General Anaesthetics (DONE) Flashcards
Minimum Alveolar Concentration definition
The minimum alveolar concentration at steady state that prevents reaction to a standard surgical stimulus in 50% of patients
Factors increasing MAC
Infancy Hyperthermia Hyperthyroidism Sympathomimetics Chronic opioid use Chronic alcohol intake Acute amphetamine intake Hypernatraemia
Factors decreasing MAC
Neonates Increased age Pregnancy Hypotension Hypothermia Hypothyroidism Sedatives Acute opioid use Acute alcohol intake Chronic amphetamine intake Lithium
Properties of ideal inhaled anaesthetic
Physical, not flammable/explosive, preservative free, stable to heat and light, cheap, pleasant odour, inert when in contact with metal, rubber and soda lime, atmospherically friendly, biochemical, low MAC, high oil: gas partition coefficient??, only affects the CNS, not metabolised, some analgesic properties, non-toxic, not epileptogenic
Blood:gas partition coefficient
Drug uptake from the lung dictated by its blood/gas partition coefficient
How the anaesthetic partitions itself between these two phases at equilibrium
Low blood: gas PC- more soluble in gas, more rapid onset/offset of anaesthesia
High blood: gas PC- more soluble in blood, slower onset/offset
Nitrous oxide
Used alongside volatile anaesthetics and in combination with oxygen (entanox)
High MAC (105%)
Manufactured by heating ammonium nitrate to 250c
Stored in blue cylinders
Nitrous oxide effects
CVS: mild cardiac depressant, increases sympathetic activity
CNS: increases cerebral blood flow, high MAC but may well cause anaesthesia in certain patients
Resp: causes small fall in tidal volume but this is offset by increase in resp rate
Isoflurane
Halogenated ethyl methyl ether
Isoflurane effects
CVS: reduces resistance, compensatory tachycardia
CNS: of all the volatiles isoflurane gives the best balance of reduced cerebral oxygen requirement and minimal increase in cerebral blood flow
Resp: respiratory depressant, rarely used for induction due to its unpleasant smell
Sevoflurane
Favourable combination of relatively low blood gas partition coefficient, relatively low MAC, pleasant smell
Sevoflurane effects
CVS: fall in resistance, heart rate unchanged, BP falls
CNS: increases cerebral blood flow, decreased cerebral oxygen requirement, post op agitation in children
Resp: respiratory depressant, pleasant odour so can be used for induction
Toxicity- evidence of compound A production in rats which is hepatotoxic
Desflurane
Boiling point of 23.5c, cannot administer via a conventional vaporiser, heated to 39c
Desflurane effects
CVS: cardiac stimulation at high MAC
CNS: increases cerebral blood flow, decreased cerebral oxygen requirement
Resp: respiratory depressant, causes coughing and breath holding so is not used for induction
Muscle relaxants
Effectively paralyse the patient
Not always needed with a general anaesthetic
Anaesthetic or surgical reason
Types of neuromuscular blockers
Depolarising: suxamethonium
Non-depolarising: rocuronium, atracurium
Suxamethonium
Introduced 1952 Profound muscle relaxation of short duration Emergency surgery Colourless solution Store in fridge
Suxamethonium mechanism of action
Mimics ACh
Attaches to ACh receptor
Causes membrane depolarisation
Hydrolysing enzyme is not present at NMJ so depolarisation persists until all ATP is used up
Suxamethonium other effects
Arrhythmias Hyperkalaemia Myalgia Anaphylaxis Prolonged block- suxamethonium apnoea Malignant hyperthermia
Non-depolarising mechanism of action
Competitively bind to the a subunit of the nicotinic ACh receptor on the post junctional membrane inhibiting the action of ACh
Polar drugs- unable to cross lipid membranes (small V)
Rocuronium
Rapid onset
Intubating conditions within 60-90 seconds
New drug for sequestration of rocuronium
Has selective relaxant binding agent which reverses effects quickly
Atracurium
Mixture of 10 stereoisomers
Intubating conditions 90-120 seconds
May precipitate histamine release
Cis-atracurium: one of the stereoisomers, possibly reduced histamine release
