A Closer Look at Local Anesthesia

Q. What is the difference between local and general anesthesia?

A. General anesthesia uses drugs that depress the brain. Termed central nervous system depressants, these drugs induce unconsciousness. If general anesthesia is used during debridement, scaling and root planing will still elicit a nerve impulse to the brain but since the patient’s brain has been depressed by the anesthetic drugs, he or she is unable to visibly respond to the pain. So although the patient is unconscious, he or she still feels pain. We know this because vital signs are monitored during general anesthesia and when a painful procedure is started, such as drilling or surgery, the patient’s blood pressure and heart rate usually increase. Local anesthetics prevent this from happening. Local anesthetics are the only drugs in medicine that actually prevent pain from occurring because they prevent the nerve impulse from reaching the brain.

THE NECESSITY OF VASOCONSTRICTORS

Q. What is the role of the vasoconstrictor in local anesthesia?

A. Local anesthetics diffuse from outside the nerve into the nerve, blocking nerve conduction. For as long as the anesthetic stays in the nerve in a high enough concentration, anesthesia persists. Outside of the nerve are blood vessels and as blood circulates through the area, the anesthetic outside the nerve gets absorbed into the blood vessels and carried away from the injection site. When the volume of local anesthetic inside the nerve exceeds the volume surrounding the nerve, the drug begins to diffuse out of the nerve. Eventually, there is not enough anesthetic inside of the nerve to prevent the nerve impulse from reaching the brain and the patient feels pain. This process is called redistribution.

All injectable local anesthetics used in dentistry are vasodilators. When you inject a local anesthetic, the blood vessels around the nerve dilate and more blood flows into the area. Therefore the plain local anesthetic is absorbed away from the nerve relatively rapidly. Plain local anesthetics have a short duration of action and also don’t produce the same depth (profoundness) of anesthesia as drugs containing epinephrine. This also means that the amount of anesthetic within the blood at any given time is going to be higher. Higher local anesthetic blood levels lead to a greater risk of adverse effects such as overdose (toxicity). The addition of a vasoconstrictor, like epinephrine, decreases blood flow into the area by constricting arterioles. As less blood flows into the region where the anesthetic was placed, it is therefore absorbed away from the nerve more slowly. This leads to several positive effects. Most notably, the drug stays in the nerve in a higher concentration longer. Also, less bleeding occurs during the procedure. And since the drug stays in the nerve longer, the anesthetic blood levels are lower, thereby reducing the risk of overdose. When you select a local anesthetic for use on a typical patient, it should always contain a vasoconstrictor unless there is a compelling reason to use a plain drug.

DRUG SELECTION

Q. What are the criteria for selecting a local anesthetic drug?

A. There are several criteria used for selecting a local anesthetic drug. The main one is the procedure being performed. What will be the patient’s need for pain control when undergoing this procedure? Once a drug is selected, you must consider any contraindications for using that drug with this patient. Another factor to consider is the need for hemostasis. Ask yourself this question: “Am I doing a soft tissue procedure where I need to inject epinephrine directly into the site where I’m doing the procedure to minimize bleeding?”

A very important consideration in drug selection is the duration of anesthesia, which is why dental practices should have a variety of local anesthetic drugs available. There are three categories based on duration—short-acting drugs providing pulpal anesthesia lasting for about 30 minutes, intermediate-acting drugs providing pulpal anesthesia for about 1 hour, and long-acting drugs that provide pulpal anesthesia from 1 1/2 to 3 hours. Because a general dental practice handles all types of procedures, from surgery to pediatrics, a short-acting drug, at least one drug from the 1 hour category, and a long acting drug are all recommended. An endodontic or periodontic practice that focuses more on surgery usually does not need a short- acting drug.

I think for a hygienist, the inclusion of a drug with epinephrine routinely makes a lot of sense because hygienists are working on soft tissue and a drug containing epinephrine provides some hemostasis. Hygienists may occasionally use a short-acting drug but most of the time an intermediate-acting drug is used because of the nature of what hygienists do. The intermediate category includes the most commonly used drugs. The American Dental Association conducts a Survey of Dental Practices each year where volunteer dentists are asked about the characteristics of their practice. The ADA found that the average dental appointment is 1 hour. Of that 1 hour, approximately 7 minutes are spent in the reception room and 44 minutes are spent in the dental chair actually being treated. If the patient is in the chair, receiving dental treatment for 44 minutes, the short-acting category of anesthesia is not appropriate while, at the same time, a drug lasting for more than an hour is not necessary, making the intermediate drug category the logical choice.

To read Dr. Malamed’s previous interviews on local anesthesia, check out the June and September 2005 issues as well as the January 2006 issue.

From Dimensions of Dental Hygiene. April 2006;4(4): 28, 30.