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. 2024 Sep-Oct;38(5):2739-2747.
doi: 10.1111/jvim.17174. Epub 2024 Aug 16.

Parasympathetic tone activity, heart rate, and grimace scale in conscious horses of 3 breeds before, during, and after nociceptive mechanical stimulation

Jorge Eduardo Mendoza Flores  1   2 , Angelica Terrazas  1 , Alma V Lara Sagahon  1 , Monica Aleman  3
Affiliations

Parasympathetic tone activity, heart rate, and grimace scale in conscious horses of 3 breeds before, during, and after nociceptive mechanical stimulation

Jorge Eduardo Mendoza Flores et al. J Vet Intern Med. 2024 Sep-Oct.

Abstract

Background: Parasympathetic tone activity (PTA) in response to nociceptive stimulus in conscious non-sedated horses is unknown.

Objectives: Study PTA, heart rate (HR), and horse grimace scale (HGS) at rest and during mechanical nociceptive stimulation.

Animals: Ninety healthy young adult horses (females, males): 30 each of Friesians, Quarter Horses, and Warmbloods.

Methods: Prospective control study. The study consisted of habituation to equipment (Day 1), baseline recordings (Days 2 and 3), and nociceptive testing applying mild pressure to the metacarpus (Day 4). Parasympathetic tone, HR, and HGS were recorded simultaneously on Days 2 to 4. Each study lasted 30 minutes and was done in triplicate at 3 different time points per day.

Results: Baseline PTA was not different among breeds. It decreased in Warmbloods and Quarter Horses during placement of the stimulus device without stimulation (P < .01). A significant decrease in PTA (P < .001) occurred during nociceptive stimulus (marked in Quarter Horses, intermediate in Warmbloods, and mild in Friesians). Heart rate and HGS increased significantly (P < .001) during the stimulus in all breeds but returned to baseline poststimulation. Friesians required higher pressure (P < .05) to elicit an aversive response to the stimulus.

Conclusions: Horses' PTA, HR, and HGS change in response to a mild mechanical nociceptive stimulus with Friesians showing less variation. Stress induced a decrease in PTA in Quarter Horses and Warmbloods but not in Friesians. Friesians appeared to be more tolerant to pain based on PTA, HR, and HGS findings compared with other breeds.

Keywords: autonomous; behavior; pain; vagal tone; welfare.

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Conflict of interest statement

Authors declare no conflicts of interest. We thank Drs. Alvaro Rioja and Moises Valderrama for assisting and providing the horses.

Figures

FIGURE 1
FIGURE 1
Horse instrumentation. PTA equipment to record PTAi, PTAm, respiratory cycle, and HR. NP, nociceptive pressure through a pressure bracelet wrapped around the metacarpal area.
FIGURE 2
FIGURE 2
(A) Protocol followed for each horse under study. Three horses of the same breed were studied per day at 3 different time points as shown for 30 minutes each. Habituation (Day 1), baseline (Days 2 and 3), nociceptive testing (Day 4). (B) Nociceptive testing (Day 4). Recordings at 5 (−5 minutes) and 10 (−10 minutes) minutes prestimulus, during nociceptive stimulus (minute 0), during aversive reaction, and 5 (5 minutes) and 10 (10 minutes) minutes poststimulus. HGS, horse grimace scale; HR, heart rate; PTA, parasympathetic tone activity.
FIGURE 3
FIGURE 3
(A) PTAm at baseline (average of Days 2 and 3) and during nociceptive testing (Day 4) by breed. Recordings of PTAm at prestimulus (−10 min, −5 min pre‐), during the nociceptive stimulus, and poststimulus (5 minutes, 10 minutes post‐). Note significant decrease in PTAm during nociceptive stimulus and 5 minutes poststimulus. By 10 minutes poststimulus, PTAm returns to prestimulus values for all 3 breeds. FRI, Friesian; QH, Quarter Horse; WB, Warmblood; ***significant difference between time events; †significant difference between Friesians and the other breeds. Data are shown as mean and CI. (B) PTAm and HR at baseline and during nociceptive testing for all breeds combined. Significance shown: ***significant difference (P < .001) between time events. Data are shown as mean and CI. (C) HGS at baseline and during nociceptive testing by breed. Note significant increase in HGS during the nociceptive stimulus at the time of aversive reaction and decrease to prestimulus scores at 1 and 5 minutes poststimulus in all breeds. Data shown as medians: Bold line within rectangle = median, boxplot = interquartile range, solid line rectangle = Friesians, large dashed rectangle = Warmbloods, small dashed rectangle = Quarter Horses, a = no significant difference, b = significant difference.
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