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Anesthesia > Injectable anesthetics and anesthesia induction > Flashcards

Injectable anesthetics and anesthesia induction Flashcards

1
Q

injectable anesthetic mechanism

A

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 (brain) for a titrable effect

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2
Q

Properties of an ideal injectable anesthetic agent

A

-Rapid onset
-smooth induction and recovery of anesthesia
- non-irritant to tissues
-good bioavailability
-short duration of action
-non-cumulative
-rapid metabolism
-no toxic or active metabolites
-does not cause histamine release
-min cardio and respt side effects
-produce some muscle relaxation and analgesia
-stable in storage and solution
-miscible with other agents
-inexpensive
-high therapeutic index (SAFE)

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3
Q

Advantages of injectable drugs

A

-little equipment needed
-easy to administer
-induction of anesthesia can be rapid and smooth
-possible relatively cheap
-no environmental pollution

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4
Q

Disadvantages of injectable drugs

A

-once give, retrieval is impossible
-patient weight needs to be accurate to calculate dose
-when used alone, need high concentration to have CNS depression
-can have profound cardio and resp depression
-not well tolerated by debilitated, hypovolemic, endotoxemic patients OR renal and hepatic disease patients
-potential human abuse link
-risk of inadvertent self administration

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5
Q

When do you use injectable anesthetics?

A

-sedation
-induction
-maintenance
-emergency

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6
Q

Sedation injectable anesthetics

A

-low doses (sub-anesthetic) can result in profound and reliable sedation
Eg. Ketamine

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7
Q

Induction injectable anesthetics

A

**Main use
-need to be at surgical plane to pass an endotracheal tube into trachea

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8
Q

Maintenance injectable anesthetics

A

-using same and/or combination of drugs
-top ups
-TIVA/PIVA

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9
Q

Emergency injectable anesthetics

A

To supplement inhalational anesthesia if animal rapidly wakes up

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10
Q

Routes of administration of injectable anesthetics

A

-IM
-SQ
-Rectal
-Oral
-Intraperitoneal
-IV

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11
Q

Intramuscular administration route

A

-less precision
-difficult to titrate effect
-best for wildlife anesthesia

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12
Q

SQ/Rectal/oral route of administration

A

-too slow
-unreliable

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13
Q

Intraperitoneal routes of administration

A

-risk of depositing drug in gut
-lab animals

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14
Q

IV route of administration

A

-preferred route!
-accurate, titratable, rapid acting (20-60 secs)
-rapidly achieves surgical plane of anesthesia (stage 3)… bypasses stage 1 and stage 2
-needs restraint and ideally IV catheter

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15
Q

Pharmacokinetics of IV bolus injection

A
  1. alpha phase- lipophilic; distribution from blood to vessel rich tissues (brain, heart, lungs, liver, kidneys)
  2. Beta phase- elimination from central compartment (blood). Drug leaves the CNS, goes back into blood and animal recovers from anesthetic effects
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16
Q

How drug movement influences anesthetic recovery?

A

-Drugs moves from central compartment (blood), to peripheral compartment 1 (distribution to brain), and to peripheral compartment 2 (redistribution to less vascular muscle and fat)

-metabolism allows for elimination of the drug in urine and bile

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17
Q

Tissue drug concentration over time

A

-modern injectables last 4-10mins

> longer procedures: use top ups or switch to inhalational drugs

> recovery: redistribution occurs: drug from brain to blood to other body systems before metabolism and elimination
*hangover effect (occurs less for rapidly metabolized drugs)

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18
Q

Drug metabolism and excretion

A

Lipid soluble drugs must be converted to water soluble compounds for efficient excretion

Phase I: oxidation, reduction, hydrolysis
Phase 2: conjugation

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19
Q

What drugs can produce active metabolites?

A

-nordiazepam
-morphine glucuronide

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20
Q

What sites are metabolizing drugs?

A

-Kidneys, lungs, gut eg. propofol

-Plasma eg. remifentanil, atracurium

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21
Q

Intermittent bolus vs. IV infusion

A

IV: constant rate of infusion (CRI)
-animal stays in therapeutic range with no pain

Bolus: variable rate infusion (VRI)
-animal comes in and out of pain as plasma concentrations spike and drop

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22
Q

Induction

A

-goal is to reach stage 3 anesthesia and bypass excitement phase (Stage 1 and stage 2)

-titrate to effect in small animals (premedication= 20-80% dose reduction depending on premed) vs. large animals where you give whole dose because want to avoid partial anesthetized horse

-consider physical status of patient

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23
Q

induction injection timing

A

slow injection (60-120 secs) or give 1/3 or 1/2 of calculated dose
-wait for max effect and then proceed further with increments until desired effect

**note large animals, give full dose

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24
Q

Types of anesthetic maintenance

A
  1. TIVA= Total intravenous anesthesia
  2. PIVA= Partial intravenous anesthesia
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25
Q

TIVA

A

-one or more drugs can be used
-can be slow to change depth of anesthesia

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26
Q

PIVA

A

-use injectable drugs to supplement inhalational anesthesia

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27
Q

What are the different injectable anesthetics?

A

1.Substituted phenols eg. propofol

  1. Neurosteroids
    eg. Alfaxalone CD-RTU
  2. Phencyclidine derivative/ aryl-cyclohexamines (dissociative)
    eg. ketamine
28
Q

Propofol

A

-no analgesia
-uses TIVA (minimal accumulation)
-can be painful upon injection
-no tissue damage if injected perivascularly

29
Q

Propofol (alkyl phenol) mechanism

A

-Acts on GABA A receptors in CNS to produce anesthesia

30
Q

Propofol timing

A

Induction: smooth, rapid, 40-90 secs, one dose lasts 5-10mins

Duration:short, because redistribution to extra-hepatic sites of action (kidneys, lungs, GI tract)

Recovery: fast and smooth

31
Q

Propofol as a lipid emulsion

A

will grow bacteria and needs to be discarded 24 hrs after opening

*Propoclear= lipid free formula

*Propoflow 28= contains preservatives and labelled for use up to 28 days after opening

32
Q

Propofol impact on cardiovascular system

A

-dose dependent
-myocardial depression
-venodiltation= BP drops
-resets baroreceptor reflex (increase in HR with a drop in BR does not occur!)

**avoid in use of hypovolemic animals or patients with cardiac disease

33
Q

Propofol impact on respiratory system

A

-respiratory depression is dose dependent
-post induction apnea and/or cyanosis
-mild bronchodilation

**need to supplement oxygen, pre-oxygenate

34
Q

Dogs on induction with propofol

A

-can result in limb stiffness, paddling movements, opisthotonus, twitching

35
Q

When is propofol important to use?

A

-normal procedures
-C-sections due to rapid redistribution out of blood
-cerebroprotection (reduces CBF and cerebral metabolic rate)
-patients with compromised liver function

36
Q

Propofol in cats

A

-reduced capacity for glucuronide conjugation
-therefore infusion can result in recovery from anesthesia delayed

**repeated administration over several days may result in:
-hemolysis and heinz body formation BUT clinical relevance in question!

37
Q

Alfaxalone CD-RTU (neurosteroid) mechanism

A

-acts on gABA A receptors in CNS

38
Q

Alfaxalone timing

A

Induction: smooth and rapid, 15-45 secs

Duration: short, 5-10 mins; depends on redistribution

Recovery: fast, quality improves with premedication

39
Q

Alfaxalone

A

-TIVA; no accumulation
-no analgesia
-no pain upon injection
-no tissue damage if injected perivascularly
-no muscle relaxation
-similar effects to propofol (longer shelf life, clear solution!)
-can be administered IM

40
Q

What animals in alfaxalone used in?

A

-fish
-reptiles
-reliable in cats IM
-other animals

41
Q

Alfaxalone effects on Cardio system

A

-cardiovascular depression is dose dependent

-hypotension- combo of myocardial depression and some peripheral vasodilation
-compensation via reflex tachycardia (baroreceptor reflex)

42
Q

Alfaxalone effects on respiratory system

A

*dose dependent
post-induction apnea is of concern

43
Q

Co-induction

A

The combination of alfaxalone or propofol with another agent to minimize the amount required
*also helps with decreasing dose dependent cardiovascular and respiratory side effects

44
Q

What are typical agents that are used with alfaxalone or propofol for co-induction?

A

-Benzodiazepines (midazolam or diazepam)
-ketamine

45
Q

Ketamine

A

-A dissociative anesthetic agent (interrupts information reaching higher centers in the brain)

46
Q

Effects of Ketamine

A

different from GABA A agonist drugs
-Cataleptoid state with slow nystagmus
-muscle rigidity with high doses
-Maintains cranial nerve reflexes (gag, swallow, palpebral and central eye in dogs and cats (no ventral medial rotation)

47
Q

Ketamine racemic mixture

A

S (+) and R(-) isomers

S(+) isomer is 2-4 times more potent

48
Q

Ketamine

A

-popular in vet med (high margin of safety)
-used for induction and maintenance and analgesia
-can be administered IM, IV, SQ, TM
-sub-anesthetic doses can be used for reliable sedation
-neither anti or pro convulsant

49
Q

Ketamine timing

A

-Slow onset (30-90 secs)

-Long duration (20-30mins)

50
Q

Ketamine and analgesia

A

-Somatic control is greater than visceral
-can be used in wind up pain management

51
Q

Ketamine mechanisms of action

A

-NMDA receptor antagonist allows for analgesic effects
-CNS voltage dependent Na, K, Ca channels
-depression of CNS Acetylcholine receptors
-some action at GABA A receptors
-depression of nociceptive cells in the dorsal horn of the spinal cord

52
Q

Ketamine in cats with poor renal function

A

-avoid because the drug is excreted as the active metabolite via kidneys

53
Q

Ketamine and muscle rigidity

A

-Ketamine causes muscle rigidity so needs to be combined with muscle relaxant

54
Q

How to avoid ketamine side effects?

A

-use other drugs (alpha 2 agonists, benzodiazepines, acepromazine) to reduce side effects

55
Q

Does ketamine cause emergence delirium?

A

Auditory and visual stimuli disturbed during recovery can cause emergence delirium

56
Q

Ketamine effect on cardiovascular systems

A

Healthy animals:
-indirect mild cardio stimulation
-sympathomimtic effects last 2-15mins
-increase in cardiac work and myocardial oxygen consumption
-avoid in cats with hypertrophic cardiomyopathy

Critically ill patients or catecholamine depleted:
-may see mild direct myocardial depressant effects

57
Q

Ketamine effect on respiratory system

A

-min respiratory depression
-bronchodilation
-laryngeal and pharyngeal reflexes are preserved
-irregular/periodic breathing pattern= apneustic breathing

58
Q

Ketamine behavioural side effects

A

-Can cause rough recoveries (head shaking, vocalization, dysphoria, salivation)

59
Q

Major goals of combining drugs with ketamine

A

-decrease the amount of ketamine needed
-eliminate unwanted side effects and improve recovery quality
-produce good skeletal muscle relaxation
-improve visceral analgesia
-prolong period of anesthesia/immobilization

60
Q

Ketamine/Diazepam (“Ket-Val”)

A

Used as an IV induction in dogs, cats, neonatal foals, horses, calves, cattle
>minimal cardio and respiratory effects (induction apnea possible)

**not to be used for C-sections because diazepam accumulates in neonates

61
Q

Ketamine/Diazepam (“Ket-Val”) timing

A

Slow onset (30-90secs)

Short acting, rapid recovery

62
Q

alpha 2 agonists + Ketamine

A

-xylazine + ketamine

-Dexmedetomidine- Ketamine- Opioid

63
Q

Xylazine +ketamine

A

-good combination in large animals (horse, cattle)
-reliable for wildlife immobilization (IM)
-Results analgesia, muscle relaxation, and narcosis.
-potent cardiopulmonary depression
-not recommended for routine use in cats and dogs

64
Q

Dexmedetomidine- Ketamine - opioid

A

“Kitty-Magic”

-wildlife, game ranch animals
-supportive care needed (provide oxygen)
-monitor closely
-alpha2 reversible (Atipamezole)

65
Q

When does Induction Apnea occur?

A

-usually observed after rapid bolus of IV induction agent, and typically resolves after the IV agent is redistributed (animal wakes uo)

66
Q

What happens during induction apnea?

A

Apnea or hypoventilation results in poor uptake of maintenance inhalant anesthetic AND poor transition to maintenance phase (stable plane) of inhalant anesthesia

67
Q

Ventilation during induction apnea

A

-Need to assist ventilation until spontaneous ventilation returns

-when induction agent wears off, need to ensure that the inhalant plasma concentration should be enough to keep patient anesthetized

Anesthesia (24 decks)

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