The air-liquid interface and the pH and PCO2 of alkalinized local anaesthetic solutions

The alkalinization of certain local anaesthetics with sodium bicarbonate hastens the onset of epidural analgesia. Increases in both the pH and PCO2 of the local anaesthetic are necessary to hasten onset. However, carbon dioxide can diffuse from local anaesthetic solutions following alkalinization with sodium bicarbonate and change both the pH and PCO2 of the mixture. This study examined changes in pH and PCO2 of three local anaesthetics reported to have a faster onset of analgesia following mixture with sodium bicarbonate and examined the effects of time and the local anaesthetic container air/liquid interface on the pH and PCO2 of the buffered local anaesthetic solutions. Bupivacaine 0.5%, lidocaine 2%, and chloroprocaine 2% were each buffered with sodium bicarbonate. The pH and PCO2 of each solution were measured at time 0 and at 5, 10, 15, 20, 30, 40, 50 and 60 min intervals. The solutions were placed in containers as follows: 30 ml in 40 ml containers, 10 ml in 40 ml containers, 10 ml in 13 ml containers, and 10 ml in polypropylene syringes. The pH and PCO2 increased following alkalinization but gradually decreased in all containers except in polypropylene syringes.