Clinical Equine Anaesthesia Flashcards
What are anatomical particularities of horses that must be considered during anaesthesia?
- obligatory nasal breathers (long soft palate)
- diaphragm inclined & arched so when animal is in dorsal, all weight including of the GIT is on the diaphragm, compressing lungs
Normal values of importance in horses
- HR: 35-45 bpm
- SAP 90-130
- DAP 60-90
- MAP 70-110
- CRT 1-2 sec
- RR 8-15
- TV: 10-15 ml/kg
- minute ventilation: 80-225 ml/kg/min
- O2 consumption: 5 ml/kg/min
Pre-anaesthetic exam in horses
- accurate p hx: prev/current dz, medications, exercise intolerance, lack of appetite, anaesthetic experiences
- physical exam: BCS, temperament, TPR
- CVS: Pulse rate/qlty/abnorm, mm colour, CRT, skin turgor, jugular vein hydration status (refill), heart murmurs
- Resp system: RR, resp pattern, resp effort, lung sounds, temp
Pre-operative assessment
- resp system: auscultate both sides, auscultate base of trachea, nasal integrity? nasal d/c?
- COPD - check environment/mgmt
Lab tests pre-anaesthesia
- haematology
- total plasma protein
- blood chemistry
- blood gas
- to correct blood chem/gas: check electrolyte disturbances, check pH, PaO2, abnormalities
Minimum blood tests in young adult, clinically healthy P
HCT, TP, fibrinogen
Minimum blood tests in clinically sick P
- HCT, TP, fibrinogen
- CBC
- organ parameters according to suspicion
- serum amyloid A
- electrolytes
- blood gas analysis
too long of starvation increases…
the incidence of post-op colic
Pre-op preparation
- groom/clean
- tie up/bandage tail
- clean Sx site
- pick out feet, remove shoes/clean
- rinse mouth, esp for intubation
- prophylactic txt for tetanus
- AB txt if needed
- place IVC
- determine the weight of the horse
What are possible complications of IVC in horses
- air embolism: anxiety, ataxia, cyanosis, collapse, loss of consciousness
- infection or thrombophlebitis
- carotid artery venipuncture
Procedures done with Sedation & standing
W/ or w/o local anaesthesia
* Diagnostic
* dental procedures
* orthopaedic procedures
* laparoscopy
* ophthalmologic procedures
sedation & standing procedure protocols depend on
procedure
temperament of horse
Sedation & standing procedures, the P should be…
- immobile
- unresponsive to manipulation (adeq analgesia, local anaesthesia technique)
Chemical restraint for sedation & standing procedures
- phenothiazines
- A-2 agonists
- opioids
- benzodiazepines
Standing sedation
Effect on P, requirements, other considerations
- Less CV depression
- Minimal resp impairment
- No recovery phase
- Cheaper?
- Less staff req’d
GA
When is it used? Risks?
- Allows longer & more complicated procedures
- Uncooperative P’s
- Procedures in dorsal recumbency
- Less stressful for horse? Analgesia, Unconsciousness
- Higher mortality &↑’d risk of post-op complications
Advantages of pre-medication
- Reduces apprehension and fear
- Helps in catheter placement
- Sparing effect – less inhalant anaesthesia
- Contributes to a calm induction
- Contributes to a calm recovery
The sedated horse has what signs
- dropped head
- eyes partially closed
- ears, lower eyelid & lower lip droop
- ataxia possible
- check rxns to noises
Acepromazine in horses
Effect, Duration, Side Effects, Avoid in, Notes
- Effect: calming effect for minor procedure, No analgesia
- Duration: up to 6 hrs
- Side Effects: Penile prolapse or priapism
- Note: increasing dose only increases length of sedative effect or incidence of side effects, not the sedation
- AVOID IN: anaemia, coagulation disorders, hypovolaemia, young foals, stallions?
A-2 agonists in horses
Effect, Additive to, Procedure
- sedation, analgesia, muscle relaxation
- additive to Acepromazine
- CV, resp, metabolic effects
- standing procedures: more profound sedation than acepromazine
Xylazine
Onset & Duration of Action
Sedation w/i 5-10 mins, lasts up to 30-45 mins
Detomidine
Peak Sedation, Lasts?, AE
Peak sedation in 5 mins, lasts 1-1.5 hr
Higher ataxia, may avoid b/c of this
Romifidine
onset, duration, Effects
- Onset: 15 min following IV admin
- Duration: up to 2 hr
- Longer-acting
- Effects: less severe ataxia, less lowering of head
Ways of administration of alpha-2 agonists?
- single bolus for short procedures
- intermittent dosing
- as CRI
- level of sedation
- alone or in combo w/ opioid
Analgesics
NSAIDs, Alpha-2s
- NSAID: flunixin meglumine, phenylbutazone
- Alpha-2 agonists: Detomidine, Romifidine, Xylazine
Opioids (Morphine, buprenorphine, butorphanol)
Response is…, Mix w/, Morphine note, Effects if unsedated/pain-free
- Response is: dose-dependent & opioid-dependent
- Mixed w/: alpha2, acepromazine
- Note on Morphine: give A-2 first, then morphine
- commonly observed effects if unsedated/pain-free: muzzle twitching, muscular spasms, shifting of limbs, vocalizing, ataxia, head pressing, reduced gut motility, dose-dependent resp depression
Benzodiazepines (Midazolam, Diazepam)
Receptor binding, effects, stage of life to use, AEs
- Receptor Binding: inhibitory GABA receptors
- Effects: anxiolytic, muscle relaxant, anticonvulsive
- combined w/ ketamine: dose-dependent resp depression
- enhance sedative effect of other anaesthetic drugs
- Stage of Life: FOALS ONLY
- AE: pronounced ataxia, esp adults; muscle relaxation up to recumbency
Aims of Induction
- Perform in quiet environment
- Controlled, slow, gentle induction & intubation
Induction options for the horse
Positioning/stabilisation of the horse
free fall, against a wall, swing gate, tilt table, sling
Ketamine Induction
- Main drug used for induction of GA
- Lateral recumbency in 1-3 min
- Effects: poor muscle relaxation,
- Combined w/: (muscle relaxants), benzodiazepines, guaifenesin
Guaifenesin induction
- req’s large vol
- give when ataxic, give induction, then keep giving until full dose given
- no analgesia, no reversal
Thiopental induction
- careful w/ perivascular injection
- rough induction/recovery
- need good premed
- quick-acting
Tracheal intubation
- usually performed blind
- oral or nasally
- gag helpful
Nasotracheal intubation
- Procedures in mouth: dental, mandibular fractures
- ETT not > 20 mm ID
- ETT should be directed over the ventral meatus into the trachea
Field anaesthesia:
- monitor: HR, RR, breathing pattern, eye reflexes/nystagmus
- Time limited: 60 min
- combo of ketamine & A-2 agonist induction
- Bolus: Butorphanol or guaifenesin to enhance analgesia &/or muscle relaxation or benzodiazepines
- CRI: triple drip or benzodrip
inhalation anaesthesia
Isoflurane, sevoflurane
Anaesthetic-related complications
- disturbances of cardiopulmonary fxns
- muscle or nerve damage
- complications during recovery phase
Disturbances of cardiopulmonary fxns
- inhalants cause: vasodilation, myocardial depression & decrease HR & CO
- hypotension causes: BP/perfusion problems in periphery
- Myopathy
Disturbances of the resp fxns
- Hypoxaemia
- hypoventilation
- atelectasis in lungs
- alveolar dead space
- V/Q mismatch
V/Q mismatch in dependent and non-dependent areas
- dependent areas: poor ventilation, good perfusion, low V/Q ratio, hypoxaemia
- non-dependent: proper ventilation, poor perfusion, high V/Q ratio, hypercapnoea, alveolar dead space
Muscle & nerve damage
- pay attention to positioning of the animal
- myopathy: hypoperfusion, risk factors incl duration of anaesthesia, intraop hypotension
Ideal recovery situation
- quiet
- keep airways patent
- non-slippery floor, padding
- sufficient analgesia
- assisted or not (ropes, sling, swimming pool)
Grading Recovery Quality
1 → Standing on first attempt, minimal ataxia
2 → one or two attempts to stand, horse calm, slight ataxia
3 → more than three attempts to rise, horse calm but moderate ataxia
4 → multiple attempts to stand, stumbling, ataxic, mild excitation
5 → as for 4, but extreme excitement, horse injures itself, possibly fatally
Problems in recovery
- apnoea
- hypoventilation
- respiratory obstruction
- excitement/dysphoria
- nerve injury
- fractures/dislocations/abrasions
- corneal ulcers
- poor qlty recoveries
- death
