General anaesthetics Flashcards
What are the targets of GAs?
GABA and NMDA receptors
What is the overall mechanisms of GAs?
Decrease in neurotransmission in the CNS, leading to loss of consciousness
What are the 3 required components of general anaesthesia?
Unconsciousness
Analgesia
Muscle relaxation
Why do we anaesthetise animals?
To perform painful surgical or diagnostic procedures
To minimise patient suffering
To reduce risk to vet adn other individuals
To facilitate the procedure by immobilising the patient
How is GA achieved?
By using a combination of drugs:
- premedicant
- induction drug
- maintenance drug
Define premedicant
- drugs given prior to GA
- typically a sedative-opiod combo
- purpose is to calm and sedate the animal
Define induction drugs
- usually IV
- used to achieve transition from consciousness to unconsciousness
- done rapidly
Define maintenance drugs
- usually inhalational agents
- maintain anaesthetic state
What are the stages of anaesthesia?
Stage 1 - voluntary movement. GA inhibit inhibitory pathways causing initial voluntary movement
Stage 2 - involuntary movement or excitement
Stage 3 - surgical anaesthesia (divided into 4 stages)
Stage 4 - medullary paralysis
What is a characteristic of GAs which enable them to get to their site of action quickly?
High lipid solubility. Can readily cross cell barriers, and willl distribute faster around the body and have a faster effect on the CNS. Brain blood perfusion is v high and has a high lipid content which means get a rapid rise in concentration of the drug.
What are IV anaesthetic agents typically used for?
- induction of anaesthesia
- occasionally to maintain anaesthesia
What are the advantages to using IV agents?
- rapid, smooth induction (avoid stage 1 and 2)
- rapid protection of the airway (intubate rapidly)
- no environmental pollution
What are the disadvantages to IV induction?
Need IV access
What is the method by which IV agents can be used to maintain anaesthesia?
Total intravenous anaesthesia (TIVA)
What are inhalational agents typically used for?
- maintenance of anaesthesia (once patient is in stage 3)
- occasionally to induce anaesthesia
What are the advantages to inhalational agents for maintenace?
- delivery/elimination depends on ventilation (patient rapidly wakes up)
- rapid adjustment of anaesthetic depth (breath by breath adjustment)
What are the disadvantages to using inhalational agents for maintenance?
- equipment is required
- environmental pollution
What are the advantages to using inhalational agents for induction?
IV access can be secured after induction
What are the disadvantages to using inhalational agents for induction?
- envrionmental pollution
- takes longer and delay in securing the airways may be a problem in some cases
Describe the pharmacokinetics of inhalational agents
- Blood:gas coefficient (low gives rapid induction/recovery)
- oil:gas coefficient (high gives high potency)
- depends on alveolar ventilation rate and cardiac output
- metabolism (liver, extenten depends on agent, potential toxic metabolites)
- elimination (primarily by exhalation, determines duration of action)
Describe the pharmacokinetics of IV agents
- Redistribution - drug will redistribute throughout the body; as it equilibrates will get high level settlign in fat stores. As drug distributes away from CNS patient will begin to wake up
- Metabolism - lipid soluble drugs aren’t easily excreted. Rate of metabolism is vital for determining whether a drug is suitable for TIVA
What are the pharmacodynamic effects of GAs on the nervous system?
- inhibits conduction of action potentials
- inhibits transmission at synapses (low doses during induction)
- reticular formation adn hippocampus are susceptible to action of GAs
What are the pharmacodynamic effects of GAs on the CVS and respiration?
- decreased contractility of isolated heart preparations
- effects cardiac output and blood pressure
- potentially arrhythmogenic (take care if administering catecholamines to counteract this)
- decreased respiration
- increased arterial CO2 tension
What are the properties of propofol?
- hindered phenol
- oil at room temp. (administered in an emulsion)
- enhances GABA transmission
- short acting, smooth rapid recovery
Describe the pharmacokinetics of propofol
- highly plasma protein bound
- large volume of distribution (>3L/kg)
- phase II metabolsim - conjugaed in the liver prior to excretion in urine
- rapidly cleared (>40ml/kg/min)
- suitable for TIVA
What are the clinical considerations when using propofol?
- pharmacological effect may be enhanced by hypoproteinaemia
- pharmacological effect is not prolonged if repeated doses are administered and in dogs with hepatic dysfunction
What are the properties of alfaxalone?
- steroid anaesthetic
- insoluble in water, presented in cyclodextrin vehicle
- enhances inhibitory action of GABA
- high therapeutic index
- rapid induction
- rapid metabolism
- suitable for TIVA
What are the properties of ketamine?
- interrupts the association between the limbic and cortical regions (inhibits NMDA receptors)
- dissociative agents produce a different qulaity of anaesthesia
- sensory loss with analgesia
- increased muscle tone
- eyes open +/- slow nystagmus
- active reflexes include laryngeal/pharyngeal
- less profound CVS and resp depression
- hallucinations/emergence delerium
- commonly given with a sedative to offset other effects
What is total intravenous anaesthesia?
Anaesthesia maintained by intermittent boluses or continuous infusion of an IV agent
What are the advantages to using TIVA?
- easy to administer
- pharmacokinetics are known
- inhalational agents may be unsuitable in some individuals
- avoids risk to people administering drug
Define minimun alveolar concentration (MAC)
The minimum alveolar concentration at which 50% of patients will not respond to a particular stimulus. Compares potency of different inhalational anaesthetics. Lower the MAC, the more potent the drug.
Describe the properties of isoflurane
- licensed in non-food producing animals
- good speed of induction/recovery/change of depth
- pungent odour (breath hold)
- minimal metabolism
- cardiac output is better maintained than haloflurane
- less arrhythmogenic
- most widely used inhalation agent
What are the properties of sevoflurane?
- licensed in dogs
- rapid induction/recovery/change of anaesthetic depth
- pleasant odour and minimal airway irritation so is suitable for induction
- low rate of metabolism
What are the properties of nitrous oxide?
- colourless gas, no tase or odour
- stored under pressure in containers
- can’t use as sole agent as can’t get full anaesthesia with it on own and couldn’t give O2 with it (MAC >100%)
- used as an adjunct
- diffusional properties (helps to concentrate the effect of volatile agent as is v rapidly taken into body)
- displaces N2 in body (is larger so will cause gas filled cavities to expand)
