Induction agents Flashcards
what are the properties of an ideal induction agent?
physiochemical
* stable at room temp and in light
* doesnt need reconstituting - soluble in water
* compatible with other anaesthetic agents and giving sets
* long shelf life , doesnt fascilitate bacterial growth
pharmacodynamics
* non painful at injection, no irriation with extravasion
* quick onset - one arm circulation
* smooth predictable end point
* anaesthesia with added benefits - PONV, antiepileptic, amnesia, analgesia
* no allergies
* minimal CVS/ resp effects
* doesnt increase ICP
* sutiable in different groups - paeds, obsetrics
Pharmacokinetics
* Can be absorbed IV or IM
* very lipid soluble so large Vd - allows it to get to effect site of brain
* metabolised rapidly and doesnt accumulate so can have short CSHT for TIVA
* no active/ toxic metabolities
* safe in hepatic/ renal impairment
environemnt
* cheap to make
* non toxic waste
what are the different classes of IV induction agent?
4 main agents
propofol = phenol derivative
ketamine = phencyclidine derivative
etomidate = imidazole with ester link
thiopentone = barbiturate
others..
barbiturates = methohexital, phenobarbital
benzo = midazolam
how do the IV induction agents work?
overall reduce neuronal excitability in the brain either by increasing inhibitory NT (GABA) or decreasing excitatory (nAChR or Glutamate)
mostly via GABA allosteric modulation
propofol also inhibits NMDA, serotonin receptors and nACHR
ketamine is a non-competitive anatagonist of NMDA
what is propofol related infusion syndrome?
rare syndrome associated with prolonged infusion of propofol, mostly seen in paediatrics and in ITU.
usually after >4mg/kg/hr for >24hrs
pathophysiology:
* mitochondrial dysfunction - impaired oxidative phosphorylation - lactatic acid, ROS, oxidative stress
* cell damage - rhabdo, hyperkalaemia
* prevents B oxidation of fats
* build up of triglycerides
results in
* myocardial depression - low BP and HR and arrhythmias
* high K+, triglycerides, lactate, CK
* renal failure
* green urine
what are the risk factors for developing propofol related infusion syndrome (PRIS)?
children
sepsis
>48 hr infusion
low card, high lipid diet e.g. starvation
how is PRIS managed?
early recognition - check CK, triglycerides and pH regularly
stop propofol and switch to alternative
supportive - ionotropes , vasopresors
how is PRIS prevented?
not in children under 16yrs
limit transfusion to 4mg/kg/ hr
why does thio have a quicker onset of action to propofol?
thiopentone is a weak acid with pKA of 7.6
propofol is also a weak acid with pKA of 11
acids ionise above
at physiological pH, propofol is more unionised
HOWEVER
propofol is heavily protein bound -98%
thiopentone 80%
so more free portion of thio to cross BBB
what happens if thiopentone is given intra-arterially?
thiopentone is a barbiturate with ketone group. when put at a pH above its pKA e.g. pH of 10.5 it ionises and becomes soluble in water as its enol from C-S- Na +
the solution thio is stored in is at this pH to maintain its water solubility however this is very alkolitic and if given intra-arterially can irritate arteries causes crystalisation, spasming and thrombosis.
this can lead to extreme pain and distal ischaemia.
how is an intra-arterial injection of thiopentone managed?
quickly dilute with saline
can give lidocaine to reduce pain
can give papaverine to dilate arteries
anti-coagulation to reduce thrombosis risk
could also do a sympathetic block to improve blood flow e..g stellate ganglion block
what happens if thiopentone is given through tissued cannula?
can precipitate and cause tissue necrosis due to high pH of solution
why would a septic patient need less thio
pH of blood is lower
hence more unionised available
also lower albumin potentially increases free portion
what did NAP5 study find
looked into awareness
increased awareness in obstetric anaesthesia because
* RSI
* junior anaesthesist
* surgical stimulation short after induction
* less opioid used due to risk crossing placenta
* use of thio - mistaken for co-amox and less familiar
why may propofol have advantages in maternity over thio when reducing risk of awareness?
less likely to be mistaken for other drug e.g. co-amox
more familiar
no pre-mixing required
what is meant by dissociative anaesthesia?
a state of anaesthesia whereby an individual feels separate from their environment and in a trance like state. they are unaware of pain and sensory input.
however differs from standard anaesthesia with other agents.
this is seen with ketamine.