The Effect of Tissue pH Values of a Wound on Pain Behaviors in an Acute Pain Model of the Rat

Background: There is a suggestion that tissue acidosis may contribute to the induction of pain in inflammation and ischemia. High local proton concentrations have been found in painful inflammatory and ischemic conditions. We assessed tissue pH and pain behavior in a rat model for postoperative pain. Methods: An incision of skin, fascia, and muscle was performed in the right foot. Pain behaviors to von Frey, blunt probe stimulation, and resting pain were evaluated on the incised feet. Then tissue pH was measured in each of the five rats at 4 hours, 1, 2, 4, and 7 days after incision. Results: The withdrawal threshold to von Frey was decreased, the response frequency to blunt probe and pain scores were elevated from 4 hours to 1-2 days after the incision. The deep tissue pH decreased from pH 7.16 in sham to pH 6.95, 6.90, 6.89, and 6.95 at 4 h and 1, 2, and 4 days after incision respectively. Conclusions: In this study, the tissue pH of the wound decreased after surgery. However, the change of tissue pH was not always consistent with pain behaviors. Our results suggest that tissue acidosis may relate partially to the induction of pain after tissue injury.