Effects of Food Deprivation on Horses' Cardiac Function

Effects of Food Deprivation on Horses' Cardiac Function

Withholding a horse's feed reduced his heart rate and increased his frequency of second-degree atrioventricular block (AVB,when the heart "misses" a beat).

Photo: The Horse Staff

A group of researchers recently evaluated how food deprivation affects a horse's autonomic nervous system and found that it slows the animals' heart rates, a conclusion opposite of the team's original hypothesis.

"We were interested in assessing ... whether fasting might reduce parasympathetic tone, (and in turn increase heart rate)," relayed James Jones, PhD, DVM, a professor of surgical and radiological sciences at the University of California, Davis. "We were concerned that fasting might result in an increased heart rate and also lead to gut stasis that can predispose a horse to colic."

The horse's autonomic nervous system (ANS), which controls a number of bodily functions, includes two subsystems that control cardiac function and digestion:

  • The sympathetic nervous system (SNS), which increases heart rate while inhibiting digestion; and
  • The parasympathetic nervous system (PNS), which generally slows heart rate and enhances digestion.

At rest, healthy horses have a high resting parasympathetic tone, meaning their PNS dominates their SNS, resulting in a slower heart rate.

In their study, Jones and colleagues randomly assigned five healthy Thoroughbreds to one of two groups: normal feeding protocol (control group) or 24-hour fasting protocol. One week later, they assigned horses to the opposite group and repeated the experiment.

They fitted each horse with surcingle carrying adhesive electrocardiogram (ECG) electrodes to monitor heart activity. Researchers measured heart rate, heart-rate variability (HRV, which occurs when the time interval between heart beats varies), and atrioventricular block (AVB, partial or complete interruption of impulse transmission from the atria to the ventricles--essentially, an irregular heart beat).

The team found that heart rate was significantly lower and the frequency of second degree AVB (when the heart "misses" a beat) was significantly higher during the fasting protocol. Jones assured that, "Second degree AVB is fairly common in horses at rest, because they tend to have very high parasympathetic autonomic activity."

"We thought that fasting would be a form of mild stress for the horses," Jones concluded. "However it actually appeared to be more relaxing, at least in the sense that their indices of parasympathetic activity increased."

The study, "Changes in heart rate, heart rate variability, and atrioventricular block during withholding of food in Thoroughbreds," appeared in April in the American Journal of Veterinary Research. The abstract is available online.

About the Author

Casie Bazay, NBCAAM

Casie Bazay holds a bachelor of science degree in education from Oklahoma State University. She taught middle school for ten years, but now is a nationally certified equine acupressure practitioner and freelance writer. She has owned Quarter Horses nearly her entire life and has participated in a variety of horse events including Western and English pleasure, trail riding, and speed events. She was a competitive barrel racer for many years and hopes to pursue the sport again soon.

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