Epidural Analgesia and Anesthesia

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Historical Aspects. 1901 use of cocaine as an epidural agent for humans and dogs reportedAlternative to general anesthesia1927 E.R. Frank describes use of procaine as a successful alternativeInterest wanes as inhalant anesthesia gains favor1979 first report of use of spinal opioids. Current Role.

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Epidural Analgesia and Anesthesia

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1. Epidural Analgesia and Anesthesia September 25, 2002 Kristy Donnelly, DVM

3. Current Role Significant role in modern veterinary and human anesthesia and analgesia Important aspect of “balanced anesthesia” concept

4. Indications Surgery caudal to the umbilicus Up to and including thoracic limb if using morphine Peritonitis Severe pancreatitis Caudal trauma High risk anesthetic candidates Dystocia Preemptive analgesia

5. Contraindications Coagulopathies Bacteremia Severe systemic infection Infection at the site of needle placement Thoracolumbar deficits Lumbosacral fractures or dislocations Uncorrected hypovolemia Inexperience of operator

6. Potential Complications Infection Epidural or intrathecal hemorrhage Spinal or nerve root trauma Persistent weakness or ataxia Rare with experienced clinician

7. Epidural Space and Spinal Anatomy Epidural space Between dura and vertebral canal Dural sac ends at L7-S2 Spinal cord Usually ends at L6-7 intervertebral space Extends to LS region in small dogs and cats

9. Landmarks for administration LS region preferred Relatively large intervertebral space Dura terminates cranial to (in most cases) Lateral recumbency Sternal recumbency

10. Techniques of Administration Lateral recumbency Ideal if lateralized orthopedic problem Surgical site down for maximal infiltration of agent (at least 5min) Sternal Potentially easier Pelvic limbs drawn cranial Stretches out ligamentum flavum Expands intervertebral space

11. Sterile Preparation Clip generous region Surgical scrub Drape Needles

12. Needle selection Spinal and Tuohy needles preferable Shorter bevels Steel stylet Longer length Duller tip

13. Needle Selection con’t…

14. Anatomical Landmarks Draw pelvic limbs cranially Palpate wings of ilia L6 just cranial to wings L7 is substantially shorter just caudal to wings LS space Palpable depression just cranial to 1st dorsal sacral process Caudal to L7

15. Injection Needle should penetrate on midline, perpendicular to skin of LS space Needle passes through: Skin Supraspinous ligament Interarcuate ligament “pop” Epidural space

16. Correct placement of needle Hanging drop technique Air leakage Loss of resistance Whoosh test

17. Hanging Drop Technique Advance needle to point near ligamentum flavum Withdraw stylet Place several drop of sterile saline in hub so liquid protrudes beyond orifice Advance needle into epidural space Negative pressure will pull drop down needle shaft Most effective in larger animals *Hansen, B. “Epidural Anesthesia,” Current Veterinary therapy, XIII. p. 129

18. Air leakage test 0.5 to 2.0 ml of air will proceed w/ no resistance No air bulge or crepitus Usually can aspirate it back in part *Hansen, B. “Epidural Anesthesia,” Current Veterinary therapy, XIII. p. 129

19. Whoosh test Stethoscope is used to auscult directly over spinal just cranial to LS Inject 0.5-2.0ml of air Proper placement results in a “whoosh” sound Outside of space produces loud crepitus *Hansen, B. “Epidural Anesthesia,” Current Veterinary therapy, XIII. p. 129

20. Injection Inject over 10-20sec Maximum injectate of 6 ml

21. Injection/dose precautions If intrathecal reduce dose by 40-75% of epidural Reduce dose by up to 75% in pregnant patients Engorgement of epidural vessels Decreases volume of space Increased absorption

22. Post injection Keep head elevated for 10 min. Place affected limb down for 5 min.

23. Agents Local anesthetics Opioids a-2 agonists Dissociatives

24. Preservatives Prudent to choose preservative free when possible No reports of neurotoxicity in animals after one dose formaldehyde phenol Avoid especially with: Repeated injections Intrathecal injections Avoid antioxidant Na metabisulfite (in local anesthetics that contain epinephrine) intrathecally Not a true preservative

25. Local Anesthetics: General Bupivicaine, lidocaine Amide-linked drugs Agents reversibly bind to neuronal voltage-gated sodium channels and block nerve impulse conduction Affect segmental nerve roots Individual pharmacodynamics of agents depend on lipid solubility, dissociation constant, protein-binding characteristics

26. Local Anesthetics con’t…. Effects based on myelination and size Smaller sensory and ANS fibers affected 1st Sensation disappears in following order: Pain Cold Warmth Touch Joint Deep pressure Recovery in reverse order

27. Local Anesthetics: Uses Minimal motor blockade when used in dilute concentrations Effective analgesia, potential complete regional anesthesia, potentiates epidurally administered opioids

28. Local Anesthetics: Disadvantages Relatively short duration of action Possibility of unwanted motor blockade Potential blockade of spinal sympathetic nerves Cause or aggravate hypotension

29. Local Anesthetics Lidocaine Quick onset of action Short-acting Bupivacaine More potent Slower onset Analgesia with minimal motor blockade High affinity for Na channels (potential for cardiotoxicity)

30. Lidocaine: Dosages Lidocaine for injection or preservative free 1.0-2.0% Single LS injection 1ml/5kg lean body weight for caudal procedures 1ml/3.5kg lean BW for abdominal procedures

31. Bupivacaine: dosages Bupivacaine with epinephrine: 0.25-0.50% Single LS injection 1ml/5kg lean body weight for caudal procedures 1ml/3.5kg lean BW for abdominal procedures Bupivacaine preservative free (no epi) 1ml/5-3.5kg lean BW +/- CRI 0.1-0.4mg/kg/day *CVT XIII p. 127, B. Hansen

32. Duration of action Lidocaine ~1 hour Bupivacaine ~4-6 hour

33. Toxicity Seizures Crossing of BBB Treat with diazepam Respiratory depression artificial ventilation and oxygen Cardiovascular depression Hypotension Myocardial depression

34. Opioids Morphine, Fentanyl, oxymorphone, meperidine, buprenorphine Selectively block pain conduction without: Motor, sensory or sympathetic blockade Central effects MAC reduction Hemodynamic stability Blockade of autonomic response to noxious stimuli

35. Opioids mechanism of action Bind at opioid receptors on interneurons of superficial laminae of dorsal horn of spinal cord segments Pre/postsynaptic inhibition of afferent transmission (glutamate & substance P) Better for dull aching post-op pain than acute intraoperative Must cross dura to CSF and spinal cord Diffuse across meninges into CSF then SC* Arachnoid mater is main meningeal diffusion barrier Travel thru perineurium of spinal nn. along n. root into SC Absorbed by spinal segmental aa. or epidural vv. and then to brain and SC *Quandt & Rawlings, “Reducing Post-operative pain for dogs: Local Anesthetic and Analgesic Techniques,” Compendium, pp. 101-111,1996.

36. Opioid Agents More lipophilic = quicker onset = shorter acting Most to least lipid soluble Fentanyl Buprenorphine Oxymorphone Morphine

37. Morphine Least lipid-soluble Peak effect ~90 min. May persist for 24 hours Cephalad migration independent of volume

38. Other Opioid Agents Fentanyl Little use as a single agent Does not extend more than 2 spinal segments from site Useful in combination with morphine Buprenorphine Local action Slightly longer acting than fentanyl Oxymorphone Has been used successfully Slightly less duration than morphine

39. Side Effects of Opioids Pruritis at affected dermatomes Especially with morphine Delayed respiratory depression Up to 24 hours with morphine <2% of humans Not clinically significant in companion animals Posterior ataxia Urine retention Detrusor m. weakness Up to 24 hrs post morphine/oxymorphone *Hansen, B. “Epidural Anesthesia,” Current Veterinary therapy, XIII. p. 128

41. Intrathecal Injections Accidental entrance into subarachnoid space May see significant central effects Dogs: sedation and miosis Cats: agitation and mydriasis Respiratory depression Hyperesthesia If aware of intrathecal location: Reduce by 30-75% of epidural dose Preservative-free, w/o epinephrine

42. Opioids and MAC Morphine proven to reduce halothane MAC in dogs 42% reduction in HL 35% reduction in FL Morphine proven to reduce isoflurane MAC in cats 31% reduction using tail clamp *J.E. Ilkiw, Balanced Anesthetic techniques in dogs and cats, pp. 31-36., 1999

43. Synergism of Opioids and Locals Post-op combination of morphine and bupivacaine Superior analgesia to morphine alone Longer than 24 hours of analgesia *Torske & Dyson, “Epidural Analgesia and Anesthesia,” Veterinary Clinics of N. America., p. 859-874, vol. 30, no. 4, July 2000.

44. a2-agonists Xylazine, medetomidine, clonidine, detomidine Cross dura to bind a2 adrenoreceptors and act at dorsal horn (similar to opioids) Work best when combined with other agents ie. Morphine *Hansen, B. “Epidural Anesthesia,” Current Veterinary therapy, XIII. p. 128

45. a2-agonists dosages Medetomidine and Morphine 0.005mg/kg medetomidine 0.1mg/kg morphine Effects for 13 hours

46. Dissociatives Ketamine Blockade of a adrenoreceptors Selective suppression of dorsal horn Interactions with opioid receptors 1-2 hour duration 2.0mg/kg

47. Combinations Opioids and ketamine Opioids and local anesthetics Alpha-2 and opioids Bupivacaine, lidocaine and opioids Etc.

48. Conclusions Epidurals are an important part of pain management Multimodal therapy Easy to implement

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