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Epidural Anesthesia

Epidural Anesthesia. Epidural Anesthesia. Presentation divided into two sections: Anatomy and Physiology Techniques. Epidural Anesthesia. A Neuraxial technique that offers a wider range of applications than a Spinal Anesthetic

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Epidural Anesthesia

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  1. Epidural Anesthesia

  2. Epidural Anesthesia • Presentation divided into two sections: • Anatomy and Physiology • Techniques

  3. Epidural Anesthesia • A Neuraxial technique that offers a wider range of applications than a Spinal Anesthetic • An Epidural block can be performed at the Lumbar, Thoracic, Cervical and Caudal level • Wide use of applications; Operative anesthesia, Obstetric Anesthesia & Analgesia, Postop pain control and Chronic Pain Management • It can be used as a “Single Shot” or with a catheter that allows intermittent boluses or a Continuous Infusion

  4. Epidural Anesthesia • One advantage of an Epidural is that the muscle blockade can range from none to complete • Everything can be regulated and changed by: • Choice of drug • Concentration of LA • Dosage • Level of Injection

  5. Anatomy • The Epidural space surrounds the Dura Mater posterior, laterally and anteriorly • Nerve roots travel in this space as they exit the spinal cord laterally • They then exit the foramen and travel peripherally to become peripheral nerves carrying both afferent and efferent pathways

  6. Anatomy • Other contents of the Epidural space include: • Fatty connective tissue • Lymphatics • Venous plexus (Batson’s) • Septa and Connective tissue bands

  7. Physiology • Local anesthetics or other solutions injected into the epidural space (steroids, narcotics) spread anatomically • Horizontal spread is to the region of the dural cuffs with diffusion into the CSF and leakage through the intervertebral foramen into paravertebral spaces • Longitudinal spread is preferentially cephalad in direction

  8. Physiology • Possible sites of anesthetic action include: • Paravertebral nerve roots • Intradural spinal roots • Dorsal and Ventral spinal roots • Dorsal root ganglia • The Spinal Cord • The Brain itself (by diffusion)

  9. Physiology • Initial blockade is PROBABLY a result of anesthetic blockade at the spinal roots within the dural sleeves • The Dural Cuffs or Sleeves have a proliferation of arachnoid villi and granulations that effectively reduce the THICKNESS of the dura mater facilitating rapid diffusion of the LA from the Epidural space, through the Dura and into the CSF surrounding the nerve roots • Then the local anesthetic diffuses into the nerve root itself, producing anesthesia to that particular dermatome

  10. Physiology • Because Epidural anesthesia is DIFFUSION dependent, relatively LARGE volumes of LA are needed to achieve a block that spans several dermatomes • The block ONLY goes as high or low as you regulate it (by volume) • It’s not like a Spinal which is EVERYTHING distal to the level of the block; it is a DIFFERENTIAL block dependent on the volume and site of injection

  11. Advantages • Consequently, Epidural techniques have the advantage of better control of level (and also of sympathetic blockade) • Epidural techniques allow for the placement of a continuous catheter which is especially useful for: • Cases of unpredictable duration • Prolonged postoperative analgesia • Chronic pain control • Obstetric Analgesia & Anesthesia

  12. Spread of Anesthesia • To be able to choose the most appropriate anesthetic dose, concentration and volume of LA, the anesthetist must be familiar with the variables that affect spread and duration of Epidural Anesthesia • The variables are more numerous than those of spinal anesthesia and Baricity plays a VERY small factor when dealing with Epidurals, whereas in a Spinal, baricity is a KEY factor in spread and distribution of the block

  13. Spread of Anesthesia • The factors that affect the level of the Epidural block are: • Injection Site • Dose • Volume • Concentration • Position • Age • Height and Weight (?) • Pregnancy (?) • Speed of injection (?)

  14. Injection Site • INJECTION SITE: Unlike Spinal anesthesia, Epidural anesthesia produces a segmental block that spreads both caudally and cranially Based on that fact, then the INJECTION SITE is arguably THE most important determinant of the spread of an epidural block The injection site should be in the MIDDLE of the range of dermatomes that needs to be anesthetized and closest to the main nerve roots involved

  15. Injection Site • Caudal epidural blocks are largely restricted to sacral and LOW lumbar dermatomes • Thoracic levels can be reached by the caudal approach only if large volumes (30cc) are given, and then the block is patchy at best because of the distance that the anesthetic has to travel

  16. Injection Site • Lumbar local anesthetic injections of 10cc tend to spread caudad to include all the sacral dermatomes • Lumbar injections of 20cc volumes produce much better quality sacral blocks and can also extend cranially to include the midthoracic levels

  17. Injection Site • Thoracic injections tend to produce a symmetric segmental band of anesthesia with minimal lumbar spread • When using a thoracic approach, it is prudent to decrease your volume by about 30-50% to prevent cranially spread • It is generally not feasible to produce surgical anesthesia in the low lumbar or sacral nerve distributions when using thoracic injection sites • Thoracic injection sites are ideally suited for procedures of the chest and upper abdomen or for relief of post-op thoracotomy pain with a catheter being placed for continuous infusions

  18. Dose, Volume & Concentration • Within the range typically used for surgical anesthesia, drug CONCENTRATION is relatively unimportant in determining block spread • DOSE & VOLUME, however, are important variables in determining both spread and quality of the Epidural block obtained

  19. Dose, Volume & Concentration • If drug CONCENTRATION is held constant, increasing the volume of LA (and thereby increasing the DOSE) results in significantly greater average spread • DOSE = Volume x Concentration (i.e. 15cc x 2.5mg/cc = 37.5mg; 20cc x 2.5mg/cc = 50mg) • The CONCENTRATION of the LA generally affects the DENSITY of the block, NOT the spread

  20. Dose, Volume & Concentration • So a small volume of a more concentrated LA will produce a very limited BUT very strong block • But take the same DOSE and double the volume, the spread will increase BUT the strength of the block may not be as intense

  21. Dose, Volume & Concentration • NOTE: The increase in block level IS NOT in direct proportion to the volume increase. Doubling the volume WILL NOT double the block spread. It is a NON-linear relationship and doubling the volume will only increase the level about 1/3-1/2 the original number of segments • The same relationship exists with DOSE; doubling the dose will usually only increase the level of block the same 1/3-1/2 of the original number of segments blocked

  22. Dose, Volume & Concentration • Recommended amounts of LA differ as to which level is being injected: Cervical/Thoracic doses are 0.7 to 1cc per segment with an initial volume of 10cc Lumbar level doses are 1.25 – 1.5cc per segment with an initial volume of 15-20cc • This is due to the narrowing of the spinal canal as it progresses cranially

  23. Concentration and Differential Block • Using a lower concentration anesthetic can sometimes give you a differential block • The lower concentration means the dose is lower and there is less LA to penetrate the nerve roots so the block acts more peripherally on the nerves, differentially blocking sensory and pain fibers over larger muscle fibers in the center of the nerves

  24. Concentration and Differential Block • An example of this is used in Obstetrics: Bupivicaine 0.25%, 20cc, usually ONLY provides a sensory block but leaves the motor fibers intact so the patient can push when needed to If Bupivicaine 0.5% is given with the same volume, then a sensory as well as motor block is obtained, paralyzing the muscles at the levels of the block so NO pushing is going to be possible • There is quite a bit of individual sensitivity and some people may end up with a purely sensory block while others may end up with significant muscle weakness or paralysis; (ooooppps!!)

  25. Position • Some people feel that the Lateral position is the preferred position to optimize spread • Others feel that the sitting position is preferred due to anatomical advantages • Studies have shown small to NO differences in spread of block when comparing the two positions; it’s your preference which one to use

  26. Age • Most (but NOT all) studies that have examined the effect of age on Epidural blocks have demonstrated a greater spread in older patients • This is thought to be related to a less compliant epidural space and Dura Mater • Even so, the clinical effect is usually AT MOST an increase of no more than three or four dermatomes

  27. Height and Weight • The correlation between patient Height or Weight and spread of epidural block is very weak at best and seems to have no clinical significance • The only instance where it may have an effect is in EXTREMELY TALL people (greater than 6’6”) or in EXTREMELY SHORT (less than 4’10”) or in MORBIDLY obese patients

  28. Pregnancy • Studies examining the effect of pregnancy on spread of Epidural blocks are conflicting • Some have shown a greater spread at TERM and early in pregnancy • Other studies have shown no significant differences in level of spread between pregnant and non-pregnant patients • ?????????????

  29. Speed of Injection • Some feel that a rapid injection will increase the level of spread or decrease the time it takes for the block to set • This has NEVER been shown to make any difference in either • Drugs should, in fact, be injected SLOWLY to avoid rapid increases in CSF pressure, headache and increased intracranial pressures • Also, incremental bolus vs. slow, steady injection has shown NO difference in level of spread in multiple studies

  30. Speed of Injection • All solutions should be injected in increments of 3-5cc every 3 minutes and titrated to the desired anesthetic level • If a catheter has been placed and you are injecting through the catheter, then the catheter needs to be aspirated prior to every injection to show no CSF is present

  31. Speed of Injection • This gradual administration of medication slows the rate of onset of the anesthetic level and controls the development of the sympathetic blockade • This is an advantage that you have with an Epidural that you DO NOT have with a Spinal • The Spinal is ALL or none, whereas the Epidural can be brought up gradually, slowing whatever hypotensive response you may have to a more manageable level (and saving you an extra pair of pants!!)

  32. Onset of Blockade • The onset of an epidural block can usually be detected within 5 minutes in the dermatomes immediately surrounding the injection site • The time to PEAK effect differs somewhat among different LA’s • Shorter acting drugs usually reach their maximum spread in 15-20 minutes • Longer acting LA’s usually reach their maximum spread in 20-25 minutes • Increasing the DOSE of LA SPEEDS the onset of both motor and sensory block

  33. Duration of Block • The DURATION of the Epidural block depends on: • The LA itself • Dose given • Patient age • Use of Adrenergic Agonists

  34. Local Anesthetics & Duration • Your choice of LA is the most important factor in determining DURATION of the block • Chlorprocaine is shortest, Lidocaine & Mepivicaine are intermediate and Bupivicaine and Ropivicaine produce the longest lasting Epidural blocks

  35. LA’s & Duration • Back to the differential block topic: ETIDOCAINE is a long acting agent that has a profound muscle relaxation effect but a weak sensory effect, so you would end up with a paralyzed patient in severe pain; it has been almost completely eliminated from use as a result of this differential blockade

  36. LA’s and Duration • On the flip side, BUPIVICAINE is the opposite of Etidocaine • In lower doses (concentrations) BUPIVICAINE seems to have a preferential sensory block with minimal motor effect • That is why it is an ideal drug for Obstetric ANALGESIA during labor, eliminating pain while preserving muscle function

  37. Dose and Age • DOSE: Increasing the DOSE of a LA results in increased duration AND density of the block • AGE: There are conflicting studies, but the majority seem to show a longer duration of action in the elderly population. The exact reason is unknown and more studies need to be performed

  38. Adrenergic Agents and Duration • Epinephrine in a concentration of 5 micrograms/cc (1:200,000) is the most common adrenergic agent added to epidural LA’s • It has been shown to prolong the blocks of Lidocaine and Mepivicaine by as much as 80% • Epinephrine has been shown NOT to significantly prolong the duration of anesthesia when added to concentrated solutions of Bupivicaine and Ropivicaine used for surgical anesthesia

  39. Adrenergic Agents and Duration • However, when added to more dilute concentrations of Bupivicaine, as used for OB Analgesia, it has been shown to increase the duration AND quality of the block • The mechanism proposed, although never proven, is that through vasoconstriction, it slows the systemic absorption and elimination of the LA • Why it does not work with higher concentrations of Bupivicaine and Ropivicaine is not clearly understood

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