Stability of buffered lidocaine in glass vials.

At the author’s institution, the use of buffered lidocaine before painful treatment procedures has increased dramatically over the past 4 years, and more than 4500 vials are now used each year. Most vials are used by the peripherally inserted central catheter (PICC) team for insertion of central lines, and the rest are used to aid in insertion of peripheral lines. The benefits of buffered lidocaine as a skin infiltrate during painful procedures relate to the rapid onset of action after injection and the longer duration of sensory blockade. The addition of sodium bicarbonate to commercial 1% lidocaine hydrochloride (at a 1:10 ratio) results in a solution containing about 50% of the local anesthetic as the free base. Solutions prepared this way have a final pH of about 8.0 (pKa 7.9). This change to the free base increases the rate of penetration of lidocaine into the nerve cell, which in turn significantly decreases the burning sensation at the time of infiltration and speeds up the onset of anesthesia. Although buffered lidocaine has been used in the clinical setting for several years, its long-term physical and chemical stability remains unclear. Two studies examined the chemical stability of buffered lidocaine with and without epinephrine. As a result of the work by Stewart and others, the author’s hospital assigned an arbitrary expiry period of 7 days to solutions of buffered lidocaine 1%; however, this was extrapolated from data generated for 2% lidocaine solutions. More recently, Pascuet and others clarified the stability of 1% and 2% buffered lidocaine with and without epinephrine packaged in plastic syringes. The period of stability, with refrigerated storage, was 7 days for solutions containing epinephrine and 28 days for epinephrinefree solutions. The purpose of the study reported here was to determine the physical compatibility and chemical stability of 1% buffered lidocaine solution packaged in glass vials and stored at room temperature with exposure to light or at 5°C with protection from light. A stock solution of 1% buffered lidocaine was prepared by adding 10 mL of sodium bicarbonate 8.4% (Hospira Healthcare Corporation, Saint-Laurent, Quebec; lot 54-202-EV, expiry June 2009) to 100 mL of 1% lidocaine solution (Astra-Zeneca Canada Inc, Mississauga, Ontario; lot 9924619-1, expiry April 2010). The solution was filtered through a 0.2-μm filter, and 3-mL aliquots were transferred into thirty-six 5-mL glass vials (APP Pharmaceuticals, Schaumburg, Illinois; lot 404079, expiry June 2011). On day zero, 6 vials were collected and frozen at –70°C for later analysis. The rest of the vials were divided into 2 groups: half were stored at 5°C with protection from light and the other half were stored at 23°C with exposure to light. On days 7, 14, 28, 56, and 91, 3 vials were removed from each storage condition and frozen at –70°C for later analysis. On the day of analysis, all vials were allowed to thaw for 2 h. The samples were further diluted, and an internal standard was added. Each sample was then analyzed in duplicate by a previously validated stability-indicating high-performance liquid chromatography (HPLC) method. The pH of each solution was measured with a calibrated pH meter, and colour and clarity were determined by the unaided eye against white and black backgrounds. The concentration of all samples remained above 90% for a total of 91 days under both storage conditions (Table 1). The pH rose slightly over the study period (from a mean of 7.89 to a mean of 8.01). All solutions remained clear and colourless for the duration of the study. On the basis of the information presented here, solutions of 1% buffered lidocaine packaged in glass vials may be stored at 23°C with exposure to light or at 5°C with protection from light for up to 91 days.