Anaesthetics Flashcards
Give some inhalational agents
Nitrous oxide IsofluraneDesfluraneSevoflurane
Give some intravenous agents
PropofolKetamine
What areas of the brain do anaesthetics have an effect on?
Reticular formation depressionThalamusHippocampus depressedBrainstem depressedSpinal cord - dorsal horn analgesia
Describe general anaesthesia
Affects the whole bodyUse inhalational and IV plus adjuvants Reversibly inhibit sensory, motor and sympathetic nerve transmission in CNSGive unconsciousness and sedation
Describe regional anaesthesia
Gives anaesthesia to large, specific regions of the bodyTransmission block between spinal cord and part of the body Occurs with spinal and epidural anaesthesiaPt remains conscious
Describe local anaesthesia
Defined peripheral nerve block e.g. tooth extraction, procedures on hands/feet.
Describe dissociative anaesthesia
Inhibition of transmission of nerve pulses between higher and lower centres of the brain. Used in kids and elderly - less susceptible to hallucinogenic effects
Generally, how do anaesthetics work?
Affect post synaptic transmission of inhibitory and excitatory ligand gated ion channels. - are weak and easily reversed reactions
How do anaesthetics bind to GABA mediated inhibitory channels?
Bind and increased sensitivity to GABA, which increased Cl- entering the cell to hyperpolarise the cell and decrease its excitability. Positive allosteric modulation
How do anaesthetics exert their action at glycine activated chlorides channels?
Bind and increase glycine sensitivity to increase Cl- entering the cell. Causes hyperpolarisation and decreases excitability. Positive allosteric modulation. V. important in inhibitory transmission in spinal cord and brainstem and reducing te response to noxious stimuli
How do anaesthetics exert their effect on neuronal nicotinic ACh receptors?
Bind and inhibit certain subtypes of receptors, by reducing excitatory Na+ currents due to ACh binding. - contributes to analgesia and amnesia rather than anaesthesia. Non-competitive antagonists
How do anaesthetics exert their action on NMDA receptors?
Bind and reduce Ca2+ currents, which are involved in synaptic responses. Nitrous oxide and ketamine exert their action here. Non-competitive antagonism
Describe the administration of inhalational agents
Fluranes are volatile liquids at room temp. Mix anaesthetic agent with oxygen, air and usually nitrous oxide. Then it is administrated via a mask with spontaneous or controlled respiration.
Define MAC
Alveolar concentration at one atmospheric pressure at which 50% of subjects fail to move to a surgical stimulus.
What does a lower MAC value indicate?
More potent anaesthetic- is more lipid soluble
What MAC is needed fro surgical depth?
1.2 - 1.5
How can a MAC be reduced for an individual agent?
Use another agent in combinatione.g. N2O or fentanyl
Define the blood:gas coefficient
The volume of gas in litres that can dissolve in one litre of blood
What does a higher blood:gas coefficient indicate?
The gas will enter blood more readily.
What determines the distribution of gas around the body?
- relative blood supply to each organ or tissue- specific tissue uptake capacity (tissue:blood coefficient)
Give some characteristics of different tissues regarding uptake of inhalational anaesthetics
Brain - slightly more than bloodSk muscle - double brainAdipose - 30 times more than brain - gives a large reservoir of anaesthetic that can redistribute during the recovery phase
Describe elimination of inhalational anaesthetics
Anaesthetist withdraws anaesthesia, with adequate oxygenation. [Blood] drops, so anaesthetic moves out of tissue into venous blood where it travels to the lungs to be eliminated from the alveoli in an unchanged form.
How does the rate of induction and the rate of recovery from inhalational anaesthetics differ?
Does not differ much - elimination is led by well perfused tissues, followed by muscle and then fat.
What factors affect then length of recovery from inhalational anaesthetics?
Length of procedureDegree of loading in fat and muscle compartments
How does the rate of induction differ with inhalational and intravenous anaesthetics differ?
IV is much quicker - seconds rather than minutes.
What are some characteristics of intravenous anaesthetics?
Need further vigilance as it is harder to reverse dose related effects once administered.
Describe propofol administration nad distribution
IV bolus, rapid distribution to CNS and less to muscle and fat. Redistribution occurs from CNS to other compartments.
Give the metabolism characteristics of propofol
Undergoes hepaic and extrahepatic conjugation- half life of 2 hours Means there is not a post-procedural “hangover” during recovery
Give some neuromuscular blocking agents
TubocurarinePancuroniumSuccinylcholine
What do neuromuscular blocking agents do?
Abolish reflexes that occur with significantly invasive procedures and induce muscle relaxation.
What are the stages of anaesthesia?
InductionMaintenance- adjuvant balance to maintain adequate anaesthetic depth Recovery - withdraw anaesthetics, monitor physiological function. - can administer antidotes to facilitate this
Give the stages of anaesthetic depth
- Analgesia2. Excitement3. Surgical anaesthesia4. Respiratory paralysis and death
Describe analgesia stage
Early effects on transmission in the spinothalamic tract
Describe the excitement stage
Delirium and aggressive behaviour are experienced- uncommon due to rapid induction with propofol
Describe surgical anaesthesia
Profound CNS depressionSkeletal muscles are fully relaxedMay need to assist breathing. MAC of 1.2-1.5Four levels of increasing depth of breathing until breathing is weak
Describe respiratory paralysis and death
Above 2.2 MAC, increased risk of this stage. Severe medullary depression which can lead to respiratory and cardiac arrest and death
