The exhilaration of a gallop through a field might make your heart race, but you’re not alone: Your horse’s heart is working hard to power every stride he takes. Along with his respiratory system, a horse’s cardiovascular system serves as the engine he needs to perform everything from day-to-day activities to high-level athletic pursuits. But both systems have limits, and it’s important to understand them.

Here, Anna M. Firshman, BVSc, PhD, Dipl. ACVIM, ACVSMR, an associate clinical professor at the University of Minnesota College of Veterinary Medicine, reviews how the horse’s cardiovascular and respiratory systems function during exercise.

But first, here are a few terms and concepts you should be familiar with:

• Heart rate (or HR) The number of times the heart beats each minute. A resting horse’s HR is generally around 30 to 40 beats per minute, or BPM, Firshman said; during exercise horses’ heart rates can increase to anywhere from 150 to more than 250 BPM, depending on the exercise intensity. Veterinarians and owners can measure HR by feeling the horse’s pulse, using a stethoscope, or via electronic means, the latter being the most practical and reliable when evaluating horses during and after exercise.

   

• Maximal HR (HR_max) The highest rate at which a particular horse’s heart can beat. Firshman said this can range from 210 to 280 BPM.

   

• Stroke volume (or SV) The amount of blood pumped during each heartbeat. At rest, an average 1,000-pound horse will have a SV of about 900 milliliters (ml), Firshman said. During exercise SV can increase roughly 33% to about 1,200 ml, she said.

   

• Cardiac output (or CO) The amount of blood the heart pumps each minute. Firshman said an average 1,000-pound horse pumps 30 to 45 liters per minute (l/min) at rest, while an exercising horse will pump up about 240 l/min.

   

• Respiratory rate The number of breaths a horse takes each minute. At rest, most horses’ respiratory rates are between 12 and 20 breaths per minute, while at exercise that number can increase to as high as 180 breaths per minute, Firshman said.

   

• Tidal volume (TV) The amount of air inhaled and exhaled with each breath. Firshman said the average 1,000-pound horse has a tidal volume of 4 to 7 liters at rest, which can increase to about 10 liters during exercise.

   

• Minute volume (MV) The amount of air that passes in and out of the lungs each minute. An average 1,000-pound horse at rest has a minute volume of about 100 l/min, while an exercising horse can have upwards of 1,500 liters pass through its lungs each minute, Firshman said.

   

•  VO_2max The maximal volume of oxygen a horse can use each minute. Most horses’ VO_2max is around 200 milliliters per kilogram body weight per minute.

Using all the aforementioned measures, the horse’s cardiovascular and respiratory systems “work in a coordinated way to transport oxygen and energy products (such as glucose and fatty acids) to the muscle fibers, where they are used for aerobic energy production, and to remove waste products (such as lactate, carbon dioxide, and water),” Firshman said.

The Cardiovascular System During Exercise

So how do the horse’s heart and related structures react when he starts to exercise? First—and sometimes even beforehand—his HR will increase.

“HR increases in anticipation of exercise,” Firshman said. “The more excitable the horse is, the more it will show an anticipatory rise in HR.”

Once the horse starts exercising, his HR increases rapidly—often to a higher rate than needed for the work being performed—before settling to a slightly lower plateau two to three minutes after work begins. Both values—the amount the horse “overshoots” the HR and the working HR—will vary depending on how fit the horse is and the work’s intensity, Firshman said. So, a fitter horse’s HR will tend to rise less than an unfit horse’s HR at the beginning of work, and a horse working intensely right off the bat will tend to have a larger HR spike than a horse eased into exercise.

“In horses working at a steady speed on flat ground, there is a liner relationship between speed and HR at speeds in the range of 350 to 700 meters per minute (or, 13 to 26 miles per hour), which are roughly equivalent to an HR of 140 to 200 BPM,” Firshman explained.

But as a horse continues to exercise and his HR approaches HR_max, the rate at which his HR increases will slow until it plateaus: “Once a horse reaches its HR_max, a further increase in speed is still possible, but it does not elevate the HR anymore,” Firshman said.

She cautioned that HR_max isn’t something trainers should seek out during workouts, noting, “Maximal heart rates should not be used as a major part of conditioning programs; rather, they should be considered a danger zone where fatigue may occur quickly.”

Once a horse stops exercising, his HR will decrease rapidly and eventually return to resting levels.

“Generally the fitter the horse, the faster HR returns to normal after a standard amount of exercise,” Firshman said.

Although the amount of work a horse can perform at a certain HR can increase with conditioning, a horse’s HR_max will never change, she said. Conditioning can also help improve the blood supply to the muscles and, hence, the horses’ ability to consume oxygen, she said.

The Respiratory System During Exercise

Horses’ respiratory systems are also designed to help them exercise; however, many veterinarians and researchers believe the horse’s respiratory tract is the “weak link” in horses’ oxygen supply—and thus performance—pathway, Firshman said.

Horses are obligate nasal breathers, meaning they must breathe through their noses. Unfortunately, their nostrils, along with their larynx (voice box), are the two narrowest parts of the respiratory tract and the most likely to deliver resistance to incoming air. Firshman said horses will flare their nostrils and dilate their larynx during exercise to try reduce this resistance.

At the walk and trot horses can select their respiratory rate based on their oxygen needs. However, once they pick up a canter or gallop, locomotor-respiratory coupling determines their respiratory rate—this means they’ll take a breath each time they take a stride.

“Expiration occurs as the forelimbs hit the ground, and inspiration occurs as the forelimbs push off the ground,” Firshman explained. This seems to be the most efficient way for a horse to breath during hard exercise.

Once the horse stops exercising, his respiratory rate slows.

“Typically the horse takes a few deep breaths, and then the respiratory rate settles in the range of 60 to 100 BPM with the horse breathing deeply until the oxygen debt is repaid,” Firshman said.

If a horse is taking quick, shallow breaths post-exercise (or panting), he could be overheated, she cautioned. At that point, she recommended evaluating the horse’s respiration rate and depth closely and taking his HR and rectal temperature to determine whether he’s overheating. If he is, take steps to cool him down or contact your veterinarian for assistance.

Similar to the cardiovascular system, conditioning can improve a few aspects of the horse’s respiratory tract. For instance, Firshman said, the muscles in the nostrils, pharynx, and larynx—all of which hold the airways open during exercise—might gain some strength with conditioning. However, she said, there is little change in the lower airway, MV, or TV based on conditioning.

Take-Home Message

The horse’s respiratory and cardiovascular systems work in concert to provide the horse with the oxygen he needs during exercise. Both systems have limitations; however, many consider the respiratory system the limiting factor in a horse’s performance ability.