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Aneasthesia > Equine Anaesthesia > Flashcards

Equine Anaesthesia Flashcards

1
Q

What is the death rate of horses associated with anaesthesia?

A

1% (1/100) ^ with anaesthetic duration

- IV and inhalation agents equally

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2
Q

Why may some drugs be indicated but not allowed in the horse?

A

Must be viewed as a food producing animal

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3
Q

What is the better alternative to performing a surgery under GA if poss?

A

Sedation and local
- but if very excited will not work despite increasing the dose (so don’t increase above recommended dose rates)
> either come back later or go for GA

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4
Q

Which drugs are used for sedation peri-operatively?

A

> Phenothiazine (ACP)

> a2 ags (romifidine, xylazine, detomidine)

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5
Q

What are the a2 ags common generic trade names? Which is the most concentrated formula? Which is longest acting?

A
  • xylazine = Rompun
  • detomidine = Domosedan (concentrated)
  • romifidine = Sedivet (longest acting)
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6
Q

Describe the effects, ROA, CIs of phenothiazines

A

> Phenothiazine (ACP)

  • on its own limited sedative, combined with opioids better
  • calms the horse, good for clipping shoeing etc.
  • may be abused in competitions
  • IV/IM/oral/transmucosal
  • Hypotensive so CI (contraindicated) with hypovolaemia (blood loss, toxic colic)
  • beware use in stallions - risj of priapism
  • very long acting (weeks!?)
  • considerably v dose of anaesthetic agents needed
  • stabilise cardiac membrane (v chance of adrenaline-induced arrhythmia, improve recovery)
  • 1/2 anaesthetic death risk
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7
Q

Describe the effects, ROA, CIs of a2 ags

A

> a2 ags (romifidine, xylazine, detomidine)

  • mainstay of equine sedation
  • deep sedatives, dose related in effect
  • HORSE CAN STILL KICK! + subanalgesic dose opioids [usually butorphanol] to v touch response and prevent kicking
  • bradycardia, v CO, biphasic BP change (up then back to normal, not hypo)
  • ^ blood glucose
  • ^ urine output
  • gut motility inhibited (more of a problem with long term use)
  • ecbolic esp. xylazine (uterine contractions) so CI in pregnancy
  • IV/IM/detomidine sublingual
  • romifidine less ataxia
  • xylazine 45 mins effectiveness, detomidine 1hr, romifidine several hours
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8
Q

Which a2 ag is not licensed in horses?

A

Medetomidine but used widely as an infusion during anaesthesia
> given on its own to sanding hroses -> severe ataxia

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9
Q

How may a2 ags be combined? Effects of this?

A

> Opioids
- ^ ataxia even with romifidine
ACP low dose
or just used as infusions for maintainence
or as premeds to reduce dose of volatile agents required

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10
Q

What effects and side effects may opioids cause?

A
  • walking or generalised excitement (dose related)
  • low dose (subanalgesic dose) increases depth of sedation of phenothiazines and a2 ags.
  • reduces response to touch
  • ataxia (may fall)
  • gut effects -> impaction colic
  • most side effects seen at supra-therapeutic doses
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11
Q

Which sedatives are banned in equine competition?

A
  • all

- except magnesium

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12
Q

Which phenothiazine is incredibly long acting? Indications? Other drugs for this purpose?

A

Fluphenazine (not seen in UK)
- used for box rest as a2s sedate too much
> Reserpine depletes norepinephrine nerve ednigns, lasts up to 3 weeks, nasty side effects

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13
Q

Why may local limb analgesia not be wanted?

A

Full weight bearing wanted? May worsen problem..

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14
Q

Are NSAIDs effective in horses?

A

Excellent - especially as often swelling causes pain esp. legs
> beware high dose flunixin esp. can mask surgical colic
- toxicity manifests as colitis and diarrhoea

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15
Q

Which opioids are licensed?

A

> Butorphanol- widely used but limited analgesia
Pethidine (merperidine) - different as antispasmodic to gut (good for spasmodic colic) IM only (may cause dramatic anaphylactic reactions IV

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16
Q

Which drugs are used for analgesia peri-operatively?

A
  • local analgesics
  • NSAIDs
  • opioids
  • a2 agonists
  • epidural analgesics
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17
Q

What are the 2 main effects of a2 agonists?

A
  • pain relief
  • sedation
    > useful for anaesthetic protocol and epidurals
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18
Q

Is epidural easy to perform? Side effects? Which drugs commonly used?

A
  • easy to perform
  • ataxia/recumbency (esp local and xylazine)
  • non-surgical analgesia detomidine (quick onset) and morphine (slow onset long action)
  • beware morphine epidurally can cause pruritis
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19
Q

Pre-op considerations and preparation?

A
  • starvation (8-12hours (2-4 probs sufficient))
  • place IV cathetier
  • consider pre-op analgesia
  • take of shoes (to protect induction box!)
  • Abx/anti-tet
  • wash out mouth
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20
Q

What could go wrong with IV injections?

A
  • subcut injection of irritant drugs
  • intracarotide injection if going too deep
  • air embolism (esp if catheter facing downwards - intrathoracic neg pressure draws air in)
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21
Q

Standard pre-med for equines?

A
  • ACP: long acintg, reduces anaesthetic death by 50%
  • a2 ags/butorphanol close to time of induction
  • further analgesia if required
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22
Q

What drugs are commonly used for IV induction?

A

> ketamine based

  • must be deeply sedated first - a2 ags [xylazine, detomidine, romifidine] as ketamine hallucinogenic and convulsant
  • sometimes given with benzodiazepine (diazepam/ midazolam)
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23
Q

How quickly does ketamine take effect? why?

A
  • slow onset PLUS a2 v CO so takes longer to disperse throughout body anyway: 1-3 mins from injection -> recumbence
    > cf. thiopental/propofol/alfaxolone
  • ensure quiet and clam on induction
  • wait 1-2mins after induction before repositioning
24
Q

How long does induction with ketamine last? What is recovery from volatile anaesthesia after ketamine induction like?

A

15 mins, effects disappear abruptly!
> if used for induction of GA with inhalational agent a2 ag used governs ease with which subsequent volatile anaesthesia kept stable
- recovery very rapid

25
Q

Which drug similar to ketamine may be used for induction/

A
  • Tiletamine-zolazepam combination
    > following xylazine sedation
  • duration of action longer than ketamine
26
Q

Are hypnotic agents ever used to induce equine anaesthesia? Eg. of drugs?

A
  • eg. propofol, alfaxolone, thiopental
    > prop and alfax have been successfully used but NOT clinically due to practical disadvantages (large volumes needed quickly, cost)
    > thiopental used to be used, no longer available
  • extremely irritant extravascularly (catheterise)
  • dose depends of premed (usually a2; can use chloral hydrate or guaiphenesin - GG/GGE)
  • blood levels v by redistribution so small doses short acting but large doses cumulative and prolonged sedation, worse recovery
  • reduces dose of volatile agent required
27
Q

What is guiaphenesin?

A

= GG, GGE

  • centrally acting muscle relaxant
  • v long lasting
  • lowers volatile anaesthetic doses needed
  • NOT AN ANAESTHETIC so always used in combo with anaesthetic agent
  • Induction given as an infusion until horse becomes ataxic then induce with ketamine or thiopentone
  • OR mix ketamine/thiopentone into GGE infusion and give to sedated horse until becomes recumbent
  • high doses cause long lasting muscle weakness and poor recovery
28
Q

What size tubes are appropriate for horses and ponies?

A
  • 25mm for 450-500kg
  • 20mm ponies
  • 30mm for heavy breeds
29
Q

Which volatile anaesthetic agents are used in the horse?

A

> isoflurane (only licensed agent)
- onset and recovery rapid
- quality of recovery may be poor esp if katmine induction used
- sedation in recovery used
- respiratory depressant (> halothane) so IPPV ofen used
- hypotension > halothane too, BUT less cardiac depression so CO maintained
halothane
- no longer available in UK or USA]
- good quality recovery
- less respiratory depressant
- v CO
sevoflurane (licensed for other species not horses) and desflurane (not licensed but used successfully)
- fast induction and recovery, ease of stabilising anaesthesia
- esp. desflurane extremely good recovery
Nitous Oxide (rarely used)
- diffuses into gut spaces -> ^ pressure on lungs
- only use if monitoring arterial oxygen tension possible

30
Q

Minimal monitoring required for equine anaesthesia?

A 
> depth of anaesthesia 
- beware dying horse looks like one that is too light!!
- response to surgery 
- CV measurements
- knowledge of inspired (+- end tidal volatile agent) 
> heart rate and rhythm 
> arterial BP 
- cannula in facial a.)
> additional
\+ end tidal CO2 (capnography) \$\$$
\+ pulse ox
\+ arterial blood gas and acid base concentrations
31
Q

Considerations when positioning horses?

A

> lateral
- under foreleg forward and hind limb back
- do NOT pull limbs over abdo (-> upper limb myopathy)
dorsal
- do not lock stifles (femoral nn. damage)
any position
- head raised (prevent nasal oedema)
- do not overstretch (RLN paralysis)

32
Q

Which drugs can be used as rescue drugs if get too light?

A
  • thiopental
  • alfaxolone
  • propofol
    > quick onset)
33
Q

Intra-operative analgesia options? What is this known as?

A

> PIVA (partial intravenous anaesthesia)

  • local anaesthetic (best)
  • NSAIDs post op but NO effect intraoperatively
  • opioids controversial (no evidence of v MAC, may cause stimulation in anaesthetised animal)
  • a2 ags by intermittent injection or infusion (medetomidine often used but ? under cascade, should use xylazine)
  • lidocinae v MAC isoflurane, but combination of CV effects not improved cf. iso alone. Must be stopped 20mins before end of anaesthesia otherwise poor recovery associated.
  • Ketamine most effective and improves CV status but too much worsens recovery
34
Q

6 main anaesthetic complications in anaesthesia with volatile agents?

A
  1. hypotension
  2. hypercarbia (hypercapnia)
  3. Hypoxaemia
  4. Cardiac arrhythmias
  5. Cardiac arrest
35
Q

When does hypotension occour and effects of this?

A
  • minimum BP ~30mins after induction, then gradually improves
    > BUT due to ^ SVR not ^CO -> poor peripheral perfusion contributing to hypoxic muscle damage.
  • Severe hypotension MAP inadequate coronary perfusion and cardiac arrest
36
Q

Tx of hypotension in anaesthesia?

A
  • reduce volatile agents, supplement analgesia with injectables
  • fluids (no evidence for this but probably useful!)
  • ionotropes to improve cardiac output (dobutamine infusion)
    > beware as BP^, HR v via vagal reflex so v debutamine infusion if this happens
    > if premed with anti-cholinergic eg. atropine then dobutamine infusion -> marked tachycardia >80bpm
    > do not use vasoconstrictors eg. phenylephidrine as pressure ^ at expense of peripheral perfusion and v CO
  • ensure NOT hyPOcarbic (if being overventilated IPPV) as HYPERcarbia excellent circulatory stimulant
37
Q

How can hypercarbia be combatted and what side effects may this have? What level of hypercarbia is actually a problem?

A
  • IPPV
  • beware v CO as ^ intrathoracic pressure
  • moderate increases (ie. 50mmHg with norm 40mmHg) stimulate CO but severe increases (75mmHg) cause muscle tremor (horse may appear to be awakening)
38
Q

How does the horse respond to hypoxaemia under GA? How does IPPV affect this?

A
  • hyperventilation when O2 sat v CO2 with O2 also v.
  • IPPV does not significantly improve arterial oxygenation as due to V/Q mismatch -> shunting
  • deoxygenating effects of the shunt worsened by low CO (which is worsened by IPPV!)
39
Q

Which horses and position are worst for ventilation?

A
  • larger horses

- dorsal recumbence

40
Q

Tx of hypozaemia due to VQ mismatch?

A
  • ask surgeon to hurry up!
  • salbutamol inhaler
  • improve CO
  • rescue manoeuvre where lungs inflated???
41
Q

Is hypoxaemia a problem in the horse?

A
  • tolerate hypoxaemia v. well (cf. hypotension, major cause of mortality)
  • oxygen dissociation curve shifted left cf. dogs so haemoglobin fully saturated at lower oxygen tension
42
Q

What cardiac arryhtmias may occour under GA in horses? Prevention or treatment?

A

> cardiac arrest
- with no prior bradycardia as v. sensitive to vagal stimulation when under inhalational anaesthesia (eg. eye, larynx, neck surgery)
- with no vagal stimulation too [detection can be difficult if inadequately monitored as 1* cardiac arrest -> hypoxia (powerful stimulant in horse) -> apparent signs of lightening (hyperventilation, nystagmus, jerking movements)]
- capnograph useful diagnostic tool
- preventative tx = anticholinergic (atropine or hyoscine)
bradycardia for no apparent reason - suspect a2 ags
tachycardia and tachyarrythmias following:
- anti-cholinergic use
- toxic
- short of fluid
- presence of adrenaline (eg. excited induction)
- non-anaesthetic IV agents eg. potentiated sulphonamide (DO NOT GIVE TO GA ANIMALS!)
- Tx = remove the cause, reduce arterial CO2, anti-arrhythmic drugs

43
Q

Resuscitation following cardiac arrest

A
  • switch off volatile agent
  • external cardiac massage (jump on chest)
  • rate as fast as practically possible
  • provide IPPV even if horse appears to be breahthing
  • drugs: epinephrine, atropine to restart heart
  • many can be brought back to life!!*
44
Q

Which nerves are commonly damaged during surgery and anaesthesia?

A
  • facial n
  • radial or obturator paralysis with limb adduction
  • spinal damage sporadically (young horses in dorsal usually, not necessarily long surgeries)
45
Q

What is equine post-anaesthetic myopathy?

A
  • major cause of morbidity
  • can result in death
  • v perfusion due to hypotension and weight of animal
  • likely to occour in any surgery >2hrs
46
Q

How can risk of myopathy be reduced?

A
  • ^ CO (try to maintain ABP >70mmHg or higher)

- reduce surgical time

47
Q

Advantages of TIVA?

A
  • BP better maintained (but not necessarily CO)

- potentially v risk cardiac arrest

48
Q

What are the 3 methods of TIVA?

A
  • short anaesthetics = induction + top up
  • medium duration = infusion of induction agents (a2/ketamine)
  • long duration = currently only experimental as v. expensive
    > propofol + analgesic (a2 ag and/or ketamine)
    > midazolam/ketamine/a2 - reverse midazolam (flumazenil) at end
49
Q

Position in recovery dependant on what?

A
  • left lateral if has been in dorsal

- if has been in lateral, leaving on same side reduces hypoxia, changing sides reduces muscle damage

50
Q

When should endotracheal tube be removed?

A
  • controversial
  • range from taking it out as ssoon as in recovery boxto protect expensive tube to leaving I tin until horse is on its feet
51
Q

When may airway obstruction occour in recovery? Management?

A
  • swelling of nasal mms. if head not elevated during surgery
  • tx: pass small nasal tube or phenylephrine intra-nasal spray
52
Q

What may happen to oxygen levels in recovery?

A

Severely decreased as no longer on 100% oxygen

- can administer o2 by nasal tube

53
Q

How may speed of recovery be altered?

A
  • more sedative can be given (eg. xylazine
54
Q

How can horse be encouraged to get up in recovery?

A

Pinch rectum!

55
Q

What is imobilon?

A

Etorphine (used for elephant darting!) + ACP
- antagonise opioid at end of procedure
> v. low therapeutic window
> not advocated

Aneasthesia (11 decks)

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