3. Injectable Anesthetics Flashcards
What are the 3 sections of the body anesthetics affect
Affects the brain and/or spinal cord and/or peripheral nerves and cause loss of sensation/pain
What are the 3 classes of anesthesia?
local
regional
general - injectable, inhalant
What is general anesthesia?
state of reversible unconsciousness produced by general anesthetic agents
central-acting, coma-like state (absence of pain sensation over entire body and is not aware of stimulus, amnesia) (varying degrees of muscle relaxation, immobility)
Always see changes in CV and pulmonary function
Temporary, so long as drug is active
What are the 4 stages of general anesthesia?
Stage 1 - Voluntary excitement/conscious fear response
Stage 2 - Involuntary (unconscious) excitement
Stage 3 - Unconsciouss (surgical anesthesia)
- Plane 1: light unconsciousness (goal of induction)
- Plane 2: Mod unconsciousness (surgical plane)
- Plane 3: deep uncon. (early OD)
Stage 4 - anesthetic overdose
What happens with Stage 1 anesthesia?
Voluntary excitement; conscious fear response
still conscious, but losing consciousness
Fear, excitement; fight-or-flight
disorientation, struggling, ic HR, inc muscle tension, inc RR, inc BP
urination, defecation, panting and/or breath-holding
What happens with Stage 2 anesthesia?
involuntary unconsciousness excitement
Unconscious fightorflight
loss of voluntary control - twitching, paddling, moaning, tremors, rigid limbs; dilated pupils, irregular breathing
Towards the end of stage 2, muscles relax, dec reflexes, slowing HR/RR
What is the goal of induction?
to get the patient through stages 1 & 2 as quickly as possible to minimize stress and to intubate
Stage 3, plane 1
What happens with stage 3 anesthesia?
unconsciousness, surgical plane of anesthesia
dec cardiopulmon func and dec response to stimulus, dec sympathetic func, breathing is regular, 3 planes of unconsciousness within stage 3
What happens with stage 3 plane 1 anesthetic?
Light surgical plane
light unconsciousness, dec muscle tone; dec reflexes; dec sensation but still react to painful stimulus (HR, RR, BP will inc if painful stimulus applied)
Movement is possible
Goal of induction
Can intubate and prep patient
inadequate for surgery
What happens with stage 3 plane 1 anesthesia?
Moderate unconsciousness, surgical plane
Adequate, ideal degree of unconsciousness for surgery, depressed CV function and resp
Vitals are steady and stable
Minimal muscle tone, no reflexes, no reaction to most painful procedures (if pulling on viscera, ex. ovarian ligament, may still see transient inc in HR, RR, BP)
What happens with stage 3 plane 3 of anesthesia?
Deep unconsciousness (aka early OD)
extensive CNS depression
significant CV, resp depression
NO muscle tone, no reflexes; limp
Early warning patient is no stable - TOO MUCH ANESTHETIC
may need to manually support ventilation; will need to support BP
What is stage 4 anesthesia?
Anesthetic OD
brainstem paralysis, CV, pulmonary collapse (aka shock)
initially will see fight or flick response (dilated pupils, inc HR/RR); rapidly followed by shutdown of CV and resp functions (Rapid dec in HR, RR, BP, MM go white)
Death will ensue in 1-5 mins if you do nothing
Stop anesthetic. Start CPR
What are the 3 parts to performing anesthesia?
Induction, maintenance, recovery
What is induction?
The process where patients move thru stages 1-2 and enters stage 3
Ie. moving from conscious to uncon
Induction should always be rapid as stage 1/2 are not nice for anyone
Induction is typically with a general anesthesia
Injectable GA provides fast induction so stage 1/2 never noticed
Stage 1 can be achieved by OD on a sedative, recall patient is still conscious in stage 1
What is maintenance in regards to anesthesia?
Where patient is consisten at stage 3 - consistent depth of unconsciousness; CNS depression. Consistent CV and resp func
If painful procedures are being performed; then the patients should be in plane 2
most commonly, patients are maintained with an inhalant anesthetics - occasions where maintenance is with an injectable anesthetic
What is recovery in regards to anesthestic?
After anesthetic turned off or no longer being administered, patient stages reverse
like induction, slow recovery can be rough
smooth recoveries are typically fast and controlled
patient returns to state of consciousness - typically, use return of vitals to pre-anesthetic normas and sternal recumbency as end of recovery period
Why is knowing drugs and how to monitor important? What can we prevent by knowing this?
Anesthetic accidents are devastating and can result in permanent injury, death, loss of license, lawsuits
Majority can be prevented by knowing drugs, proper dosing and admin, knowing patient, appropriate monitoring (know normals), accurate communication b/w vet and tech, using highest standards of care
Always keep meticulous records
What are some unique things about injectable general anesthetics?
drugs given by IV route
centrally-acting depressants (all of them) - must reach brain to work properly, designed to cross BBB; usually very lipophilic
Used as part of balanced anesthetic, with or w/o an inhalant anesthetic
require inactivation/elim by liver/kidneys
Why are injectable anesthetic drugs the preferred method of induction
Preferred for all species over gas - more rapid onset of stage 3 and considered standard in all med to lg dogs and LA. Except in small exotics, cats and very sm dogs where inducing inhalation anesthetic may be considered acceptable in specific situations
may be single or combo of drugs
always given IV to effect - except in lg animals where entire calculated dose should be given (a lg animal that is not fully anesthetizes is dangerous)
What are the limitations for injectable anesthetics
- cause CNS, CV and resp depression - always risk of debilitating or fatal OD, must monitor HR, ventilation and BP
- Do not provide (sufficient) analgesia - MUST use w/ analgesics for painful procedures
- Do not provide muscle relaxation - combine w/ muscle relaxants, inhalant anesthetics
- Very low TI; no error for mistakes - must dose accurately for the individual patient
- Cannot be reversed/removed
- Have longer recovery period than inhalants - reqs liver metab and/or elim, have more excitement and/or hallucinations
What are the steps to using injectable anesthetics?
- Always dose for lean body weight
- Always administer “to effect” IV (SA)
- Do not administer too rapidly
- Always use a catheter
- Can use alone for short procedures
- Check and double-check doses
What is the purpose of administering to effect iv (SA?)
calculate & draw up the full dose for the patient
using IV cath, only give 1/4-1/2 of total dose > flush > watch for effect. If more drug needed give 1/4 of total dose > flush > watch for effect, repeat until animal is in stage 3 then stop
Purpose: only give as much drug is req; dec risk of OD
Why do we not want to administer injectable anesthetics too fast?
giving too fast can cause arrhythmias and induction apnea (breath holding)
Wait 15-90 after each increment
What will we always use a catheter with injectable anesthetics?
Can give to effect easier
ensures venous access since drugs will cause some degree of hypotension
Why should we always doublecheck out injectable anesthetic doses?
low TI, cannot reverse
What is total intravenous anesthesia?
TIVA: method where only injectable anesthetic drug(s) are used to maintain general anesthesia for any duration of time but typically for procedures <60 mins
^^ including induction AND maintenance. Ex. no inhalant used
Drug is by IV infusion (CRI)
currently used in equine field work (ex. triple drip)
What are the 5 types of injectable drugs? There are 2 others with more specific uses, what are they?
- Barbiturates - thiopental, pentobarbital
- Dissociative anesthetics - ketamine
- Guaifenesin
- Propofol
- Alfaxalone
Others include 1. synthetic opioids like fentanyl, sufentanil, etorphine (distinct as these provide algesia and are reversible) - wildlife - Etomidate
What are barbituates?
WERE commonly used as gen anes., but newer injectables/inhalants and the loss of thiopental are now only used for specific indications
GABA agonist
Thiopental - no longer available
Phenobarbital - anticonvulsant
Pentobarbital sodium - Humane euth (controlled drug, DO NOT dilute for use as seizure control or anesthesia as it contains toxic preservatives which cause RBC hemolysis), dyes may be added when opened to prevent accidental misuse
What are dissociative anesthetics?
They disrupt nerve transmission responsible for windup pain, and stimulate other areas of the brain
Inhib NMDA receptors in CNS responsible for windup = an exaggerated response to low intensity pain
Provides some somatic analgesia but no visceral analgesia
no reversal
What does dissociative anesthesia create?
A distinctive trancelike state
What is ketamine?
Dissociative anesthetic?
Controlled drug - phencyclidine derivative (PCP, angel dust, Date-rape drug)
Complex mechanism - disrupts nerve transmission in some parts of brain and stims other parts of brain
What are the physiological effects of ketamine
- Immobility - muscle tone is unchanged or rigid
- Dissociative anesthesia - somewhat aware but immobilized
- Temp to long-term amnesia
- Moderate, brief control of somatic pain (pain originating from muscles, skin, bone)
- Less effects on CV and resp func than other anesthetics - Apneustic breathing pattern
- NOTE: flexes remain intact and cats have centrally located, dilated pupils
What are some contraindication of ketamine?
Recovery occurs due to rapid redistribution of drug out of brain. Avoid/use w/ caution if liver function compromised in dogs, kidney dz in others. Prolongs recovery and/or inc toxicity
No reversal
Contraind: seizure, brain trauma, neurotoxins (strychnine, street dogs, insecticides)
Ketamine metabolized by liver in dogs; excreted in active form in all other species
What is different from apneustic breathing pattern?
Apnea: Breath holding
Apneustic breathing pattern: Breath in, pause and then breath out
What are the adverse effects of ketamine?
inc resp to sensory stimuli during recovery
may cause seizures and/or convulsions in dogs if given alone - avoid using along if Hx or risk of seizure
Behavioural change may last for a few days
abnormal nystagmus in cats
Pain if given IM, use IV
What are the indications of ketamine?
chemically restrait-only time used on its own, sprayed orally in difficult cats
induction or general anesthesia when combined w/ other drugs (diazepam)
has fairly high TI compared to other injectable anes.
How is ketamine used?
IV use (only) in all species, IM painful
Give to effect, onset of 30-90s, duration of 10-20 min
Mutiple routes in cats for chemical restraint
IM (painful), PO (tastes horiffic), duration 10 min immobility
Is it ethical to use ketamine alone for surgery? Why or why not?
No, bc they can still feel what you’re doing
No visceral analgesia so 0 pain control
Provides muscle rigidity, not fully unconscious so partially aware
If we’re on the field and anesthetizing an equine, how might ketamine be used?
pre-sedate w/ an A2 agonist + an opiod (often butorphanol)
once sedated induce w/ ketamine alone or ket/val
What is ketval?
mixture of ketamine + diazepam (can be replaced with midazolam in older/sick animals
induction agent, commonly used
induction of dogs, cats, calves, foals and horses
mix in same syringe: with diazepam will NOT precipitate w/ ketamine - only exception currently
Give IV to effect in SA but not Eq; can take up to 1min to see effects but allow for 5-20 mins for GA
What are the advantages of Ketval?
rapid induction
provides true unconsciousness
less CV and resp depression compared to other induction agents
Diazepams provides very good muscle relaxation and loss of reflexes that would be seen w/ ketamine alone
some analgesia (due to ketamine)
BUT, rougher recoveries in cats
What is kitty magic?
Ketamine in combo with dexmedetomidine (xylazine) and an opioid
prods state of sedation to anesthesia depending on dose (neuroleptanalgesia - can be used for minor surgical procedures)
provides supplemental O2 where possible
should always lube eyes to prevent drying of cornea
similar combo can be used in dogs
What is guaifenesin?
not controlled
Only used in LA (for balanced anesthetic)
A muscle relaxant!
On its own, centrally-acting skeletal relaxant at recommended dose
Acts on CNS; blocks motor pathways, exact mechanism of action is unknown, some sedative properties
not anes on its own
What are the clinical indications of guaifensis?
on its own; muscle relaxant, expectorant - loosens airway secretions so easier to cough, very mild sedation
Why might guaifenesin be used as part of a balanced anesthesia in LA?
Inc CNS depressant effects of other premeds/anes drugs - dec dose of other drugs req
Relaxes pharyngeal and laryngeal muscles so easier to intubate
skeletal muscle relaxation during surgery
smooth induction and recovery
What are the adverse effects of guaifenesin?
minimal CV and rep effects on own at therapeutic dose - if 3-4x OD, will cause muscle rigidy and Cardioresp arrest
Perivascular injection will cause pain and tissue damage - always use cath)
inflamm to veins @ Injection site - must flush
possible hemolysis if not diluted
What is triple dip?
Ketamine/xylazine/guaifenesin
In eq/cattle/sheep - significanly lower xylazine dose in cattle/sheep
Indications infusion to achieve heavy sedation, IV induction followed w/ gas anesthetic, TIVA (up to 1hr) for equine field work
Good CV and resp stability compared to other IJ Anes. very smooth induction and recover compared to most gases
What is propofol?
Mechanism of action: GABA receptor agonist (inhibs CNS)
CNS depression, NO analgesia, good muscle relaxation
max duration of action is approximately 10 min depending on dose; can be shorter - considered ultra short acting
Not controlled- however, potential for abuse exists - some clinics keep bottles locked up
no reversal
How do we ID propofol?
Very distinct looking - like milk
the ONLY milk emulsion given IV
How do we store/handle propofol?
emulsion contains soy/egg - ideal medium for bact growth
use aseptic technique when handling bottle
shake b4 use, store btw 4-25C, don’t freeze
Label advises to discard “after procedure” ~6 hrs
longterm storage inc risk of bact contamination therefore patient risk of septicemia
practices typically use for the day or 24hr max and refrig after opening
What is the pharmacokinetics of propofol?
1 admin IV (rapid loss of consciousness)
2. unbound drug is very lipophilic - brain has high volume blood flow and high fat content; so drug distributes to and enters brain rapidly, crosses BBB and prods induction in <30s
drug is only active while in the brain
3. drug leaves brain
4. drug redistributes to fat stores
5. drug moves from fat to liver for metabolism
6. metabolite elimed by kidneys
WHat are the clinical uses of propofol?
commonly used
SA, exotics, neonates of all species (limitation is cost, $$$), also limited by conc/V in Eq
Induction - can give as IV bolus - better to give or titrate to effect (onset 20-40s)
maintenance (TIVA) up to 20 min
anti-convulsanrt (stop seizures)
can give as IV bolus for emerg tx of seizure or CRI for status epilepticus
What are some adverse effects and warnings of propofol?
- CV and resp depression
- Induction apnea causing cyanosis (stop breathing in min’s; occurs if given too fast), offset risk by pre-oxygenating prior to inducing, less likely if giving slowly to effect vx bolus
- Excitement + twitching/tremors/paddling + opisthotonus + abnormal nystagmus (occurs after induction in 12% dogs/cats, can look like seizure but not true one. Likely to occur w/o premed
- Repeated used in cats over several days can cause hemolysis and possible CNS (drug remains in body longer bc of poor glucuronyl transferase metabolism)
- risk of bacterial sepsis if not handled properly
What do we need to know about inducing with propofol?
pre-oxygenate patient due to risk of induction apnea (flow by 100% O for 3m prior to induction)
Give IV to effect
Induction is rapid and smooth
can give repeated small boluses for maintenance - doesn’t accumulate
How do we manage induction apnea with propofol? How is recovery?
Monitor C0/0 LVLs closely
most spontaneously resume breathing but may take 1-2 mins
Provide O (bag patient) IF needed
Recovery is rapid and smooth and most completely recover in 20-30 min
What do we need to keep in mind when using propofol with cesareans?
one of the preferred anesthetic for C-section in dogs. Can use alone IV for rapid induction w/ no other premed
Drug will enter fetus so we must wait 10-20 min after admin before detaching placenta bc the fetal liver is still immature and less able to metabolize drug, allows frug to leave fetus and be metabolized by maternal liver
What is alfaxalone?
ultra short-acting injectable IV anesthetic agent
also a GABA receptor agonist
provides rapid, smooth induction and good muscle relaxation with rapid recovery thru redistribution (also rapid metabolism - slow IV to effect), poor analgesia, non-controlled, no reversal
What makes alfaxalone ideal for infusions?Duration of action?
Lack of accumulation (even in cats) makes it ideal for infusions
dogs and cats. large animal use is limited (mainly due to concentration issues)
duration, 5-10min in dogs, 15-30 in cats
often drug of choice for induction for c-sections (rapid metabolism), very little effect on neonates
What are the major effects and adverse effects of alfaxalone?
dose dependent CNS depression (sedation to general anesthesia)
minimal CV depression - HR may inc
hypotension especially when used w/ inhalant anesthetics
Resp depression including apnea especially after rapid injection or high doses
muscle relaxation
excitement may occur during recover (especially if not good premed or surroundings are noisy
What should we know with using alfaxalone?
labelled for induction and short-term maintenance in D+C
always give IV, to effect, for induction
5-10 min anesthesia in dogs, 15-30 in cats
causes induction apnea - patients should be pre-oxygenated, will cause more resp depression if given too fast IV-give to effect/titrate to effect
Why is alfax different than propofol?
can store multidose vials 28d @room temp after opening, can give IM to cats for sedation
What is unique about alfax sedation in cats
labelled fr IM sedation in cats
used in combo w/ butorphanol
will sting on initial injection
gives approx. 40min of good sedation (variable results) = improved if you add midazolam
have 100% O on hand in case of hypoxia
