Session 9 Intro To Anaesthetics Flashcards
What is ‘conscious sedation’?
Use of small amounts of anaesthetics or benzodiazepines to produce a ‘sleepy-like’ state
I.e. you maintain verbal contact but patient feels comfortable
What 4 things is anaesthesia a combination of?
- Analgesia
- Hypnosis (loss of consciousness)
- Depression of spinal reflexes
- Muscle relaxation (insensibility and immobility)
What are the 4 stages of ‘Guedel’s signs’?
Comment of muscle tone and eye movement in each stage
Stage 1: analgesia and consciousness
* muscle tone = normal
* eye movement = slight
Stage 2: unconscious, breathing erratic but delirium could occur leading to an excitement phase
* muscle tone = normal to markedly increased
* eye movement = moderate
Stage 3: surgical anaesthesia, with four levels describing increasing depth until breathing is weak
* muscle tone = slightly relaxed to markedly relaxed
* eye movement = slight to none
Stage 4: respiratory paralysis and death
* muscle tone = flaccid
* eye movement = none
What do we use to describe potency (for volatile gases)?
MAC = minimum alveolar concentration
= the concentration of a vapour in the alveoli of the lungs that is needed to prevent movement in 50% of subjects in response to surgical stimulus
What is MAC?
Alveolar concentration (at 1atm) at which 50% of subjects fail to move to surgical stimulus (unpremeditated breathing oxygen)
At equilibrium [alveolar] = [spinal cord]
What is the anatomical substrate for MAC?
Spinal cord
What factors affect induction and recovery?
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 and lower in elderly Hyperthermia = increased Hypothermia = decreased Pregnancy = increased Alcoholism = increased Central stimulants = increased Other anaesthetics and sedatives = decreased Opioids = decreased
What are the critical targets in terms of anaesthetics?
GABAa receptors
= major inhibitory transmitter
AND
NMDA glutamate receptors
= central excitation
What are the main IV anaesthetics used?
Propofol (rapid) - potentiates GABAa
Barbiturates (rapid) - potentiates GABAa
Ketamine (slower) - potentiates NMDA
Can be used as sole anaesthetic in TIVA (total intravenous anaesthesia)
What are some examples of local anaesthetics?
lidocaine
Bupivacaine
Ropivacaine
Procaine
Describe the characteristics of local anaesthetics
Lipid solubility: higher = greater potency Dissociation constancy (pKa): lower pKa = faster onset Chemical link with metabolism Protein binding: if higher = longer duration of action
How does bupivacaine work for wound analgesia?
Block is use dependent
Blocks small myelinated (afferent) nerves leading to nociceptive and sympathetic blocking
Adrenaline increases the duration
Compare the potency, speed, metabolism and duration of lidocaine, bupivacaine, ropivacaine and procaine
- from most to least
Potent = bupivacaine > ropivacaine > lidocaine > procaine - from longest to shortest
Duration of action = bupivacaine > ropivacaine > lidocaine > procaine
Metabolism
- bupivacaine, ropivacaine and lidocaine = amide metabolised (longer acting)
- procaine = esterase metabolised (shorter acting)
pKa (lowest to highest)
* lidocaine > bupivacaine + ropivacaine > procaine
What is ‘regional’ anaesthesia?
Selectively anaesthetising a part of the body
- often described as a block of a nerve an hence patient remains awake
Uses local anaesthetic or an opioid
Administration: Extramural, Intrathecal or combined
