Local Anesthetic Techniques

Question 1

What are the main benefits to local and regional anesthesia?

Answer 1

-allows for examination and procedures without general anesthesia (bypasses risk of induction and recovery)
-provides analgesia without systemic side effects
-can be combined with sedation to provide balanced analgesia

Question 2

What is the main local anesthetic used in veterinary medicine and what are its main characteristics? What are the other options?

Answer 2

-2% lidocaine is used most commonly
-duration is around 1-2 hours
-onset in 2-5 minutes
-very inexpensive

Other options: mepivacaine which lasts 1-2 hours or bupivacaine which lasts 4-6 hours

Question 3

T/F: IV lidocaine is a good thing to add when performing painful procedures to provide analgesia

Answer 3

False- does not help
- actually delays recovery

Question 4

Why is sedation and local anesthesia required for examination, diagnostic and minor surgical procedures in the eyes of horses?

Answer 4

Horses often clamp their eyelids really tight together making it impossible to do an exam let alone operate on their eyes
-without sedation they will not tolerate the exam and will be fighting everything you are doing to them

Question 5

What drug is recommended for topical anesthesia for the eyes of horses?

Answer 5

Proparacaine: onset 15 s, duration 15-30 min
-not appropriate for ongoing pain relief

Question 6

What block is used to block the motor innervation to the eyelids to perform an eye exam?

Answer 6

Auriculopalpebral
- use 25 ga 1 inch needle, and inject 1-2 mLs
-infiltrate the caudal aspect of the zygomatic arch

Question 7

What sensory block can you add if you want to desensitize an area of the eyelid prior to a procedure?

Answer 7

Supraorbital block will desensitize most of the upper eyelid
- inject in and over the supraorbital foramen (just dorsal to the medial canthus)

Question 8

If you want to desensitize the entirety of the upper eyelid, what blocks would you perform along with the supraorbital block? What if you want to desensitize the lower eyelid?

Answer 8

Lacrimal- gets the lateral 25% of the upper lid
Infratrochlear- gets the 25% of the medial upper lid

For lower lid, perform the zygomaticofacial block

Question 9

What does the infraorbital block 1 desensitize? What are the landmarks?

What about the infraorbital block 2?

Answer 9

1: Provides analgesia of the upper lip and nose
- inject over the infraorbital foramen
-palpate the nasal notch and facial crest and inject in between the two (use 4-5 mL of local)

2: desensitizes teeth to first molar, maxillary sinus, roof of the nasal cavity, skin to medial canthus
- thread the needle into the infraorbital foramen
-standing horses do not tolerate this well

Question 10

What does the maxillary block desensitize? How is it performed?

Answer 10

Blocks all upper teeth, sinuses and nasal cavity
-insert a 3.5 inch needle ventral to the zygomatic process, dorsal to the vessels at 90 degree angle
-inject 15-20 mLs

Hard to perform on standing horse

Question 11

What does the mandibular block desensitize? How is it performed?

Answer 11

Blocks all mandibular structures
-blocks everything up to the lateral canthus down
-palpate mandibular notch and slide needle under mandible
-use 20 ga 6 inch needle, inject rostral to angle of mandible (insert 10-15 cm) and inject 15-20 mL of local

Question 12

What does the mental block 1 desensitize? How is it performed?
What about the mental block 2?

Answer 12

1: Provides analgesia of the lower lip
-inject 5 mL over the mental foramen
-palpate by pushing the tendon of the depressor labi inferioris dorsally

2: provides analgesia of the lower incisors and caudal to the premolars
-must thread the needle into the foramen
-inject 5-10 mL

Question 13

What are the main indications for performing an epidural?

Answer 13

For procedures involving the rectum, vagina, perineum, urethra and bladder
-obstetric manipulations (if foal is dead)
-analgesia of stifles and hocks
-intraop - can decrease MAC

Question 14

What are the contraindications for performing an epidural?

Answer 14

-infection at the puncture site
-+/-sepsis
-uncorrected hypovolemia
-anticoagulation reaction (if on aspirin)
-anatomic abnormalities

Question 15

Where is the epidural space?

Answer 15

Within the spinal canal but outside the visceral layer of the dura matter
- not the subarachnoid space- deeper (if you hit this, would be a spinal)

Question 16

Why are caudal epidurals performed more in horses than lumbosacral epidurals?

Answer 16

Lumbosacral epidurals are hard- often will hit subarachnoid space. Want to use this for flank procedures. You need special equipment and aseptic technique

Caudal epidurals are simple to perform, no special equipment is needed, and you can preserve the locomotor function of the hindlimb

Question 17

How should you prep a horse for an epidural?

Answer 17

-need proper restraint
-horse should be standing squarely
-clip and prep the skin
-use sterile gloves and supplies
-can block the skin with 2% lidocaine

Question 18

What are the landmarks for a lumbosacral spinal?

Answer 18

-same landmarks as for CSF collection
-1-2 cm caudal to a line drawn from the cranial edge of the tuber sacrale and dorsal midline
-need a 17.5 cm 17 g spinal needle- being vertical in all planes is important

Question 19

What are the landmarks for a caudal epidural?

Answer 19

• palpate co-1 and co2 as the first midline depression caudal to the sacrum
• also the first moveable coccygeal articulation when the tail is raised and lowered

Question 20

Describe the 2 different techniques for performing a caudal epidural

Answer 20

Technique 1: use 18 ga 1.5 inch needle
-enter the center of the space perpendicular to the skin
-may feel popping as the interarcuate ligament is penetrated

Technique 2: a 5-7.5 inch 18 ga spinal needle
- insert at the caudal part of the interspace at above 30 degrees parallel to horizontal plane
-can be useful if the horse has had multiple epidurals

Question 21

Compare and contrast the caudal and lumbosacral epidurals

Answer 21

Caudal: preferred site, easier and safer, no risk of dural puncture or CSF tap, less risk of motor blockade and ataxia

Lumbosacral: less commonly used, more difficult, can result in dural puncture and CSF tap, high risk of motor blockade and ataxia

Question 22

What equipment do you need to perform an epidural?

Answer 22

-needles, spinal needles, or epidural catheters
-needles are recommended for one time administration
-catheters recommended for repeated or long-term use

Question 23

Describe the two techniques to ensure you are in the right spot for an epidural

Answer 23

1: hanging drop- Go through just the skin with 18 ga 1.5 inch needle, once through skin full hub with saline and then advance until the drop is sucked in

Question 24

When would you use an epidural catheter?

Answer 24

When continuous epidural analgesia is needed
-best for repeat dosing
-use 17 ga huber point directional needle with stylet
-be aware that catheters can cause local inflammation and necrosis

Examples: pelvic fractures, hindlimb fractures, septic joints

Question 25

What medications can you use for epidurals?

Answer 25

Local anesthetics, alpha 2 agonists, opioids (morphine, butorphanol), ketamine

Question 26

Describe the block that occurs with 2% lidocaine or mepivicaine given epidurally

Answer 26

-can result in a motor blockade
-give 0.26-0.35 mg/kg (6-8 mL per 500 kg horse) - can increase volume with saline if you want it to diffuse more cranially
-rapid onset (6-10 min), 60-90 min duration of action (dose dependent)

Question 27

Describe the effects of alpha 2 agonists given epidurally

Answer 27

Effects are similar to opioids
-can act systemically as well as locally
-when combined with locals, they act synergistically to prolong anesthesia

Question 28

What is the mechanism of action of opioids in the epidural space?

Answer 28

They inhibit the pain transmission in the dorsal horn of the spinal cord
-result in minimal motor nerve blockade, minimal dose requirements and few systemic side effects
-provides analgesia without motor blockage
-usually used for intra and post operative analgesia of the perineum and hind limbs
-morphine is the most effective

Question 29

Describe how you use morphine epidurally

Answer 29

-can use preservative and preservative free versions
-pure agonist
-slow onset (1-5 h) and long duration (6-16 h)
-can see mild systemic opioid effects in the awake horse
-reduces MAC of halothane for hindlimb surgeries by 14% (more profound reduction if given several hours prior to surgery)

Question 30

Compare and contrast the preservative and preservative free forms of morphine

Answer 30

Preservative free: 1 mg/mL formulation, larger volume needed, more expensive, better for repeat administration

Regular: 15 mg/mL, smaller volume, inexpensive, good for a one time use

Question 31

Describe the considerations for using butorphanol epidurally

Answer 31

-variable reports on efficacy
-often does not reduce the MAC
-can improve analgesia
-morphine is often preferred

Question 32

Why would you use ketamine epidurally?

Answer 32

-it provides good somatic but poor visceral analgesia
-adding a local anesthetic improves visceral analgesia
-has been used for standing flank surgery (ovarectomies)
-has a fast onset (10 min) and short effect (30-75 min)

Question 33

What epidural agents are often combined?

Answer 33

Opioids and alpha 2s (morphine and xylazine)- good for long term pain management

Alpha 2s and locals- useful for standing surgery as lasts longer than local alone

Question 34

What are some complications that can occur from epidurals?

Answer 34

-failure to achieve analgesia
-hypoventilation
-bradycardia (from systemic effects of alpha 2s)
-pruritis (occurs with morphine, not as bad with local added)
-upward fixation of the patella
-sepsis
-recumbency