7. Anaesthetics Flashcards
why does anaesthesia require multiple drugs?
- pre-medication (e.g. benzodiazepine) - hypnotic, anxiolytic
- induction (usually IV but can be inhalational):
- analgesic (e.g. fentanyl) - for intubation
- sedation
- muscle paralysis - for intubation
- maintenance (IV and/or inhalational)
- muscle paralysis reversal and recovery, inc. post-op analgesia (opioid/NSAID/paracetamol)
- anti-emetic to prevent PONV (post-op nausea and vomiting)
what are the 2 main groups of general anaesthetics?
- volatile
2. intravenous
describe the 4 stages of general anaesthesia
GUEDEL’S CLASSIFICATION
Stage 1: analgesia
- from induction start to loss of consciousness, i.e. P is conscious
- normal muscle tone and breathing
- slight eye mov.
Stage 2: excitement
- from loss of consciousness to automatic breathing onset
- normal to markedly increased muscle tone
- erratic breathing and moderate eye mov.
Stage 3: surgical anaesthesia
- from start of automatic breathing to resp. paralysis, 4 sub-stages
- progressive decrease muscle tone
- progressive respiration depression
- no eye mov.
Stage 4: respiratory paralysis and death
- flaccid muscle tone
- no breathing or eye mov.
how is the potency of volatile anaesthetics measured?
MAC (minimum alveolar conc.): agent alveolar conc. (at 1 atm) at which 50% of Ps fail to move to surgical (apin) stimulus.
I.e. low MAC = high potency
name 3 factors affecting the MAC of an agent
- age (decreases with age - so less agent required in elderly)
- hyperthermia (increased) and hypothermia (decreased)
- pregnancy (increased)
- alcoholism (increased)
- central stimulants (increased)
- other anaesthetics and sedatives (decreased)
- opioids (decreased)
why is nitrous oxide often added to other volatile agents
Combinations of anaesthetic agents reduce MAC (and thus agent side effects) - so lower volatile agent conc. required when used in combo. with nitrous oxide.
name 2 properties of volatile agents that affect induction and recovery
Partition coefficients (i.e. solubility):
- blood:gas partition (solubility in blood)
- low value = fast induction and recovery - oil:gas partition (solubility in fat)
- high value = greater potency
why do obese Ps have longer recovery?
agent accumulates in fat (esp. those with high oil:gas partition) so has longer effect
how is potency of IV anaesthetics measured?
plasma conc. required to achieve specific end point (e.g. loss of eyelash reflex)
describe the 2 possible modes of action of general anaesthetic agents
Target brain:
- potentiate GABAa Rs (most agents): increase Cl- influx through LGIC… neurone hyperpolarisation/inhibition… depress CNS activity
- inhibit NMDA glutamate Rs (Xe, N2O, ketamine): prevent Ca2+ (and Na+) influx through channel… prevent neuronal excitation… depress CNS activity
name 3 examples of IV anaesthetics
- propofol (rapid)
- barbituates (rapid)
- ketamine (slow)
describe the basic structure of a local anaesthetic - how does this affect duration of action?
Aromatic ring linked to an amine:
- ester link (short acting as many esterases in plasma)
- amide link (longer acting)
name the key factors affecting potency, time of onset and duration of local anaesthetics
- potency: lipid solubility (high solubility = high potency)
- time of onset: pKa dissociation constant (lower pKa = faster onset)
- duration: protein binding (high binding = longer duration)
explain how local anaesthetics, e.g. bupivacaine, act to promote pain relief/analgesia
i. uncharged agent diffuses through plasma membrane…
ii. becomes charged in plasma (depending on pKa)…
iii. absorbed into inner core of voltage-gated Na+ channel…
iv. blocks transmission of AP
Blocks small myelinated (afferent) nerves in preference, so nociceptive and sympathetic block.
how can duration of action of local anaesthetics be increased?
Also inject adrenaline - causes local vasoconstriction to reduce spread
