Equine anaesthesia, sedation Flashcards
What to give for a Premed in horses? What if a colic?
ACP (40 mins before start prepping horse)
- still use in breeding stallions, just warn owner of risk
- don’t use in colics as causes hypotension and likely already hypovolaemic (and lasts long time and not reversible)
Romifidine when prepping (e.g in stocks)
IM detomidine at same time as ACP if very difficult horse
(A2s not ideal for colics as reduce CO by 40-50% but still use as need good sedation - use xylazine as shorter acting so shortest negative CV effects)
NSAIDs or opioids
Which a2 agonist is licensed for IM injection?
Detomidine only
What to pre-med and induce a foal with if suspected porto-systemic shunt?
Just morphine pre-med
Induce with alfaxalone
Can’t use ACP as affects blood flow to liver
Benzodiazpeines contraindicated as gut already makes benzodiazepines which are not being broken down by liver so would have too much going to brain -> seizures
Induction: Licensing? Onset of action?
Ketamine and thiopental licensed in horses
Usually ketamine + benzodiazepine (midazolam licenced)
Ketamine takes 90s-2m to cause recumbency
Thiopental takes 20s for recumbency (more ataxia?)
Maintenance agents used in horses: Options? Side effects?
Isoflurane Sevoflurane - not licensed in horses (only dogs, cats and rabbits) - but poss better recovery - more expensive? Both cause vasodilation so HR goes up Ketamine bolus if light Thiopental bolus if waking up
What are the minimum alveolar concentrations (MAC)s of isoflurane and sevoflurane?
Isoflurane: 1.3
Sevoflurane: 2.3
But monitor individual horse to see if needs more or less than this
How much oxygen to give a horse during anaesthesia?
4-10ml/kg
A bit is used by capnography (accounted for if use 10ml/kg)
E.g. give 700kg horse 7L oxygen
Opioids in horses: Which ones are licensed? Pros and cons?
Licensed: - pethidine - butorphanol - buprenorphine Butorphanol: - poor analgesia - excitable effect when given with sedation (so just don't use) - good for antitussive for BAL - don't use when doing NG tube as don't cough when going into trachea Buprenorphine: - only partial mu agonist Fentanyl: - full mu agonist - licensed in other species - but need infusion as very short acting (very lipophilic) Methadone: - full mu agonist - licensed in other species - but can cause profound facial twitching in horses (can still use but avoid for head surgery etc) Morphine: - full mu agonist - not licensed in any species but used under cascade
A2 agonists in horses: Pros and cons?
Xylazine:
- good for standing sedation continuous infusion as shortest onset and action so will respond quicker to changes of rate and will recover quicker when infusion stopped
Detomidine:
- only one that is licensed IM
Romifidine:
- less ataxia than detomidine
All cause diuresis due to ADH inhibition and action of DCTs and collecting duct
All cause hyperglycaemia
Initial vasoconstriction and bradycardia - reduced CO but MAP about normal
How does the analgesic effect of A2 agonists compare to their sedative effect?
Analgesic effect lasts 1/2 - 1/3 time of sedative effect
NSAIDs in horses: Licensing? Pros and cons?
Licensed: - phenylbutazone - suxibutazome - flunixin - ketoprofen - carprofen - aspirin - meloxicam - firocoxib ...... others Meloxicam: - most palatable - 72h FEI withdrawal (others are 8 days) - good in shetlands as syringed amounts more practical than bute sachets Bute and flunixin same efficacy but flunixin more potent
Epidurals in horses: Which type? What drugs used?
Lumbosacral very painful so generally just do sacrococcygeal
Don’t use local (except for caudal epidural for tail/perineum) as risk going off back legs
Can put a2 agonists in but..
So just opioids
Usually combo of methadone and morphine as different lipid solubilities:
- methadone has quicker effect but only lasts 4 hours
- morphine takes longer to work but lasts for 24h
What is phenylephrine used for in horses? What is it?
= A1 agonist
During GA to cause vasoconstriction to increase BP (lots - aim for MAP 160-180) e.g. to make spleen smaller for nephrosplenic entrapment colic surgery
What can be used for euthanasia on the surgical table, other than somulose? Why?
KCl (dilute salts in tap water)
Cheaper than somulose
Do get lots of agonal breathing
What is atracurium?
Neuromuscular blocker
May be given when on ventilator
What can be done to help improve ventilation and perfusion under GA?
All dorsal recumbencies put on a ventilator as poor lung perfusion
Salbutamol (B2 agonist) inhaler (2 puffs per 100kg) - causes vasodilation and bronchodilator to open alveoli
Recruitment manoeuvre = artificially large break to inflate lungs (inspiratory pressure of 40-45 2-3 times)
What is important to know about soda lime colour?
Changes colour during use but will change back so not reliable assessment at beginning of a surgery Use FI (inspired concentration)
How to assess anaesthetic depth in horses?
HR and RR often don’t change with depth or pain so don’t rely on these
What should BP be under anaesthesia?
SBP >80
MAP >60/70 (>70 if higher risk of myopathy e.g. big horse)
DBP >40
If HR low (e.g. 20) but BP high then don’t worry too much
What can be used if BP falling under GA?
Dobutamine - increases heart contractility (and HR)
What is dampening of BP monitoring?
Underestimation of systolic pressure
Flush arterial line with heparin saline as can be caused by blood clots
So if low BP, check trace before giving dobutamine etc
What causes the anacrotic notch on a BP trace?
Recoil of aorta
Which arteries are used for invasive BP monitoring in horses under GA?
Facial (just briefly wipe and spirit as can cause spasm if scrubbed)
Transverse facial
Metatarsal (sterile prep as infection would be v bad)
Don’t move art lines as interferes with trace
Complications of arterial catheters?
Trauma
Haematoma
Emboli
Infection
How does the level of the transducer for BP monitoring under anaesthesia affect the trace?
Must be at level of heart
If above, will falsely reduce BP
If below, will falsely increase BP
Why do we want the MAP >60 under GA?
If MAP <60, blood stops faint to skin, muscles etc
Horses often get myopathy - can panic from this and get fracture etc
What is distal pulse amplification?
Further away the arterial catheter is from the heart, there is a false increase in SBP and false decrease in DBP but proportional so MAP stays about the same
Why may horses have a bifid P wave?
Large atria so not quite synchronised atrial contraction as takes longer to depolarise
(Biphasic different thing and not normal, goes above and below axis)
Which lead is used for equine ECGs? Where are the leads placed?
Lead I (base-apex)
+ve lead: yellow, heart apex
-ve lead: red, right jugular furrow
Grounding lead: green, anywhere
Pulse oximetry: What does it measure? How does it work?
Percentage of peripheral Hb saturated with oxygen (SpO2) = indirect measure of arterial oxygen saturation (SaO2)
Optical device that measures the difference in absorption of infrared and red light between oxygenated Hb (HbO2) and non-oxygenated Hb (Hb) by emitting red and infrared light to a photodetector
HbO2 absorbs more infrared light
Hb absorbs more red light
Clip placed on non pigmented area: tongue, axilla, inguinal folds, prepuce, vulva
What is pulse oximetry affected by?
Light Movement Pigmentation Poor peripheral perfusion - relies on tissue perfusion so won't work if: - hypovolaemia - low BP - hypothermia - vasoconstriction e.g. from a2 agonists
Why is pulse oximetry not very sensitive for PaO2 when on oxygen therapy?
If on 100% oxygen, 5 times the amount of oxygen of normal 21% air
PaO2 should be 100mmHg with normal 21% oxygen from air
So PaO2 is 5 times greater the inspired oxygen fraction (FiO2)
So if on 100% oxygen, PaO2 should be 500mmHg
PaO2 of 100mmHg = 100% SpO2
So SpO2 will not detect changes in PaO2 past 100mmHg
Therefore, important to do blood gas analysis when on oxygen therapy (could drop quickly from 500mmHg to 100mmHg but pulse oximeter will still have SpO2 of 100%)