Anaesthetics Flashcards
How can anaesthetic be applied?
Local or general
If general, inhalational or IV
And if local, it could be regional
And all of these can be combined
How many different drugs are normally applied for a procedure with anaesthetic used?
Premedication (calming) (hypnotic-benzodiapine)
Induction (usually intravenous but may be inhalational)
Intraoperative analgesia (usually opioid)
Muscle paralysis-facilitate intubation/ventilation
Maintenance (iv or inhalational)
Reversal of muscle paralysis and recovery
- Includes analgesia (opioid/NSAID/paracetamol)
- Provision for post-op nausea and vomiting (PONV)
So many (interacting) pharmacological agents Could be up to nine!!!
How safe is anaesthesia?
Deaths caused by anaesthesia are very rare
What is a crude classification of general anaesthetics?
Gases-Volatiles-Delivered via lungs (wide range of these)
or
Intravenous
What are the Guedel’s signs?
Stage 1: analgesia and consciousness
Stage 2: unconscious, breathing erratic but delirium could occur, leading to an excitement phase
Stage 3: surgical anaesthesia, with four levels describing increasing depth until breathing weak
Stage 4: respiratory paralysis and death
How does the concentration of anaesthetic affect different factors?
Memory goes first, then consciousness shortly after
Movement next
Then cardiovascular response
It is very steep as well, small change in anaesthetic conc. big changes in response
How is the potency of a volatile anaesthetic measured?
As MAC
Maximum Alveolar Concentration
It is the alveolar concentration at 1atm at which 50% of subjects fail to move to surgical stimulus (Like move at a knife cut)
At equilibrium [alveolar] = [spinal cord]
Anatomical substrate for MAC is spinal cord
- In animal models if section cord (i.e. remove connection
to brain) MAC is unchanged
What factors affect induction and recovery in volatile anaesthetics?
Partition coefficients (solubility)
Blood:Gas partition (in the blood)
- Low value, fast induction and recovery
Oil:Gas partition (in fat)
- Determines potency and slow accumulation due to
partition into fat
What affects MAC?
Age (high in infants, lower in elderly) Hyperthermia (increased) Hypothermia (decreased) Pregnancy (increased) Alcoholism (increased) Central stimulants (increase) Other anaesthetics and sedatives (decreased) Opioids (decreased)
How can nitrous oxide be used in anaesthetics?
By adding nitrous oxide to other anaesthetics, you can reduce the MAC and reduce ADRs
This means that the anaesthetics require a lower dosing
How do most anaesthetics work?
By activating GABAa receptors Potentiates GABA activity - Anxiolysis - Sedation - Anaesthesia
With the exception of Xe, N2O and ketamine
Which probably act at NMDA receptors
In very simple terms consciousness is a balance between excitation (glutamate) and inhibition (GABA)
Anaesthetics modulate this balance by either:
- Increasing inhibition or
- Decreasing excitation
What are the targets of anaesthetics in the CNS?
Reticular formation (hindbrain, midbrain and thalamus) depressed
Reticular system often called “activating system” due to ability to increase arousal
Thalamus transmits and modifies sensory information
Anaesthetics stop connectivity between centres
Hippocampus depressed (memory) Brainstem depressed (respiratory and some CVS)
Spinal cord-depressed
Dorsal horn (analgesia)
Motor neuronal activity (MAC)
What are the main intravenous anaesthetics?
Propofol (rapid),
Barbiturates (rapid)
Ketamine (slower)
How are intravenous anaesthetics usually used?
Given intravenously for ‘induction’
Then inhalational used
But can be used as sole anaesthetic in TIVA (Total IntraVenous Anaesthesia)
Target sites as for inhalational
With exception of ketamine (NMDA) all potentiate GABA
How do we describe intravenous anaesthetic potency?
Plasma concentration to achieve a specific end point (loss of eyelash reflex etc…)
For induction in mixed anaesthesia - Bolus to end point then switch to volatile
TIVA uses a defined PK based algorithm to infuse at a rate to maintain a set point. Pre-ceded by bolus
