Challenges in evaluation of pain and a pre-incisional line block

Background: Our objective was to test the effectiveness of a local anesthetic line block administered before surgery in reducing postoperative pain scores in dogs undergoing ovariohysterectomy (OVHX). This study was a prospective, randomized, blinded, clinical trial involving 59 healthy female dogs. An algometric pressure-measuring device was used to determine nociceptive threshold, and compared to three subjective pain scales. Group L/B received a line block of lidocaine (4 mg/kg) and bupivacaine (1 mg/kg) subcutaneously in the area of the proposed incision and saline subcutaneously as premedication; group L/BM (positive control) received a similar block and morphine (0.5 mg/kg) subcutaneously for premedication; and group SS (negative control) received a saline line block and saline premedication. Criteria for rescue analgesia were defined before the study. Dogs were assessed prior to surgery, at extubation (time 0) and at 2, 4, 6, 8 and 24 hours post-recovery. The data were analyzed with one-way ANOVA, and a repeated measures ANOVA with one grouping factor and one repeat factor (time). P < 0.05 was considered statistically significant. Results: Pain was so subtle that there were no significant differences between treatment groups with any assessment method, and no significant difference between positive and negative controls. Conclusions: Pain in non-verbal responders is subtle, even in animals with a known painful stimulus. Pre-emptive, intraoperative and post-operative analgesia is necessary regardless of pain score within the first 24 hours. None of the pain scales evaluated were sensitive enough to determine pain in all animals in this study.