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

Soli Deo Gloria

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.

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.

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.

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

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.

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.

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

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

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.

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.

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.

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

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

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

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.

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.

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.

Spread of Local Anesthetics- Longitudinal

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.

Spread of Local Anesthetics- Horizontal

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

Distribution, Uptake & Elimination Takes 6-8 times the dose of a spinal

anesthetic to create a comparable block.

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.

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.

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.

Factors Affecting Height of Epidural Blockade

Volume of local anesthetic Age Height of the patient Gravity

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!

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.

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

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

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

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.

Local Anesthetics used for Epidural Anesthesia

Considerations in choosing

Understanding of local anesthetic potency & duration

Surgical requirements and duration of surgery

Postoperative analgesic requirements

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

Categories according to duration of action Short Acting: 2-chloroprocaine Intermediate Acting: lidocaine and

mepivacaine Long Acting: bupivacaine, etidocaine,

ropivacaine, levobupivacaine

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

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

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

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

Short Acting 2-chloroprocaine

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

Intermediate Acting Lidocaine

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%

Intermediate Acting Mepivacaine

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

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

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

Long Acting Bupivacaine

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

Long Acting Levobupivacaine

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

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

Long Acting Ropivacaine

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%

Long Acting Etidocaine

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

Epidural Additives

Epi vs phenylephrine Epi is more effective in reducing peak

blood levels Phenylephrine does not appear to

reduce the peak blood levels

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

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

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 > 6.8

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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