Equine anaesthesia and challenges

risk factors of horses for anaesthesia

Increasing ASA grade

Increasing age and foals

Surgery type and position

Duration

Time of day, OOH provision

Agents used (premed/TIVA)

Recovery quality ?

Why do horses die during or after anaesthesia?

Fractures in recovery (highest mortality)

Abdominal problems (colic)

Neuro and spinal cord malacia

Intra operative cardiac arrest

Respiratory obstruction

Post anaesthetic myopathy (PAM)

neuropathies

why si there a risk of fractures in recovery

Increased amt of pressure and strain going through limb and still having drugs on board

how can horses be sedated?

standing of recumbent

how are sedatives asnd anagesics and local anaesthetics given?

in combination with each other

why can horses urinate a lot and what needs to be done

alpha 2 agonists make them make more urine

 urinary catheters to help keep sterile

GA prep the night before

• Admit the night before

• Complete physical exam

  • Murmurs?

• Blood sample

  • isnt routine

• Starve

  • Starve as little as possible (longer starve -> incrs post op ileus -> colic

• Remove shoes

• Ensure horse is clean

  • minimal contam into theatre

what should be done on the morning of surgery

IV cannula- ALWAYS

Flush out mouth

• So no food, intubate blind to don’t want to push food down trachea

Tail bandage/plait

• Cleanliness in theatre. Don’t want to stand on tail in recovery-> scare

Clip if possible

• Do before to reduce time if poss

Weigh

Make an anaesthestic plan

Pre-medicate the horse

• Can settle, examine them, make sure no worries, tick off things on list

• Can tape iv In recovery can hit into themselves so more likely to cause damage. So helps protect horse

Preparation for Anaesthesia

1. prepare monitoring equipement (prime anaesthetic circuit with isoflourane (cap on end))

2. select et tubes

3. change soda lime

4. label and prepare drugs

5. leak test anaesthetic machine and ventilator set tidal volume

6. prepare fluids and table

what type of circuit is used and FGF?

• Rebreathing circuit- CIRCLE

• fgf= Start 6-8L/min

  • After 15min reduce to 2-4L/min

premedication for horses

Acepromazine

alpha 2 agonists

opioids

why is acepromazine used as a premed in horses

•Anxiolytic (reduces anxiety, but not a true sedative)

•Reduces anaesthetic gas requirement (MAC)

•Improved recoveries (CEPEF 4)

Historical reduction in mortality (CEPEF2)

when to use acepromazine?

•IM 40minutes before sedation

•IV 20minutes before sedation

what are risks with acepromazine?

•Penile prolapse

•Hypovolaemia

why do we get hypovolemia with acepromazine

Bc vasodilation

Dcrs bp

Fall over

why do we need to discuss with the owner abt using acepromazine if male

Penile prolapse- if breeding stallion need to talk abt this to owner as can affect fertility with it. Is rare tho.

 

what do alpha 2 agonists do and used for?

•Sedation for induction

•Analgesia

•Vasoconstriction and bradycardia

• So need to wait long enough (abt5mins) before giving ket

chose based on how long you need them to last

why are opiods used?

•Analgesia

•Improved sedation quality

•Respiratory depression

chose based on the level of pain you expect them to be in

when to give pre meds?

•In induction box

Give enough time to take effect!

how do we want the horse to present when they are ready for induction?

head down, wide based stance. Horses are insensible to surroundings - they are ready for induction...... GIVE KETAMINE

head between knees give ketamine

if a horses head isnt between their knees, can we give ketamine

NO
 NEED MORE premed

• Start lower dose and give more till you get this

what are typical equine premeds?

ASA 1&2:

+/- Acepromazine (wait 20-40mins) + alpha 2 agonist +/- opioid

Remember NSAID +/- Antibiotic at this time too

what is used for induction?

• IV ketamine

• A benzodiazepine (midazolam can be combined with the ketamine)

  • Bc midazolam is licensed

• Occasionally guaifenesin IV can be used instead of an alpha 2 agonist

what is guaifenesin

centrally acting m relaxant

give an example of an anaesthetic protocol for horses?

•Acepromazine (sedation) IM, wait approx 30-45 minutes

•Detomidine (alpha 2) IV, wait 5 minutes, adequate sedation must be apparent

•Methadone (opioid) IV, HR>20 bpm, head between knees and non-responsive...

•Midazolam/ketamine IV for induction (or ketamine)

what 3 ways can we get a horse to the ground after induction

• free drop

• squeeze door

• assist → not really done now

how cna we assist a horse down after premed

• someone in box with them

• guide them down with ropes

• not done now

what is a free drop

• horse falls down int he padded room by themselves

• Safe for humans

  But may not be as safe for horses

how is a squeeze door used

• it comes out of wall

• Can trap the horse between padded gate and wall

• Guides the horse down

• So is best for horse and humans

• But not in most places

what are next steps after induction? (4)

Intubate (ET tubes, cuffs checked, gag, lubricant)

• Gag in mouth to keep open

• Cant see where going

• So if cany food -> can be pushed into lungs -> is why we wash mouth

Attach hobbles and move horse

Place on table and connect to anaesthetic machine

Maintain horse in dorsal or lateral recumbency on oxygen in isoflurane

when intubating what does resistance mean

Should be no resistance in trachea

If in oes will get resistance

what to do once horse is on table?

Monitor as usual, place arterial line for ABP, ECG, Et CO2, pulse oximetry

Catheterise (urinary) and bucket

Connect IV fluids, consider PIVA

what reason on recovery is it better to catheterise patient in surgery

• if bladders full more likely to stand up

• Also helps them to lie down for longer in recovery so exhale more of the anaesthetic gas before standing

which v is used for venous cannula and IVFT

jugular v

which a is used for arterial cannula

facial a

what is used for maintenance?

Volatile anaesthetic agent (isoflurane or sevoflurane; only isoflurane licensed, isoflurane has a bigger carbon footprint

what is the problem in recovery that hypotension in surgery can lead to

myopathy

what is the problem with the maintenance used in horses?

§Hypotension!

Need lowest dose that ensures anaesthesia

what is the MAC for isoflurane?

1.3-1.6%

what is the MAC for sevoflurane?

2.3-2.8%

what can we use to guide us to know how much volatile agent to give in surgery

Use end tidal isoflurane concentration and monitoring parameters

what do we monitor on the horse

• palp pulses

• reflexes and eyes

• m tone

what machines can we use to monitor

multiparameter

pulse ox

blood gases

how do horses eyes respond differently

• Can have palp resp

• Track the trends -> didn’t now does this is a difference

• If not had one all time that’s ok

if we get nystagmus what could this mean

• Nystagmus (horizontal) -> either v deep or getting lighter and waking up -> happens just before the animal starts to move on the table

why do we use m tone and not jaw tone, how do we do this

Jaw tone difficult-> do just the m tone

• Palp pectorals or side of neck

• Does it feel more tense

• Muscles tenses just before they start to move

how can TIVA be achieved?

1)Top-up bolus injections (ketamine e.g. 0.5mg/kg)

• shorter

2)Continuous rate infusions ('triple drip' in equine anaesthesia - useful for field anaesthesia)

• for Bit longer procedures (45-hr)

what is included in a triple drip in horse anaesthesia?

guaifenesin, ketamine, detomidine

what does TIVA allow for?

Produces a much diminished anaesthesia stress response compared with inhalation agents and is therefore considered as a physiologically superior method of anaesthesia

what can we use for PIVA

Ketamine infusion

Lidocaine CRI

Alpha 2 agonist infusion

what does piva allow for

• we use MAC reducing drugs so can reduce the inhalation agent needed

Problems during anaesthesia

•Hypotension

•Hypoxaemia

•Hypercapnia

•Bradycardia

•Tachycardia

•Neuropathy

•Eye problems

what can hypotension lead to?

myopathy on recovery

what is used to improve prevention of hypotension?

Use positive inotrope (dobutamine)

Reduce isoflurane

• This is causing the low bp. Get to min level we can

Use PIVA

Increase fluids (if hypovolaemic)

• Improve vasc vol

what MAP is considered hypotensive

<70mmHg using invasive arterial monitoring

what does dobutamine (+ve inotrope) do

Incrs contractiltiy of heart

Incrs co

Incrs bp

why does low bp lead to myopathy

Low bp

Dcrs perfusion to m

Not enough o2

Respire anaerobically

Myopathy bc run out of o2 to function and produce lactate which damages muscles

when do we see Spinal cord malacia

•Becomes apparent in recovery period

• but occurs in the maintenance

what is Spinal cord malacia

Can use fl but have paralysis of hl

Fatal

what horses get Spinal cord malacia more

•Usually heavy breed horses & young

what is cause of Spinal cord malacia

• unknow

• potentially hypotension → but Can still have a good bp and get this

• •More common in dorsal recumbency

  •Not related to duration of anaesthesia

how does hypoxaemia present on machines?

•Shown by low sats on SPO2% and low SaO2 on arterial blood gases

•PaO2 <60mmHg

why does hypoxamia happen?

•O2 failure→ so not getting enough o2

•V/Q mismatch...

The animal is turned on its back, RR is reduced (by drugs), CO is reduced (by drugs) = low O2 delivery

• Parts of lungs ventilated and parts are perfused, don’t match up, poor gas exchange, not enough o2, co2 incrs

• Get resp acidosis too

What is hypoxemia?

Low O2 tension in blood

what to do in case of hypoxamia? (7 steps)

1.Check the SPO2% reading - some pulse oximeter probes don’t work well on horse tongues’, so check it first

2.Take blood gas - if PaO2 is much lower than it should be (remember is should be 4-5 x FIO2%) – act

3.Switch down the isoflurane/sevoflurane

4.Ventilate/IPPV… increase resp/tidal volume

5.Ensure adequate circulating blood volume (IVFT)

6.Tilt table (head up) slightly if possible

7.Give beta agonist (down the ETT) eg salbutamol (is bronchodilator)

and hurry up and finish and get horse back into sternal!

what is hypercapnia?

excess carbon dioxide in the blood

why does hypercapnia happen?

•The animal is turned on its back, RR is reduced (by drugs and position), so CO2 rises in the body

how to reduce co2 in cases of hypercapnia?

•Reduce the volatile agent (isoflurane)

•Instigate ventilation

why do we tolerate a bit of hypercapnia

•Spontaneous breathing in recovery

•Increased oxygen dissociation in tissues

• If slightly acidic, Hb releases o2 better, so get more oxygen to tissues -> reduce myopathy

what to do in cases of bradycardia?

•Consider cause... Volatile agent, vagal tone, opioid, alpha 2, toxaemia, hypoxia - address those first

•THEN, if HR still low use drug.... Hyoscine N-butylbromide (BUSCOPAN!) →incrs hr

•Other drugs include atropine or glycopyrrolate

what do you need to consider with bradycardia

§Breed/fitness of horse and their HR prior to anaesthesia

§If blood pressure or oxygen delivery is compromised too – you need to act

what do you need to consider with tachycardia

Breed/fitness of horse and their HR prior to anaesthesia

Evaluate ABP and reflexes too, especially nystagmus and palpebral reflex, may need to act fast if horse moves

what to do with tachycardia?

Consider the causes... sympathetic stimulation (nociception is most likely), but can be CO2, acid base disturbance, drug reaction, cardiac issue, hypotension...

•Depth not adequate... Administer ketamine (or thiopentone) bolus IV and consider the % volatile agent - may need to increase too (increase FGF too to speed this rate of change up, remember the horse is on a circle)

•More analgesia?

•Check arterial blood gas... hypercapnia?

BP Low.... Treat as for hypotension

why does neuropathy occur?

Poor positioning +/- hypotension

• Pressure placed onto nerve so get problems post op and recovery

when does neuropathy become evident?

when horse wakes up, may cause poor recovery

but happens in the maintenance

what is the most common neuropathy?

Radial nerve, facial nerve (headcollar left on during sx)

what has happened here

• neuropathy

  • Radial n -> dropped elbow

    Cant weight bare

    From being in lateral

what does neuropathy resemble

myopathy → not as painful as myopathy tho

when are eye problems seen?

when horse has been in lateral recumbency

so Eye rubbed against something

how to prevent eye problems?

Must protect lowermost eye with padding

Lubrication for eye at start of surgery

what occurs in the recovery period: part 1

•Volatile agent switched off

•Reconnected to hoist

•Head supported

•Positioned in RLR in recovery box (if dorsal or RLR on table), LLR if LLR on table

• Flipping can make them stop breathing due to the lungs

•Pull dependent limb forwards

• Reduce risk of radial n neuopathy

•Demand valve can be used to stimulate breathing

•Nasal tubes (obligate nasal breather)

•Extubate when swallowing or just before

•Exit recovery box, watch, can recover with ropes outside

• Ropes don’t pull them up, and doesn’t support weight but just gives them some support/ stabiliser

What are the characteristics of an optimum recovery environment?

Quiet, Dark

What should be ensured regarding the bladder in an optimum recovery environment?

Empty bladder

What is important for pain management in the recovery environment?

Analgesia

What should be allowed time for in the recovery environment?

Anaesthetic drug elimination

What type of sedation can be used in recovery?

Low dose alpha 2 agonists or acepromazine

When should romifidine be administered in recovery?

Once in recovery and breathing spontaneously

When should xylazine be administered in recovery?

Once signs of reduced anaesthetic depth occur

what are the optimal timings in recovery

•Lateral: 30 minutes

•Sternal: 10-15minutes

•Standing: 15 minutes then taken back to stable

what are some complications in recovery?

UAO, Laryngeal obstruction, nasal oedema

what are signs of upper airway obstruction

Stridor or stertor following tracheal extubation

Nostril flaring on inspiration

Abdominal respiratory effort

Exaggerated thoracic excursion

Absence of airflow at the nostrils

where can upper airway obstruction occur

Obstruction tends to occur within the nasal passages or at the level of the larynx

what can cause laryngeal obstruction

Dorsal displacement of soft palate

Epiglottic retroversion

what can be done to reduce laryngeal obstruction

Select appropriate ET tube size and insert gently

Following laryngeal surgery may leave ET tube in place for recovery (secured)

what can be done if laryngeal obstruction occurs

would need to pass another endotracheal tube to fix this

or

Emergency tracheostomy

how can we reduce nasal oedema

Common usually resolves as horse stands

Use nasopharyngeal tubes, or phenylephrine or both

Can recover horse to standing with ET tube in place

what has happened here

very occasionally they can throw their heads around with the nasal tubes in place and you can get bleeding

but as long as that horse can breathe, that's fine, the bleeding will stop