Mechanical nociceptive thresholds using four probe configurations in horses

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Processo FAPESP: 2010/08967-0

To examine the relationship between probe tip size and force readings of mechanical nociceptive thresholds (MTs) to identify appropriate probes for horses. Randomized, crossover study. Eight adult, mixed-breed horses aged 5-10 years, weighing 268-460 kg. Four probe configurations (PCs) were used in random sequence: 1.0 mm diameter (SHARP); 3.2 mm (BLUNT); spring-mounted 1.0 mm (SPRING), and 3 × 2.5 mm (3PIN). A remote-controlled unit on the horse increased force (1.2 N second(-1) ) in a pneumatic actuator on the metacarpus. Mean MT for each PC was calculated from 10 readings for each horse. Data were log-transformed for analysis using mixed-effects anova/linear regression (p < 0.05). Variability of data for each PC was assessed using the coefficient of variation (CV). Mean ± standard deviation MTs were: SHARP, 5.6 ± 2.3 N; BLUNT, 11.4 ± 3.4 N; 3PIN, 9.6 ± 4.6 N, and SPRING 6.4 ± 1.8 N. Mean MT for SHARP was significantly lower than for BLUNT (p < 0.001) and 3PIN (p < 0.001), but not different from SPRING (p > 0.05). Mean MT was significantly higher for BLUNT than for 3PIN (p < 0.05) and SPRING (p < 0.001). Mean MT for 3PIN was significantly higher than for SPRING (p < 0.001). Larger contact area PCs produced higher MTs than smaller PCs, but the relationship was not linear. BLUNT (area: 10-fold greater) gave a MT two-fold higher than SHARP. 3PIN (area: 20-fold greater) produced more variable MTs, less than two-fold higher than SHARP. SPRING was similar to SHARP. CVs were: SHARP, 22.9%; BLUNT, 72.3%; 3PIN, 44.2%, and SPRING, 28.7%. The PC has nonlinear effects on MT. Therefore, it is important to define PC when measuring MT. Smaller probe tips may be preferable as MT data are less variable.