Injectable Anesthetics & Anesthesia Induction Flashcards
What are injectable anesthetics?
- majority of these drugs MECHANISM OF ACTION is via potentiation or facilitation of GABA, by their actions at GABA’A’ receptors in the CNS
- need a high concentration of drug to rapidly reach the site of action (THE BRAIN) for a titratable effect
What are the properties of an ideal injectable anesthetic agent?
- rapid onset of action
- smooth induction of anesthesia
- smooth recovery from anesthesia
- non-irritant to tissues
- good bioavailability by ALL routes of administration
- short duration of action
- non cumulative
- rapid metabolism
- no toxic or active metabolites
- does not cause histamine release
- minimal cardiorespiratory side effects
- produces a degree of muscle relaxation
- produces a degree of analgesia
- stable in storage
- stable in solution
- miscible with other agents
- inexpensive
- high therapeutic index (SAFE)
What are the ADVANTAGES of injectable drugs?
- little equipment needed
- usually easy to administer
- induction of anesthesia can be rapid & smooth
- possibly relatively cheap
- no environmental pollution
What are the DISADVANTAGES of injectable drugs?
- once given, retrieval is impossible
- patient must be weighed accurately in order to calculate dose
- when used as the sole anesthetic agent: high dose is necessary to produce sufficient CNS depression to prevent response to surgical stimulus; profound cardiovascular and respiratory depression
- not well tolerated by: debilitated, hypovolemic, or endotoxemic patients; patients suffering from renal or hepatic disease
- some drugs have potential for human abuse
- risk of inadvertent self administration
When do we use injectable anesthetics?
- SEDATION: low doses (sub-anesthetic) can result in profound and reliable sedation ex: “ketamine stun”
- INDUCTION: MAIN USE; need to be at surgical plane to pass an endotracheal tube into the trachea
- MAINTENANCE: using the same &/or combination of drugs; “top-ups”; TIVA/PIVA
- EMERGENCY: to supplement inhalational anesthesia if animal rapidly ‘wakes’
What are the routes of administration of drugs?
- IM: less precision; difficult to titrate effect; best for wildlife anesthesia
- SUB Q, rectal, oral: too slow; unreliable
- Intraperitoneal: risk of depositing drug in gut; lab animals
What is IV drug administration?
- ACCURATE, TITRATABLE, RAPID-ACTING
- injectable anesthetics act in 20-60 secs following injection
- rapidly achieves surgical plane of anesthesia (Stage 3)
- bypasses stage 1 & 2 (excitement) w/ correct dose
- requires restraint & ideally an IV catheter
- preferred route of administration if possible
What are the pharmacokinetics of IV bolus injection?
- alpha phase: distribution from blood to vessel-rich tissues; heart, brain, lungs, liver, kidneys; lipophilic so uptake into CNS is rapid
- beta phase: elimination from central compartment (blood); drug leaves the CNS, goes back into blood & animal recovers from anesthetic effects
How does drug movement influence anesthetic recovery?
How does the drug concentration in the tissues change during distribution and redistribution over time?
- modern injectable anesthetics have an anesthetic effect lasting 4-10 mins
- for longer procedures, more injectable drug is given (‘top-ups’) or switch to inhalational drugs
- recovery from injectable bolus: initially, REDISTRIBUTION - drug from brain to blood & other body systems; ‘hangover effect’ - rapidly metabolized drugs produce little hang over
How are drugs metabolized & excreted?
- anesthetic drugs are handled the same as other exogenous compounds
- lipid soluble drugs must be converted to water soluble compounds for efficient excretion: PHASE I - oxidation, reduction, hydrolysis; PHASE II - conjugation
- can produce ACTIVE metabolites: nordiazepam; morphine glucuronide
- drugs can also be metabolized in other sites as well as the liver: kidney, lungs, gut - PROPOFOL; plasma - REMIFENTANIL, ATRACURIUM
What is CRI?
constant rate infusion = not changing rate
What is VRI?
variable rate infusion = changing rate
How do we administer drugs to INDUCE anesthesia?
- goal is to achieve stage 3 anesthesia & bypass the excitement phase (anesthesia stages 1 & 2)
- titrate “to effect” in SMALL ANIMALS: consider premedication - 20-80% dose reduction depending on premedication
- consider physical status of your patient
- slow injection (60-120 seconds) OR give 1/3 to 1/2 of calculated dose: wait for maximum effect; proceed further w/ increments until desired effect
- in LARGE ANIMALS - give whole dose: this is for safety purposes - you DO NOT want a partially anesthetized Eq
What is TIVA and PIVA?
- Total IntraVenous Anesthesia (TIVA): one or more drugs can be used; can be slow to change depth of anesthesia
- Partial IntraVenous Anesthesia (PIVA): use injectable drugs to supplement inhalational anesthesia
What are the different injectable anesthetics?
- substituted phenols (propofol)
- neurosteroids (alfaxalone CD-RTU)
- phencyclidine derivative/aryl-cyclohexamines (dissociative) (ketamine)
What is Ketamine?
Phencyclidine derivative
- is a DISSOCIATIVE ANESTHETIC AGENT - interrupts information reaching higher centers of the brain
- different from GABA’A’ agonist drugs: cataleptoid state w/ slow nystagmus; muscle rigidity w/ higher doses, maintains cranial nerve reflexes - gag, swallow, & palpebral & central eye positions in Ca & Fe (no ventral-medial rotation)
- racemic mixture: S(+) & R(-) isomers; S(+) isomer is 2-4x more potent
What effects does Ketamine have on the respiratory system?
- minimal respiratory depression
- bronchodilation
- laryngeal & pharyngeal reflexes are preserved
- irregular or periodic breathing patterns -> apneustic breathing
