Thermal and mechanical nociceptive threshold testing in pregnant sheep

Abstract

OBJECTIVE: Analgesic regimes were compared in pregnant ewes after laparotomy by measuring thermal (TT) and mechanical (MT) nociceptive thresholds. STUDY DESIGN: Prospective randomised experimental study. ANIMALS: Pregnant ewes at 121 days gestation underwent laparotomy as part of another research project. METHODS: Thermal and mechanical thresholds were measured before, and 2, 6, 24 and 48 hours after surgery. Thermal stimuli were delivered to the lateral aspect of the metatarsus via a skin-mounted probe, and mechanical stimuli to the contralateral site via a pneumatically driven 1.5 mm diameter pin. Each test was performed five times, alternating thermal and mechanical stimuli, with ten minutes between thermal stimuli. At the end of surgery ewes received either: 75 μg hour(-1) transdermal fentanyl patch (medial thigh) (group FP) (n = 8), or 3 μg kg(-1 ) hour(-1) intra-peritoneal medetomidine via an osmotic pump (group IPM) (n = 8) inserted immediately prior to closure. Data were analysed using the Kruskal-Wallis RS Test (p < 0.05). Once a significant effect was identified, pairwise comparisons were performed using paired Wilcoxon RS tests. To compensate for multiple hypotheses testing, p < 0.005 was considered significant. RESULTS: Prior to surgery mean ± SD TT was 56.1 ± 5.0 °C (FP) and 55.6 ± 5.0 °C (IPM); MT was 5.3 ± 2.6 N (FP) and 8.0 ± 5.0 N (IPM). In FP there was no significant change in either TT or MT over time. In IPM there was no significant change in MT over time but TT increased at two hours to 59.2 ± 3.0 °C (p = 0.003). Skin temperature (ST) ranged from 33.0 to 34.7 °C and did not change over time. There were no significant differences between groups in TT, MT or ST. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of intra-peritoneal medetomidine (3 μg kg(-1 ) hour(-1) ) by an osmotic pump increases the thermal nociceptive threshold in the immediate post operative period in pregnant sheep, suggesting that this agent may have a role in providing post-operative analgesia.