13. Introduction To Anaesthetics Flashcards
What are the 7 steps of anaesthesia?
Hint: P double I, double M, R, P
1) Premedication - hypnotic-benzodiazepines for the anxiety
2)Induction - usually IV or inhalational given prior to surgery
3) intraoperative analgesia - same time as induction, usually an opioid e.g. fentanyl, causes resp rate to drop
4) muscle paralysis - makes patient floppy so can intubate them and ventilate and keep them still
5) maintenance - with a mask to keep asleep. IV/inhalational
6) reversal of muscle paralysis - give neostigmine to reverse the muscle relaxants.
7) provision for post-operative nausea and vomiting (PONV)
3 classes of anti-emetics
- centrally acting dopamine antagonists
-histamine receptor antagonists
-5 HT receptor antagonists
And also want to provide postoperative analgesia e.g. Opioid/NSAID or paracetamol.
What are Guedel’s signs?
It is a means of assessing the depth of anaesthesia and the levels of consciousness. Stages 1 - 4
Stage 1: analgesia (conscious) - normal muscle tone, slight eye movements
Stage 2: unconscious (may get paradoxical excitement), increased muscle tone, moderate eye movements , and breathing can be erratic
Stage 3: surgical anaesthesia - where we want to be and there’s 4 levels. Go from slightly to markedly relaxed and eventually no eye movements.
Stage 4: respiratory paralysis and death. Would be flaccid
How do we measure the potency of volatile anaesthetics?
MAC - minimum alveolar concentration
This is the concentration of alveolar vapour at 1 atm at which 50% of patients fail to move to surgical (pain) stimulus (unpremeditated breathing 02/air).
Any other agents on board will affect MAC
What is MAC BAR?
The concentration required to block autonomic reflexes to nociceptive stimuli.
What is MAC-awake?
The concentration required to block voluntary reflexes and control perceptive awareness.
Why might obese patients be asleep for longer with volatile (gas) anaesthetics?
- greater solubility in fat for most general anaesthetics
- partitions in fat and plasma membrane
- prolonged and less predictable effect
- become like a human vaporiser
What affects MAC?
- age: higher in infants, lower in elderly
- pregnancy: increases
- alcoholism: increase
- central stimulants: increases
- hyperthermia (increase)
- hypothermia (decrease)
- other anaesthesia and sedatives (decrease)
- opioids (decrease)
If 1 MAC of halothane is 0.75% how many MACs are required to Make sure 100% of patients are anaesthetised?
- 2 MACS
- this is because 1 MAC at 075% would be enough to anaesthetise 50% of patients
Which structure in fat correlates with anaesthetic potency?
The GABA receptor is a critical target
- major inhibitory transmitter
- Ligand gated ion channel (Chloride conductance)
- with the exceptions of xenon, ketamine and nitrous oxide, all anaesthetics potentiate GABA mediated gloried conductance to depress CNS activity
How is the brain affected by anaesthesia?
- the reticular formation (midbrain, hindbrain and thalamus)
- is usually able to increase arousal
- the thalamus transmits and modifies sensory information
- the hippocampus is involved in memory
- brainstem contains the respiratory and CVS centres
Anaesthesia goes to brain as sensitive to volatile so cant remember (hippocampus) and reduced depth of breathing.
Name some intravenous anaesthetics
- propofol
- barbiturates
- etomidate
- ketamine
They all act on GABA except Ketamine which acts on NMDA receptors. Can be given IV for induction.
How do we describe the intravenous anaesthetic potency?
Plasma concentration to reach a specific end point e.g. the loss of the eyelash reflex
When do we use local/regional anaesthesia?
- dentistry
- obstetrics
- regional surgery with patient awake
- chronic pain management
Give examples of some local anaesthetics
- lidocaine
- procaine
- Bupivacaine
How do local anaesthetics work?
- local anaesthetics are not charged
- can pass easily through the plasma membrane
- becomes charged in water environment
- gets sucked into the inner core of VgNa channels
- therefore will block an action potential
