It has been thought that the more local anesthetic containing a vasoconstrictor (i.e., epinephrine) that is administered, the faster and more profound the local anesthetic effects will be for the patient. However, this methodology for administering an anesthetic with only a vasoconstrictor can actually delay the uptake of anesthetic in patients.16
Epinephrine is acidic and therefore lowers the pH of the injection site. This lowered pH will enable fewer amounts of base to be available for nerve sheath penetration. Also, although epinephrine acts on both alpha receptors (vasoconstriction) and beta receptors (vasodilatation, bronchodilation, and increased heart rate and contraction), its effect on beta receptors is equal to its effect on alpha receptors.17 Therefore, the amount of vasoconstriction that will actually occur in the injection site will be affected.
When giving a regional block, either an IANB or SANB, the dentist should administer 1 carpule of an anesthetic with a vasoconstrictor and then wait a few minutes to observe whether the patient reports any subjective signs (“feeling numb” or “feeling my lip or cheek is fat”). The first injection with an anesthetic with vasoconstrictor will help maintain the anesthetic in the region of the block.
If the patient states that he or she feels the anesthetic is taking effect, the clinician did not miss the anatomical block and should proceed with an anesthetic with no vasoconstrictor. If the patient does not report “feeling numb” or any other subjective equivalent, it is a sign that either the anatomical block was missed or there was not enough concentration of base “onboard” to penetrate the nerve sheath. Either way, the second carpule of anesthesia should consist of 3% mepivacaine with no epinephrine. The subsequent injections with 3% mepivacaine will not further lower the surrounding pH and therefore enhance the amount of base to be available to penetrate the nerve sheath. After administering the second carpule of anesthetic, if the patient begins to feel numb, it was due to anesthetic concentration; if not, the clinician must reevaluate the anatomical placement of the anesthetic.
Since the introduction of articaine into the US market, there have been studies that have demonstrated there is no significant difference between 4% articaine with 1:100,000 epinephrine and 2% lidocaine with 1:100,000 epinephrine in IANB anesthesia.18,19 It is important to note that studies have reported a higher incidence of paresthesia when articaine has been administered for IANB. Although the exact etiology of the paresthesia is unknown, it is hypothesized that the neurotoxicity may be due to the higher concentration of local anesthesia used: 4% articaine as compared with 2% lidocaine.20,21 The clinician must consider these risks along with the benefits of administering 4% articaine for IANB anesthesia.