The ability of dental professionals to perform routine endodontic procedures successfully relies primarily on the adequacy of the local anaesthetic (LA) administered. However, LAs tend to cause pain on mucosal infiltration, which adds to patient anxiety during procedures (1). More painful skin and subcutaneous infiltration has been reported with adrenaline containing lidocaine (2). The lower pH of an anaesthetic solution containing adrenaline is primarily responsible for the greater pain sensation. A weak basic amide, lidocaine being unstable at a pH of 7.9, is made in an acidic preparation to enhance its solubility and prolong its shelf life. Epinephrine is added to lidocaine to extend the duration of action of the aesthetic, decrease toxicity and achieve haemostasis. As adrenaline is stable for lengthy phases in an acidic environment, the pH of commercially available lidocaine with epinephrine (pH 3.3-5.5) is lower than that of plain lidocaine (pH 5.7-6.5). The acidity can give rise to tissue irritation, which may be felt by patients as a stinging or burning pain (3,4).

The most common method for buffering is by alkalinization of the lidocaine with sodium bicarbonate just prior to the injection. Buffering with sodium bicarbonate (NaHCO₃) 8.4% in a 10:1 or 9:1 ratio (10 or 9 parts lidocaine-epinephrine 1% containing 5 µg/ml to 1part sodium bicarbonate containing 8.4 g/l) more closely resembles the neutral pH (~7.4) in human tissues. Buffered solutions are known to cause less pain than unbuffered lidocaine (1,3-14).

The present study aimed to evaluate the pain profile of nerve blocks following the injection of the LA 2% lidocaine with adrenaline; with and without buffering using 8.4% sodium bicarbonate. The objectives of the present study were to compare the pH levels of anaesthetic solutions in buffered and non-buffered form, and to evaluate the perception of pain during LA administration, while using buffered and non-buffered forms.

The score obtained for the buffered and non-buffered LAs was compared using one-way ANOVA. The mean (± SD) pH value of the buffered solution was 6.9±0.34, while that without bicarbonate was 3.4±0.26 (Table I). 65 % of the 20 subjects felt that the non-buffered lidocaine was more painful than the buffered solution. Lidocaine with adrenaline had a very low pH; however, when buffered with sodium bicarbonate, the solution approached that of the normal body tissue pH. In addition, the adrenaline-containing solutions (approximate pH 3.5) were markedly more acidic than those without adrenaline (approximate pH 6) (Table I)

The pain scores obtained during the LA injection with and without sodium bicarbonate are presented in Table II. There was a trend towards lower pain scores when the LA contained sodium bicarbonate. The mean (± SD) pain scores (0-10) in the bicarbonate-containing groups were 1.4±0.68 compared to 3.15±1.27 in the groups not administered bicarbonate (P=0.001). The mean difference in pain perception with the buffered solution was almost one-half compared with the non-buffered solution.

Hence, lidocaine with adrenaline buffered with sodium bicarbonate yielded statistically significant differences in terms of the pH achieved and the score of pain perception when compared with the non-buffered solution.

The buffering of LAs is well recognized and established in medicine. In a systematic review by Davies (14) on the buffering of LAs, a significant reduction in the pain of the injection was found with buffering with sodium bicarbonate, while not affecting efficacy. The use of pH-adjusted LA solutions (pH 7.4) in epidural analgesia, peripheral nerve blocks and regional anaesthesia was investigated by Galindo (15) and it was concluded that basic solutions established anaesthesia of better quality. The addition of 1 ml of 8.4% sodium bicarbonate to 10 ml LA is the most common method of buffering lidocaine. The 10:1 ratio of the LA to bicarbonate ratio raises the pH of the solution to a more physiological range (16).

Following the buffering of the anaesthetic with sodium bicarbonate, the LA injections have been shown to be associated with a decreased intensity of pain, and reduction of the stinging quality of pain (1,5). Immediately after the administration, the pH of the injected solution would rapidly approach the physiological pH of the tissue. Buffering raises the concentration of the non-ionized component of the anaesthetic solution, resulting in an enhanced diffusion through the neuronal membrane resulting in a more rapid onset of action (17).

Buffering with bicarbonate leads to the production of carbon dioxide (CO₂) and water as a by-product. It was previously confirmed by Catchlove (18) that the presence of CO₂ tends to lower the interstitial fluid pH within the nerve sheath. This further enhances the ionization of the LA that has diffused into this region. Moreover, CO₂ relative to lidocaine may cause an instant form of analgesia as it rapidly diffuses through the nerve sheath and reaches the axon before the LA (18). While this initial effect may be as beneficial as gas, however, buffered anaesthetics in a glass carpule may be considered unstable. As the excessive alkalinity can cause precipitation in the solution, the buffered mixture freshly prepared should be administered immediately following preparation. Tissue damage from an unstable mixture or precipitate could also be of clinical concern (18). Nerve conduction blockade being significantly more in the presence of CO₂ was also confirmed by Condouris and Shakalis (19) in an isolated rat sciatic nerve model. Momsen et al (20) established the stability of buffered lidocaine adrenaline solution for up to 24 h following preparation.

The difference in pain scores being statistically significant confirms the use of buffered lidocaine as a means of reducing pain on injection.

In conclusion, anxiety and phobia are perhaps the most frequently encountered issues in the clinical practice of dentistry and are associated with the painful stimulus of the LA injection. Ensuring adequate anaesthesia in patients with minimum discomfort is paramount, particularly in endodontic procedures, such as pulp extirpation, enlarging the canal in vital teeth and obturation. The present study demonstrates that this a simple, inexpensive method that can easily be performed by dentists shortly before the LA injection to deliver improved patient care.