9a – Induction and Injectable Anesthetics Flashcards
Injectable anesthetics: mechanism of action is via
- Potentiation or facilitation of GABA by their actions at GABAa receptors in the CNS
- *need a high concentration of drug to rapidly reach the site of action (THE BRAIN) for a titratable effect
Properties of an ideal injectable drugs
- Rapid onset of action
- Smooth induction and recovery
- Non-irritant
- Good bioavailability by ALL routes
- Short duration of action
- Non-cumulative
- Rapid metabolism
- No toxic or histamine release
- Minimal cardiorespiratory side effects
- Degree of muscle relaxation and analgesia
- Stable in storage and solution
- Miscible with other agents
- Inexpensive
- High therapeutic index (SAFE!)
Advantages of injectable drugs
- Little equipment needed
- Usually easy to administer
- Induction can be rapid and smooth
- Possibly cheaper
- No environmental pollution
Disadvantages of injectable drugs
- Once give, retrieval is IMPOSSIBLE
- Need accurate weight to calculate dose
- Not well tolerated in all patients
- Some have potential for human abuse
- Risk of inadvertent self-administration
Disadvantage of injectable drugs when used as a sole anesthetic agent
- High doses necessary to produce sufficient CNS depression to prevent response to surgical stimulus
- Profound CV and respiratory depression
Injectable drugs not well tolerated by all patients
- Debilitated, hypovolemic or endotoxemia patients
- Patients suffering from renal or hepatic disease
When do you use injectable anesthetics?
- Sedation: low doses can result in profound and reliable sedation (ex. ‘Ketamine stun’)
- Induction=MAIN USE (surgical plane to pass a endotracheal tube)
- Maintenance
- Emergency (supplement inhalation anesthesia if animal rapidly ‘wakes up’)
Routes of administration for injectable drugs
- IM
- Subcutaneous/rectal/oral
- Intraperitoneal
- IV
IM: injectable drugs
- Less precision
- Difficult to titrate effect
- Best for wildlife anesthesia
Subcutaneous/rectal/oral: injectable drugs
- Too slow
- Unreliable
Intraperitoneal: injectable drugs
- Risk of depositing drug in gut
- Laboratory animals
IV: injectable drugs
- Accurate, titratable, rapid-acting
- *act in 20-60s: Rapidly achieves surgical plane (Stage 3)
- Bypasses stage 1 and 2 (excitement) with correct dose
- Requires restraint and ideally an IV catheter
- **PREFERRED route of administration if possible
Pharmacokinetics of IV bolus injection: alpha phase
- DISTRIBUTION phase
- Distribution from blood to vessel-rich tissues
- Heart, brain, lungs, liver, kidneys
- Lipophilic=uptake into CNS is rapid
Pharmacokinetics of IV bolus injection: beta phase
- ELIMINATION phase
- Elimination from central compartment (blood)
- Drug leaves CNS, goes back into blood and animal recovers from anesthetic effects
- Also have some redistribution (more into fat=quicker wake up and then excrete it)
How drug movement influences anesthetic recovery
- IV injection into central compartment (highly vascular organs) then redistribution to less vascular (ex. fat) tissue, move into blood slowly
- metabolized and eliminated
Modern injectable anesthetics that have an anesthetic effect lasting:
- 4-10 mins
- For longer procedures: more injectable drug is given (‘top-ups’) or switch to inhalation drugs
Recovery from injectable bolus
- Initially: REDISTRIBUTION=drug from brain to blood and other body systems
- ‘hangover effect’=rapidly metabolized drugs produce little hang-over
Anesthetic drugs (lipid soluble) metabolism and excretion
- Phase I: oxidation reduction, hydrolysis
- Phase II: conjugation
- *can produce ACTIVE metabolites
Examples of ACTIVE metabolites
- Nordiazepam
- Morphine glucuronide
Drugs metabolized in other sites as well as the liver
- *Propofol (ex. a patient with liver failure): kidney, lungs, guts
- *Remifentanil, Atracurium: plasma
- DURATION of action=short as they don’t need to go to the liver
Constant rate infusion (CRI)
- Not changing rate
Variable rate infusion (VRI)
- Changing rate
IV infusion vs. bolus
- Give bolus and then use IV fusion
- *maintain plane better
- **infusions are nicer than boluses
How do we administer these injectable drugs?
- INDUCE anesthesia
- To MAINTAIN anesthesia
Induction goal
- Achieve stage 3 anesthesia and bypass the excitement phase
Induction
- Consider physical status of patient
- Slow injection OR give 1/3 to ½ calculated dose
o Wait for max effect
o Proceed further with increments until desired effect
Induction in small animals
- Titrate ‘to effect’
- Consider premedication (20-80% dose reduction depending on premedication)
Induction in large animals
- Give whole dose
o SAFETY! (do NOT want a partially anesthetized horse)
Maintenance using injectable drugs options
- TIVA
- PIVA
TIVA: total IV anesthesia
- One or more drugs can be used (multimodal)
- Can be slow to change depth of anesthesia
PIVA: partial IV anesthesia
- Use injectable drugs to supplement inhalation anesthesia
What are the different injectable anesthetics?
- Substituted phenols (Propofol)
- Neurosteroids (alfaxalone CD-RTU)
- Phencyclidine derivative / Aryl-Cyclohexamines (dissociative) (Ketamine)
