holding during induction means sevoflurane is preferred)

Question 29

What causes inhalational agent uptake?

Answer 29

‘Pressure gradient’ fromt he vaporiser to the brain (vaporiser - anaesthetic system - alveoli - blood - brain).

Question 30

What does brain concentration of an inhalational anaesthetic approximate?

Answer 30

Alveolar concentration (high partial pressure in lungs = high partial pressure in brain. The speed of induction (and recovery) is slower for more SOLUBLE agents since the blood hangs on to the drug more).

Question 31

What is the blood: gas partition coefficient?

Answer 31

Number of parts of gas in blood versus alveolus. High value means that the gas is very soluble in blood.

Question 32

Which anaesthetics have the highest partition coefficient?

Answer 32

Halothane > isoflurane > sevoflurance > nitrous oxide > desflurane

Question 33

What monitoring is needed during induction?

Answer 33

CVS
RespS.
Depth of anaesthesia
Rapid changes in these systems.

Question 34

What happens after induction of anaesthesia? Why? 4

Answer 34

Endotracheal intubation generally recommended:
deliver oxygen
deliver inhalational anaesthetic (if required)
protect airways (occlusion or aspiration)
allow ventilation to be supported if depressed