Equine Anesthesia Flashcards
general info
Most commonly anesthetized of all large animals
temperment and size pose a challenge
Stallions: penile prolapse/priapism from ace
Xylazine abortion in 3rd trimester
Common procedures
Colic
arthroscopy
castration
upper airway
Prep
Clinical exam
Lab tests
Clinical examination
hx
basics:
HR, RR, mm (color & CRT)
hydration status
Lab tests
Basic hematology and biochem:
PCV, TP, urine SG
compromised patients require more specific laboratory tests
Venous catheterization
usually jugular vein Aseptic prepping of site Local anesthetic infiltration Catheter placement, 12 gauge (adult horse) Suture catheter to skin with nylon
Preanesthetic preparation
Starve for 4-6 hours
Withhold water for at least 2 hours
Pull out all horse shoes
Rinse mouth with pressurized water to clear foreign matter that might attach to endotracheal tube and possibly contaminate/infect lungs
Premedication
Analgesia Butorphanol Methadone/morphine A2 adrenergic drug (xylazine, romifidine, detomidine) Flunixine meglumine 1.1 mg/kg Meloxicam
Considerations during premedication
preferably in an induction (-cum-recovery) box
Padded stall, swing door, tiltable table
Restrain horse properly during induction
quiet environment
effective communication within the team
be sure sedation (including muscle relaxation) has taken full effect before administering induction drug (ketamine)
Induction
Advisable to administer a muscle relaxant before the induction agent
-GGE
-Benzodiazepine: diazepam or midazolam
Ketamine 2.2 mg/kg
Intubation
26mm diameter tube for average 500 kg horse mouth gag to pull jaws apart neck extended lubricate tube intubate blindly- easy inflate cuff
Positioning on theatre table
Depends on type of sx
cushion pressure points (head, shoulders, pelvis)
stretch and spread apart limbs- to encourage blood flow
Protect eyes from trauma, pressure
Turn while changing body positions slowly
Keep in only one lateral recumbency throughout
Maintenance
Isoflurane, sevoflurane
Large animal circle anesthetic circuit required
Oxygen flow rate: 20ml/kg/min soon after induction (30-60min) and towards recovery; 10ml/kg/min during maintenance
Mechanical ventilation (IPPV) recommended esp if anesthesia time longer than 45 min
IV drugs for partial or total anesthesia
Make use of local anesthetic drugs and techniques as much as possible
PIVA and TIVA
most IVA protocols in horses combine:
A2 adrenergic drugs (xylazine, detomidine, romifidine, medetomidine)
Muscle relaxant (GGE, midazolam)
Hypnotic agent (ketamine)
Maintenance PIVA
combination of inhalational and IV drugs attainment of balanced anesthesia Goals: MAC (inhalation agent) reduction Analgesia provision Vital system adverse effects reduction
Maintenance: TIVA advantages
Less cardio system depression
Superior analgesia
less likelihood of movement in response to surgical stimulus
decreased surgical stress
lower morbidity and mortality
better quality of recovery from anesthesia
TIVA: field maintenance
TIVA protocols commonly known as
Triple drips
Double drips effective as well
Include any possible local anesthetic techniques for maintenance
TIVA
alternatively top up with intermittent boluses
Ketamine about 0.4 mg/kg
Diazepam or midazolam 0.1 mg/kg
a2 adrenergic agonist (xylazine, deromidine, romifidine)
Monitoring
Anesthetic depth Resp Cardio Regular arterial blood gas Thermoregulatory
Anesthetic depth
CNS
Surgical: weak palpebral reflex/anal tone
too light: lacrimation, nystagmus, movement
Respiratory system
RR and depth (TV)
hypoventilation likely: IPPV mostly needed; asses ETCO2 (expired CO2)
Cardiovascular system
Hypotension common: direct blood pressure
ECG lead I as for ruminants
Regular arterial blood gas measurements
Hypercapnea (high PaCO2)
hypoxemia (low PaO2)
Ideal anesthetic depth
weak palpebral reflex no nystagmus weak anal tone Stable autonomic reflexes non-responsive to noxious stimulus
Too light
Blinkng lacrimating nystagmus corneal reflex present tachypnea, tachycardia movement of limbs/neck in response to surgical stimulus
Too deep
Absent palpebral reflex
absent corneal reflex
dry cornea
bradypnea, apnea, bradycardia, hypotension
Recovery from anesthesia
Highest risk period of equine anx- characteristically excitable ‘flight’ animals and myopathy risk
soft-padded, non-slip recovery boxes
dim light and quietness in recovery boxes
sedate with a2 adrenergic agonist (romifidine) to delay standing attempts
nasotracheal intubation
supplement o2
keep same lateral recumbency from in sx- sacrifice only one lung side to inevitable atelectasis
may be supported with robes or left alone in recovery box
Analgesia
temperature control
Techniques used in assisted recovery
manual assistance within recovery box head and tail rope recovery -deflating air pillow -sling recovery -pool recovery
Complications
sudden heart failure hypotension hypoventilation hypoxemia myopathies: postanesthetic rhabdomyolysis Myelopathies: esp radial n, obturator n, facial n excitement and ataxia nasal obstruction jugular vein thrombosis
Hypotension: treatment options
MAP < 70 mmHg; SAP <90 mmHg
treat specific underlying cause
reduce administration rate of anesthetic drugs, if indicated
volume resuscitation (crystalloids 10 mL/kh; colloids 5mL/kg; blood- if hemorrhagic)
cardio-active drugs: dobutamine, ephedrine etc
Hypoventilation management
mechanical ventilation
set RR and volume (pressure)
target 35-45 mmHg expired CO2
Myopathy in horse
Most common cause of morbidity/mortality during horse anesthesia various types of myopathies arising from different causes Anesthesia associated myopathies with rhabdomyolysis -compartmental myopathy -malignant hyperthermia Without rhabdomyolysis -hyperkalemic periodic paresis
Myopathy– clinico-pathological changes
myoglobinuria & hyperglobinemia muscle enzymes (CK, AST, LDH elevated) electrolytes (P & K elevated, Na, Cl, Ca decreased BUN and creatinine elevated CBC: inflammatory leukogram
compartmental myopathy- predisposing factors
muscle and nerve tissue ischemia associated with poor perfusion
- heavily muscles horse: >600kg
- fit or nervous horse
- persistently low MAP: <70 mmHg
- improper positioning: perioperatively
- prolonged anesthesia (recumbency) time: > 2hours
- halothan maintenance
compartmental myopathy- signs and symptoms
6 ps (pain, paresis, paralysis, palloe, pulselessness, poikilothermia)
failure to stand on specific muscle group: forelimb, hindlimb
muscle feels hard (tense and firm) and is severely painful
Myoglobinuria and subsequent renal failure
compartmental myopathy- treatment
symptomatic:
-fluids
-analgesics, muscle relaxants &/or sedatives
-o2 radical scavengers:DMSO
-PT
-tender love and care
Fasciotomy-decompression
Malignant hyperthermia: predisposing factors
life-threatening skeletal muscle pharmaco-genetic disease
humans, pigs, horses susceptible- rare
mutations in ryanodine gene causes dysfunctional release of excessive calcium into sarcoplasm
-halothane
-stress
-depolarizing muscle relaxants (succinyl choline)
-improper positioning: laying on hard surface, legs compressive each other for too long, etc
Malignant hyperthermia: signs and symptoms
hyper-metabolic state: hyperthermia, hypercapnea, lactic acidosis
- profuse sweating
- tachypnea
- tachycardia and arrythmias
- hypertension
- prolapse of third eyelid
- flared nostrils
- contracted masseters
- muscle rigidity and twitching
- myoglobinuria
- death and per acute rigor mortis
Malignant hyperthermia: prevention
correct positioning and padding
pre-anesthetic dantrolene in susceptible
maintain normal blood pressure
Malignant hyperthermia: treatment
dantrolene water/alcohol baths ACP for vasodilation Na+ bicarb for acidosis TLC
Hyperkalemic period paralysis (HYPP)
Rare genetic trait that affects quarter horses (most commonly), less commonly in Appalosas and their crosses
Mutation results in failure of Na+ channels to deactivate- excessive Na+ influx and K+ outward flux in mm cells
Episodes can be triggered by:
-transportation
-stress
-sedation
-anesthesia
Hyperkalemic period paralysis (HYPP)- signs and symptoms
can be a challenge to intubate might develop myopathy at recovery -respiratory distress, laryngeal paralysis -swaying -staggering -dog-sitting -recumbency -prolapse of eyelid
Hyperkalemic period paralysis (HYPP)- treatment
increase K+ excretion by:
- ace
- dex
- Ca gluconate
- TLC
Neuropathy- general info
commonly concurrent with myopathy
Ischemia, pressure and stretching possible causes
nerves on dependent weight bearing parts
-facial, radial, spinal malacia (draught horses- always fatal)
Neuropathy- prevention and tx
correct positioning and padding Remove head halter during anesthesia Maintain normal blood pressure not painful nerves may revitalize once swelling subsides symptomatic treatment TLC
Nasal edema
due to gravitational (hypostatic) fluid extravasation which then restricts diameter of nostrils
place nasal tube until horse stands- otherwise risk horse will panic at threat of asphyxiation and try to jump up prematurely
oxygen supplementation
Excitement during recovery
Minimize stimulation during recovery
sedate with romifidine/detomaidine/xylazine to delay attempts at standing. romifidine causes least ataxia
excited horses may suffer bone fractures
re-anesthetize to repair fracture or euth
Foal
Presents a different challenge from adult
be aware of physiological differences
Premed in presence of mother
Benzos are v effective sedative in foals
may combine with butorphanol for analgesia
Foal induction and maintenance
inhalation induction: through nasotracheal tube or face mask
inductionL propofol, ketamine
isoflurane, sevoflurane, for maintenance
Foal-recovery
usually requires some human physical support during recovery
- delay attempts at getting up by keeping down manually
- support head and pull on tail base once up
Donkey
Stoic animal
fight rather than fright and flight
thick skin over jugular furrow
cut down for venous catheterization
donkey- drugs
less prone to excitement during induction or recovery periods
more resistant to sedatives, anesthetics and analgestics- inc dose by 30%
ketamine half life also shorter
anticipate more frequent top ups
more sensitive to GGE
donkey-recovery
smoother than the horses
rarely noticeable emergency delirium/excitement
nasal tube may be used to counter adverse effects of nasal edema
human physically assisted recovery: watch out for kicks
