Quality Resource Guide

Local Anesthetics 5th Edition

www.metdental.com

The optimal concentration of epinephrine in local

anesthetic solutions has long been debated. A

concentration of 5 to 10 mcg/ml (or 1:200,000

to 1:100,000) provides a useful extension of the

duration of action of such commonly used amide

anesthetics as lidocaine, articaine and prilocaine

with low risk of adverse effects. The additional

benefit gained by higher concentrations of

epinephrine,

e.g.

, 20 mcg/ml (or 1:50,000) is seldom

justified. Although a vasoconstrictor only marginally

increases the duration of intrinsically long-acting

agents,

e.g.,

bupivacaine, epinephrine is still utilized

with such agents to reduce the risk of toxicity.

In addition to increasing the duration of anesthesia,

the slower rate of removal of drug from the site

of injection resulting from a vasoconstrictor may

also contribute to the safety of local anesthetics

in two ways. First, by prolonging the period of

anesthesia, a lower total dose of drug may suffice

to complete the operative procedure. Second,

slower removal from the site of injection decreases

both the rate of rise of drug concentration in

plasma as well as the peak concentration reached.

This may allow the administration of somewhat

greater total doses of anesthetic should it be

necessary in extensive or lengthy procedures. In

the interest of safety, however, the maximal doses

of local anesthetics approved by the Food and

Drug Administration with the concurrence of the

ADA (

Table 2

) are, with the exception of lidocaine,

independent of the presence of a vasoconstrictor.

Side Eﬀects and Toxicity

The use of local anesthetics is enormous, and

these drugs are among the most widely used in

the world. It is indeed fortunate that the therapeutic

importance of currently available local anesthetics

is matched by an impressive record of safety.

However, local anesthetics and the often present

vasoconstrictors can cause serious adverse effects

on the brain and cardiovascular system that may

have a fatal outcome. It is unfortunate that the

acknowledged safety of local anesthetics together

with their daily routine use may lead the unwary

practitioner to complacency and errors in drug

selection or administration with adverse outcomes.

The most common reactions encountered in the

delivery of local anesthesia occur in anxious

patients and are psychogenic in nature,

i.e.,

consequence of the administration of the drug

rather than the drug itself. Psychogenic responses

are usually manifested by nervousness and

agitation, sweating, pallor, hyperventilation or

fainting (vasodepressor syncope). It is essential

for both prevention and management that the

practitioner be able to distinguish these reactions

from those resulting from the response to the

injected drug or drugs themselves.

Localized reactions to local anesthetics at the site

of injection are uncommon and may be difficult to

distinguish from tissue injury resulting from needle

penetration.

However, prolonged neurosensory

disturbances can occur following local anesthetic

injection, suggesting a neurotoxic effect of the

local anesthetic itself.

A recent study indicated that

trigeminal neurosensory disturbances following

mandibular blocks were highly associated with

the use of articaine 4%, compared to lidocaine,

mepivacaine and prilocaine. Whether this finding

was related to the chemical nature of these agents

or a concentration effect is unknown.

Serious adverse systemic reactions to local

anesthetics are also uncommon. The majority are

due to either direct toxicity or allergic reactions.

2,3

Toxic reactions are by definition due to an overdose

of drug, and should be largely preventable.

Systemic toxicity occurs when the level of local

anesthetic in plasma exceeds a certain level.

Although all the injected dose ultimately appears in

blood, several factors determine whether the level

reached will be toxic. The most important of these

are the total drug dose injected and the rate of

uptake from the site of injection into blood.

These two parameters are inversely related; the

slow uptake of a large dose may be less dangerous

than rapid uptake of a smaller dose. Obviously,

intravascular injection of a local anesthetic solution

is equivalent to immediate transfer of the drug from

the site into blood. Since most amide anesthetics

(articaine is an exception) are largely metabolized

in the liver, toxic levels of such agents may be

reached at lower doses of drug in patients with

severely impaired liver function, and downward

adjustment of dosage should be considered.

Just as local anesthetics suppress conduction

in peripheral nerves, these agents similarly

impair function in other excitable tissues.

These

include the cardiovascular and central nervous

systems. The latter is more sensitive to the

effects of local anesthetics than the former and

will generally manifest signs and symptoms of

toxicity first. The initial symptoms are tinnitus,

blurred vision, dizziness, tongue paresthesias

and circumoral numbness.

With further rise in

drug level, agitation, excitement, nausea, tremor

and ultimately convulsions occur.

If the drug

level rises still higher, convulsions will cease,

followed by unconsciousness and respiratory

depression.

Respiratory and cardiac arrest will

occur with further increase in drug levels.It is

important to note that previously existing hypoxia

increases local anesthetic toxicity by lowering the

plasma level that causes convulsions. In a similar

dose-related fashion, these drugs also cause

peripheral vasodilation and depress conduction in

the myocardium both directly and through central

nervous system depression, and may lead to

circulatory failure and cardiac arrest.

A most important factor in reducing the risk of

local anesthetic toxicity is determination of dosage

according to body weight. Dosing guidelines based

on weight have been established by clinical trial

and experience for approved agents used in dental

anesthesia (with the exception of bupivacaine) and

should be followed. There are also recommended

maximum total doses for each agent that should not

be exceeded during a single dental treatment visit.

For example, the recommended dose of prilocaine

is 4 mg/lb body weight and the maximum total dose

is 600 mg. This means that for patients weighing

50, 100, 150 or 200 pounds, the maximum doses

would be 200, 400, 600 and 600 mg, respectively.

Dosing guidelines specific for a particular agent

should be applied to

pediatric patients as well,

unless otherwise specified by the manufacturer.

For example, the recommended dose of articaine

for patients over 12 years of age is 3.2 mg/lb.,