Transcript Epidural Anesthesia: Factors Affecting Height and Local Anesthetic

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EPIDURAL ANESTHESIA: FACTORS
AFFECTING HEIGHT AND LOCAL
ANESTHETIC USED
Developing Countries Regional Anesthesia Lecture Series
Lecture 11
Daniel D. Moos CRNA, Ed.D.
U.S.A
[email protected]
Disclaimer
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Spread of Local Anesthetics- Local anesthetics
gain access to CSF via arachnoid granules
Distribution, Uptake & Elimination
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Takes 6-8 times the dose of a spinal anesthetic to
create a comparable block.
This is due to:
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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
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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
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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
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Volume of local anesthetic
Age
Height of the patient
Gravity
Volume
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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
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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
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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
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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
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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
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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
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Understanding of local anesthetic potency &
duration
Surgical requirements and duration of surgery
Postoperative analgesic requirements
Local Anesthetics for Epidural
Anesthesia
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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
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Short Acting: 2-chloroprocaine
Intermediate Acting: lidocaine and mepivacaine
Long Acting: bupivacaine, etidocaine, ropivacaine,
levobupivacaine
Short Acting 2-chloroprocaine
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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
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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
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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
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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
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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
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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
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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 2chloroprocaine
Long Acting Bupivacaine
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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
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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
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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
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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
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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
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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
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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
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Epi vs phenylephrine
Epi is more effective in reducing peak blood levels
Phenylephrine does not appear to reduce the peak
blood levels
Epidural Additives
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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
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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
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Generally 1 meq of bicarbonate is added to 10 ml
of local anesthetic (i.e. lidocaine, mepivacaine, 2chloroprocaine)
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
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Mixing long acting and short acting local anesthetics
may not have much advantage for epidural
anesthesia
Many choices for local anesthetics and additives
References
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Brown, D.L. (2005). Spinal, epidural, and caudal anesthesia. In R.D. Miller Miller’s 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.
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Reese CA. Clinical Techniques of Regional Anesthesia: Spinal and Epidural Blocks. 3rd edition. AANA Publishing, 2007.
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Visser L. Epidural Anaesthesia. Update in Anaesthesia. Issue 13, Article 11. 2001.
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Warren, D.T. & Liu, S.S. (2008). Neuraxial Anesthesia. In D.E. Longnecker et al (eds) Anesthesiology. New York: McGraw-Hill
Medical.