Epidural Anesthesia: Factors Affecting Height and Local Anesthetic Used

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Epidural Anesthesia Factors Affecting Height and
Local Anesthetic Used
Soli Deo Gloria

• Developing Countries Regional Anesthesia Lecture
  Series
• Daniel D. Moos CRNA, Ed.D. U.S.A
  moosd_at_charter.net

Lecture 11
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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.

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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.

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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.

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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

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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.

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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.

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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

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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

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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.

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Epidural Anesthesia Disadvantages

• Normal alteration in the patients 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.

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Epidural Anesthesia Disadvantages

• Continuous epidural catheters should not be used
  on the ward if the patients vital signs are NOT
  closely monitored.
• Risk for infection, resulting in serious
  complications.

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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

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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

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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

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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.

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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.

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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.

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Spread of Local Anesthetics- Longitudinal
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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.

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Spread of Local Anesthetics- Horizontal
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• Spread of Local Anesthetics- Local anesthetics
  gain access to CSF via arachnoid granules

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Distribution, Uptake Elimination

• Takes 6-8 times the dose of a spinal anesthetic
  to create a comparable block.

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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.

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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.

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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.

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Factors Affecting Height of Epidural Blockade

• Volume of local anesthetic
• Age
• Height of the patient
• Gravity

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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!

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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.

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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

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Height

• The shorter the patient the less local anesthetic
  required.
• A patient that is only 53 may require 1 ml per
  dermatome while someone who is 63 may require
  the full 2 ml per dermatome

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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

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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.

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Local Anesthetics used for Epidural Anesthesia
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Considerations in choosing

• Understanding of local anesthetic potency
  duration
• Surgical requirements and duration of surgery
• Postoperative analgesic requirements

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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

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Categories according to duration of action

• Short Acting 2-chloroprocaine
• Intermediate Acting lidocaine and mepivacaine
• Long Acting bupivacaine, etidocaine,
  ropivacaine, levobupivacaine

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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

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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

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Short Acting 2-chloroprocaine

• Other problem, in the past, was patient
  complaints of back pain after large doses of gt 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

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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

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Short Acting 2-chloroprocaine
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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

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Intermediate Acting Lidocaine
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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

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Intermediate Acting Mepivacaine
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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

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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

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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

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Long Acting Bupivacaine
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Long Acting Levobupivacaine

• S isomer of bupivacaine
• Used in the same concentrations
• Clinically acts just like bupivacaine with the
  exception that it is less cardiac toxic

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Long Acting Levobupivacaine
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Long Acting Ropivacaine

• Long acting amide local anesthetic
• Mepivacaine analogue
• Used in concentrations of 0.5-1 for surgical
  anesthetic
• Used in concentrations of 0.1-0.3 for analgesia
• Ropivacaine is unique among local anesthetics
  since it exhibits a vasoconstrictive effect at
  clinically relevant doses

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Long Acting Ropivacaine

• Similar to bupivacaine in onset, duration, and
  quality of anesthesia
• Key differences include in doses for analgesia
  there is excellent sensory blockade with low
  motor blockade and it is less cardiotoxic than
  bupivacaine

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Long Acting Ropivacaine
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Long Acting Etidocaine

• Long acting amide local anesthetic
• Not used clinically very often due to the
  profound motor blockade it induces
• When used for surgical anesthesia it is used in a
  concentration of 1

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Long Acting Etidocaine
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Epidural Additives

• Epinephrine will increase the duration of action
  of all epidurally administered local anesthetics.
• There is a large variability among local
  anesthetics as to the degree of increase
• The greatest effect is found with lidocaine,
  mepivacaine, 2-chloroprocaine.
• Lesser effects found with bupivacaine,
  levobupivacaine, etidocaine
• Minimal effects have been found with ropivacaine

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Epidural Additives

• Epi vs phenylephrine
• Epi is more effective in reducing peak blood
  levels
• Phenylephrine does not appear to reduce the peak
  blood levels

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Epidural Additives

• Carbonation of local anesthetics has been touted
  to improve the quality of epidural blocks due to
  increased penetration of connective tissue and
  intraneural diffusion
• Studies are ambivalent
• Carbonation may not improve quality or onset may
  lead to increased blood levels of local
  anesthetic result in a higher incidence of
  hypotension when compared to non carbonated local
  anesthetics

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Epidural Additives

• Sodium bicarbonate can be added to lidocaine,
  mepivacaine, and 2-chloroprocaine
• Addition will increase the amount of free base
  which increases rate of diffusion and speeds
  onset
• Studies have found that when added to 1.5
  lidocaine speeds onset of blockade and results in
  a more solid block

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Epidural Additives

• Generally 1 meq of bicarbonate is added to 10 ml
  of local anesthetic (i.e. lidocaine, mepivacaine,
  2-chloroprocaine)
• The addition of bicarbonate to bupivacaine is not
  as popular. Usually 0.1 ml of bicarbonate is
  added to 10 ml of bupivacaine
• Bupivacaine precipitates occurs at a pH gt 6.8

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Epidural Additives

• Mixing long acting and short acting local
  anesthetics may not have much advantage for
  epidural anesthesia
• Many choices for local anesthetics and additives

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References

• Brown, D.L. (2005). Spinal, epidural, and caudal
  anesthesia. In R.D. Miller Millers Anesthesia,
  6th edition. Philadelphia Elsevier Churchill
  Livingstone.
• Burkard J, Lee Olson R., Vacchiano CA. Regional
  Anesthesia. In Nurse Anesthesia 3rd edition.
  Nagelhout, JJ Zaglaniczny KL ed. Pages
  977-1030.
• Kleinman, W. Mikhail, M. (2006). Spinal,
  epidural, caudal blocks. In G.E. Morgan et al
  Clinical Anesthesiology, 4th edition. New York
  Lange Medical Books.
• Niemi, G., Breivik, H. (2002). Epinephrine
  markedly improves thoracic epidural analgesia
  produced by small-dose infusion of ropivacaine,
  fentanyl, and epinephrine after major thoracic or
  abdominal surgery a randomized, double-blind
  crossover study with and without epinephrine.
  Anesthesia and Analgesia, 94, 1598-1605.
• Reese CA. Clinical Techniques of Regional
  Anesthesia Spinal and Epidural Blocks. 3rd
  edition. AANA Publishing, 2007.
• Visser L. Epidural Anaesthesia. Update in
  Anaesthesia. Issue 13, Article 11. 2001.
• Warren, D.T. Liu, S.S. (2008). Neuraxial
  Anesthesia. In D.E. Longnecker et al (eds)
  Anesthesiology. New York McGraw-Hill Medical.