Anaesthetics Flashcards
What is anaesthesia
Loss of sensation
What are the effects of anaesthesia
Sedation up to unconsciousness Amnesia Muscular relaxation Spinal reflex suppression Anxiolysis Analgesia
Why does anaesthesia involve poly pharmacy
No single agent can produce all the effects of analgesia so require polypharmacy of short acting adjuvant Drugs
What are the types of anaesthesia
General: IV, Inhalational
Regional: spinal, epidural
Local
Dissociative
What is dissociative anaesthesia
Conscious sedation by inhibiting transmission between higher and lower brain centres
What are the steps in the procedure of anaesthesia
Premedication: anxiolytic, anti emetic Induction Intra-operative analgesia Muscle paralysis Maintenance Reversal of muscle paralysis Post op analgesia Anti-emetic
What are the anatomical targets for general anaesthesia and their effects
CNS: Spinal cord - immobilisation Brainstem - Respiratory depression Reticular formation - sedation Hippocampus - amnesia
What are the molecular targets of GA
GABA receptors
NMDA Glutamate receptors
What is the mechanism of action of GA on GABA receptors
GABA receptor agonist
GABA receptors are LGICs on post synaptic membranes that cause Cl influx
Increase Cl influx and hyperpolarisation
Decrease neuronal excitability
What GA agents act on GABA receptors
Most except:
N2O
Ketamine
Xe
What is the mechanism of action of GA agents on NMDA glutamate receptors
NMDA glutamate receptor antagonist
NMDA receptors are LGICs in post synaptic membrane for Ca
Prevent Ca influx and depolarisation
Decrease neuronal excitability
How is consciousness controlled in GA
Balance between neuronal excitation and inhibition by balance between NMDA and GABA receptor activation
Give examples of inhalational GA
N2O Desflurane Isoflurane Sevoflurane Halothane Xenon (paediatric)
How is potency measured for volatile GA
Minimum alveolar concentration
What is MAC
% of inhaled anaesthetic in alveoli that abolishes response to surgical stimuli in 50% of patients
What is potency
Concentration of drug required to achieve 50% of its maximal response
What is MAC of volatile GA dependent on
Lipid solubility: determines ability to dissolve into cell membranes and access target LGIC receptors
What factors increase/decrease MAC
Age: infants (increase), elderly (decrease)
Hypothermia (decrease), Hyperthermia (increase)
Pregnancy (increase)
Opioids (decrease)
Other anaesthetics (decrease)
CNS stimulants (increase)
Alcoholism (increase)
What effect does N2O have on MAC of other volatile agents
Decreases MAC
Thus acts as a MAC sparing agent, allows reduced dosing of agents when given together
What is absorption of volatile agent from alveoli to CNS affected by
Alveolar concentration
Blood:gas coefficient
Distribution
Tissue:blood coefficient
What does alveolar concentration of volatile agent indicate
It is proportional to CNS concentration of agent
What is blood:gas coefficient
Degree of absorption of agent from alveoli into blood (solubility in blood)
What is distribution dependent on
Relative blood supply of organs
What is tissue:blood coefficient
Which tissue has highest coefficient
Degree of uptake of volatile agent from blood to tissues
Fat > muscle > CNS
How is volatile GA eliminated
Withdrawal of anaesthesia after surgery
Decreased concentration of agent in blood
Agent moves out of cell into blood
Agent moves out of blood into alveoli
Agent breathed out unchanged
Eliminated from lungs with first order kinetics
What is re-distribution of volatile GA agent
When proportion of volatile agent not removed from lungs and re-enters circulation and re-enters CNS
