Pharmacology of Induction Agents Flashcards
describe injectable agents (6)
- can be given IM, IV, or both
- rapid and smooth production of unconsciousness
- used for induction and maintenance of anesthesia
- no waste anesthetic gases
- cannot be controlled or eliminated once administered
- include: propofol, alfaxolone, etomidate, ketamine, telazol (tiletamine + zolezepam)
how do injectable induction agents work? (2)
- augmentation of inhibitory pathways (GABA receptors): propofol, alfaxolone, etomidate, barbiturates
-fall asleep, beautiful slow natural induction - suppression of excitatory pathways (NDMA receptors): ketamine, tiletamine
-rigid, different induction
describe propofol (6)
- binds at GABA receptors; MUST be administered IV
- no patient is refractory to propofol, check catheter if not going to sleep
- short duration of action, generally smooth recovery
- old cheap version has shorter shelf life (6 hours), propofol 28 has longer shelf life (28 days) so if using mult bottles a day just get normal propofol or throwing away money
- metabolism via hepatic AND extrahepatic sites; great for patients with hepatic dysfunction; will wake up bc not just liver metabolizing it
- not federally (DEA) controlled, but may be controlled by state
describe propofol pharmacodynamics (CNS, cardiovascular, respiratory, miscellaneous)
CNS:
-decreases cerebral oxygen consumption
-decreases cerebral blood flow
-lowers intracranial pressure; a good choice for patients where elevated ICP is a concern
cardiovascular:
-dose dependent decrease in heart rate and contractility; not usually relevant at clinical doses
respiratory:
-causes significant respiratory depression!!!
-apnea and transient cyanosis occur frequently; preoxygenate patients!! give more time to intubate patient before desaturation (5 min versus 60 seconds)
-effect is dose related and more severe when propofol is administered rapidly
-DO NOT use propofol if you cannot intubate and ventilate if need!! especially cats!!
miscellaneous:
1. NOT analgesic, may even cause pain on injection
2. occasionally causes myclonus (muscle twitch)
3. anti-emetic
4. rapidly metabolized by fetus= drug of choice for c section
5. oxidative injury to RBC and heinz body anemia after repeated use in cats
describe clinical use of propofol (3)
- commonly used induction agent in human and small animal
- administered IV to effect, ideally after premed providing analgesia
- extravascular admin = NO effect
describe alfaxolone/alfaxan (5)
- binds to GABA receptors; can be given IV or IM!
-IM use is major difference between propfol (no IM) and alfaxolone)
-IM use = can use alfaxolone as premed - cyclodextrin carrier allows for 28 day shelf life
- smooth and rapid induction with short duration of action
- recovery generally smooth and minimal hang over effect
- hepatic metabolism!!
- DEA schedule 4!!
describe alfaxolone pharmacodynamics (CNS, cardiovascular, respiratory)
CNS:
-decreases cerebral oxygen consumption
-decreases cerebral blood flow
-lowers ICP
-produces myoclonus that is difficult to distinguish from seizure!! (difference from propofol)
cardiovascular:
-like propofol
-does NOT cause anemia in cats
respiratory:
same as propofol! and effect is more severe when alfaxolone is administered rapidly
all effects are much milder when given IM as a premed!!! IM as premed = major perk
describe alfaxolone clinical use (3)
- significantly (10x) more expensive than propofol and not commonly available in private practice
- most utility in small patients requiring heavy premedication that are not candidates for pehnothiazines or alpha-2 agonists
- a good choice for cats requiring multiple anesthetic events
describe etomidate
- binds to GABA receptors
- formulated in propylene glycol which may cause erythrocyte damage
- must be administered IV like propofol
- rapid smooth induction and short duration of action
- metabolized by hepatic enzymes and hepatic esterases
describe etomidate pharmacodynamics
CNS:
-decreases cerebral metabolic requirement for oxygen
-decreases cerebral blood flow
-decreases ICP
cardiovascular:
-minimal to no change in heart rate, blood pressure, stroke volume, and cardiac output (real life = heart failure, NAVLE = any heart patient)
respiratory:
-dose dependent respiratory depression
-can cause apnea!
endocrine:
-inhibition of 11-B hydroxylase for 6-8 hours (enzyme responsible for conversion of cholsterol to cortisol = make them addisonian for 6-8 hours and can cause addesonian chrisis!
-depresses cortisol stress response
-avoid etomidate in patients with pre-existing addison’s disease
-multiple doses and CRIs should be avoided even in healthy patients
miscellaneous:
-no analgesia
-provides poor muscle relaxation
-may see myoclonus
-may see pain on injection
describe clinical use of etomidate
- induction agent of choice for patients in heart failure
- not commonly used on patients without pre-existing heart disease due to endocrine adverse effects
- not commonly available in SA private practice
describe PCP derivatives
- bind at NMDA receptors- antagonizing them
- produce a cataleptoid state via dissociation of the thalamus and limbic system
- ketamine: formulated as a 10% solution with benzethonium chloride preservative
- tiletamine: formulated as telazol in combination with zolazepam
- can give IV or IM
- metabolized in the liver
describe PCP derivative pharmacodynamics
CNS:
-when given alone without heavy premedication, leads to increased cerebral metabolic requirement for oxygen, increased cerebral blood flow, and increased intracranial pressure
-effect is less when heavily premedicated or patient already under anesthesia
cardiovascular:
-sympathetic nervous stimulation results in increased heart rate, blood pressure, and cardiac output
-may cause tachycardia and arryhthmias due to endogenous catecholamine release
respiratory:
-do NOT cause significant respiratory depression
-can exacerbate the respiratory depressant effects of other drugs
-laryngeal function is maintained
-may cause apneustic breathing patter
-causes bronchodilaion = great for asthmatic patients
describe the analgesia effects of PCP derivatives
- profound analgesic effect
- antagonism of NMDA receptors prevent wind-up, hyperalgesia, allodynia, and chronic pain
- also works at opioid receptors providing analgesia via this pathway as well
- a very good choice for patients with traumatic injury or undergoing painful ortho surgery
describe miscellaneous effects of PCP derivatives
- provides very poor muscle relaxation so commonly administered with a benzodiazepine (ketamine is friends with everyone)
- crosses placental barrier and causes severe fetal depression = WORST choice for C section anesthesia
- weird NAVLE fact: telazol has been associated with severe adverse reactions including death in tigers and renal necrosis in New Zealand White Rabbits
