Equine Anaesthesia Flashcards
What is the death rate of horses associated with anaesthesia?
1% (1/100) ^ with anaesthetic duration
- IV and inhalation agents equally
Why may some drugs be indicated but not allowed in the horse?
Must be viewed as a food producing animal
What is the better alternative to performing a surgery under GA if poss?
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
Which drugs are used for sedation peri-operatively?
> Phenothiazine (ACP)
> a2 ags (romifidine, xylazine, detomidine)
What are the a2 ags common generic trade names? Which is the most concentrated formula? Which is longest acting?
- xylazine = Rompun
- detomidine = Domosedan (concentrated)
- romifidine = Sedivet (longest acting)
Describe the effects, ROA, CIs of phenothiazines
> 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
Describe the effects, ROA, CIs of a2 ags
> 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
Which a2 ag is not licensed in horses?
Medetomidine but used widely as an infusion during anaesthesia
> given on its own to sanding hroses -> severe ataxia
How may a2 ags be combined? Effects of this?
> 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
What effects and side effects may opioids cause?
- 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
Which sedatives are banned in equine competition?
- all
- except magnesium
Which phenothiazine is incredibly long acting? Indications? Other drugs for this purpose?
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
Why may local limb analgesia not be wanted?
Full weight bearing wanted? May worsen problem..
Are NSAIDs effective in horses?
Excellent - especially as often swelling causes pain esp. legs
> beware high dose flunixin esp. can mask surgical colic
- toxicity manifests as colitis and diarrhoea
Which opioids are licensed?
> 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
Which drugs are used for analgesia peri-operatively?
- local analgesics
- NSAIDs
- opioids
- a2 agonists
- epidural analgesics
What are the 2 main effects of a2 agonists?
- pain relief
- sedation
> useful for anaesthetic protocol and epidurals
Is epidural easy to perform? Side effects? Which drugs commonly used?
- 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
Pre-op considerations and preparation?
- 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
What could go wrong with IV injections?
- subcut injection of irritant drugs
- intracarotide injection if going too deep
- air embolism (esp if catheter facing downwards - intrathoracic neg pressure draws air in)
Standard pre-med for equines?
- ACP: long acintg, reduces anaesthetic death by 50%
- a2 ags/butorphanol close to time of induction
- further analgesia if required
What drugs are commonly used for IV induction?
> ketamine based
- must be deeply sedated first - a2 ags [xylazine, detomidine, romifidine] as ketamine hallucinogenic and convulsant
- sometimes given with benzodiazepine (diazepam/ midazolam)
How quickly does ketamine take effect? why?
- 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
How long does induction with ketamine last? What is recovery from volatile anaesthesia after ketamine induction like?
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
Which drug similar to ketamine may be used for induction/
- Tiletamine-zolazepam combination
> following xylazine sedation - duration of action longer than ketamine
Are hypnotic agents ever used to induce equine anaesthesia? Eg. of drugs?
- 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
What is guiaphenesin?
= 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
What size tubes are appropriate for horses and ponies?
- 25mm for 450-500kg
- 20mm ponies
- 30mm for heavy breeds
Which volatile anaesthetic agents are used in the horse?
> 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
Minimal monitoring required for equine anaesthesia?
> 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
Considerations when positioning horses?
> 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)
Which drugs can be used as rescue drugs if get too light?
- thiopental
- alfaxolone
- propofol
> quick onset)
Intra-operative analgesia options? What is this known as?
> 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
6 main anaesthetic complications in anaesthesia with volatile agents?
- hypotension
- hypercarbia (hypercapnia)
- Hypoxaemia
- Cardiac arrhythmias
- Cardiac arrest
When does hypotension occour and effects of this?
- 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
Tx of hypotension in anaesthesia?
- 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
How can hypercarbia be combatted and what side effects may this have? What level of hypercarbia is actually a problem?
- 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)
How does the horse respond to hypoxaemia under GA? How does IPPV affect this?
- 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!)
Which horses and position are worst for ventilation?
- larger horses
- dorsal recumbence
Tx of hypozaemia due to VQ mismatch?
- ask surgeon to hurry up!
- salbutamol inhaler
- improve CO
- rescue manoeuvre where lungs inflated???
Is hypoxaemia a problem in the horse?
- 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
What cardiac arryhtmias may occour under GA in horses? Prevention or treatment?
> 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
Resuscitation following cardiac arrest
- 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!!*
Which nerves are commonly damaged during surgery and anaesthesia?
- facial n
- radial or obturator paralysis with limb adduction
- spinal damage sporadically (young horses in dorsal usually, not necessarily long surgeries)
What is equine post-anaesthetic myopathy?
- major cause of morbidity
- can result in death
- v perfusion due to hypotension and weight of animal
- likely to occour in any surgery >2hrs
How can risk of myopathy be reduced?
- ^ CO (try to maintain ABP >70mmHg or higher)
- reduce surgical time
Advantages of TIVA?
- BP better maintained (but not necessarily CO)
- potentially v risk cardiac arrest
What are the 3 methods of TIVA?
- 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
Position in recovery dependant on what?
- left lateral if has been in dorsal
- if has been in lateral, leaving on same side reduces hypoxia, changing sides reduces muscle damage
When should endotracheal tube be removed?
- 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
When may airway obstruction occour in recovery? Management?
- swelling of nasal mms. if head not elevated during surgery
- tx: pass small nasal tube or phenylephrine intra-nasal spray
What may happen to oxygen levels in recovery?
Severely decreased as no longer on 100% oxygen
- can administer o2 by nasal tube
How may speed of recovery be altered?
- more sedative can be given (eg. xylazine
How can horse be encouraged to get up in recovery?
Pinch rectum!
What is imobilon?
Etorphine (used for elephant darting!) + ACP
- antagonise opioid at end of procedure
> v. low therapeutic window
> not advocated
