Thermoregulation and Electrolyte Management in the Endurance Horse

Dr. Kerry K. Ridgway

Heat Dissipation
One of the most formidable challenges an endurance horse faces is the task of ridding the body of the tremendous heat load that exercise generates. The horse, like many mammals, is not the most efficient converter of fuel-to-work. Only between 20-25% of the fuel (food, water, and oxygen consumed) is transferred into work, most of the remainder, i.e. about 80%, ends up as heat that needs to be eliminated to keep the animal from literally cooking in its own juices.

To give you some idea of the incredible heat production occurring, a horse racing at 13 meters per second (about 28 miles per hour) produces enough heat to bring eight liters of water from room temperature to a boil. That is enough heat to bring over two gallons of water to a boil after just a two minute race. Even more astounding, an endurance horse traveling at about 17 km per hour sweats approximately 12.5 liters every hour and produces enough heat in that hour to generate 30,000 BTUs of heat and bring approximately 25 gallons of water from room temperature to a boil.

Evaporation
The single most important means the horse has for getting rid of this enormous heat load is evaporation, accounting for about 65% of the heat dissipation. Sweat is evaporated off the skin surface and cools the horse. The lungs account for about another 25%. This capacity of the respiratory tract in dissipating heat from the body becomes very important under conditions of high humidity and high temperature when evaporation conditions are not favorable.

Sweating can only occur at a significant level when the horse is not dehydrated and has plenty of fluids in its body with which to produce sweat. When the horse is dehydrated, the lungs play an even more important role. Even so, the dehydrated horse is severely handicapped in ability to rid the body of any excess heat. One can readily see the important of keeping endurance horses in the best possible state of hydration and why veterinarians place so much emphasis on hydration when deciding whether a horse is fit to continue.

Other means of Cooling
Other means of cooling include conduction, e.g. the transfer of heat into the cool water put on the horse; convection, e.g. the effect of wind in increasing evaporation; and radiation, e.g. the hotter surface, such as skin, radiates and disperses the heat into the cooler air surrounding the animal.

To top it off, horses are not as effective at managing to get rid of heat by sweating as are humans. Why? Because when you calculate the ratio of human body surface area to weight, you find that the human athlete has about a three times greater ratio than the horse. It takes work, and as previously stated, oxygen consumption to move a mass (body weight). The more mass moved, the more heat produced. Surface area that is covered with sweat glands provides for fluid to be evaporated away, the more surface area, the more sweat glands and the better the cooling. With a poorer ratio of surface area to mass, the horse is disadvantaged in comparison with the human athlete.

Let's complicate this process one more step and take a look at what is happening to the circulation. The blood that the horse has available to circulate is what gets the nutrients and oxygen to the individual cells and takes away the waste products of that cellular metabolism, i.e. the wastes, toxins, and heat. It gets rid of much of the waste through the kidneys. In order to get rid of the heat, the circulation to the skin is greatly increased and brought closer to the surface. The surface veins dilate so they can bring even more of the hot blood to the surface for cooling. This is all well and good up to a point; but suddenly we have a strong and literally "heated" competition going on for who gets the blood the skin for cooling or the muscles and organs for metabolism and work required to uphold the requested level of performance.

Add dehydration to the picture and there is even less circulating fluid volume available to accomplish the demands for both metabolism and heat dispersion. At this point, the skin loses out and sweating rapidly diminishes, even though the need for heat dissipation remains critical. If work continues, the animal experiences heat exhaustion, or heat stroke and the likelihood of death.

The horse's ability, with regard to minimizing heat production, can be significantly improved by conditioning. Every litre of oxygen consumed by the horse results in the production of 5 Kcal of heat (one Kilocalorie is the amount of heat necessary to raise the temperature of one litre of water from 17C to 18C). The relationship between increasing speed and heart rate is referred to as being linear. This means that a graph showing heart rate on the vertical axis and speed on the horizontal axis will increase in a basically straight line. The relationship between oxygen consumption and speed or between heart rate and oxygen consumption is also a linear relationship. What this all means is that the conditioned horse can travel further and/or faster on a given amount of oxygen. Said a different way, a conditioned horse can travel at a given speed with less oxygen consumption and less heat produced than can an unconditioned horse. Or stated in yet another way, conditioning increases the aerobic threshold, and lowers the heart rate.

Electrolytes
Earlier I mentioned that a horse trotting at 17-18 km/h (11.2 mph) loses about 12.5 litres of sweat per hour in order to manage its heat load. This fluid isn't just water - it contains a lot of salt. These salts, when they are broken down into their chemical components, are referred to as electrolytes. There are typically groups of different salts that contain such electrolytes as sodium, potassium, chloride, magnesium and calcium. Each one of these individual components or electrolytes carries an electrical charge (when they are considered or discussed in terms of charged particles, they are referred to as ions). By means of these electrical charges, electrolytes govern the transfer of water through cell membranes into or out of the cells. Thus, they function in getting the nutrients in and the waste products out. They are responsible for getting nerves to fire and muscles to contract. Essentially all of the physiological actions in the body require electrolytes. And importantly, they need to be present in the fluids in the appropriate amounts for these biochemical reactions to proceed in an orderly manner.

If the horse sweated out these electrolytes in the same percentages in its sweat as in its circulating fluids, the body's concentrations of these electrolytes would remain in balance even though there would exist a depletion of both total fluids and electrolytes. However, this balance is not maintained. The horse's sweat is more concentrated in electrolytes than the electrolytes in its circulating fluids. This is referred to as being hypotomic, i.e. containing a lesser percentage or concentration of electrolytes than in the circulating fluids. This difference between the two species is a compensating mechanism for the poorer ratio of body surface to mass in the horse. In normal (isotonic) concentrations in the body, there is about one teaspoon of salts in every pint of circulating fluid. So when sweating is occurring at the level stated in the example, the horse loses more than one teaspoonful of salts (electrolytes) for every cup of fluid it sweats. Those fluids lost (about 12.5 litres per hour at a fast trot) translate to about three-fourths of a cup to one cup of sweat produced every minute or in essence a pint every two minutes. That in turn means the loss of over a teaspoonful of electrolytes every two minutes. Multiplication reveals this is 30 teaspoonfuls an hour. There are six teaspoons per ounce. That means there is a loss (in this example) of over five ounces (140 grams) of electrolytes every hour by the horse working at this level.

Electrolyte and Fluid Balance
Needless to say, that is an amount that we seldom, if ever, need to replace during the course of an endurance event. However, if we don't provide at least a minimum replacement, horses show up with such medical conditions as metabolic alkalosis, inefficient transport of oxygen and energy substrates, poor tissue perfusion, thumps, muscle spasms, exertional rhabdomyolysis, cardiac arrhydrosis, kidney impairment, and poor recoveries. (Actually, poor hear and respiratory recovery is one of the key signs that can lead you to recognize the problems associated with the task of accomplishing thermoregulation.) The point is, most of these problems mentioned stem from the resulting dehydration and electrolyte imbalance. It is nothing short of amazing how much loss and imbalance the horse can endure not only without dying, but often they even continue at a credible performance level.

The real question becomes, how much better could the horse do if it were in a state of ideal electrolyte and fluid balance? How many of the horses who fade in the last third, or have prolonged recoveries after the event, could be winners if their electrolytes and fluids were balanced and at appropriate levels? The problem is it is very difficult to tell an individual just how much electrolyte mixture needs to be administered. There are simply to many variables. These include how high the temperature is. How humid is it? What pace is the rider choosing to maintain? Is the horse well conditioned? What is the horse's body type, i.e. how much subcutaneous and internal fat does it carry? Is it thick or thin skinned? Is it a calm horse or very nervous and high strung? How well has the horse been drinking and eating? Is there plenty of water at appropriate intervals to bathe and cool the horse? Is dehydration already present?

Fortunately, the task sounds more formidable on paper than it proves to be in the actual case if you use what you have learned in the first part of this paper. Simply recognize that if the horse is experiencing conditions that increase its level of sweating, it is going to need more electrolytes and at more frequent levels. If you are able to take precautions that minimize sweating, then the horse will lose less fluids and less electrolytes because it won't have to sweat as much.

Environmental factors
Let me pose a series of questions to which you implicitly already know the answers. How do terrain and climatic conditions affect sweating? We all realize that if we are going to ride difficult terrain instead of easier terrain, the horse will work harder, sweat more and lose more electrolytes, simple so far! Other environmental factors that will obviously affect how sweating will occur are the environmental temperature, the humidity factor and the wind factor.

We easily understand that the horse sweats more if the temperature is high. Conditions of high humidity produce special problems. If the humidity produce special problems. If the humidity is high, the ability to dissipate body heat is made more difficult. The percent of oxygen is reduced in humid air, so the horse has less oxygen available, and therefore must increase its respiration and increase the heart rate. This means an increased work load. Evaporation becomes ineffective with riding humidity, especially if the temperature is high. The animal responds by sweating even more copiously in response to the rising body temperature and thus, proportionally more electrolytes are lost. A cooling breeze will help keep the hard working horse cooler, therefore, it won't build as much body heat and won't lose as much electrolyte containing fluid. Wind also creates convection and aids evaporation.

We have already alluded to the fact that a conditioned horse sweats less than one that lacks adequate conditioning. As an animal conditions, several additional adaptations take place. The threshold of sweating is lowered and so is the threshold of shivering. This means that the horse will start sweating sooner (even in cold weather) and also will shiver and chill more easily. Another adaptation alters the sweat composition so that it contains lower levels of salts (electrolytes). So, although the well conditioned horses start to sweat sooner, they lose less electrolytes, and will require less replacement.

Pace
Pace (previously discussed in reference to heart rate and speed) markedly affects the work load. Obviously, the faster you intend to ride your horse the greater the work load and the more heat buildup that needs to be dissipated by sweating. It is then likely that the horse who is running for the front will lose more electrolytes than a moderately paced horse trying for completion only.

Acclimatization
How does acclimatization affect heat production? Especially in the face of heat and humidity, acclimatization becomes extremely important. A very well conditioned horse who has not trained in, and has not experienced high levels of humidity, will build much more body heat than a horse who is acclimatized to such conditions. Such a horse will risk an even greater level of dehydration, electrolyte loss and electrolyte imbalance than a moderately conditioned horse who is well acclimated to heat and humidity. Periods for acclimatization to humidity have been estimated at between one to three months.

Body and muscle type
Body type, mass (weight), skin thickness and muscle type all affect heat dissipation. We now recognize, from previous discussion, that it takes energy to move mass. So the bulky, very large, or over weight animal will expend more calories to move that larger mass over a given distance. This can become a significant factor in considering the use of heavier horses in, for example, eventing/combined training. If, in addition, the animal is unconditioned, not acclimated, or worked fast over difficult terrain, it will likely require an enormous amount of fluid and electrolyte replacement. More, perhaps, than can be managed! No wonder veterinarians cast a jaundiced eye on letting such animals start an endurance ride or leave the ten minute box of a three-day event.

What about skin thickness or horses with more subcutaneous (under the skin) fat? this type of horse, likely, genetically originated in regions where it was very cold and survival dictated adaptations to preserve body heat. We are using horses that have just the opposite need, i.e. to easily dissipate body heat. Thicker skinned horses are not as able to dissipate heat by the method of radiation so will tend to sweat more and thus lose more electrolyte containing fluid. Lastly, in regard to body type, we need to consider muscle fiber composition. Slow twitch fibers are capable of thousands of repetitive contractions, are more easily nerve stimulated, and consume less fuel then the more powerful and more easily fatigued fast twitch and explosive fast twitch type IIb fibres. Thus, the classic bull dog type horse will produce more heat over long distances at speed than a more flat, sloping muscled type of horse.

There also exists the matter of individual variation. Some animals are highly nervous and sweat significantly more than the normal horse. It is not known whether the electrolyte composition of normal sweat versus nervous sweat is different. It is also recognized that horses with tendencies toward exertional rhabdomyolysis (tying up syndrome) or that exhibit subclinical tying up have higher need for certain electrolytes. Subclinical means that the signs are so subtle and mild that one does not realize the disease entity is present. (The only signs of subclinical tying up may be excessive sweating!) Likewise, horses prone to Synchronous Diaphragmatic Flutter (S.D.F. known as thumps) also have greater need for certain special electrolyte supplementation.

Transportation
An area of special consideration and extreme importance is the nature of the trailering conditions. Many times in this sport we need to hall long distances, often under conditions of high temperature and humidity. We need to realize that most metal trailers act much like a Dutch oven when in direct sunlight! One only needs to step into a parked trailer on a hot day to realize that the interior temperature may be 10 - 15 degrees higher than the outside temperature. Adequate ventilation (for both heat and carbon monoxide dispersion) is imperative! Without adequate preventive measures, it is entirely possible for your horse to arrive at an event dehydrated and in electrolyte imbalance.

Electrolyte replacement
Let's consider some other factors that will govern your decisions about electrolyte replacement. We need to consider the availability of fluid and electrolyte replacements through the water the horse drinks and the food it eats during an endurance ride. We also need to see how alternative ways of dissipating body heat affects electrolyte needs. Additionally, the state of hydration also must be considered before replacing the electrolytes.

Availability of natural replacement sources: If your horse has ample time and access to natural forage or hay, the horse will pick up a significant amount of some electrolytes. Most notably, potassium will be taken in. If you are feeding alfalfa, a significant amount of calcium and other electrolytes will be consumed. Sodium and chloride may still be lacking, however, unless the horse is consuming loose salt at rest or check points. It is not likely, under ride conditions, that it will take in an adequate amount from a salt block.

Perhaps the most significant reason that we don't need to replace salt at the enormous level at which it is lost is the amount of electrolyte containing foodstuffs that are present in the "hind" gut of the horse (cecum and large bowel). This source of electrolytes may become available to the horse during the course of extended performance. Much more study needs to be done in this area.

Though water sources may contain some electrolytes, one cannot be sure of the content and the water should be considered only as a water source and not a source of significant amounts of electrolytes. However, the water one puts on a horse can be as important as the water the horse consumes, since one way to minimize electrolyte loss is to bathe or sponge the horse at every opportunity. Water removes heat by the conduction principle (transferring the heat from the skin into the cool water) as well as providing fluid to evaporate. Water has been shown to be 20 times as effective as air for removing heat from the skin surface. (One should observe the usual precautions in the use of cold water on the large muscles - especially the back, shoulders and thighs.)

Other methods of minimizing sweat/electrolyte loss involve moving air over the horse (removing heat by convection) by such means as fanning or pumping the saddle up and down etc. It follows that if heat can be frequently dissipated by alternate methods, the horse does not need to sweat as much and won't lose as much fluid and electrolytes.

A critical factor in deciding whether to use an electrolyte preparation is the degree of dehydration present. It does no good, and can do significant harm, if the animal is significantly dehydrated and electrolytes are forced into the system. If the animal is dehydrated, any electrolytes administered orally will stay in the gastrointestinal (GI) tract. They will not be absorbed, and will in fact draw fluids from the already dehydrated horse into the GI tract. This further reduces the extracellular fluid volume and circulating plasma. The horse in this state must consume or receive liquids before administering significant amounts of electrolytes.

What about the opposite side of the coin: water replacement without electrolyte replacement? For best athletic performance, the fluids in the body need to be balanced with the proper amount of electrolytes. Replacement of fluids without correcting the electrolyte deficit will not restore the horse to normal and will not bring its performance back to an optimum state. When the horse drinks well it is OK and appropriate to administer electrolytes, if the conditions we discussed earlier dictate their need. In fact, I feel it is good to start electrolytes before any significant fluid deficit occurs in order to stimulate thirst and get the horse drinking sooner during the event. In this instance, the horse may drink without necessarily having lost a significant quantity of electrolytes. Consider the horse's probable loss via sweat!

The signs associated with electrolyte loss and imbalance are difficult to separate clinically from those associated with dehydration since the loss of fluids and the loss of electrolytes go hand in hand in the horse. (Do not try to transpose human physiology to the horse in this area.) Early loss will produce few obvious clinical signs, yet as little as 3% loss affects performance level. As the loss progresses, performance decline becomes obvious, and deterioration occurs in the parameters of skin response, mucous membranes, capillary refill, gut sounds and jugular refill. Commonly in endurance competition, the first thing a rider may become aware of is a loss of brilliance to the gait, i.e. a loss of elasticity; movement becomes stiff, more jarring and less floating. This is typically followed by: dullness; lack of interest in food, water and surroundings. This horse has little interest in forward progress.

As dehydration progresses, abnormal sweating patterns began to occur. The sweat glands have begun to fatigue, and sweating becomes less and less. Sweat patterns may appear patchy since not all glands cease or slow function simultaneously. The sweat itself will change character and become sticky or gummy feeling to the touch. By this time things are serious! One should act before the horse reaches this stage.

What should we use? There are myriads of commercial electrolyte preparations being touted and sold. Though some are adequate, many are not designed to treat the type of electrolyte loss seen in submaximally exercising endurance or combined training horses. Most are geared to replacement for animals sick with infectious diseases or in surgical situations and postoperative conditions. Most commercial preparations are formulated to treat a state of acidosis, while, with the exception of racehorses, most horses with electrolyte loss are in a state of alkalosis. For the most part, commercial preparations are useable, even if not ideal. However, some containing bicarbonate or lactated products can actually help induce "thumps." Most are between relatively and downright expensive. Some are claimed to be formulated with endurance stress in mind but fail to show the concentration of electrolytes on the label. Even some that do show the concentrations, when compared with loss needs, contain less than 10% of the quantities needed for a single dose. One might have to administer six to 10 tubes to get one ounce of salt. If in doubt, check with a knowledgeable sports medicine oriented veterinarian.

According to research done by Carlson, and Tomlinson, losses can be quite nicely replaced in proper proportion by administration of a combination of regular table salt and Morton Lite® Salt. Lite Salt is a reduced sodium salt that contains potassium as replacement for a portion of the sodium. It also contains a small amount of calcium and magnesium. The salt/Lite salt combination will handle most endurance exercise type of induced electrolyte loss scenarios. However, if one desires to refine the formula, Dolomite (calcium_magnesium-carbonate) can be added. Your mixture then should be 2 parts table salt, 2 parts "Lite" salt and 1 part dolomite.

Administration: This combination can be made up in bulk and administered an ounce (28 grams) at a time in apple sauce using a large syringe. It helps to cut the tip off of the syringe and make a larger opening. Horses do not generally take this concentration of salts well when they are mixed into the horse's drinking water. Tip: 35 mm film canisters make an easy way of measuring and carrying electrolytes as they hold one ounce or approximately 30 grams.

How often should electrolytes be given? From the previous discussion it is easy to see that those factors that create loss and natural replacement need to be reviewed. Certainly you want to supplement before the horse is actually in trouble. Check the parameters that deal with fluid loss, i.e. skin pinch, mucous membranes, capillary refill, jugular distention and gut sounds. It becomes obvious that one must know what is normal, with regard to the above mentioned parameters, in order to distinguish deterioration of the parameters. If these parameters are deteriorating, replenishing fluids and electrolytes is in order. How frequently to repeat electrolytes depends on repeated assessment of the parameters.

You now have the tools to make that decision. You need only to take a few moments to calmly assess the situation! The only thing that remains is to ascertain how much should be administered at any given time. Unfortunately, in the absence of laboratory values one can only rely on good clinical judgment. A good rule of thumb administer 28 grams (one ounce) of the above mentioned electrolyte mix at any given time. Electrolytes are absorbed quite rapidly into a horse that is not dehydrated. In about 20 minutes, re-evaluate the parameters and note how the horse feels. If indicated, administer another 28 grams. Think back to the example of how much salt is lost every hour in a horse trotting at 11 miles in an hour. Expressed in this light, it is unlikely that you will overdose any endurance horse by administering 28 - 56 grams of electrolyte preparation every hour or two.

Some horses require even higher amounts of supplementation than described, and a more increased frequency of administration. This includes horses who have recurrent problems with muscle cramps or tying up, or those prone to "thumps." It also pertains to certain horses who have prolonged recoveries toward the end of rides and after most rides. In these cases, I strongly recommend that one should, under the supervision of a veterinarian knowledgeable in this field, work out a specific electrolyte program.

Proper electrolyte supplementation can and frequently does make the difference between a winner and an "also ran."

For more information contact: Kerry Ridgway, DVM Equisport Enterprises, 4120 Meadowbrook Road, Garden Valley, CA. USA 95633

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