Noninvasive assessment of autonomic activity for evaluation of pain in calves, using surgical castration as a model

The role of the autonomic nervous system (ANS) in mediating eye temperature responses during painful procedures was examined in thirty 4-mo-old bull calves randomly assigned to 4 treatments: 1) sham handling control (C; n = 8), 2) surgical castration (SC; n = 6), 3) local anesthesia with sham handling (LAC; n = 8), and 4) local anesthesia with surgical castration (LASC; n = 8). Maximum eye temperature (°C), measured by infrared thermography, heart rate (HR), and heart rate variability (HRV) were recorded continuously from 25 min before to 20 min after castration. The HRV was analyzed by examining segments of 512 interbeat intervals before and after treatments and comparing the root mean square of successive differences (RMSSD), high and low frequency (HF and LF, respectively) power, and the ratio of LF and HF powers (LF:HF). Jugular blood samples were analyzed for norepinephrine and epinephrine in C and SC treatments and for cortisol during all treatments. There was an immediate increase in HR following castration in SC (+15.3 ± 2.8 beats/min) and LASC (+6.3 ± 2.4 beats/min) calves. Eye temperature increased during the 20-min observation period in SC and LASC calves (+0.47 ± 0.05°C and +0.28 ± 0.05°C, respectively), and there was a small increase in C calves (+0.10 ± 0.05°C). Following castration in SC calves, there was an increase in RMSSD (+25.8 ± 6.4) and HF power (+11.0 ± 6.5) and LF:HF decreased (−2.1 ± 0.7). Following castration in LASC, there was an increase in RMSSD (+18.1 ± 4.9) and a decrease in LF power (−10.2 ± 5.0). Cortisol increased above baseline within 15 min following treatment in both castrated groups, but was greater for SC calves (+18.4 ± 2.3 ng/mL) than for LASC calves (+11.1 ± 1.9 ng/mL). After castration, norepinephrine increased 3-fold and epinephrine increased by half in SC calves but not in C calves. There were no changes in HR, HRV, or cortisol responses to C or LAC treatments. Local anesthetic reduced, but did not eliminate, responses to surgical castration. The synchronized increase in catecholamine and HR responses immediately following SC treatment suggests the initial response was mediated by the sympathetic branch of the ANS. The subsequent changes in RMSSD, HF power, and LF:HF ratio indicated this was followed by an increase in parasympathetic activity. The use of HR, HRV, and infrared thermography measurements together provide a noninvasive means to assess ANS responses as indicators of acute pain in cattle.