Field Anesthetic Techniques

Question 1

Why are induction and recovery from anesthesia especially difficult in horses?

Answer 1

• recumbency is unnatural for them
• they try to escape the unfamiliar

Question 2

How long can IV anesthesia be administered safely in the field?

Answer 2

Up to an hour
- if longer, refer to hospital where they can be hooked up to gas anesthesia and ventilator

Question 3

What horses have a higher mortality rate compared to other species when under anesthesia? What is the average risk?

Answer 3

Age: foals <10 days, and old patients at higher risk

Duration of surgery: longer procedures have worse prognosis (try to keep elective procedures under 3 hrs)

Drugs for sedation: decreased risk with Ace, increased risk with inhalants? Depends more on individual case and anesthesia team

*Average mortality around 0.5-1%- should not be barrier if surgery provides better prognosis for survival

Question 4

What are the primary goals of pre-medications for field anesthesia?

Answer 4

-Premeds should result in sedation, analgesia, and muscle relaxation
-pre-anesthetics must induce a state of calmness
-NEVER anesthetize an excited horse

Question 5

How should you prepare before an anesthetic procedure?

Answer 5

-complete physical exam with special intention to the respiratory and cardiovascular systems
-subclinical respiratory disease can become overt after anesthesia (may need to rebreathe to evaluate)
-evaluate clinical signs, perform bloodwork if needed
-careful auscultation of heart

TRY TO ONLY ANESTHETIZE HEALTHY HORSES IF POSSIBLE

Question 6

What are the most common arrythmias in horses? What about murmurs?

Answer 6

1st and 2nd degree AV block, Atrial fibrillation (irregularly irregular-sounds like tennis shoes in dryer)
- afib is the only one that warrants much further examination

Murmurs: Innocent murmurs (from PDA), VSD, or mitral/aortic insufficiency

Question 7

Why does HYPP or tying up increase anesthetic risk?

Answer 7

These horses have hyperkalemia which can cause heart problems, especially under anesthesia
- they are easily worked up
- need to heavily sedate these horses pre-op

Question 8

How long should you fast horses prior to field anesthesia?

Answer 8

-4-6 hours is ok in most cases
- some surgeries require 24-48 hours, such as abdominal procedures

Question 9

T/F: The medications that we use for standing sedation in horses are different than the medications we use for pre-meds prior to anesthesia

Answer 9

False- we use the same medications (alpha 2 agonists, opioids, benzodiazepines)

Question 10

What are the routes that alpha-2 agonists can be given? What are its effects and when is their peak effect?

Answer 10

PO, IM, or IV
-results in sedation, muscle relaxation and analgesia
-results in the 5 point stance
-peak effect 3-5 min after being given IV, 10-15 min after being given IM

Question 11

Describe some of the effects that alpha 2s have on the heart and other organs.

Answer 11

-Decreased HR and CO, can cause first and second degree AV block- be sure to monitor HR when under
-increased urination (osmotic diuresis from hyperglycemia, inhibition of ADH)

Question 12

Which is more potent: detomidine or xylazine?

Answer 12

Detomidine

Question 13

What are the reversal agents for alpha 2s?

Answer 13

Yohimbine, atipamazole, tolazoline
-not often used due to risk of getting too excited post op leading to injury
-only use in emergency situations

Question 14

T/F: you need to restrain horses under the effects of alpha 2s

Answer 14

True- they can always react and kick even while sedated

Question 15

T/F: you can give detomidine orally

Answer 15

True
- injectable form can be given PO with applesauce, molasses or alone
- also a gel is available, but this has fallen out of favor

Question 16

Describe how you would give a detomidine CRI

Answer 16

-Give to effect (dont need fancy syringe pumps)
-using a catheter is ideal
-good option if you dont have a lot of help

Question 17

Describe the features of phenothiazines (aka acepromazine)

Answer 17

-Given PO, IM, IV
-results in a calm and relaxed horse which can be aroused
-minimal muscle relaxation or ataxia
-no analgesia
-there is a potential for hypotension when combined with inhalants

Question 18

What is the onset of action and duration of action for acepromazine?

Answer 18

-onset of 25-30 min after being given IV
-duration of action of 6-10 hours

Question 19

When is acepromazine indicated?

Answer 19

Usually combined with alpha 2 agonists to enhance sedation

Question 20

What is a reason that you should use caution when using acepromazine in breeding stallions?

Answer 20

Priapism is a potential side effect

Question 21

Describe the common opioids used in horses and when they should be used.

Answer 21

Butorphanol, Morphine, Transdermal fentanyl
-use as adjuncts to tranquilizers (can cause excitement when used alone)

Question 22

What are the mainstays for induction in horses?

Answer 22

Dissociatives: Ketamine and telazol
- can add on muscle relaxants (such as benzodiazepines and guaifenesin) or propofol

Question 23

Describe the features of dissociatives

Answer 23

-onset of action is rapid
-induction and recovery is generally smooth when used in conjunction with alpha 2 agonists
-duration of ketamine-alpha 2 anesthetics is generally 15-20 minutes
-alpha 2s are more effective as an adjunct to dissociatives

Question 24

Describe the xylazine ketamine protocol for induction.

Answer 24

Xylazine followed by ketamine (1 mg/kg IV xylazine, then 2 mg/kg ketamine IV)- don’t give together
-success depends on xylazine producing sedation, not giving ketamine to an excited horse, not disturbing the horse until recovered and only giving ketamine IV
-muscle relaxation can be poor, apneustic breathing is common
-can be used for minor surgeries
-adding diazepam can lead to smoother induction and greater muscle relaxation

Question 25

How can telazol be added onto a field induction protocol?

Answer 25

Telazol = tiletamine and zolazepam
-can be combined with xylazine/butorphanol or with detomidine
-combining with xylazine has the potential for prolonged recoveries
-provides a longer duration of anesthesia

** best protocol is still xylazine ketamine

Question 26

What is TKD and what are its main considerations for use?

Answer 26

TKD= telazol- ketamine- detomidine
- very small volume per horse
-induces a light plane of anesthesia
-will induce recumbency in one min, be sternal in 36 min and standing in 40 min
-lower doses of all drugs
-should not be used alone for surgery

Question 27

Describe the features of guafenesin (GG)

Answer 27

centrally acting muscle relaxant
-does not have analgesia so should not be used alone
-can decrease doses of other drugs (less CV depression)

Question 28

What are the signs of GG overdose?

Answer 28

Extension of forelimbs, labored breathing, respiratory and cardiac arrest
- opposite of what you might think

Question 29

What are the main considerations when using benzodiazepines?

Answer 29

-includes diazepam and midazolam
-midazolam is only one appropriate for IM use (water soluble)
-do not use alone except in young foals
-provides additional muscle relaxation in conjunction with other agents

Question 30

What are the main considerations when using propofol?

Answer 30

-cost may be prohibitive in equines (often need around 2 bottles)
-dose can be decreased by combining with GG or ketamine for induction
-respiratory depression may limit its usefulness in the field

Question 31

What are your options when a procedure is taking longer than expected?

Answer 31

use longer acting drugs or repeat the administration of the shorter acting drugs
-can also consider giving repeated boluses or setting up a CRI

If using xylazine ketamine, can give 1/3 to 1/2 of the original dose, can add on GG/ketamine or GKX, can use detomidine instead of xylazine or can give 1/4-1/2 ketamine and valium

Question 32

What is “triple drip”?

Answer 32

1 L 5% GG + 500 mg xylazine + 1-2 g of ketamine
- can be used for induction in foals
- signs of overdose look similar to a light horse
- recovery is influenced by total dose administered
-can be used to induce/maintain anesthesia safely for up to an hour
-need a large bore catheter (rapid delivery to effect)
-should give supplemental O2 if surgery is longer than 30 min
-can substitute midazolam for GG

Question 33

How can you monitor horses in the field while under anesthesia?

Answer 33

-this is challenging in non-hospital situations
-proper padding and positioning is important (remove halter and protect eyes)
-IV catheter recommended for TIVA. Should give to effect
-eye signs: brisk palpebral, tearing are signs of being too light, occasional nystagmus is normal
-assess muscle relaxation
-CV: assess digital pulses, CRT, mucous membrane color, observe respiratory rate and rhythm. HR usually changes very little due to anesthesia, though RR may become more rapid and forceful as horse gets lighter (may be better than eye signs)
-it is possible to monitor SAP with doppler- useful for compromised patients
-recumbent horses are hypoxic to some degree and healthy horses can tolerate this for short periods

Question 34

How can you supplement oxygen in the field?

Answer 34

There are portable oxygen concentrator with pulsed delivery for trt of hypoxia during surgery
- there is also a commercial foot pump available
-also can use oxygen tank with regulator and a demand valve

Question 35

Describe how to avoid injuries during recovery

Answer 35

-warn owners about risk of catastrophic injury
-more common when horse tries to stand early so must combine physical/chemical restraint with analgesia
-field should be free of debris, use tail assist if needed, prevent the horse from gaining too much momentum (use long rope), make sure they have good footing (assess area), covering eyes with cloth can be helpful to avoid stimulation