Chp. 52 Epidural Anesthesia

Question 1

What are epidural/extradural anesthesia techniques?

Answer 1

• Epidural = anesthetic solution “upon the dura mater”
• Extradural = anesthetic solution “outside the dura mater”
• Used interchangeably

Question 2

What are intrathecal/subarachnoid/spinal anesthesia techniques?

Answer 2

• Used interchangeably; refer to placement of anesthetic solution under spinal arachnoid, but above pia mater (within SAS)
• When SAS entered, CSF will usually drip from needle hub

Question 3

At what level does the subarachnoid space end in the dog?

Answer 3

• Interindividual, interspecies differences
• Spinal cord usually ends ~L6, SAS ~L7
• Young animals, small breed dogs: SAS may extend into sacral region
• In dogs, ~5% chance of obtaining CSF from an inadvertent subarachnoid puncture at LS junction
• If SAS penetration has occurred, dose of anesthetic should be reduced by 50%

Question 4

At what level does the subarachnoid space end in the cat?

Answer 4

• Both SC and SAS extend slightly caudal to their respective levels in the dog - usually terminate at level of sacrum
• Higher probability of subarachnoid puncture in cats
• If subarachnoid penetration has occurred, dose of anesthetic should be reduced by 50%

Question 5

Will I damage the spinal cord by doing an epidural injection?

Answer 5

• In dogs, SC tissue does not usually lie directly below LS space –> probability of direct spinal cord trauma is low
• 90% of dogs: SC terminates by level of 7th vertebral body
• Cats: SC usually terminates at level of sacrum

Question 6

What is within the epidural space?

Answer 6

• Extensive venous network
• Semisolid epidural fat that surrounds/supports SC
• Bony spinal canal has fixed volume, when changes occur in the volume of the contents (such as decreased epidural fat in emaciated animals or increased blood volume during pregnancy), volume available for drug solutions will change + cranial movement of drug will vary

Question 7

What types of drugs can be administered into the epidural space?

Answer 7

• Local anesthetics
• Opioids
• Alpha 2 adrenergic agonists

Question 8

What are the absolute contraindications to epidural administration?

Answer 8

• Coagulopathies
• Septicemia
• Infection in area of needle insertion
• Uncorrected hypotension, esp with LAs
• Lumbosacral pathologic lesion that would make needle placement difficult (?)

Question 9

What are the relative contraindications to epidural administration?

Answer 9

• Bacteremia

- Neurologic disease that would be aggravated by epidural drug administration

Question 10

Why do I get blood back during epidural needle placement?

Answer 10

• Vessel punctured during insertion of needle through extra spinal muscle, connective tissue
• More commonly, due to inadvertent entry of needle into venous sinus in epidural space
• Injection into venous sinus = IV injection, not recommended –> withdraw needle, try again

Question 11

What signs can I use to be confident of correct needle placement?

Answer 11

• Hanging drop technique: aspiration of fluid into hub of spinal or tuohy needle following penetration of epidural space, since at slightly atmospheric pressure
• Tail twitching, pelvic limb movement as needle touches nerve roots
• Characteristic pop as needle penetrates ligaments flavum
• Loss of a resistance to a small volume of air or fluid
• Characteristic change in respiratory pattern during injection of solution
• Presence of CSF = SAS -> reduce dose accordingly

Question 12

What effect does injection of air into the epidural space have?

Answer 12

Inj of small volume of air (<1.0mL) will generally not have any effect on patient -> absorbed from epidural space, causes no adverse effects

Inj of large volumes of air can contribute to patchy or inconsistent blocks in humans

Question 13

Can epinephrine be combined with LA?

Answer 13

• Can be combined at 1:200,000 to prolong duration/slow absorption of LA
• A 1:200,000 mixture would be 1mg of epinephrine for every 200mL LA

Question 14

Why do spinal needles or tuohy needles have a stylet?

Answer 14

• Occlude end of needle during puncture of skin
• Occasionally, needle without stylet will cut a plug as passes through the skin and can carry it into epidural space
• Skin piece may serve as nidus for infection/inflammation, or rarely will continue to grow to form tumor-like structure’
• Will also block ability to administer epidural

Question 15

Can preservative-containing solutions be placed in the epidural space?

Answer 15

• Several reports of histologic changes occurring in the spinal cord following administration of preservative-containing solutions
• Repeated administration would increase exposure, result in greater change
• PF solutions should be used for epidural, spinal anesthesia/analgesia
• Clinical evidence suggests that single epidural administration of preservative-containing solutions does not result in observable changes on neurological exam

Question 16

What are the landmarks for LS epidural anesthesia?

Answer 16

• Needle inserted in line with DSPs of lumbar and sacral vertebrate btw L7 and S1
• Commonly located just caudal to a line connecting the cranial wings of the ilia
• Also palpate DSPs in area bc DSPs of lumbar vertebrate are separate, taller whereas those of the sacrum are fused and shorter

Question 17

Which patient positioning is best: lateral or sternal recumbency?

Answer 17

• Lateral: more amenable to positioning patients with pelvic/hind limb fractures –> hanging drop technique cannot be used, needle placement is verified by feel
• Sternal: facilitate alignment of landmarks, comparable with hanging drop technique

Question 18

What is the hanging drop technique?

Answer 18

• Method of verification of epidural needle technique
• Once needle has been passed through skin, remove stylet and add anesthetic/analgesic solution to hub until meniscus forms
• Slowly advance needle until needle felt to enter epidural space –> sub atmospheric pressure in epidural space should draw fluid into hub of needle

Question 19

Why might an epidural injection not work?

Answer 19

• Can have incorrect needle placement with misleading signs (eg loss of resistance in epidural fat)
• Leakage of solution outside of nearby intervertebral foramen, inadvertent IV injection in a venous sinus, direction of solution away from site of action by fibrous tissue or epidural fat

Question 20

How are spinal needles different than hypodermic needles?

Answer 20

Spinal needles have different bevel, duller than that of hypodermic needles
–dullness allows operator to have better feel for tissue layers, accentuates “pop” as needle penetrates ligament flavum

Question 21

What supplies are required for epidural injection?

Answer 21

• Spinal or Touhy needle
• Sterile syringe
• Drape
• Loss of resistance syringe
• PF injectate solution (single use or new vials)

Question 22

What side effects are commonly encountered?

Answer 22

• Motor weakness immediately following recovery from GA - continuation of desired effect during surgery, typically resolves within few hours but may take up to a day depending on drug/doses used
• Urinary retention (LA, opioids)
• Pruritis (opioids)
• Nausea/vomiting (opioids, alpha 2 serene

Question 23

What side effects are commonly encountered?

Answer 23

• Motor weakness immediately following recovery from GA - continuation of desired effect during surgery, typically resolves within few hours but may take up to a day depending on drug/doses used
• Urinary retention (LA, opioids)
• Pruritis (opioids)
• Nausea/vomiting (opioids, alpha 2 adrenergic agonists)
• Respiratory depression (LAs, opioids, a2s)
• Hypotension (LAs, a2s_
• Delayed hair growth over needle insertion site
• Meningitis from contaminated needle insertion (rare)