Many variables influence the degree of pain experienced with local anesthetic infiltration. Slow infiltration of the anesthetic (30 s per mL) with a 27- or 30-gauge needle will decrease the pain compared to rapid injection with a larger needle, probably because of less rapid distension of local tissue. Numerous clinical studies have demonstrated attenuation of pain on infiltration with buffered lidocaine. It is prepared by mixing 9 mL of 1 percent lidocaine (with or without epinephrine) with 1 mL of 8.4 percent (1 meq/mL) sodium bicarbonate solution. Bicarbonate can cause precipitation of bupivacaine and should not be used with this anesthetic unless it can be used immediately. It is prepared by mixing 29 mL bupivacaine (0.25 percent) with 1 mL (1 meq/mL) NaHCO3 solution.
The exact mechanisms by which buffering of lidocaine reduces pain is not clear. Buffering to a higher pH may reduce direct tissue irritation by reducing the acidity of the agent. Additionally, buffering increases the concentration of lidocaine in the uncharged form, which theoretically may allow more rapid tissue dispersion resulting in more rapid sensory nerve blockade. Because buffered lidocaine undergoes biodegradation at room temperature, its shelf life is limited. One study suggests 7 days, while another suggests that efficacy is maintained up to 30 days. Each ED must compare the cost and utilization of premixed solution with single dose preparation.
Studies evaluating the efficacy of warm lidocaine (anywhere from 37 to 42°C) to decrease injection pain have yielded equivocal results. It is postulated that heating the anesthetic causes faster diffusion into tissues and reduces or avoids stimulation of cold receptors, increasing the rate of onset of neuronal block. Lidocaine can be warmed either in dry heat (blanket warmers) or in temperature-regulated water baths at 37°C. From a practical standpoint, anesthetics should at least be administered at room temperature. Unlike buffered lidocaine, heated lidocaine undergoes no chemical denaturation, placing no limitations on shelf life.
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