Anesthetic Considerations in the Horse Flashcards
general considerations of anesthesia in a horse
- challenging
- higher morbidity and mortality
- potential for injury to themselves and staff
special features of horse anesthesia
- size
- behavior
- CV function
- resp function
- GI system
- recovery
- pain management/anesthetic protocols
why is horse size challenging with anesthesia?
- handling/restraint
- moving/positioning
- neuro/myopathy
- ventilation/perfusion mismatch
- different drug response
horse behavior w anesthesia
- fight or flight response: self destructive nature, affects induction and recovery
- quiet environment!
- experienced handling to minimize risk
what effects do anesthetics have on horse CV system
- depressant: decrease myocardial contractility, decrease CO by 40% = decreased BP
- poor tissue perfusion: post anesthetic myopathy
- inotropic support is key! dobutamine is most common
what kind of support is key in horses to support CV function?
inotropic support: want to support heart contractility: dobutamine is most common
anesthesia’s effects on resp fx in horses
- resp depressant
- HYPOVENTILATION: common
- abdominal distension
- position: head down
- mechanical ventilation often needed
how do anesthetics affect oxygenation of horses?
- severe V/Q mismatch: physiological shunt, low PaO2, hypoxemia
- positioning matters! esp when pt on side or back
- often need mechanical ventilation and oxygen
how can you help severe V/Q mismatch in recovery with an anesthetized horse?
- O2 insufflation thru endotracheal tube: can stand with it; easy way to supplement O2
- demand valve: provides high flow: 100% O2 to give horse a breath, can inflate lungs in recovery
why are horses at risk of obstruction in recovery?
- they are obligate nasal breathers
- nasal congestion (why we recover with tube in)
- keep intubated! wait for swallow reflex and standing and then pull tube
- can spray phenylephrine on nares (horse is vasodilated from drugs, venous congestion can occur, get discharge, phenylephrine is a alpha 1 agonist and will cause vasoconstriction)
a 2 year old healthy quarter horse mare is anesthetized with sevoflurane for arthroscopy. in recovery the horse has a difficult time rising and you notice that the gluteal muscles are firm, swollen and very painful. which of the following is the most like cause if it occurred for an extensive period:
a. HR of 50 bpm
b. MAP of 50mmHg
c. PaCo2 of 50 mmHg
b.
A: little high but not enough to impact filling. if at 70, would worry
B: we worry bc don’t want myopathies to happen! need to ensure that muscles are perfused. between 75-85 is ideal range
C: horses can be hypercapneic: this # is not uncommon
what is the ideal BP for an anesthetized horse?
between 75-85mmHg. small: 75. draft: 80-85.
how does the horse GI system affect anesthesia?
- large colon = fermentation vat
- distension from gas or feed
- compromised cardiopulmonary function!
- preoperatory fasting? to decrease food, less to ferment and decrease pressure: didn’t show complications tho and had faster return to normal in non-fasted patients…
Co2 will be increased and O2 decreased: not able to ventilate well
anesthetic risk with GI in horses
- decreases GI motility: stress, drugs, transport, PAIN
- increases risk of ILEUS! opioids risky
blocks can help
recumbency in horses
- long periods of recumbency is not normal: increases risk of myopathies, neuropathies, and get V/Q mismatch
- good padding is very important!!
- need to stand within 1 hr of anesthesia?
when do we want to see horses stand after anesthetic procedures?
stand within 1 hr after anesthesia! horses MUST stand after anesthesia
recover with ET tube in place to prevent airway obstruction!!
recovery complications in horses
- complications: fractures and injuries from attempts to stand, upper airway obstruction
- quest for the perfect recovery technique may not exist
what can we do to improve recovery for horses?
- maintain airway (ETT)
- prevent hypoxemia (O2 insufflation)
- padded recovery stall
- additional sedation
- assist recovery if indicated
what are common sedation protocols for horses?
- alpha 2 agonists: xylazine, detomidine, romifidine, dexmedetomidine
- acepromazine (10-20 min prior)
+/- opioid: butorphanol, hydromorphone, morphine. give a lil sedation FIRST, then opioid because opioid will deepen their sedation.
butorphanol is most commonly used in horses, good sedative but not great for pain. hydro is best for pain: lasts longer and less impact on GI than morphine
what opioid is best for painful procedures in horses?
hydromorphone: lasts longer and less impact on GI than morphine
what is common induction protocols for horses?
- ketamine: can give AFTER sedation. doesn’t have muscle relaxation!
- ketamine + diazepam/midazolam (these are muscle relaxants)
- ketamine + propofol: preferred
- ketamine + guaifenesin
- ketamine + guaifenesin + diaz/midaz
ketamine and muscle relaxant is best: get relaxation, and decreases amount of ketamine needed. don’t do propofol alone! harder to modulate
what is the preferred induction protocol for horses?
ketamine + propofol
how do we maintain anesthesia in a horse?
- TIVA: total intravenous anesthesia
- inhalant anesthesia (IA)
what is total intravenous anesthesia?
- way of maintaining it in the horse
- IV top ups (ketamine/xylazine): 1/2 to 1/3 of induction dose
- “triple dip” or GKX/MKX: for longer anesthestic
^ ketamine + muscle relaxant + alpha 2 agonist
^ ketamine + guaifenesin or midazolam + xylaxine
what is GKX or MKX?
“triple dip” way for maintaining longer anesthesia IV
- ketamine + muscle relaxant + alpha 2 agonist
= ketamine + guaifenesin/midazolam + xylazine
CRI: ketamine + xylazine
pros of total IV anesthesia in horse
- anesthesia machine not required
- generally good quality recoveries
- time limit of 1 hour
- avoid accumulation of drugs and prolonged recoveries
inhalants used in horses?
isoflurane, sevoflurane, desflurane
- better muscle relaxation than TIVA
- procedure > 1 hr duration
- requires LA anesthesia machine
- oxygen supplementation
- ventilation can be assisted
common anesthetic protocol/combo in horses
- PIVA/balanced anesthetic technique
what adjuncts can be added to inhalant anesthetics in horses?
- alpha 2 agonists, ketamine, lidocaine
- analgesia, decreases anesthetic requirements
- may influence recovery
how do you monitor TIVA?
- pulse rate, RR and pattern, MMC
- anesthetic depth: eyes more active; always a palpebral reflex
- blinking, tearing, brisk palpebral reflex = light
- nasal oxygen insufflation recommended
on ketamine: maintain reflexes: will blink
you have a horse anesthetized with ketamine on board who starts to show a palpebral reflex. what are your thoughts?
this is normal as ketamine causes them to maintain their reflexes. ketamine is a dissociative drug that doesn’t cause the same CNS depression
when you have a horse on inhalant anesthetics, what MUST you monitor?
arterial blood pressure!!
why must arterial blood pressure be monitored when you have a horse on inhalant anesthetics?
- marked contractility depression + vasodilation
- hypotension can cause myopathy!!
- hypotension MUST be treated: decreased drug depth, inotropic drugs, fluids
what should the MAP of an anesthetized horse on inhalant anesthetics be?
MAP should be >70 mmHg
how should you monitor inhaled anesthetics on a horse?
- ECG for HR and rhythm
- periodic arterial blood gas analysis
- pulse oximetry and capnometry are less reliable in horses
- IPPV frequently indicated (intermittent positive pressure ventilation) esp with newer inhalation agents
what is the most typical way to manage pain in horses in anesthesia?
NSAIDS
T/F: pulse oximetry and capnometry are less reliable in horses
true
alpha 2 agonists for pain management in horses?
- yes: excellent analgesia
- heavy sedation (Sedation > Analgesia)
- cardiovascular depression
- limited long-term use
- adjunct to general anesthesia (CRI)
lidocaine CRI in horses for pain management?
- anti inflammatory GI effects
- mild analgesia
- improves GI motility (but not a prokinetic)
- decreases anesthetic requirement
- BUT DOES NOT IMPROVE CO
T/F: lidocaine CRIs are useful to improve CO in horses under anesthesia
false: does not improve CO!!
ketamine CRI?
- NMDA antagonist (wind up pain)
- decreases anesthetic requirement (dose dependent)
- improves CO !! rare
- prolonged infusions may affect recovery: neg behaviors
what is one of the few adjuncts that actually improves CO in horses for pain management?
ketamine CRI
systemic opioids for pain management in horses?
- may cause excitation similar to cats, non painful
- increases locomotor activity: pacing in stall, etc
- usually used with sedation
- decreased GI motility; just be mindful of this
cons of systemic opioids in horses?
- controversial: pain x MAC reduction: doesn’t cause MAC reduction like in SA; won’t decrease your inhaled dose: maybe bc of central excitation?
- doesn’t consistently reduce IA requirement
- hydromorphone good bc longer duration and lower GI effects, morphine effective for moderate to severe pain, and butorphanol for mild pain
what re the most commonly used opioids in horses?
morphine and butorphanol
morphine: pure mu agonist: effective for moderate to severe pain
butorphanol: mu antagonist/kappa agonist: mild/sedation pain
local anesthetics in horses
- distal extremities, thoracic/lumbar (caudal intercostal, paravertebral)
- proximal blocks + epidural: may interfere with motor function. femoral nerve comes out of L3/L4! don’t want to block that bc horse needs to be able to stand/get up
- intra articular: chondrotoxicity. damages cartilage
be mindful of motor function! horse has to be able to stand up!
opioid local and regional blocks
- used in joints
- intra articular morphine
- epidural is long lasting, and can be combined with alpha 2 agonists like detomidine
long lasting and pain can be days, do catheter
