intravenous anesthetic agents Flashcards
what are primary uses of IV anesthetic agents?
- IV induction that allows for rapid recovery
- used for sedative/hypnotic allowing for a deeper sleep but not unconsciousness
ex: during a breast biopsy, used for injection of local anesthetic which is painful - TIVA
what is the MOA of IV anesthetics?
changes the level of consciousness by depressing the reticular activating system either by:
- enhancing the inhibition properties of GABA
- or inhibiting the NMDA excitatory synapses
what are the different IV anesthetic agents?
- propofol
- ketamine
- thiopental
- methohexital
- etomidate
- benzodiazepines
what is the primary mechanism of terminating the central effect of IV anesthetic agents?
redistribution from the central, highly perfused compartment (blood, brain) to the larger and less-well perfused “peripheral” compartments (skeletal muscles)
*with drugs like pentothal, may still be in the body, but not at effect site
what are characteristics of the IDEAL IV anesthetic?
- water soluble with chemical stability and IV fluid compatible
- rapid onset w/o unwanted movement or unpredictable CV or CNS side effects
- rapid (lipid soluble) and predictable recovery
- anticonvulsant, antiemetic, analgesic (ketamine), amnesic
- no renal/liver impairment, steroid synthesis (etomidate), teratogenicity
- no pain on injection (propofol hurts)
- low histamine release (pentothal causes histamine release)
- rapidly metabolized to inactive substances with minimal accumulation
- “dialable” or rapidly responsive to titration (propofol)
- decreases cerebral metabolism proportional to CBF and no increase in ICP (pentothal)
- no hangover or headache (pentothal causes “hangover”)
Describe thiopental (Pentothal)
- barbiturate (thiobarbiturate)
- 2.5% sodium salt preparation is water soluble (25 mg/cc)
- highly alkaline (10.5 pH) making bacteriostatic
- unstable once reconstituted (last 6 days at rm temp, 2 weeks in refrig)
- no pain on injection
- precipitate if mixed with opioids, catecholamines, NMB, and other acidic fluids (LR) so flush well after!
what is the mechanism of action for thiopental?
- increases duration of GABA activating its receptor causing the chloride ion channels to remain open longer, allowing more influx and hyperpolarization and inhibition of the cell
- mimics GABA at its receptor to directly cause Cl- channels to open
- also activates other receptors (glutamine, adenosine, nACh) to depress SNS ganglia transmission causing hemodynamic effects and desensitize nACh receptors to ACh with large doses
what are CV effects of thiopental?
- minimal BP decrease
- increase in HR unless on beta blockers or hypovolemic and have greater drop in BP
- peripheral vasodilation: venous pooling, decreased venous return (possible decreased CO) BUT offset by normal sympathetic activation
- possible histamine (greater drop in BP d/t arterial and venous dilation)
- effects prominent in hypovolemia
- myocardial depression less than volatile agents
- CV effects considered MINIMAL
what are respiratory effects of thiopental?
- decrease sensitivity of medullary ventilator center to stimulation of CO2
- respirations return as small TV and slow RR (drop in MV)
- reflexes remain intact with apnea, allowing possibility of laryngospasm (not good with LMA or intubation w/o NMB)
- apnea more likely when other depressant drugs, opioids, and benzos are also used (so not good with sedation cases)
what are CNS effects of thiopental?
- decreases ICP by decreasing cerebral blood volume d/t cerebral vasoconstriction
- decreases cerebral metabolic oxygen requirements by 55% (decreases demand more than decrease supply)
- careful of hypotension with large doses which would decrease CPP (MAP-ICP) below adequate pressure
- tolerance and physical dependence may occur
what are renal and liver effects of thiopental?
no damage
what is the effect of thiopental on heme?
increases the production of heme, which may exacerbate acute intermittent porphyria
**do not give to porphyria patients
what dose of thiopental is safe for the fetus?
up to 4 mg/kg maternal dose
what may occur with thiopental d/t histamine release?
anaphylactoid and anaphylaxis reactions
what happens to thiopental at physiological pH?
becomes highly lipid soluble
how much of thiopental is protein bound?
85%
*decrease dose with elderly who have decreased albumin
what is the distribution 1/2 life and elimination 1/2 life of thiopental?
- distribution 1/2 life: 2-4 min
- elimination 1/2 life: 10-12 hrs (reason have hangover effects even though desired effects last only minutes)
what is the dose of thiopental?
3-5 mg/kg
*decrease with elderly or if using benzos or other depressant drugs
what is the onset and duration of thiopental?
onset: 30 sec
duration: 5-10 min
what are the basic characteristics of thiopental?
- “gold standard”
- minimal CV effects
- good cerebral protection
- minimal pain on injection
- hangover
- moderate emetic (makes nauseated)
- precipitation
- don’t give with porphyria patients
describe methohexital (Brevital)
- barbiturate (oxybarbiturate)
- less lipid soluble than thiopental
- 1% solution (10 mg/cc; more potent d/t higher % nonionized at blood pH 7.4)
- stable when reconstituted (refrigerate up to 6 weeks)
- MOA like thiopental and distribution limits central effects like thiopental
describe metabolism of methohexital compared to thiopental?
- 3-4x faster d/t decreased lipid solubility
- smaller VOD allows more to stay in plasma and go past liver more allowing quicker elimination
- more rapid awakening with no hangover effects or groggy, but not a startling wake up like with propofol
what is the elimination 1/2 time of methohexital?
3.9 hours (10-12 with thiopental)
what is the advantage of methohexital?
a more rapid recovery
- good with cataract procedures; allows for retrobulbar block needle insertion
- good with elderly when don’t want benzo or opioids
