Painful treatment procedures are frequently required during visits to the emergency department, particularly for younger children who need IV therapy immediately. Among adults, painful emergency procedures include placement of central IV lines. To help ease the pain associated with such procedures, lidocaine is commonly used to infiltrate the skin because of its rapid onset of action and its long duration of sensory blockade. Lidocaine acts by blocking the sodium (Na+) channels in the nerve endings during both initiation and conduction of nerve impulses, which prevents the depolarization of neurons and leads to the anesthetic effects of this drug.
Buffering the pH of the lidocaine solution with sodium bicarbonate can reduce the pain associated with infiltration and increase its duration of action. The pH of plain lidocaine is about 6.3—6.4, but the addition of sodium bicarbonate converts about 50% of the drug to the free base, at a pH of about 8.0 (pKa 7.9). This increases the rate of penetration of the anesthetic into the nerve cells, which substantially decreases the burning sensation 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. Larson and others examined the stability of buffered lidocaine combined with epinephrine and stored at room temperature (23°C) or under refrigeration (0°C to 4°C), using chromatography to measure the remaining concentrations over time. At room temperature the epinephrine concentration dropped by 27% in the first week and by 73% at 2 weeks. In contrast, the refrigerated solution remained more stable over time, and the authors concluded that buffered lidocaine with epinephrine could be stored for up to 2 weeks if refrigerated. Similar results were obtained by Stewart and others, who used liquid chromatography to examine the epinephrine concentration remaining in stored buffered lidocaine with epinephrine.
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They found that the epinephrine concentration decreased by about 25% per week with storage at room temperature. The authors of that study suggested an expiry date of 7 days for buffered lidocaine stored at room temperature. However, it is inappropriate to extrapolate this information to buffered 1% lidocaine stored in unspecified forms of packaging. Also, flaws in the methodology of the earlier study raise questions about the validity of the assay methods that were used. The purpose of the study reported here was to conduct a physical compatibility and chemical stability study for buffered 1% and 2% lidocaine, with and without epinephrine, stored in polypropylene syringes under refrigeration with protection from light.