Is His Heart In It?

Do you know why a Thoroughbred can deliver that great burst of speed to eat up a mile in 1 1/2 minutes, or how an endurance horse can maintain the strength and stamina to cover long distances at steady speeds? Of course you do: Conditioning. Conditioning builds sturdy bones, develops strong muscles, and maximizes efficiency in the cardiovascular system. Although all these components are needed to deliver peak performance, like a souped-up engine that drives the sports car, it's the "tuned-up" heart that drives the athlete, and makes everything else happen. That's because the amount of oxygen required by muscular tissues is extremely high during exercise compared to rest. It's the heart's job to increase delivery of blood during exercise in order to meet those high oxygen demands.

What this mighty organ can do is amazing. When a horse is at rest, the heart pumps about 10.6 gallons (40 liters) of blood per minute; during maximal exercise, a conditioned heart can pump 79 gallons (300 liters) a minute. (Compare that to the comparatively meek human heart, which only manages to move around 1.0-1.3 gallons, or four to five liters, per minute at rest.) During exercise, the equine heart can increase from around 35 beats per minute up to a whopping 240 beats per minute! And yet, this organ only constitutes about 1% of the horse's overall body weight, coming in at 3.3-8.8 pounds (1.5-4 kg).

Anatomy of a Heart

The heart's purpose is to act as a mechanical pump--squeezing blood out of the heart to the lungs, where it takes up oxygen, and from there to the body to provide nutrients and oxygen, says Daniel F. Hogan, DVM, Dipl. ACVIM, an assistant professor of cardiology in the Department of Veterinary Clinical Sciences at Purdue University and a researcher in equine cardiovascular disease.

As in all mammals, the equine heart has four chambers--two pumping chambers (left and right ventricles) and two receiving chambers (left and right atria)--and four valves that prevent blood from flowing backward. Explains Janice M. Bright, DVM, Dipl. ACVIM, professor of cardiology at Colorado State University, "The right side of the heart receives the blood returning from the body tissues and pumps this de-oxygenated blood through the lungs, where the blood picks up oxygen and eliminates carbon dioxide. The left side of the heart receives oxygenated blood from the lungs and pumps it to the body tissues. The right and left sides of the heart comprise a continuous circuit. Through its rhythmic contractions, the heart acts like a double suction and pressure pump to move the blood through the blood vessels, thereby delivering oxygen and nutrients to the tissues and removing wastes from the tissues.

"Because horses are highly adapted aerobic athletes (they have evolved to run at great speeds), the amount of oxygen required by the muscular tissues is extremely high during exercise compared to rest," continues Bright, who has a special interest in studying the cardiac mechanisms that allow adequate filling of the left ventricle during heavy exercise in horses. "The equine heart must be able to increase delivery of blood during exercise in order to meet the high oxygen demands and to dissipate heat. In other words, the amount of blood pumped by the heart, which is referred to as cardiac output, must increase quickly and substantially in exercising horses. The ability to substantially increase cardiac output during exercise is fundamental to the athletic performance of the horse. This is especially true in events lasting more than one minute, which necessitate the use of oxygen by the exercising skeletal muscle."

The heart has two quick, primary responses to exercise: A rise in blood volume pumped and dilation of the blood vessels. "The heart rate increases and the heart beats stronger, both of which allow more blood to be pumped out of the heart," Hogan explains. "The blood vessels to the exercising muscles dilate, allowing more blood to go to these muscles."

The stroke volume is the volume of blood ejected by the heart during each heartbeat. "In horses, stroke volume may increase by 20-50% during exercise," says Bright. "Heart rate recovery (return toward normal pre-exercise values) is usually very rapid in the first minute after cessation of exercise. Heart rate then decreases more gradually toward normal resting values with rates less than 60 beats/minute by 30 minutes post-exercise. Recovery heart rates may fluctuate, however, due to excitement."

Different Strokes

Cardiac response varies somewhat depending on the intensity of exercise. "The major difference is the heart rate," says Hogan. "The heart rate increases more dramatically with short, high-output bursts, as in one-mile racing, than with prolonged exercise, such as endurance racing."

Adds Bright, "There is a direct, linear relationship between heart rate and intensity of exercise. The heart rate increases in proportion to work speed up to a certain maximal velocity. The horse is then exercising at a maximal or peak heart rate, and the speed at which he is running is referred to as the velocity of maximal heart rate (VHRmax).

"In exercise requiring a short, demanding, full-out burst, the heart rate usually increases quickly and continuously to maximal heart rate," Bright says. "In horses doing sustained, submaximal exercise, an 'overshoot' in heart rate at the commencement of exercise is often seen. In other words, at the onset of exercise the heart rate initially increases to levels above the submaximal steady-state heart rate. Shortly thereafter, however, the heart rate decreases to a lower, steady state for that intensity of exercise. Also, during prolonged strenuous submaximal exercise at a constant work effort, as with endurance racing, a gradual rise in heart rate (called cardiovascular drift) may occur with an absence of acceleration. For example, after 30 minutes of sustained, constant-speed trotting, heart rate may rise from 150 to 170 beats per minute without an increase in cardiac output. This 'drift' is inconsistent, however, and probably reflects hydration status and the environmental conditions."

Conditioning

Conditioning improves cardiac performance--an important aspect for athletic horses. "One of the most accurate measurable parameters that determines a horse's athletic performance is termed the maximal oxygen consumption," says Hogan. Maximal oxygen consumption, which can increase as much as 35-40 times in exercise than at rest, occurs with exercise by the following mechanisms:

  • Increased efficiency of gas exchange within the lung;
  • Increased cardiac output;
  • Increased oxygen-carrying capacity of the blood related to spleen contraction, which ejects stored blood into the circulatory system during exercise;
  • Improved delivery of oxygen to the exercising tissues; and
  • Improved utilization of oxygen by the exercising tissues.

However, Bright notes that while exercise training in people produces predictable cardiac changes (reduced resting and submaximal exercise heart rates, increased cardiac mass and left ventricular chamber size, and improved filling and contractile responses), the effects of training on the equine heart are less clearly understood. "Variability in results of these equine studies is probably due to differences in durations and intensities of training," she says. "Trained horses have slightly higher relative heart weights compared to untrained horses. However, this increase in cardiac muscle mass occurs early in the training period."

Bright says most studies show that a favorable effect of training on cardiac output during exercise results primarily from increased stroke volume during exercise. "There is also general agreement that heart rate in resting horses does not decrease as a result of training, as in human athletes. Furthermore, the maximal heart rate is not affected by training. However, some studies have shown that heart rate in horses is lower during submaximal exercise as a result of training. It is likely--but not yet proven--that, as in people, exercise training has a favorable effect on the heart's ability to fill during exercise. The improvement in filling as a result of exercise training would most likely be from a training-induced improvement (increase) in the rate at which the relaxation of the heart muscle can occur. If so, training may enhance performance by providing improved filling of the pumping chambers at high heart rates, translating ultimately to increased stroke volume and cardiac output."

The rate of heart rate recovery after maximal exercise does not appear to vary as a result of training, Bright says. "But," she says, "the analysis of fitness using post-exercise heart rates may be limited because of the rapid cardiac deceleration at the cessation of exercise and by the effect of excitement on rate."


FURTHER READING

Geor, R. Is Your Horse Fit for the Task? The Horse, April 2002, 91-96. Article Quick Find #3406 at www.TheHorse.com.

Geor, R. Getting Your Horse in Shape. The Horse, February 2002, 71-76. Article Quick Find #3263.


DID YOU KNOW? Heart Facts

  • The lowest relative heart weight (heart weight to body weight ratio) is only 0.6% and applies to heavy draft horses with massive body structure.
  • The relative heart weight ranges from 0.62% to 0.99% in the lighter breeds with the highest value being 1.04% in Thoroughbreds.
  • The absolute weight of the heart in non-draft breeds of horses ranges from approximately 3.3-8.8 pounds (1.5 to 4 kg) in healthy adults.
  • One Thoroughbred's heart, which has been kept since 1927 at the Institute of Veterinary Anatomy of the University of Giessen in Germany, weighs 19.6 pounds (8.9 kg) empty of blood!
  • A healthy, adult horse's heart pumps approximately 10.6 gallons (40 liters) of blood per minute at rest.
  • During maximal exercise in a conditioned horse, cardiac output might reach 79 gallons (300 liters) per minute, an increase of six- to eight-fold.
  • The increase in cardiac output during exercise in a conditioned horse is two to three times greater than the increase in cardiac output achieved by an elite human athlete.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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