General anaesthetics - injectable drugs

Question 1

When are they used?

Answer 1

Used to induce anaesthesia before maintenance of inhaled anaesthetics

Question 2

Advantages of these?

Answer 2

Inexpensive equipment needles and syringes
Minimal technical requirements
Safe for the operator

Question 3

Disadvantages?

Answer 3

Easy to overdose
Hard to reverse effects
May take long for induction and recovery is slow
Many in this category are controlled substances (S8 drugs)

Question 4

What are some commonly used injectables?

Answer 4

Barbiturates (phenobarbital, hexobarbital, thiopental, thiamylal)
Propofol, Etomidate, Guaifenesin
Alfaxalone
Benzodiasepines (Midazolam and Diazepam)
Dissociative anaesthetics - NDMA receptor antagonists (ketamine, Phencyclidine HCl, Tiletamine HCl)

Question 5

What are the pharmacological effects of majority of these drugs?

Answer 5

Reversible dose-dependent depression of CNS, mild muscle relaxation and a decrease in cerebral blood flow, minimal or no analgesia except the dissociative anaesthetics
Dose-dependent decrease blood pressure, peripheral vascular resistance and myocardial contractility
Dose-dependent depression for respiration and apnea

Question 6

What are the 2 safest injectable drugs?

Answer 6

Alfaxalone and ketamine

Question 7

What is the MOA of barbituates (thiopental, midazolam, etomidate, alfaxlone, propofol)?

Answer 7

Act by enhancing the efficiency of the GABA at different sites of GABAa receptors (causes CNS depression) –> enhances Cl- conductance and cellular hyperpolarisation

Question 8

What is the MOA of ketamine (dissociative anaesthetic) and what effects does it have systemically?

Answer 8

Glutamate antagonist
Binds non-competitively to the NMDA receptor to inhibit the excitatory effects of glutamate which causes CNS depression
Only had weak action on GABA receptors
Increases HR, BP and CO

Question 9

What type of drugs should you combine ketamine with?

Answer 9

Combine with drugs that relaxes muscles

Question 10

How is ketamine used in practice?

Answer 10

Can be used as an induction agent but also as a sole agent

As a sole agent it maintains normal laryngeal and pharyngeal reflexes and skeletal muscle tone (catalepsy)

Question 11

What protective reflexes are maintained when using ketamine?

Answer 11

Cough, swallowing, pedal and corneal open eyes

Question 12

What anaesthetic properties does ketamine have?

Answer 12

Dissociation from environment and amnesia, immobility, catalepsy and analgesia

Question 13

Eliminate of ketamine?

Answer 13

Liver and kidney function very important for elimination

Question 14

What are some adverse effects/disadvantages of ketamine?

Answer 14

Eyes remain open
Causes muscles to be stiff and not relax
Stimulates salivation
Increases intracranial pressure can produce seizures
Patients with glaucoma and deep corneal ulcers or susceptibility to tachycardia, elevated sympathetic tone (hyperthyroidism), hypertrophic cardiomyopathy, malignant hyperthermia –> shouldn’t be used in

Question 15

What is the MOA of Propofol?

Answer 15

Acts by enhancing the efficacy of GABA at different sites of GABAa receptors.

Question 16

What are some differences of Propofol compared to thiopental?

Answer 16

Myocardial depression and peripheral vasodilation may occur - baroreflex not supressed as much as with thiopental

Question 17

What are some effects pf Propofol?

Answer 17

Antiepileptic effects

Minimal nauseas, vomiting and foetal depression

Question 18

What kind of duration is Propofol used for?

Answer 18

Has a rapid onset and a short duration of action

Question 19

What are some adverse effects of Propofol seen in the cat?

Answer 19

Recovery can be prolonged

Cats lack glucuronidase activity and there may be Heniz body formation in RBCs due to repeated admin

Question 20

Should Propofol be used for long surgery’s?

Answer 20

No it shouldn’t because even when used for short procedures the patient has a prolonged recovery time

Question 21

What is the MOA of Alfaxalone?

Answer 21

Acts by enhancing the efficacy of GABA at different sites of GABAa receptors.

Question 22

Why should Alfaxlone be infused slowly?

Answer 22

Should be given slowly because it has a very short duration time. Up to 50% protein bound and undergoes rapid hepatic glucuronidation and clearance

Question 23

What is the effect of alfaxolone of blood pressure?

Answer 23

Decreases BP but may increase HR and minimal effects on CO

Question 24

Drug combinations are used in anaesthesia what are the 4 main groups and what they achieve?

Answer 24

Anxiolytic premedication (Diazepines)
Autonomic stabilization (atropine: prevents visceral reflexes)
Analgesics (opioids: Fentanyl) - mostly analgesia
Muscle relaxants (pancuronium)

Question 25

What drugs should ketamine be combined with and why?

Answer 25

Benzodiazepines e.g diazepam
Alpha2-agonists e.g Medetmidine (IV) and Xylazine (IM) +- Butorphanol
Phenothiazines e.g Acepromazine (painful procedures)
General anaesthetics e.g Isoflurane or Propofol
Needs to be combined because Ketamine does not cause muscle relaxation