Transcript Pharmacology of drug that used in epidural anesthesia

Dr.H-Kayalha
Anesthesilogist
Successful selection of drug for epidural anesthesia
requires an understanding of the local anesthetic's
potency and duration, as well as estimation of the
surgical requirements and duration and postoperative
analgesia requirements.
As with any regional anesthetic, the surgeon,
anesthesiologist, procedure, and anesthetic
technique must all be included in the drug choice
equation.
Drugs available for epidural use can be categorized as
short-, intermediate-, and long-acting local
anesthetics, and with the addition of epinephrine to
these agents, surgical anesthesia ranging from 45 to
240 minutes is possible.
Chloroprocaine, an ester local anesthetic, is a shortacting agent that was associated with neurotoxicity
(adhesive arachnoiditis) when unintentionally
injected in large volumes into the subarachnoid space
before a change in formulation.
since 1987, bisulfate-free 2-chloroprocaine has been
available; and since 1996, preservative-free 2chloroprocaine has been available. Since these
formulation changes, there have been no reports of
neurotoxicity attributable to 2-chloroprocaine until
recently.
In the era of ever-increasing numbers of surgical
outpatients, combining 2-chloroprocaine and a
catheter technique allows efficient matching of the
surgical procedure and duration of epidural analgesia
and enables patients to spend minimal recovery
time in the facility.
 2-chloroprocaine is available in 2% and 3%
concentrations, with the latter preferable for surgical
anesthesia and the former for techniques not requiring
muscle relaxation.
There is evidence that the back pain developing after
larger volumes (>25 mL) of 2-chloroprocaine was related
to the ethylenediaminetetraacetic acid used as a
preservative in the chloroprocaine.
It appears that preservative-free chloroprocaine is not
associated with back pain in larger doses at the same
frequency.
Lidocaine is the prototypical amide local anesthetic and
is used epidurally in 1.5% to 2% concentrations.
Mepivacaine is similar to lidocaine in the concentrations
necessary for epidural anesthesia and lasts 15 to 30
minutes longer.
Epinephrine prolongs the duration of surgical anesthesia
by approximately 50% with lidocaine and mepivacaine.
Another additive to these drugs that may influence
clinical use is fentanyl, an intermediate-acting amide
drug for epidural use.
When fentanyl was added to mepivacaine, it
accelerated the onset of analgesia and enhanced
the analgesic effect during epidural anesthesia.
One technique receiving more attention to minimize
the length of an epidural motor block after surgery is
completed is the use of saline in 15- to 30-mL
volumes through the epidural catheter before it is
removed.
Bupivacaine remains the most widely used longacting local anesthetic; it is used in 0.5% and 0.75%
concentrations for surgical anesthesia. Analgesic
techniques can be performed with concentrations
from 0.125% to 0.25%.
Its duration of action is less consistently prolonged by the
addition of epinephrine, although up to 240 minutes of
surgical anesthesia can be obtained when epinephrine
is added.
bupivacaine and etidocaine are more likely than other
local anesthetics to impair myocardial performance
and conduction when systemic toxicity occurs.
Ropivacaine is increasing in use as an epidural agent. It
is used in 0.5% to 1.0% concentrations for surgical
anesthesia and 0.1% to 0.3% concentrations for
analgesia.
Ropivacaine has less impact on cardiac conduction
and the frequency of arrhythmias than local
anesthetics do at blood levels producing systemic
toxicity.
the clinical effect of ropivacaine is difficult to separate
from a similar effect from bupivacaine, although it
appears to produce less motor block and has a
slightly shorter duration of action than bupivacaine
does.
However, it is the only local anesthetic still in use that
has a vasoconstrictive property at clinically used
concentrations.
Levobupivacaine is also used as an epidural local
anesthetic in 0.5% to 0.75% concentrations for surgical
anesthesia, and analgesic techniques can be performed
with concentrations of 0.125% to 0.25%.
In an individual patient, the clinical anesthetic effect
from the drug is indistinguishable from that of
racemic bupivacaine.
levobupivacaine has less impact on cardiac
conduction and the frequency of arrhythmias than
local anesthetics do at blood levels producing systemic
toxicity.
Etidocaine is an infrequently used epidural anesthetic,
principally because of the perception and some data
supporting that motor block is more profound than
sensory block with the drug.
The reason may be that etidocaine is less effective than
bupivacaine in producing small-fiber blockade.
Additives
combining agents with local anesthetics to make
epidural anesthesia last longer, to improve the
quality of blockade, or to accelerate the onset of
blockade.
Epinephrine increases the duration of useful anesthesia
with all the agents, although the proportional effect is
greatest with: lidocaine, mepivacaine, and 2chloroprocaine
And lesser effect with :bupivacaine, levobupivacaine,
and etidocaine and has a limited effect with
ropivacaine.
Phenylephrine has been used in epidural anesthesia
less widely than in spinal anesthesia, perhaps because
it does not reduce peak blood levels of local anesthetic
as effectively as epinephrine does during epidural use.
Carbonation of the local anesthetic solution has
increasing the speed of onset and quality of the block
by producing more rapid intraneural diffusion and
more rapid penetration of connective tissue
surrounding the nerve trunk.
But some disadvantages may occur more rapidly
because peak blood levels of the drug are higher after
carbonation of the local anesthetic and blood pressure
decreases.
The addition of bicarbonate has increasing the pH of
the local anesthetic solution and increasing the
concentration of nonionized free base, which
theoretically increases the rate of diffusion of the drug
and the speed of onset of the block.
Clinically, the addition of 1 mEq of sodium bicarbonate
to each 10 mL of commercially prepared 1.5% lidocaine
solution produces a significantly faster onset of
anesthesia and more rapid spread of sensory block.
In addition to speeding onset of the block, there is
evidence that a more complete block may be
produced.
Another alteration of drugs for epidural use involves
combining long- and short-acting drugs,
theoretically to gain the benefits of each.
This practice does not seem necessary or prudent
because familiarity with the local anesthetics and
additives available allows a spectrum of block lengths
to be produced.
Moreover, the purported advantage of faster onset with
combinations of local anesthetics seems clinically
inconsequential.
Have a nice day