The addition of epinephrine delayed the time required for lidocaine concentration in the palatal mucosa and maxillary bone to reach the peak. The concentration of lidocaine in the mucosa peaked at around 10 min after application, although when epinephrine was added, it did not peak until 40 min. If the lidocaine concentration in the mucosa is correlated with its topical anesthetic effect, the addition of epinephrine extends the duration of this effect.
Conversely, the slower rate of increase in lidocaine concentration in the mucosa may delay the onset of its effect as a topical anesthetic. However, studies have found that an adequate anesthetic effect is achieved within 1–3 min of its application to the gingival mucosa.20,21 In this study, the addition of epinephrine did not significantly affect lidocaine concentration up to 2 min after application. Therefore, we considered it unlikely that the addition of epinephrine delayed the onset of lidocaine’s topical anesthetic effect.
With the addition of epinephrine, lidocaine still reached the maxillary bone after 40 min of application. The prolongation of the time for which lidocaine was localized in the mucosa may have increased the amount that penetrated the maxillary bone. This suggests that clinicians must wait sufficiently after application of epinephrine containing topical anesthetics before the initiation of invasive procedures, such as injection needle puncture and deciduous tooth extraction.
As the oral mucosa is rich in capillaries, topical anesthetics are rapidly absorbed into the bloodstream. Although no studies have been performed on the human oral mucosa, when lidocaine is sprayed on the upper airway mucosa, the concentration in blood reportedly peaks after 5–30 min.22–25 This rapid rise of lidocaine concentration in blood is more likely to cause toxicity than a gradual rise.10,26,27 In our study, the time required to reach the peak blood lidocaine concentration was increased from 20 min to 50 min after application due to the addition of epinephrine. This indicated that epinephrine inhibits the absorption of lidocaine into the bloodstream, thus lowering the rate at which the blood concentration rises. However, the peak concentration in the blood was higher when epinephrine was added. These findings warrant further studies to investigate the effect of epinephrine on the risk of local anesthetic toxicity. It has also been reported that it may affect the synthesis of methemoglobin, which occurs as a result of the metabolism of local anesthetics.28
Campbell et al.10 and Adrian et al.26 reported that the addition of epinephrine to the topical anesthetics neither prolonged the duration of anesthesia nor inhibited their absorption. Several other studies have reported that the anesthetic action of lidocaine applied to the mucosal surface peaks after 2–5 min, that epinephrine has low tissue permeability, and its addition to topical anesthetics has no effect on its duration of action.7 As epinephrine is strongly polarized in aqueous solution, it is unable to pass through the cell membrane.29 Since the cell membrane is composed of a lipid bilayer, it is more easily penetrated by fat-soluble substances, and as epinephrine is a water-soluble hormone with a receptor on the cell membrane surface, it has low fat solubility. This is considered to be the reason for its low tissue permeability.
The reasons for the difference between these reports and our results are unknown. However, it is possible that, when applied to the surface, epinephrine has no effect on the capillaries running through the deep mucosa but constricts the vessels running immediately beneath the mucosa to prolong the localization of lidocaine. This suggests that it may not be necessary for epinephrine to permeate the deep mucosa to inhibit the absorption of topical anesthetics into the bloodstream. We also accurately measured the lidocaine concentration in tissues using radioisotopes, whereas all previous reports were of clinical studies, and it is conceivable that epinephrine’s effect may not be sufficient to affect the clinical action of anesthetics. As the addition of epinephrine delayed the increase in lidocaine concentration in the mucosa from 4 min after application, the possibility that previous studies may have measured its effect before the complete manifestation of its anesthetic action cannot be excluded.
This study has some limitations. We did not directly measure the anesthetic effect, nor did we investigate the suppression of toxicity or methemoglobinemia. Further clinical research to evaluate the anesthetic effect with epinephrine and animal studies to determine the decreasing effect for systemic toxicity of anesthetics and blood concentration of methemoglobin are required.
The results of this study demonstrated that epinephrine prolongs the localization of lidocaine applied to the oral mucosa and inhibits its absorption into the bloodstream. Therefore, further studies are required to investigate the clinical application of epinephrine-containing topical anesthetics on the oral mucosa. Use of epinephrine-containing topical anesthetics could make oral injections less painful and safer during dental procedures.