Small Animal Anesthesia Flashcards
4 phases of general anesthesia. what are they, and which are desired?
- induction
- excitement
- surgical plane/ maintenance
- overdose
> only 2 are desired!
what happens during the induction phase of general anesthesia?
¡ Administration of induction anesthetic agent and loss of consciousness
¡ Goes from sedation/analgesia with awareness from premedication agents to amnesia and unconsciousness
> Hearing remains initially
> Lose hearing and consciousness with some muscle relaxation
¡ Patient is endotracheally intubated in this stage
> Once appropriate signs achieved
> ET tube for airway protection, ventilation and maintenance delivery
what is the excitement phase of general anesthesia? what happens? what will we see?
a potential phase that we want to avoid
¡ Loss of consciousness but marked excitement occurs
> Rough induction, resistance, unable to intubate
¡ Will see vomiting, dilated pupil, tachycardia, irregular respiration, spastic movements, not deep enough
¡ Vomiting potential and with unprotected airway could result in
significant consequences
¡ Not as common with newer induction agents and previous sedation
> With proper dosing and sedation
what factors can contribute to creating a bad excitement phase?
¡ Inhalant induction (mask/tank)
¡ Barbiturates induction
¡ Inadequate dosing or poor sedation/excited animal
¡ Loud environment
what occurs during the maintenance or surgical anesthesia phase?
- patient is unconscious
-muscle relaxation present
-periodic ocular rotation for the species
what are the 3 levels of depth of the maintenance phase of anesthesia?
light, medium, deep
what is the overdose phase of anesthesia? what occurs?
too deep
-significant hypoventilation to apnea
-significant reduction in CV function
>progress to arrest if not corrected
=> greater potential in critical ASA 4-5 patients
how can we avoid the overdose stage?
monitoring and patient stabilization
what will offset the decrease in resp and heart rate during the maintenance phase of anesthesia?
surgery
what is the response to surgery over the course of the 4 phases of anesthesia?
induction: +++
excitement : moving/rigid
maintenance:
>light +/-
>medium -
>deep -
overdose: N/A, ventilatory and cardiac arrest
what is the jaw tone over the course of the 4 phases of anesthesia?
induction: +
excitement : +++
maintenance:
>light +
>medium -
>deep -
overdose: N/A, ventilatory and cardiac arrest
what is the palpebral reflex over the course of the 4 phases of anesthesia?
induction: +
excitement : ++
maintenance:
>light +
>medium -
>deep -no eyelid tone; eye more central and dilated
overdose: N/A, ventilatory and cardiac arrest
???????what is the eye position over the course of the 4 phases of anesthesia????? not sure about this one
induction: central/up to ventromedial
excitement : nystagmus
maintenance:
>light: medial
>medium: lateral
>deep: central, big pupil
overdose: N/A, ventilatory and cardiac arrest
what is the RR and HR over the course of the 4 phases of anesthesia?
induction: depends on pre-med and induction agent
excitement : up
maintenance:
>light: -
>medium: - -
>deep: - - -; slow, shallow to apnea, decreased BP too
overdose: N/A, ventilatory and cardiac arrest
why should we fast small animals for anesthesia?
Most anesthetics reduce tone of Lower Esophageal Sphincter
>gastro-esoph-reflux (GER)
don’t want aspiration pneumonia, which has a high mortality rate
what factors impact gastro-esoph-reflux (GER)?
age - older increases GER
abdominal surgeries increase GER
> patient positioning in dorsal or lateral does not
fasting guidelines for small animal anesthesia
general guideline is 8 hours in dogs and cats
-food 6-12 hours
-water 0-2 hours (more important for big dogs who may try to gorge on water)
what happens if an animal is fasted for too long before anesthesia?
¡ Increase incidence of reflux and acidity has been shown in dogs
fasting guidelines for neonates/pediatric
<12 weeks old do not take away food > 1-2 hrs
fasting guidelines for toy breeds
< 2 kg do not take away food >3-4 hours
* Even if adult age
pre-anesthetic conditions requiring stabilization
-Significant dehydration (>5%)
-Blood loss
-Acidemia - pH < 7.2
-Hypokalemia (<2.5 mmol/L)
-Hyperkalemia (>6 mmol/L)
-Significant pleural disease
-Oliguria;anuria
-Congestive heart failure, Arrhythmias
-Seizures, High ICP
-Diabetes, Hyper/Hypothyroidism, Hypo/hyper Adrenocorticism
why should we stabilize a patient before anesthetic if they have certain conditions?
-required to reduce chance of arrest or significant morbidity
* Fluids, blood work, correction of electrolytes, medications
ways we can monitor a patient as they are stabilizing pre-anesthetic
- CRT, HR, BP, bloodwork
- correct deficits as much as possible
what can we do that will impact the initial dose of injectable anesthetic required?
Level of sedation achieved with pre-medication
>leads to injectable dose reduction
what drugs can reduce injectable anesthetic dose required and by how much?
- Alpha2-agonists – 60-75%
what drugs can reduce inhalant anesthetic dose required and by how much?
(MAC reduction)
- Alpha2-agonists – 50%
- Acepromazine – 20-30%
- Pure-MU opioid agonists – 50%
> Butorphanol – 0-8% ; Buprenorphine – 9% - Ketamine (IM or CRI) – 25%
5 main induction agent categories for SA
- Propofol
- Alfaxalone
- Ketamine:Benzodiazepine
¡ Midazolam or diazepam - Mask/Tank Inhalant
- Opioid and Benzodiazepine
¡ In very critical cases
> ASA 4-5
¡ Will not work in healthy patient
¡ Barbiturates - not as common now
¡ Etomidate; Telazol - not available in Canada
propofol advantages as induction agent
¡Sedation achieved at low doses
¡Titration to effect possible**
> Excitement phase not as common due to this advantage
¡Rapid onset /short duration
¡ decreased CMRO2 (cerebral metabolic oxygen consumption)
>can be used to treat seizures
¡Can be given as constant rate infusion (CRI) – TIVA
>to maintain general anesthesia
¡Non-irritant if injected perivascular
¡Cardio-respiratory effects minimized with clinical doses in stabilized patients
¡Can be used in liver disease patients
¡Can be used in neonates
>Propofol clearance exceeds hepatic blood flow
¡Can be used in renal disease patients
¡Can be used in C-section cases
propofol disadvantages as induction agent
¡ Potential negative CV effects of lowered BP, HR and cardiac output in unstabilized or very critical patients
¡ Apnea, reduced minute ventilation and PaO2
>with high doses and rapid admin
>when oxygen not available
¡ Paddling and rigidity can be seen with induction
>cholinergic effect
¡ Heinz body formation with repeated daily use in cats
¡ Pain, irritating with injection
¡ Large volumes required in larger animals
¡ Have to discard/waste unused volumes after 6 hours
Ketamine & Diazepam or Midazolam Advantages as induction agent
¡Titration to effect is possible
>not as fast acting as propofol or alfaxalone
¡Lowered doses can be used for sedative effect
¡Sympathomimetic effects of ketamine (in some cases)
> Which maintains or increases HR, BP and CO
Ketamine & Diazepam or Midazolam Disadvantages as an induction agent
¡ Sympathomimetic effects of ketamine
>Possible increase in HR may not be ideal in cases already tachycardic or with certain cardiac diseases
¡ Salivation potential
¡ Increases CMRO2 (cerebral metabolic oxygen consumption)
¡ A sick patient without remaining sympathetic stores will have
myocardial depression and reduced CO from the ketamine
>Noted in sick compromised animals – not healthy
¡ Both Ketamine and Benzodiazepines are scheduled drugs
Alfaxalone Advantages as an induction agent
¡Titration to effect is possible IV
>Similar to propofol – large advantage
¡Rapid onset /Short duration
¡Cardio-respiratory effects minimized with clinical doses in stabilized patients
¡Non-irritant if injected perivascular
¡Can be administered IM
¡No significant effects on hepatic or renal function
>Used in patients with disease – less information
¡Can be used in C-section cases
¡(Minimal information on CMRO2)
¡Can be given as constant rate infusion (CRI) – TIVA
>To maintain general anesthesia
>Still optimal recovery times
Alfaxalone Disadvantages as an induction agent
¡Excitement in recovery without premedication
¡Potential negative CV effects of lowered BP, HR and cardiac output
>Unstabilized or very critical patients
¡Apnea, reduced minute ventilation and PaO2
> With high doses and rapid administration
> When oxygen not available
what are do-induction agents and what are the most common ones?? what are they used with? what do they do?
-Used with propofol, or alfaxalone
>Benzodiazepines most common (Midazolam or Diazepam)
>lidocaine or ketamine can also be used
¡ Goal is to reduce dose and volume of and potentially the negative cardiorespiratory effects of propofol and or alfaxalone
¡ Co-induction does smooth the induction process allowing ET intubation
> Minimize cough
¡ Promotes smooth transition from stage 1-3 GA
when do we administer a co-induction agent?
Given right before or after initial IV bolus of primary induction agent (propofol, alfaxalone)
why is pre-medication recommended even if face mask is used for unduction
smoothes process of induction
reduces stress
Reduce duration of phase 2 excitement
Isoflurane and Sevoflurane for Induction - advantages
÷Administered with oxygen through mask or in tank
÷Pre-oxygenation…..
÷IV access not required (??)
§ Minimal metabolism by liver or kidneys
Isoflurane and Sevoflurane for Induction - disadvantages
§ Dose rises quickly
§ Titration to effect NOT possible
§ Stressful and with Excitement phase
§ No IV access
§ No airway support or protection as go through excitement phase
§ Require the use of costly anesthetic machine, vaporizer, breathing system
§ Must have equipment knowledge to be able to use safely
* Health and Safety of Staff
* Scavenging required to prevent pollution
§ Dose dependent cardiovascular and respiratory depression
÷ Significant
÷Requires careful monitoring during induction***
Induction Agents for the Very Sick Patient? why?
Opioid /Benzodiazepine Combinations
- On their own to enable ET intubation
- Most Cardiovascular Sparing in Compromised Dog
÷Will not work in healthy dog or cat
÷Does not allow for ET intubation even in critical CATS
Opioid /Benzodiazepine Combinations as induction agent drawback for critical cats
÷Does not allow for ET intubation even in critical CATS
eye position, jaw tone, resp signs after using Opioid /Benzodiazepine Combinations for induction
÷Eye does not rotate ventrally; more jaw tone; animal panting
Thiopental - what kind of drug is this
barbiturate
thiopental advantages for induction
cheap, short onset
thiopental disadvantages for induction
¡–ve CV effects-reduced BP, CO, arrhythmias likely
> Bigeminal rhythm – normal complex, then PVC
¡Respiratory effects-apnea, hypoventilation
¡Excitement on induction possible
¡ Irritating if given perivascular = tissue slough
¡Scheduled - records required
¡Currently low availability
¡Prolonged recovery if repeated doses
¡Prolonged recovery in sight hounds
cardiovascular effects of propofol (HR, CO, contractility, SVR, BP, arythmia potential)
HR: +/-down
CO: down
contractility: down
SVR: very down
BP: very down
Arythmia potential: +
cardiovascular effects of barbiturate (HR, CO, contractility, SVR, BP, arythmia potential)
HR: +/- up
CO: down
contractility: very down
SVR: NC +/- down
BP: down
Arythmia potential: ++++
watch those arythmias!!!
cardiovascular effects of alfaxalone (HR, CO, contractility, SVR, BP, arythmia potential)
HR: NC or up
CO: down
contractility: down
SVR: down
BP: down
Arythmia potential: +
cardiovascular effects of ketamine and diazepam or midazolam (HR, CO, contractility, SVR, BP, arythmia potential)
HR: very up
CO: up, NC, or down (related to symp. tone)
contractility: up, NC, or down (related to symp. tone)
SVR: NC
BP: NC or up, can go down in sick
Arythmia potential: ++ mostly if high rate produced
cardiovascular effects of inhalant inductors (HR, CO, contractility, SVR, BP, arythmia potential)
HR: NC +/- down
CO: super down
contractility: super down
SVR: very down
BP: super down
Arythmia potential: less with iso or sevo, compared to older drugs
respiratory effects of propofol (RR, TV, incidence of apnea, ventilatory pattern)
RR: down
TV: down
incidence of apnea: ++
ventilatory pattern: apnea with fast injection
respiratory effects of barbiturates (RR, TV, incidence of apnea, ventilatory pattern)
RR: super down
TV: down
incidence of apnea: ++
ventilatory pattern: apnea with fast injection
respiratory effects of alfaxalon (RR, TV, incidence of apnea, ventilatory pattern)
RR: down
TV: down
incidence of apnea: ++
ventilatory pattern: apnea with fast injection
respiratory effects of ketamine/BZD (RR, TV, incidence of apnea, ventilatory pattern)
RR: NC or down
TV: NC
incidence of apnea: +
ventilatory pattern: apneustic, irregular
respiratory effects of inhalant induction (RR, TV, incidence of apnea, ventilatory pattern)
RR: super down with high depth apnea
TV: very down
incidence of apnea: ++ increased depth causes apnea which is protective
ventilatory pattern: shallow and poor with increasing depth
steps of induction process
- Assess Sedation level; Attain IV access; Equipment prepared
- Assess cardiorespiratory status
*HR, RR, MM colour
*Critical cases- additional monitors also attached (ECG, BP)
*Decide if pre-oxygenation indicated - Give appropriate initial first “bolus” volume (mls) of injectable anesthetic agent
*Each drug has a dose range with general guidelines
*Assess depth level to proceed to ET intubation - Give appropriate additional incremental IV boluses to intubate, perform ET cuff inflation, transfer to inhalant/maintenance anesthesia and permit positioning
sign required for endotrancheal intubatoin
¡ Relaxation & lowering of head
¡ Eye rotation
¡ Loss of lateral palpebral
¡ Relaxed jaw tone
¡ No tongue movement
¡ No response to handler
¡ No response to laryngoscope placement
Assessing Proper Placement of Endotracheal Tube
-Direct Visualization as you intubate
-See condensation, or feel breath at end of ET tube
-See anesthetic bag movement
>Once attached to circuit
-When you ‘bag’ or breathe for animal
>Chest moves
-Can perform ET cuff inflation
>If you cannot get a seal and still hear air leakage – you are in the esophagus not trachea
considerations for tubing felines
Oral cavity -Overall Small Size
¡ Less ability to open mouth
¡ Tongue and tissue very Friable – be gentle
Larynx is Sensitive – prone to Laryngospasm
¡ Use laryngoscope - DO NOT touch epiglottis
¡ Lidocaine spray used before attempt
advantages to isofluoranve and sevofluorane for maintenance phase
- Added patient safety
> Administered with oxygen through endotracheal tube (ET)
> May be delivered with mask (but less safe) - Act rapidly
> Quick changes of anesthetic depth and recovery - Minimal metabolism by liver or kidneys
- Produce less cardiac arrhythmias compared to older inhalants
disadvantages to isofluoranve and sevofluorane for maintenance phase
¡Require the use of costly anesthetic machine, vaporizer, breathing system and ET tubes
÷Must have equipment knowledge to be able to use safely
¡Health and Safety of Staff
÷Scavenging required to prevent pollution
¡Dose dependent cardiovascular and respiratory depression
÷Significant
÷Requires careful monitoring ***
Factors Affecting MAC
¡ MAC determined in healthy with controlled conditions but…. This is NOT the clinical situation
Isoflurane and Sevoflurane decrease MAC Level with:
¡ Pre-medication, intravenous agents, analgesics
¡ High PaCO2 ( if poor ventilation and >90mmHg under GA)
¡ Increasing age – geriatric
¡ Hypothermia
¡ Pregnancy
¡ Concurrent illness
Factors Affecting MAC
¡ MAC determined in healthy with controlled conditions but…. This is NOT the clinical situation
Isoflurane and Sevoflurane decrease MAC Level with:
¡ Pre-medication, intravenous agents, analgesics
¡ High PaCO2 ( if poor ventilation and >90mmHg under GA)
¡ Increasing age – geriatric
¡ Hypothermia
¡ Pregnancy
¡ Concurrent illness
how do we prepare for the recovery phase?
¡ Want quiet, smooth and slow recovery
¡ Prepare for it:
>Do not necessarily turn the animal off anesthetic or wean them down when closing skin
>Keep them at an appropriate depth even with closing
¡ Assess last analgesia given
> Top up dose of opioid; NSAID potential
¡ Decide based on personality of patient if sedatives required
¡ Check airway
¡ Prepare anesthetic machine
indications for extubation in the dog
STRONG medial palpebral & swallow reflex indicate patient ready to extubate
¡ May also see dogs stretch move legs or head
¡ Return of swallow important if you have seen gastro-esophageal reflux
> Or have not seen!
¡ Blowing/yelling in ear; flipping the dog not necessary
Brachycephalic dogs have different criteria for extubation:
¡ Need to be more awake and holding head up especially in cases with extreme upper airway noise
indications for feline extubation
Extubate when a medial palpebral reflex present
§ Look also for ear flick, whisker reflex, tongue curl
§ Extubate earlier than return of swallow reflex, or movement to prevent laryngospasm at recovery
§ Look also for ear flick, whisker reflex
