General anaesthetics - injectable drugs Flashcards
When are they used?
Used to induce anaesthesia before maintenance of inhaled anaesthetics
Advantages of these?
Inexpensive equipment needles and syringes
Minimal technical requirements
Safe for the operator
Disadvantages?
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)
What are some commonly used injectables?
Barbiturates (phenobarbital, hexobarbital, thiopental, thiamylal)
Propofol, Etomidate, Guaifenesin
Alfaxalone
Benzodiasepines (Midazolam and Diazepam)
Dissociative anaesthetics - NDMA receptor antagonists (ketamine, Phencyclidine HCl, Tiletamine HCl)
What are the pharmacological effects of majority of these drugs?
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
What are the 2 safest injectable drugs?
Alfaxalone and ketamine
What is the MOA of barbituates (thiopental, midazolam, etomidate, alfaxlone, propofol)?
Act by enhancing the efficiency of the GABA at different sites of GABAa receptors (causes CNS depression) –> enhances Cl- conductance and cellular hyperpolarisation
What is the MOA of ketamine (dissociative anaesthetic) and what effects does it have systemically?
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
What type of drugs should you combine ketamine with?
Combine with drugs that relaxes muscles
How is ketamine used in practice?
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)
What protective reflexes are maintained when using ketamine?
Cough, swallowing, pedal and corneal open eyes
What anaesthetic properties does ketamine have?
Dissociation from environment and amnesia, immobility, catalepsy and analgesia
Eliminate of ketamine?
Liver and kidney function very important for elimination
What are some adverse effects/disadvantages of ketamine?
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
What is the MOA of Propofol?
Acts by enhancing the efficacy of GABA at different sites of GABAa receptors.
What are some differences of Propofol compared to thiopental?
Myocardial depression and peripheral vasodilation may occur - baroreflex not supressed as much as with thiopental
What are some effects pf Propofol?
Antiepileptic effects
Minimal nauseas, vomiting and foetal depression
What kind of duration is Propofol used for?
Has a rapid onset and a short duration of action
What are some adverse effects of Propofol seen in the cat?
Recovery can be prolonged
Cats lack glucuronidase activity and there may be Heniz body formation in RBCs due to repeated admin
Should Propofol be used for long surgery’s?
No it shouldn’t because even when used for short procedures the patient has a prolonged recovery time
What is the MOA of Alfaxalone?
Acts by enhancing the efficacy of GABA at different sites of GABAa receptors.
Why should Alfaxlone be infused slowly?
Should be given slowly because it has a very short duration time. Up to 50% protein bound and undergoes rapid hepatic glucuronidation and clearance
What is the effect of alfaxolone of blood pressure?
Decreases BP but may increase HR and minimal effects on CO
Drug combinations are used in anaesthesia what are the 4 main groups and what they achieve?
Anxiolytic premedication (Diazepines) Autonomic stabilization (atropine: prevents visceral reflexes) Analgesics (opioids: Fentanyl) - mostly analgesia Muscle relaxants (pancuronium)
What drugs should ketamine be combined with and why?
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
