1/8 General Anaesthesia Flashcards
Name two main general anaesthetic targets and describe how they work.
GABA receptor: chloride channel, when open it allows Ca2+ into the cell causing hyperpolarization –> has inhibitory action
NMDA receptor: excitatory amino acid receptor, allows sodium and Ca2+ into cell, ketamine and nitrous oxide will antagonize the receptor (blocking etc.)
Name some other potential targets of general anaesthetic.
Two-pore domain K+ channels
Glycine receptors
Na+ channels
What is the overal mechanism of general anaesthetics?
Decrease in neurotransmission in the CNS –> loss of consciousness
What is anaesthetsia?
Loss of feeling
What is affected by general anaesthesia?
The whole body
What is affected by local anaesthesia?
Affects part to which it is applied (blocks local sodium channels to prevent excitation)
What is general anaesthesia?
A state of reversible unconsciousness with reduced sensitivity and response to stimuli.
What are the three required components needed to achieve general anaesthesia?
Unconsciousness
Analgesia
Muscle relaxation
Why do we use anaesthesia in animals?
To preform painful surgical/diagnostic procedures
To minimise patient suffering
To reduce risk to the vet and other individuals
To facilitate the procedure by immobilising the patient
Anaesthesia is typically acheived using more than one drug, name the three different categories of drugs used.
Premedicant drugs
Induction drugs
Maintenance durgs
What are premedicant drugs?
Drugs given prior to a general anaesthetic
Typically a sedative-opioid combination
What are induction drugs?
Usually IV agents
Used to achieve the transition from consciousness to unconsciousness
What are maintenance drugs?
Usually inhalents
Drugs used to maintain the anaesthetic state
What are the stages of anesthesia?
Stage 1: voluntary movement
Stage 2: Involuntary movement/excitation
Stage 3: Surgical anesthesia
Stage 4: Medullary paralysis (death)
What stages of anesthesia do modern drugs aim to avoid?
Stage 1 and 2
Almost all general anesthetics have a narrow therapeutic index (careful- want to avoid stage 4 too)
What system do you want to target with anesthetics?
CNS
What are the routes of administration for anesthetics?
Need to get drug to site of action quickly- crossing cell barriers
- IV, inhalation (other routes too slow)
Lipid solubility a key characteristics in allow drug across cell membranes and around body.
What makes the lungs a good drug delivery route?
Vast surface area, well perfused with blood supply
Can be used in systemic administration or locally.
What characteristics are important in a drug to allow for transport throughout the body (lipid theory)?
Receptor/protein targets
Lipid solubility
What are the advantages and disadvantages when using IV anesthetic agents for induction of anesthesia?
Advantages:
- rapid smooth induction (minimal excitment)
- rapid protection of the airways
- no environmental pollution
Disadvantages:
- IV access required
What is it called and what is required when using IV anesthetics for maintenance?
Total intravenous anesthesia (TIVA)- meaning that the entire anesthesia process is achieved through IV
Suitable pharmacokinetics are required
What are inhalation anesthetic agents typically used for?
Maintenance and occasionally to induce anaesthesia
What are the advantages and disadvantages of using inhalation agents for maintenance?
Advantage:
- delivery/elimination depends on ventilation
- rapid adjustment of anesthetic depth (breath-by-breath)
Disadvantages:
- equipment required (endotracheal tube, carrier gas (O2), vaporiser, breathing system etc.)
- environmental pollution
What are the advantages and disadvantages to using inhalation anesthetics for induction?
Advantages:
- IV access can be secured after induction
Disadvantages:
- Environmental pollution
- Takes longer and delay in securing airway can be issue
Pharmacokinetics of inhalational agents:
What drug properties determine speed of induction/recovery?
Blood:gas partition coefficient
- a low b:g gives rapid induction/recovery
- indicating whether drug wants to move from alveolar air into blood
Oil:gas partition coefficient
- high o:g gives high potency
Pharmacokinetics of inhalational agents:
What physiological factors determine speed of induction/recovery?
Alveolar ventilation rate
Cardiac output
How does metabolism affect anesthetic agent?
Liver- extent depends on agent
Potential toxic metabolites can be produced
- risk to patient and staff
How is an inhaled anesthetic agent eliminated?
Exhaled- determines duration of action
What factors affect the duration of action of IV agent?
Redistribution- after being administered, the drug will eventually be redistributed throughout the body, often times in the fat and therefore won’t have any affect on the CNS any more.
Metabolism- liver/elsewhere
What effects do general anesthetics have on the nervous system?
Inhibit conduction of action potentials Inhibit transmission at synapses: - decrease transmitter release, action of transmitter, and excitability of post-synaptic cell Brain regions: - reticular formation - hippocampus
What effects do general anesthetics have on CVS and respiration?
Decrease contractility of isolated heart preparations
Whole animal:
- effects on cardiac output and blood pressure
- potentially arrhythmogenic
- decrease respiration
- increase arterial pCO2
List some IV anesthetic agents used in vet med.
Propofol Steroid anesthetics (alfaxalone) Dissociative agents (ketamine)
What are some characteristics of propofol?
Oil at room temp
Enhance GABA transmission
Short acting, smooth and rapid recovery
Suitable for TIVA
What condition may enhance propofols pharmacological effects?
hypoproteinaemia
What situations do not prolong propofols pharmacological effects?
If repeated IV doses administered in dogs (good for mainenance)
In dogs with hepatic dysfunction
prolonged effect may be seen in cats
What are some characteristics of Alfaxalone?
Enhances inhibitory action of GABA
High therapeutic index
Rapid induction and metabolism
Suitable for TIVA
How does Ketamine work?
Interrupts NMDA receptors
What are some of the side effects of ketamine and what can be done to avoid them?
Increased muscle tone
Eyes open/slow nystagmus
Less profound CVS and respiratory depression
Hallucinations
Administer with another drug (sedative)
What is Total Intravenous Anesthesia (TIVA)?
Anesthesia maintained by intermittent boluses or continuous infusion of an IV agent
Why would you use TIVA?
Easy to administer
Pharmacokinetics are known
Inhalation anesthetics may be unsuitable for some
Avoids risk to people administering drugs (environmental pollution)
List some inhalation anesthetics.
Halogenated compounds:
- halothane
- isoflurane
- sevoflurane
- desflurane
Nitrous oxide
What is the minimum alveolar concentration (MAC)?
Describes the minimum alveolar concentration at which 50% of patients will not respond to a particular stimulus.
Compares the potency of different inhalation anesthetics.
Relates to the percentage of drug in the inspired air.
