Pain Management Injections: Guidelines



The application of any injectable substance can lead to
allergic, idiosyncratic, or adverse side effects. Previous
suspicious or unfavorable responses may be verified
through prior hospital or office records. In some cases,
a small amount of the substance in question may be
injected subcutaneously to test the patient's reaction to
exposure.

Safe and effective use of local or regional anesthesia
requires thorough knowledge of the pharmacology of
local anesthetics (LAs). Local infiltration for neural
blockade can be accomplished by using dilute
concentrations of LAs, as they rapidly penetrate the
various tissues around targeted nerve endings. When
large-diameter nerves are targeted, the quantity of drug
reaching the central axonal core is reduced because of
incomplete penetration of surrounding epineurium,
perineurium, endoneurium, fat, blood vessels, and
lymphatics, which can constitute as much as 40% of the
peripheral nerve diameter.

Some of the injected substance is absorbed by local
blood during its diffusion, which acts as another
important mechanism for reducing the amount of drug
that actually reaches the nerve axon. Higher
concentration of LAs may cause local vasomotor
paralysis, which increases local blood flow and enhances
systemic absorption. Blood flow through injected tissues
can be reduced by using an LA solution mixed with
epinephrine, which decreases systemic absorption and
improves penetration of the anesthetic to its target.
Therefore, the vascularity of various tissues should be
considered when deciding on the LA concentration and
amount of injectate. Absorption into the bloodstream not
only reduces potency of the injected material at the
target site, but also increases systemic side effects. Low
concentrations of LAs typically are used to block
smaller, lightly myelinated and unmyelinated nerve
fibers, such as C, A-delta, and B-preganglionic
sympathetic fibers.

Choosing A Local Anesthetic

Several clinical characteristics should be considered
when choosing an LA. The latency of onset of anesthetic
action is an important clinical property; however,
concentration, total dose, distance between the injection
site and target, and relative penetrance of the
compound also should be considered. Penetrance
depends on target-tissue characteristics, including the
thickness of superimposed, fibrous, and other
intervening tissues. Tissue penetrance of specific LAs
determines latency of onset and intensity of induced
anesthesia. Duration of LA action depends on
pharmacodynamic properties of the anesthetic,
concentration, total dose, and vascularity of the region
under scrutiny. LA toxicity relates to all of these factors
and also to biotransformation.

All LAs have the same basic chemical structure with an
aromatic and amino end joined by an intermediate
chain. The amino esters use an ester link between the
aromatic and intermediate chain. These drugs include
cocaine, procaine, 2-chloroprocaine, and tetracaine.
Cocaine was the first anesthetic used clinically and
continues to be used for topical airway anesthesia
because it is unique among LAs in also being a
vasoconstrictor. The amino amides contain an amide
link between the aromatic and intermediate chain.
These medications include lidocaine, mepivacaine,
prilocaine, oral pivacaine, bupivacaine, and etidocaine.



Popular Local Anesthetics

Lidocaine is a widely used LA because of its rapid onset,
potency, and tissue penetration. Within this group
bupivacaine is also a popular and frequently used LA for
peripheral nerve block and epidural or spinal
anesthesia. Commercially available concentrations of
this drug range from 0.125-0.75%. Altering the
concentration of bupivacaine can elicit a separate
sensory or motor neural blockade, ie, lower
concentrations primarily induce a sensory block,
whereas higher concentrations cause motor blockade.
Bupivacaine alters myocardial conduction more
dramatically than lidocaine; therefore, the need for
cardiorespiratory monitoring during the use of LAs
should be emphasized.

Several agents are used to prolong or modify the action
of LAs. As already discussed, epinephrine causes
vasoconstriction, which reduces vascular and systemic
absorption of the drug from the intended site of action,
lowers the risk of systemic toxicity, and enhances LA
efficacy on the target tissue. Epinephrine is the agent
most often combined with LAs, which have a short to
moderate duration of action. Epinephrine is
contraindicated in some patients because of side effects
or drug sensitivity or when a compromise of blood flow
should be avoided (ie, when used in distal portions of
the extremities, especially with coexisting peripheral
vascular disease). Phenylephrine and norepinephrine
(NE) also have been used as vasoconstrictors for spinal
anesthesia; however, they do not appear to provide any
significant advantage over the more commonly used
epinephrine.