General anaesthetics Flashcards
General anaesthesia purpose
- Loss of consciousness
- Analgesia
- Muscle relaxation (associated with a loss of reflexes).
Stages of analgesia
Stage 1: analgesia (misnomer) • Drowsiness Stage 2: Excitement • Loss of consciousness Stage 3: Surgical aneasthesis • Regular respiration • Decline in muscle tone • Loss of reflexes Stage 4: Medullary paralysis
Give examples of injection/intravenous anaesthetics
- Thiopental
- Propofol
- Etomidate
- Midazolam (benzodiazepine)
- Ketamine (dissociative anaesthetics)
Effect on GABAa receptor function
Enhance GABAa receptor function
- Volatile anaesthetics – bind at the interface between alpha and beta subunits
- Intravenous anaesthetics – bind to beta subunits
Effect on potassium channels, and the type of general anaesthetics that do this
Volatile and gaseous anaesthetics Activate potassium channels
Effect on NMDA receptors and the type of general anaesthetics that do this
The following block NMDA receptors
Ketamine
Nitrous oxide
Effect of GAs on other channels
Inhibit voltage – sensitive Ca2+ channels
Inhibit voltage-sensitive sodium channels
Inhibit Glycine, nicotinic and 5-HT ligand gated ion channels
Overall Effect of general anaesthetics on the nervous system (i.e neurotransmitter release and excitability)
Inhibit excitatory synaptic transmission:
a. Decreases neurotransmitter release (presynaptic)
b. Decrease action of neurotransmitter (postsynaptic)
c. Decrease excitability of postsynaptic neurones
Advantages of injectable anaesthetics
Easy to administer
Rapid induction
Disadvantages of injectable anaesthetics
Complex pharmokinetics
Slow elimination
Side effects
Give examples of dissociative anaesthetics
Ketamine
Tiletamine (phencyclidine)
What is the action of ketamine?
NMDA channel blocker
What are the advantages of dissociative anaesthetics
No cardiovascular or respiratory depression
Non-cumulative effects
Analgesic
High therapeutic index
What are the disadvantages of dissociative anaesthetics
Psychic disturbances
Involuntary movements
What are the Inhalation Anaesthetics?
Gas:
-NO2
Volatile liquids:
- Halothane – rarely used, metabolised to toxic metabolite
- Enflurane – may cause convulsions
- Isoflurane
- Desflurane/sevoflurane – rapid induction
- Methoxyflurane
What are the Inhalation Anaesthetics useful for
Maintaining surgical anaesthetics
Route of entry of Inhalation Anaesthetics
Lungs, so problem if these ar diseased
Inhalation Anaesthetics process of entry/ mechanism
Small lipid soluble molecule, easily cross alveolar membrane.
Absorbed and excreted unchanged from lungs
The differences in each Inhalation Anaesthetics arises from?
Solubility of different agents in blood and fat
Chemical properties governing the speed of induction
Blood:gas partition coefficient (solubility in blood)
Oil:gas partition coefficient (solubility in fat)
Physiological factors governing the speed of induction
Alveolar ventilation rate
Cardiac output
How the Loading Dose governs the speed of induction in inhalation anaesthetics
Increasing initial concentration of agent in inspired air will increase rate of onset
