When horses are working hard, especially in hot weather, they are particularly prone to dehydration and electrolyte imbalances. The official term for one symptom of these problems is a mouthful: synchronous diaphragmatic flutter (SDF). Horse owners and veterinarians who have dealt with this condition through the years generally identify it with a simpler, more direct term: thumps.

Thumps is not a common condition. Many highly experienced veterinarians have never encountered a case, unless they have been involved in endurance races. The same is true of many horse owners.

"I have seen only three cases of thumps in my career," says Wes Schroeder, DVM, a Maple Plain, Minn., practitioner who has officiated at a number of endurance races and competitive trail rides. "All of them were at endurance races, and fortunately all three of them recovered."

The condition, when it does occur, normally appears in a horse that has not been properly conditioned for an endurance race or one which has refused to ingest sufficient quantities of water during competition on a hot day.

While no one is positive about everything that goes on within the horse's body when thumps occurs, it is known that the phrenic nerve becomes hyperexcitable, possibly due to electrolyte imbalance. When hyperexcitable, this nerve (which runs across the heart) is easily stimulated by the electrical impulses that normally travel across the heart. The phrenic nerve controls contraction of the diaphragm (which controls lung movement and breathing), and thus hyperstimulation of this nerve results in hyperstimulation of the diaphragm--which then contracts simultaneously with the heartbeat. A "thumping" horse breathes in time with its heartbeat.

"In essence," says Schroeder, "the horse's diaphragm is slamming up against its lungs." This is a clear sign that liquids and electrolytes need to be given right away. While the horse is not in danger of oxygen starvation, his electrolyte and water balance definitely needs to be restored.

The condition is somewhat analogous to hiccups--it's aggravating but not life-threatening. However, the dehydration and electrolyte imbalance that thumps indicates can be a serious problem if not treated.

Cases in Point

"One of the thumps cases I recall," Schroeder says, "involved a rider in an endurance race who reached the final checkpoint with a horse that was borderline as to whether it should be withdrawn from competition. Because the final check was close to the finish line, I told the rider he could continue if he would promise to walk the horse to the finish line. He promised, but as soon as he was out of our sight, we found out later, he moved the horse out at a gallop. The horse had thumps shortly after finishing."

I have seen only one case of thumps, and that too was at an endurance race. It happened when a group of us organized the first endurance race in the midwestern United States. We were long on enthusiasm and short on knowledge and experience.

For starters, we had only one veterinarian to officiate at the 50-mile ride. Early in the contest, one of the veterinarian's clients, who, unknown to any of us, was competing on a pregnant mare, got into trouble several miles from the key checkpoint. The mare was showing signs of fatigue, but not thumps.

The veterinarian went to her aid and we put in a desperate call to another vet, who came immediately to the rescue. In the meantime, however, some horses were not checked by a veterinarian and continued on. Apparently, one of the horses should have been withdrawn from the race.

After the race, this horse had thumps. He stood with his front legs splayed out, head down, breathing rapidly and shallowly.

Although dehydrated, the horse refused to drink. Fluids laced with electrolytes were provided intravenously. After a time, the horse, though still appearing exhausted, showed some signs of returning to good health--a brighter eye and more alert ears. Eventually he began to drink and later to eat. During his recovery, the thumps dissipated.

When the next year's competition rolled around, we had adopted strict rules and regulations that, among other things, forbade pregnant mares competing; prevented horses under the age of five from competing; and also helped the veterinarians establish strict parameters and frequent checks for monitoring the competing horses. Any horse not showing appropriate pulse and respiration recovery after resting a short time at a checkpoint would be eliminated from competition.

Today, endurance racing and competitive rides are governed by national organizations' rules. Fortunately, contestants also have become much more knowledgeable and sophisticated in training, conditioning, and feeding their horses properly.

To understand, at least to some degree, what happens when dehydration and electrolyte stress occurs, it is necessary to examine the forces at work when a horse is subjected to long periods of exercise, such as an endurance race.

It's like working with building blocks. When all of the blocks are in place, there is a sound structure. However, when one block is askew, it can put others out of whack, and the entire structure is compromised. The key building blocks are the muscles, respiratory system, cardiovascular system, and thermoregulatory system.

We'll begin with the muscles.

Building the Body

A key component of muscular metabolism is a tiny molecule called adenosine triphosphate (ATP), which is used to produce muscular activity. The horse can't store enough ATP to provide sufficient fuel for even short bursts of energetic activity. As a result, his body must manufacture this fuel at the same rate as it is used.

There are two different methods of fuel production: The process where fuel is generated in the presence of oxygen--aerobic--and that which is generated without oxygen--anaerobic.

The process of breaking down carbohydrates, fats, and protein into energy (fuel) is called oxidative phosphorylation. This is the aerobic process. The anaerobic process where glucose or glycogen is broken down to form lactic acid is known as glycolysis.

Endurance horses generally travel at speeds where energy is supplied aerobically unless they undergo additional stress, such as in climbing a steep incline. Conversely, racehorses are much more apt to depend on anaerobic glycolysis as an energy supply.

In the endurance horse, fatigue usually results from dehydration and electrolyte imbalance. However, because of the difference in work intensity and primary metabolism, the racehorse or other horses in high-intensity work, fatigue might result from multiple factors, including increased body temperature, changes in electrolyte balance, changes in several steps of muscle metabolism, increase in hydrogen ion concentration, potassium, and decreased fuel reserves. Obviously, no one really knows which factors are the most important in producing fatigue symptoms.

The second building block is the respiratory system. In order for the horse to have sufficient oxygen to fuel its exercise energy requirements, it must be capable of taking large quantities of air into its lungs.

The difference in a horse's oxygen needs varies greatly between at rest and exercise. When discussing this, there are three terms that must be understood--tidal volume, respiratory frequency, and minute volume. Tidal volume refers to the amount of air inspired or expired with each breath. Respiratory frequency simply refers to the number of breaths taken per minute. Minute volume is the product of respiratory frequency times tidal volume. It is estimated that minute volume in the horse--measured in gallons of air expired per minute--can increase from 39 gallons (150 liters) per minute at rest to 390 gallons (1,500 liters) per minute at a fast gallop.

When comparing a horse at the peak of exertion with a normal horse at rest, there is a vast difference in respiration rate. The horse at rest normally will take only eight to 16 breaths per minute, while during exercise he might take 150 breaths per minute.

Assisting the horse in its breathing, particularly at the gallop, is a coupling of stride and respiration. At the gallop, the horse is often taking a breath with each stride. The horse expels air as the front legs strike the ground and breathes in as it pushes off with the rear limbs. Thus, we can conclude that this coupling of stride and respiration might help reduce fatigue in the respiratory muscles.

Getting air into the lungs is only part of the job. Oxygen from the air must be transported quickly and efficiently to working muscles. Enter the next building block--the cardiovascular system. As energy demands increase, so does heart rate, so that oxygen-laden blood can be rushed to the muscles. The amount of blood that the heart can pump per minute is known as cardiac output, which is a combination of heart rate times stroke volume.

When a horse is at rest, it has a relatively slow heartbeat. It might be as low as 25 beats per minute and normally isn't higher than 45 or 50 beats per minute. The average equine heart rate is in the low 30s.

However, when a horse exerts himself to the maximum, his heart rate will climb rapidly and drastically, reaching a maximum heart rate of up to 250 beats per minute.

The horse is unique in that it stores extra red blood cells in its spleen. When more red blood cells are needed to help carry oxygen to the muscles, the spleen contracts and a new supply of red blood cells enters the bloodstream. Horses have the ability to store as many as half of their red blood cells in the spleen. For more information on the spleen, see "Spleen Problems" in the August 2000 issue of The Horse, online at http://www.thehorse.com/ViewArticle.aspx?ID=167

Thermoregulatory System

A horse generates a significant amount of metabolic heat when exercising. For the most part, the horse gets rid of heat through sweating. In fact, it is the only athletic mammal other than man that cools itself primarily through sweating.

Of course, there is more to it than just sweating. One of the key requirements is that there is a sufficient blood supply to convey heat from the core of the body to the blood vessels just beneath the skin, where it can be dissipated. This means that a significant volume of blood is required.

This can bring up something of a catch-22. The hotter a horse gets, the greater the blood volume required to help dissipate heat. However, when a horse sweats during heavy exercise, some of the liquid used in the cooling process is drawn from the blood, thus lowering blood volume. This can mean that as the demand for blood volume increases, the loss of fluid from the blood can decrease its volume.

If this reaches a critical stage, the horse becomes dehydrated. And, as mentioned earlier, one of the chief components found in thumps is dehydration. To stave off dehydration, the horse must ingest copious quantities of water especially when exercising so that the blood volume can remain at an appropriate level.

It is possible for water intake needs to increase by 300% during heavy, prolonged exercise on a hot day. When one considers that a horse can lose 12 liters (3.2 gallons) of moisture per hour through sweating, it becomes apparent that the horse must ingest a great deal of water to compensate.

It becomes just as readily apparent that if a horse is competing in an endurance race and doesn't drink frequently, it will become dehydrated. This sounds like a problem that can be solved quickly and simply--just provide the horse with water and an opportunity to drink.

Unfortunately, it doesn't always work that easily. It has been found that exercise can suppress drinking behavior in humans, dogs, and horses. In other words, they do not have a desire to drink, even though their body is in need. A human, gifted with cognitive thinking, can force himself or herself to drink when exercising, but the horse, minus that mental gift, might simply turn away from water when it is offered.

This is a serious problem, especially when we consider that the loss of fluid from a horse's body under some conditions can equal 7-10% of his body weight.

There is more to the dehydration problem than just loss of fluid from the blood. There also is a loss of electrolytes, such as sodium, potassium, and chloride, which are essential for fluid balance, nerve and muscle function, and acid-base balance. If the lost electrolytes are not replaced, the horse's ability to perform is impaired.

It is not difficult to determine when a horse has become dehydrated. His skin loses elasticity, his mucous membranes are dry, and his eyeballs appear sunken.

Dehydration, with the attendant loss of electrolytes, can cause the horse to exhibit signs of fatigue, weakness, trembling, pain, stiffness, and even tying-up. There often is an accompanying loss of calcium in the blood, along with an acid imbalance in the horse's body. It is at that point that thumps might occur.

Use Common Sense

Along the way we have talked about the various building blocks that are involved in allowing a horse to perform--muscles, respiratory system, cardiovascular system, and thermoregulatory system. By making these building blocks strong and resilient, we can stave off problems such as fatigue, exhaustion, dehydration, and thumps.

This means that sound, common-sense conditioning and nutritional programs are essential in preparing a horse for something as strenuous as an endurance race.

Fitness brings with it most of our required building blocks. As a horse becomes more fit, for example, stroke volume of the heart tends to increase, while heart rate at a particular speed will decrease. Thus, the fit horse is capable of sending a larger volume of oxygen-carrying blood through its body when faced with long-term exertion than an unfit horse, and can do it with less strain on the heart.

We can also have a positive effect on the muscles through an appropriate training regimen. Training can result in an increase in capillaries that can deliver more oxygen to the muscles, and at the same time, increase the number of mitochondria (energy-producing parts of cells) in the muscle so that it can make better use of the oxygen it is receiving.

A sound conditioning program is also of benefit to the respiratory system. A fit horse will be far less apt to suffer from fatigue of respiratory muscles than one which has not been properly conditioned.

Finally, we come to nutrition. The more a horse works, the more dietary energy it requires. It is analogous to fuel in a tractor--if the tractor is only idling, it burns very little fuel, but if it is pulling a plow that is digging deep furrows, it will burn a substantial amount of fuel.

Nutritional fitness means that the horse is ingesting the appropriate amounts of energy in its diet and, if it is to be competing in something like an endurance race, that it is receiving additional electrolytes. This might involve supplementing them in the horse's drinking water.

Thumps is only one aspect of fatigue, dehydration, and electrolyte imbalance in the horse. The good news is that thumps and other problems caused by fatigue and exhaustion can usually be prevented by first understanding the building blocks that enable a horse to perform, then feeding and conditioning the animal in such a way that they all fit snugly into place.

A good dose of common sense is also required. Even the fittest horse can be ridden to a state of exhaustion without proper intervals of rest, along with the ingestion of an insufficient amount of water, especially on a hot and humid day.

For more information on cooling horses, see "Cooling Hot Horses."

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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