Parasympathetic tone, cardiovascular variables, and behavioral alterations in conscious horses before and after castration and during anesthesia: A pilot study.

Overview

• This study investigated how parasympathetic tone activity (an index of vagal nervous system activity), heart rate, blood pressure, and behavior change in horses before, during, and after castration under anesthesia.
• The research aimed to understand autonomic nervous system modulation and behavioral alterations related to castration and anesthesia in conscious horses.

Background and Objectives

• Parasympathetic tone activity (PTA) reflects parasympathetic (vagal) nervous system influence on the heart and can provide insight into autonomic balance and stress response.
• In horses, the relationship between PTA, behavioral responses, and physiological parameters before, during, and after anesthesia for castration was not well known.
• This pilot study aimed to:
• Measure PTA, heart rate (HR), mean arterial pressure (MAP), and minimum alveolar concentration (MAC) of anesthetic during castration surgery.
• Evaluate PTA, HR, behavioral measures including the Horse Grimace Scale (HGS), locomotor activity (LA), and other behaviors in conscious horses before and after surgery.

Methods

• Seven healthy adult horses were included in the pilot study.
• Study periods were divided into:
• Habituation phase (days 1–3) to allow horses to acclimate.
• Pre-surgery evaluation: days 4 and 5 and one hour prior to anesthesia.
• Post-surgery follow-up: 6 hours after surgery and on days 7 and 8.
• Measurements included:
• PTAm (mean parasympathetic tone activity), heart rate (HR), MAP, MAC during anesthesia.
• Behavioral assessments in conscious horses — HGS (to measure pain or discomfort via facial expressions), locomotor activity, head movements, limb retraction frequency, appetite, and water intake.

Key Findings

• During anesthesia for castration:
• PTAm significantly increased over time (from 27.4% at 15 minutes to 55.6% at 30 minutes), indicating rising parasympathetic (vagal) tone during anesthesia.
• Mean arterial pressure (MAP) significantly increased shortly after emasculation — notably at 1 and 5 minutes post-procedure, reflecting a physiological response to the surgery.
• In conscious horses post-surgery:
• Horse Grimace Scale (HGS) scores increased, suggesting pain or discomfort.
• PTAm increased by approximately 16%, indicating ongoing parasympathetic modulation after surgery.
• There was a delay before horses raised their heads again (increased head up latency) and more frequent limb retraction, both potentially indicating discomfort or altered behavior.
• Locomotor activity decreased significantly, showing reduced movement post-surgery.
• Appetite and water consumption decreased after surgery, signs associated with stress, pain, or recovery effects.

Conclusions and Implications

• Castration under anesthesia leads to dynamic modulation of parasympathetic (vagal) tone, observed as increased PTA during surgery.
• Physiological changes such as increased blood pressure shortly after surgery are consistent with transient autonomic responses to surgical stress.
• Behavioral changes in the days following surgery—including increased pain indicators, reduced activity, and decreased feeding/drinking—reflect a stressed or altered state in horses post-castration.
• The study suggests preservation and modulation of autonomic homeostasis occur post-surgery, with continued vagal tone adjustments during recovery.
• This pilot data can help guide pain management, anesthesia protocols, and welfare assessment during and after equine castration.
• Further research with larger samples can elucidate detailed mechanisms and improve clinical outcomes for equine surgical care.