PHA3801 Lecture Notes - Lecture 11: Skeletal Muscle, Cerebral Circulation, Spasm
24 Jul 2018
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General Anaesthesia
Mandrakes (sleep), Opiates (pain), Curare (muscle relax), cocaine (LA).
Anaesthesia: Admin of (combinations) of drug(s) to achieve 5 conditions for loss of pain
sensation → hypnotic, analgesic, muscle relaxant, LA.
Hypnosis: Being asleep – put then keep – inhalation, IV or IM.
General anaesthesia: Inhibit synaptic transmission (excitatory + inhibitory)
Stages: Analgesic, excitement, surgical anaesthesia, overdose
- Avoid gaseous induction in most cases – only if vein easily seen – most = IV induction
Ventilates (lungs) → crosses alveolus → dissolves into blood → L heart → brain – need Pp
gradient (over-concentrate agent in lungs).
Want least soluble agent (not taken up by other tissues). In blood quicker → allows rapidly
sleep/wake rapidly + allows to titrate the level of A up or down,
Minimal alveolar concentration (MAC): The amount of inhaled A at 1 atm which prevents skeletal
muscle movement in response to a noxious stimuli – different agents have different MAC’s.
- If young, warm or chronic alcoholic – need more GA to be effective
- If acutely intoxicated, cold or old – don’t need much
- If you want to get GA agent to brain quickly → deliver in high [], inc ventilation, low solubility,
low CO (more blood flow preferentially to brain).
Inhalation agents: Gas (NO), -fluorane = halogenated compounds.
- Dose related EEG changes (eventually flatline, dec response to local potentials, dose related
changes in BP, CO, dec PVR.
- If catecholamines (e.g. adrenalin) can get arrhythmias.
- Inc rate of breathing, dec tidal V, dec response to inc CO2 levels, dec response to dec O2
levels, dec airway resistance, dec blood flow to kidneys + liver (dec metabolism of other drugs),
dec uterine SM contractions.
Methoxyfluorane: Dec pain response, long exposure at high [] → renal/liver failure.
Ideal properties of inhalation: Low solubility (adjust level of A’ easily), easy admin, non-irritant when
inhaled, minimal SE’s/toxicity, minimal metabolism, inert, safe for all age groups, highly potent,
analgesic, non-flammable, no cardiac arrhythmias, environmentally friendly.
NO: C/loss, some can smell it, not flammable but supports combustion, analgesic. MAC =
104%, low solubility, rapid on/offset, minimal metabolism, non-irritant, can cause nausea +
vomiting (fast before surgery).
IV agents: Barbituates, benzo’s, opioids.
Barbituates: Sulphur group – lipid soluble – rapid BBB, metabolised in liver.
- Rapidly into Brain (BBB) – by about 4 mins, redistributes → saturates fat.
Thyopentones: Potent cerebral depressant, dec cerebral blood flow, dec intracranial P, good
anticonvulsant, potent CVS depressant (dec BP, contractility, CO), mild dep of ventilation.
- Used for induction of A – cant maintain w it.
Propofol: Short acting IV hypnotic
- Minimal nausea/vomiting, rapid on/offset, safe all ages, no active metabolites, no significant
allergies.
Ketamine: rapid onset, 2nd/3rd world.
Questions:
- Ideal properties of an IV GA: works rapidly, short duration (pharmacokinetics allow rapid
awaking – quick response once drug stopped), no active metabolites (drowsy afterwards,
hangover effect), economics, minimal haemodynamic effects (minimal effect of BP, pulse rate),
minimal histamine release (all drugs have the potential to cause anaphylactic reactions), long
Document Summary
Mandrakes (sleep), opiates (pain), curare (muscle relax), cocaine (la). Anaesthesia: admin of (combinations) of drug(s) to achieve 5 conditions for loss of pain sensation hypnotic, analgesic, muscle relaxant, la. Hypnosis: being asleep put then keep inhalation, iv or im. General anaesthesia: inhibit synaptic transmission (excitatory + inhibitory) Avoid gaseous induction in most cases only if vein easily seen most = iv induction. Ventilates (lungs) crosses alveolus dissolves into blood l heart brain need pp gradient (over-concentrate agent in lungs). Want least soluble agent (not taken up by other tissues). In blood quicker allows rapidly sleep/wake rapidly + allows to titrate the level of a up or down, Minimal alveolar concentration (mac): the amount of inhaled a at 1 atm which prevents skeletal muscle movement in response to a noxious stimuli different agents have different mac"s. If young, warm or chronic alcoholic need more ga to be effective. If acutely intoxicated, cold or old don"t need much.
