Epidural Anesthesia: Factors Affecting Height and Local Anesthetic Used

1. Soli Deo Gloria Epidural Anesthesia: Factors Affecting Height and Local Anesthetic Used Developing Countries Regional Anesthesia Lecture Series Daniel D. Moos CRNA, Ed.D. U.S.A moosd@charter.net Lecture 11

2. Disclaimer • Doses are only general recommendations. There are several factors that may result in either an inadequate or high epidural block. • Every effort was made to ensure that material and information contained in this presentation are correct and up-to-date. The author can not accept liability/responsibility from errors that may occur from the use of this information. It is up to each clinician to ensure that they provide safe anesthetic care to their patients.

3. Introduction to Epidural Anesthesia • Epidural anesthesia produces a reversible loss of sensation and motor function much like a spinal with the exception that local anesthetic is placed within the epidural space. • Larger doses of local anesthetic are required to produce anesthesia when compared to a spinal anesthetic. • Doses must be monitored to avoid toxicity.

4. Introduction to Epidural Anesthesia • An epidural catheter allows the versatility to extend the duration of anesthesia beyond the original dose by the administration of additional local anesthetic. • Epidural catheters may be left in place for postoperative analgesia.

5. Epidural Anesthesia Indications • Cesarean section • Procedures of the uterus, perineum* • Hernia repairs • Genitourinary procedures • Lower extremity orthopedic procedures • Excellent choice for elderly or those who may not tolerate a general anesthetic

6. Epidural Anesthesia • Should NOT be used in patients who are hypovolemic or severely dehydrated. • Patients should be pre-hydrated with .5 – 1 liter of crystalloid solutions (i.e. ringers lactate) immediately prior to the block.

7. Epidural Anesthesia • Higher failure rate for procedures of the perineum. • Lower lumbar and sacral nerve roots are large and there is an increased amount of epidural fat which may affect local anesthetic penetration and blockade. • This is known as sacral sparing.

8. Epidural Anesthesia Advantages • Easy to perform (though it takes a bit more practice than spinal anesthesia) • Reliable form of anesthesia • Provides excellent operating conditions • The ability to administer additional local anesthetics increasing duration • The ability to use the epidural catheter for postoperative analgesia

9. Epidural Anesthesia Advantages • Return of gastrointestinal function generally occurs faster than with general anesthesia • Patent airway • Fewer pulmonary complications compared to general anesthesia • Decreased incidence of deep vein thrombosis and pulmonary emboli formation compared to general anesthesia

10. Epidural Anesthesia Disadvantages • Risk of block failure. The rate of failure is slightly higher than with a spinal anesthetic. Always be prepared to induce general anesthesia if block failure occurs. • Onset is slower than with spinal anesthesia. May not be a good technique if the surgeon is impatient or there is little time to properly perform the procedure.

11. Epidural Anesthesia Disadvantages • Normal alteration in the patient’s blood pressure and potentially heart rate (generally slower onset with less alteration in blood pressure and heart rate than with a spinal anesthetic). It is essential to place the epidural block in the operating room/preoperative area with monitoring of an ECG, blood pressure, and pulse oximetry. Resuscitation medications/equipment should be available. • Risk of complications as outlined in Introduction to Neuraxial Blockade chapter. There is an increase in the complication rate compared to spinal anesthesia.

12. Epidural Anesthesia Disadvantages • Continuous epidural catheters should not be used on the ward if the patient’s vital signs are NOT closely monitored. • Risk for infection, resulting in serious complications.

13. Absolute Contraindications Epidural • Patient refusal • Infection at the site of injection • Coagulopathy • Severe hypovolemia • Increased Intracranial pressure • Severe Aortic Stenosis • Severe Mitral Stenosis • Ischemic Hypertrophic Sub-aortic Stenosis

14. Relative Contraindications • Sepsis • Uncooperative patients • Pre-existing neuro deficits/neurological deficits • Demylenating lesions • Stenotic valuvular heart lesions (mild to moderate Aortic Stenosis/Ischemic Hypertrophic Sub-aortic Stenosis) • Severe spinal deformities

15. Controversial • Prior back surgery • Inability to communicate with the patient • Complicated surgeries that may involved prolonged periods of time to perform, major blood loss, maneuvers that may complicate respiration

16. Mechanism/Site of Action • Administered at a physiologic distance when compared to spinal anesthesia. The intended targets are the spinal nerves and associated nerve roots. • Several barriers to the spread of local anesthetic to the intended site of action results in the requirement of larger volumes of local anesthetic when compared to spinal anesthesia.

17. Barriers • Dura mater between the epidural space and spinal nerve and nerve roots act as a modest barrier. • The majority of the solutions is absorbed systemically through the venous rich epidural space. • Epidural fatty tissue acts as a reservoir. • The remainder reaches the spinal nerve and nerve roots.

18. Spread of Local Anesthetic in the Epidural Space • Local anesthetic injected into the epidural space moves in a horizontal and longitudinal manner. • Theoretically the longitudinal spread could reach the foramen magnum and sacral foramina if enough volume was injected.

19. Spread of Local Anesthetics- Longitudinal

20. Spread of Local Anesthetics- Horizontal • Horizontally the local anesthetic spreads through the intervertebral foramina to the dural cuff. • Local anesthetics spread through the dural cuff via the arachnoid villa and into the CSF. • Blockade occurs at the mixed spinal nerves, dorsal root ganglia, and to a small extent the spinal cord.

21. Spread of Local Anesthetics- Horizontal

22. Spread of Local Anesthetics- Local anesthetics gain access to CSF via arachnoid granules

23. Distribution, Uptake & Elimination • Takes 6-8 times the dose of a spinal anesthetic to create a comparable block.

24. This is due to: • Larger mixed nerves are found in the epidural space when compared to the subarachnoid space. • Local anesthetics must penetrate arachnoid and dura mater. • Local anesthetics are lipid soluble and will be absorbed by tissue and epidural fat. • Epidural veins absorb a significant amount of local anesthetic with blood concentrations peaking in 10-30 minutes after a bolus.

25. Distribution, Uptake & Elimination • Local anesthetics absorbed in the epidural veins will be diluted in the blood. • The pulmonary systems acts as a temporary buffer and protects other organs from the toxic effects of local anesthetics. • Distribution occurs to the vessel rich organs, muscle, and fat.

26. Distribution, Uptake & Elimination • Long acting amides will bind to alpha-1 globulins which have a high affinity to local anesthetics but become rapidly saturated. • Amides are metabolized in the liver and excreted by the kidneys. • Esters are metabolized by pseudocholinesterase so rapidly that there are rarely significant plasma levels.

27. Factors Affecting Height of Epidural Blockade • Volume of local anesthetic • Age • Height of the patient • Gravity

28. Volume • Can be variable • General rule: 1-2 ml of local anesthetic per dermatome • i.e. epidural placed at L4-L5; you want a T4 block for a C-sec. You have 4 lumbar dermatomes and 8 thoracic dermatomes. 12 dermatomes X 1-2 ml = 12-24 ml • Big range! Stresses importance of incremental dosing!

29. Volume • If you require only segmental anesthesia than the dose would be less. • Volume of local anesthetic plays a critical role in block height. • Dose of local anesthetics administered in thoracic area should be decreased by 30-50% due to decrease in compliance and volume.

30. Age • As age increases the amount of local anesthetic to achieve the same level of anesthesia decreases. A 20 year old vs 80 year old • This is due to changes in size and compliance of the epidural space

31. Height • The shorter the patient the less local anesthetic required. • A patient that is only 5’3” may require 1 ml per dermatome while someone who is 6’3” may require the full 2 ml per dermatome

32. Gravity • Position of patient does affect spread and height of local anesthetic BUT not to the point of spinal anesthesia. • i.e. lateral decubitus position will “concentrate” more local anesthetic to the dependent side will a weaker block will occur in the non-dependent area. • A sitting patient will have more local anesthetic delivered to the lower lumbar and sacral dermatomes

33. Gravity • L5-S2 sometimes will have ‘patchy’ anesthesia due to sparing. By having the patient “sitting” or in a semifowlers position one can concentrate local anesthetic to this area. • Trendelenberg or reverse trendelenberg may help spread local anesthetic cephalad or alternatively limit the spread.

34. Local Anesthetics used for Epidural Anesthesia

35. Considerations in choosing • Understanding of local anesthetic potency & duration • Surgical requirements and duration of surgery • Postoperative analgesic requirements

36. Local Anesthetics for Epidural Anesthesia • Use only preservative free solutions • Read the labels, ensure that it is preservative free or prepared for epidural/caudal anesthesia/analgesia

37. Categories according to duration of action • Short Acting: 2-chloroprocaine • Intermediate Acting: lidocaine and mepivacaine • Long Acting: bupivacaine, etidocaine, ropivacaine, levobupivacaine

38. Short Acting 2-chloroprocaine • Ester local anesthetic • Initially associated with disconcerting neurotoxicity (adhesive arachnoiditis) when administered in the intrathecal space (inadvertently) • Attributed to bisulfate concentrations

39. Short Acting 2-chloroprocaine • 1985 bisulfate content decreased • 1987 preservative free solution introduced • 1996 bisulfate free solution available • Since the change in formulation no more reports of neurotoxity. • However the other preparations may be available so you need to read labels! • Large volumes of local anesthetic injected inadvertently into the subarachnoid space may still cause neurotoxicity

40. Short Acting 2-chloroprocaine • Other problem, in the past, was patient complaints of back pain after large doses of > 25 ml of local anesthetic • Formulations contained EDTA, thought that it “leached” calcium out of the muscle and resulted in hypocalcemia. • The preservative free formulations do not appear to cause back pain after large doses have been used

41. Short Acting 2-chloroprocaine • Best suited for short procedures • Good agent for the outpatient • Available in concentrations of 2% (for procedures that do not require absolute muscle relaxation) and 3% which provides for dense muscle relaxation. • 2-chloroprocaine will interfere with the action of epidurally administered opioids

42. Short Acting 2-chloroprocaine

43. Intermediate Acting Lidocaine • Prototypical amide local anesthetic • 1.5-2% concentrations used for surgical anesthesia • Epinephrine will prolong the duration of action by 50% • Addition of fentanyl will accelerate the onset of analgesia and create a more potent/complete block

44. Intermediate Acting Lidocaine

45. Intermediate Acting Mepivacaine • Similar to lidocaine • Amide local anesthetic used in similar concentrations • Lasts about 15-30 minutes longer than lidocaine • Epinephrine will prolong the duration of action by 50%

46. Intermediate Acting Mepivacaine

47. Long Acting Bupivacaine • Long acting amide local anesthetic • 0.5-0.75% concentrations used for surgical anesthesia • 0.125-.25% used for epidural analgesia • Epinephrine will prolong duration of action but not to the extent of lidocaine, mepivacaine, and 2-chloroprocaine

48. Long Acting Bupivacaine • 0.75% concentration should not be used in OB • In 1983 the FDA came out with this recommendation • There were several cardiac arrests due to inadvertent intravascular injection in OB patients • Bupivacaine (as well as etidocaine) are more likely to impair the myocardium and conduction system with toxic doses than other local anesthetics

49. Long Acting Bupivacaine • Bupivacaine has a high degree of protein binding and lipid solubility which accumulate in the cardiac conduction system and results in the advent of refractory reentrant arrhythmias

50. Long Acting Bupivacaine