Principles of Anaesthesia Flashcards
3 main subgroups of Anaesthesia
General
Regional
Local
What is General Anaesthesia?
A loss of sensibility in the whole body, and global lack of consciousness. Unrousable, reversible and usually drug induced:
- Use centrally acting drugs - hypnotics / analgesics.
- GA agents that are inhaled or IV provide unconsciousness as well as a small degree of muscle relaxation.
What is Regional Anaesthesia?
- Numbs an area or region of the body. Local anaesthetic is injected into nerves supplying the relevant area.
- Simple blocks include finger blocks, ankle blocks, etc; more complex blocks include plexus blocks, and `major regionals’ mean epidural or spinal anaesthesia.
- May be used as the sole mode of anaesthesia or combined with general anaesthesia or sedation.
What is Local Anaesthesia?
How do these agents work?
- Producing insensibility in only the relevant part of the body.
- Local anaesthetics are applied directly to the tissues.
- They provide a bit of analgesia and muscle relaxation (block motor nerves and sensory nerves). Block Na+ ions.
- They are toxic - act on excitable membranes all over the body
Regional Anaesthesia Vs Regional Analgesia
- Regional anaesthesia - the patient should have little or no sensation of any sort from the blocked area, so warmth, proprioception, light touch and vibration sense will all be largely gone as well as pain sensation.
- Regional analgesia - only pain sensation is removed or reduced. Other sensation may be retained to varying extents.
The border between these two is very blurred.
General anaesthesia consists of a triad of….?
- Analgesia - Pain relief
- Unconsciousness - sleep/hypnosis
- Relaxation
An individual anaesthetic may consist of varying contributions from all three but does not require all three.
For instance – a simple anaesthetic for dental extraction may consist of hypnosis alone or better, with an element of analgesia. Local anaesthetic for dental extraction consists of analgesia alone. A spinal anaesthetic will consist of analgesia and relaxation with no hypnosis. However, obviously, you could not have an anaesthetic consisting of relaxation alone.
A general anaesthetic (GA) can be subdivided what 3 key stages?
- Induction - getting the patient to sleep
- Maintenance - keeping the patient asleep during the operation
- Emergence/awakening - waking the patient up at the end of the procedure.
What is the idea behind ‘balanced Anaesthesia’?
- It is the concept of using different drugs to do different jobs and reach the individual ‘targets’ as it were keeping them in the correct plane of anaesthesia
- It allows a great degree of control over the individual components of the triad.
- Avoids overdose
- E.g. can use much less general anaesthetic agent to provide hypnosis for abdominal surgery if we use a muscle relaxant to provide the muscle relaxation needed for access and immobility
- Additionally we could use less of each of these if we added in an epidural anaesthetic (regional local anaesthesia) to provide additional analgesia and muscle relaxation
To avoid problems, what must be done before giving a patient a muscle relaxant?
- Airway management must be in place
- And artificial ventilation
Failure to do this can and does result in severe morbidity or death
What is ‘Termed awareness’?
- When patients regain consciousness during surgery (the anaesthetic wears off) so they are awake, however, they are still paralysed due to muscle relaxant.
- They are unable to communicate to the medical team
- This is due to the separation of hypnosis from muscle relaxation making the latter possible without the former.
How do General Anaesthetic Agents work?
They suppress neuronal activity in a dose dependent fashion.
This is largely done by opening chloride channels which hyperpolarise the neurons, suppressing excitatory synaptic activity (reversible). The neurons cannot reach their threshold potential and so they don’t fire AP to next neuron.
- Inhalational agents - direct physical effect
- IV agents - allosteric binding - GABA receptors - open chloride channels
Effects of GA on cerebral function
- Cerebral function is lost ‘from the top down’ - the most complex processes are interrupted first (consciousness)
- Reflexes are relatively spared
How quickly do IV aneasthetic agents take effect?
- Rapid onset of unconsciousness - basically as soon as they reach the brain
- They are highly fat soluble drugs and cross basement membranes extremely quickly - they therefore cross the BBB rapidly and get into the neural tissues very quickly
Do IV anaesthetics leave the circulation quickly?
Yes, there is rapid recovery from IV anaesthetics due to disappearance of the drug from the circulation and consequently the brain. This rapid fall in blood conc. is due mainly to the drug leaving the circulation and redistributing to other parts of the body.
However, following a lengthy procedure there will be large amounts of anaesthetic drug stored in fatty tissue because the drugs are fat soluble. This will leach out slowly over a long period of time.
All inhalalational anaesthetic agents are of what type now?
Halogenated hydrocarbons
- Taken up and excreted via the lungs (undergo very little metabolism in the body)
- Moves down the pressure gradient from the lungs - blood - brain
- Cross the alveolar BM easily
How does General Anaesthesia affect the CVS?
Centrally: Depresses the cardiovascular centre
- reduce sympathetic outflow
- negative inotropic/chronotropic effect on heart
- reduced vasoconstrictor tone → vasodilation
Direct effect on the myocardium and vascular smooth muscle - reducing tone.
MAP = CO x SVR - In this case, cardiac output is reduced due to venodilation and negative inotropic effect and the stroke volume is reduced due to arteriolar vasodilation (reduced peripheral resistance)
Needs to be managed when under GA to prevent these effects - do this by giving fluids, vasopressor drugs + sometimes give inotropes etc too
