Evaluation of pH changes in inflammation of the subcutaneous air pouch lining in the rat, induced by carrageenan, dextran and Staphylococcus aureus.

The lowering of tissue pH in inflammation is generally accepted as the important mechanism for the failure of local anaesthesia in inflamed tissues. However, studies relating to pH changes in inflammation are scant in the literature and more importantly the findings are inconsistent. The purpose of this study is therefore to re-evaluate the pH changes using more reliable models of inflammation and pH measurement techniques. Wistar rats were used in the study. The inflammatory model used was based on the air pouch granuloma technique. Carrageenan, dextran and staphylococcus aureus were used as irritants and the pH measurements were made with a miniaturised glass tissue pH electrode designed by Roche. Results showed only a small lowering of pH in inflammation, of the order of about 0.5 pH unit. It may be possible that even such a small pH reduction would affect the buffer capacity of the tissues rendering the local anaesthetics less effective, but this needs to be confirmed with further studies.