Pharmacology of drug that used in epidural anesthesia

Dr.H-KayalhaAnesthesilogist

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 short-acting 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 2-chloroprocaine 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 long-acting 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 2-chloroprocaine

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.

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