Fluids and Electrolytes: Life's Curious Brew
How would you feel if somebody told you that you are little more than just a bunch of salt water? Well, it's almost true--we and our favorite companion, the horse, are approximately 60% (by body weight) salty water. So now, with the help of a little arithmetic, we can calculate the following: A generic adult horse (named Salty) weighing 450 kilograms (990 pounds) multiplied times the 60% gives us 270 kilograms of water! Great, but what kind of volume are we talking about? The real question is, how much does water weigh? The easy way to remember this is the rhyme used by many a student : "A pint's a pound the world around." So, a quart weighs two pounds, and therefore a gallon is eight pounds, which is equal to 3.6 kilograms.
With that information, we see that there are approximately 75 gallons or 283 liters of water to a horse (total body water). Now, as sore a subject as the metric system is in America (we seem to have stalled out somewhere in the middle of adopting it), we will talk from this point on in those terms. The only thing to remember is that a liter is almost a quart (0.946 of one to be exact), and that a kilogram is about two pounds (2.2 to be exact).
The Body's Water
With that bookkeeping out of the way, it becomes more complicated. The total body water is essentially divided in two compartments--the intracellular fluid compartment and the extracellular fluid compartment.
The intracellular fluid is the fluid that exists within the cells of our body; it adds up to about two-thirds of the total body water and is, therefore, 40% of the body weight, which is 180 kilograms or 189 liters (50 gallons) for our generic 450 kilogram horse.
The extracellular fluid is that which is technically outside the cells; this is made up of the non-cellular part of the blood (plasma), spinal fluid, joint fluid, fluid inside the eye, etc., and makes up about one-third of the total body water (about 94 liters for the generic horse). About 25% of the extracellular fluid exists within the vascular system as plasma. The remaining 75% of the extracellular fluid exists surrounding the(dot)cells of the body and is called the interstitial fluid.
The best way to understand interstitial fluid is by dropping a sponge into a bucket of water--the water that is sucked up into the sponge is the interstitial fluid.
The extracellular fluid volume is slightly higher in foals when compared to adult horses. The total blood volume, including the red blood cells, is approximately 100 ml/kilogram for "hot-blooded" horses (45 liters!) to 60 ml/kilogram for draft breeds.
Water is extremely important for the maintenance of life; it is considered a nutrient and in that respect an animal with a deficiency of water in its diet would die more quickly than if it had a deficiency of any other nutrient.
The average daily water consumption of a horse is reported to be between 20-30 liters per day. There is variability among individual horses and seasonal variation, with water consumption being slightly greater in the hot summer months and decreasing in the cold winter months. I think it is very important to have an idea what your individual horses drink on a daily basis, as changes in drinking habits can be a subtle clue to illness, just as subtle changes in eating habits can alert you to a potential problem.
The Salt Of Life
So now where does the salt come into play? When we talk about the salts of the body, we are referring to the electrolytes. The definition of an electrolyte is "a substance that dissociates into ions (individually charged particles) in a solution, thereby becoming electrically conductive." This fact will become extremely important later on.
Electrolytes, when dissolved in water, generally are called solutes. The main solutes in the body are sodium, chloride, potassium, bicarbonate, calcium, magnesium, and phosphate. These solutes are as common in our own home as table salt (sodium-chloride), Lyte table salt (potassium-chloride), baking soda (sodium-bicarbonate), and the stomach medicine Tums (calcium-carbonate) .
Blood electrolytes commonly are measured in sick animals (and people) as an aid to diagnosis and a guide to treatment; the electrolytes typically measured are sodium, chloride, potassium, calcium, and bicarbonate. The electrolytes are measured in a unit of weight per liter of blood--the milliequivalent. The extracellular fluid component of blood is relatively rich in sodium and chloride and has significantly lower concentrations of potassium, calcium, and bicarbonate. Conversely, the intracellular fluid has relatively high concentrations of potassium and significantly lower concentrations of sodium and chloride.
It was mentioned before that the definition of an electrolyte included a solute that was electrically conductive when in solution. This is an important fact since the electrical activity of the body is highly dependent on normal electrolyte concentrations. The normal contraction of a muscle (including the heart) and transmission of nerve signals are dependent on the electrical activity in part created by the electrolytes. It is the rapid changing of electrolyte concentrations across the cell membrane that "depolarizes" a cell and creates electrical activity. It is not by chance that the electrolyte concentrations between the intracellular and extracellular fluid differ as they do--there are many structures imbedded in the cell membranes that act as pumps (requiring energy) that move potassium into the cell and sodium out of the cell. In addition, there are channels or "gates" also imbedded in the cell membranes that when signaled to open, allow a flood of sodium or calcium into the cell (which will subsequently be moved back out of the cell).
For example, the conduction of a signal along a nerve is mediated by the movement of sodium across the nerve. This is the basis for local anesthetic function. When local anesthetics are injected around a nerve, they block the sodium channel and thus block nerve conduction.
Another example is the collection of pace-maker cells in the heart; the beat of the heart is triggered by the rapid movement of calcium with the contraction signal being "conducted" throughout the heart by movement of sodium, and finally the actual contraction of the muscle tissue dependent on calcium movement. It is these "gradients" or differential concentrations of electrolytes that create the electrical activity of the body. These sorts of electrolyte exchanges occur in almost every cell in the body, from muscle to kidney tissue.
Fluid Balance In The Body
Speaking of the kidney, this organ plays an important role in both water and electrolyte balance. If the body is overhydrated, the kidney has the ability to remove excess water from the body, and if the body is dehydrated, it can conserve great amounts of water. In addition, the kidney can conserve or allow the elimination of the electrolytes as necessary to maintain a perfect balance via very complex physiology.
In addition to the blood electrolytes' function in the body's electrical activity, the bicarbonate plays an important role in buffering the whole system. The function of a buffer is to maintain a certain pH or level of acidity. The pH of the body's tissues and blood is very tightly regulated to the "neutral" range; a pH of approximately 7.4 is considered to be normal physiologic pH.
As something becomes more acidic, the pH goes down (the pH of stomach acid is about 2!). The opposite of acidic is alkaline or "basic." The pH of baking soda in water is high. The normal physiologic processes of the body do not work well outside of the "normal" pH range, especially if they vary toward the acidic side. The normal range for the horse is 7.35-7.44; this is a tight range, with a pH much below 7.3 having a negative impact on cell function and a pH below 6.9 generally being considered incompatible with life. Living cells do not tolerate a low pH and start to die when the pH drops.
The acid-base status is important as many disease processes, as well as exercise-produced acid by-products, cause a loss of bicarbonate from the body. If dehydration is moderate to severe, the blood vessels in the "periphery" of the body constrict in an effort to maintain blood pressure and the flow of blood to the "vital" organs. During these conditions, the peripheral tissues of the body can be deprived of oxygen. As a result, the cells switch to an anaerobic (without oxygen) metabolism to survive, but in the process produce lactic acid. This also is what happens during intense exercise and explains the difference between aerobic (with oxygen) and anaerobic exercise.
If the level of exercise is low, then the oxygen demand by the cells (mainly muscle) is still below the supply being delivered by the red blood cells--this is considered aerobic exercise. Conversely, if the level of exercise is intense and the cellular oxygen demand exceeds the supply, the cells switch to the less energy-efficient anaerobic metabolism and produce lactic acid as a by-product.
There are some disease processes that cause "endotoxemia" (the presence of endotoxin in the blood stream). Endotoxin is a part of certain bacteria, and the negative effects of endotoxemia can be very significant on the peripheral blood flow and also can lead to lactic acid production. The build-up of lactic acid in the blood stream is referred to as lactic acidosis.
There is a delicate balance of pressures between the fluid inside the vascular system (intravascular) and the rest of the fluid (extravascular). Some of this pressure is created by the pumping action of the heart and the normal blood pressure balanced by this type of pressure in the tissue. Another type of pressure that also is very important is called oncotic pressure. The concept of oncotic pressure is abstract, but the essential idea is that the particles of a substance in a solution will create a force or pull on the solution; the greater number of particles, the greater the pull on the solution.
The major players or "oncotically active particles" in the extracellular fluids are sodium and the blood protein. If the concentration of blood sodium (or more importantly protein) falls, there is less of a "pull" on the water component of the blood. The decrease in oncotic pressure in the blood vessels relative to the surrounding tissue will allow the movement of fluid from the intravascular compartment across the blood vessel wall and collect in the tissue causing edema (excessive fluid in the interstitial tissue). This is one common cause of edema.
The first aspect of fluid therapy is determining the degree of dehydration. This generally is done as a subjective clinical assessment. The general classification is as mild, moderate, or severe dehydration and is categorized as 5-7%, 8-10%, and greater than 10% total water loss, respectively.
With mild (5-7%) dehydration, there typically is a decrease in skin turgor or an increased amount of time the skin stays tented after pinching. The area of skin is standardized, as with competitive trail horse veterinary inspections, to the point of the shoulder. Adding to the subjectivity of this test is the fact that older animals, due to a loss of elasticity of the skin, can normally have a mild decrease in skin turgor.
With moderate (8-10%) dehydration, in addition to a decrease in skin turgor, the eyes will appear sunken, and the animal is generally depressed. As the animal progresses to severe dehydration, in addition to the aforementioned signs, the ears and legs will be cold, and the animal might be recumbent or moribund. Other clinical signs that could support moderate to severe dehydration are sticky or dry oral mucous membranes and an elevated heart rate. It must be kept in mind that there can be other causes for an elevated heart rate, such as pain.
The blood electrolytes can be evaluated directly by analysis. Given the disease process, there are typical electrolyte derangements that occur and unique clinical signs associated with certain electrolyte aberrations. These will be discussed individually with some case examples later in this article.
The method of fluid administration depends on the degree of dehydration, the disease process being treated, the limitations of work environment (in the hospital versus on the farm), and economic concerns.
For a case with mild dehydration and a disease process where large and ongoing fluid losses are not anticipated, oral supplementation can be an easy and inexpensive way to correct the dehydration. If there are low blood electrolytes, these can be given orally along with the water. The degree of dehydration multiplied times the body weight in kilograms is the volume deficit--the volume of fluid that would need to be administered in order to correct the dehydration.
So, for our generic horse at a dehydration level of 5%, it would require 22 liters (about 5.5 gallons) of fluid to correct the dehydration. If the horse would drink this, things would be much easier, but you know the old saying...
By oral administration, we mean via a nasogastric tube inserted by a veterinarian. An owner's attempting to drench a horse with large volumes of water is not recommended due to the risk of aspiration pneumonia. In addition, passage of a nasogastric tube by an untrained person can lead to the same complication. This volume of water typically is given over three doses two to three hours apart due to the size of a horse's stomach.
Oral fluid therapy can be very beneficial for the minimally dehydrated horse. These cases can include mild diarrhea, horses with respiratory disease, fever, or virtually any mild illness that causes the animal to become slightly dehydrated from lack of drinking.
For horses which are more severely dehydrated and are showing signs of shock, the intravenous route of administration is preferable. Intravenous catheters allow for the safe and continuous administration of fluids to a sick horse.
If our generic horse is 10% dehydrated, that would correspond to 45 liters of fluid necessary to correct the dehydration; if the horse had an equal loss of fluid from diarrhea, that horse might require up to 90 liters (about 23 gallons!) of fluid on the first day of therapy.
A potential complication of giving that volume of intravenous fluids through a catheter is called thrombosis--the clotting of blood in a vein. Any time there is a catheter in a vein, there is a risk of thrombosis, with very sick horses at higher risk. This can be a severe complication due to the need of high-volume fluid therapy for horses with severe diarrhea--if the main jugular veins in the neck are not usable, it is very difficult to maintain high-volume fluid therapy.
There are many types of commercial intravenous fluid preparations available containing various combinations of electrolytes. In addition, extra amounts of any of the electrolytes can be added to these preparations to allow for custom formulation to meet the needs of an individual animal. Blood plasma also is commercially available for the horse if plasma proteins need to be administered.
Now that all the foundation for understanding body fluid and electrolytes has been presented, we need to understand how this delicate balance can be disturbed.
Almost every disease process has the potential to affect fluid and electrolyte balance. The simple fact is that when a horse does not feel well, there usually is a decrease in water consumption that often leads to some degree of dehydration. In the next section of this article, we will discuses some relatively common disease processes that have the potential to affect water and electrolyte balance in the horse.
One of the more common diseases requiring fluid therapy is diarrhea. As has been pointed out in previous articles in The Horse, the causes of diarrhea in the equine species are variable, and the severity to which an individual horse suffers from an individual cause also can be quite variable. Some cases might be very transient, and if the horse remains feeling good, despite the diarrhea, and continues to drink, therapy might not be necessary. If these horses have a large volume of diarrhea, extremely large quantities of water can be consumed per day, and it is very important that the horse always has access to water. If the illness hits hard and fast, the horse might become severely dehydrated and go into shock (sometimes in a matter of hours) and require large amounts of intravenous fluids. Some of the more severe causes of diarrhea that often require intravenous fluid therapy are salmonella infection or Potomac horse fever.
The exact mechanisms by which diarrhea is produced by various infections are variable, but the net effect is the loss of large volumes of water into the gastrointestinal tract and out of the body. In addition to potentially large losses of fluid, there generally are losses of sodium, chloride, potassium, and bicarbonate in the adult horse. These electrolytes can be lost in large quantities in the diarrhea, and the blood concentrations of them often become low. Also, in severe cases of diarrhea, there can be a significant loss of protein. Remembering how protein functions in helping keep fluid inside the vascular compartment explains the development of edema in animals with low blood protein.
There are compounding factors that can lead to an even greater decrease in the blood concentration of bicarbonate; as the dehydration progresses, the potential to develop lactic acidosis becomes greater. Remember that the main function of bicarbonate in the body is to buffer acids so that the body's pH remains normal. When bicarbonate buffers an acid, it is consumed in the chemical reaction (like adding baking soda to vinegar), so high levels of lactic acid will decrease the concentration of bicarbonate. This will be compounded by the direct loss of bicarbonate in the diarrhea. If the loss and consumption of bicarbonate become great enough, the body's pH can become dangerously low.
As the horse develops acidosis, one of the early clinical signs is depression, which can further decrease water consumption and therefore worsen the dehydration which, in effect, worsens the acidosis--it can become a negative spiral.
In advanced cases of diarrhea, the blood electrolytes and protein often are measured in order to determine what type of fluid and electrolytes need to be added for correction of the disturbances. These fluids and electrolytes typically are administered intravenously until the horse is corrected and stable. Many times after the horse is stable, it can be maintained by orally consuming water that has electrolytes added to it; the horse always should have access to plain water along with water that has electrolytes added (including bicarbonate). Many horses will preferentially drink the electrolyte water.
In some horses with mild diarrhea, the oral electrolyte water approach might be all that is necessary. If your horse develops diarrhea and shows any sign of depression or fever, it should be evaluated by your veterinarian. He or she can advise you on how to prepare the oral electrolyte water if it is appropriate for the case.
Extreme sweating can be a significant cause of dehydration and electrolyte loss, typically for the competitive trail, endurance, or three-day event horses which work hard for extended periods of time. The degree of sweating can be greatly affected by environmental temperatures. Sweat obviously contains water, and the degree of dehydration can be quite high from just sweat loss. Keep in mind that in addition, sweat is very rich in many of the electrolytes. Equine sweat has a higher concentration of sodium, potassium, and chloride than blood (almost double the chloride and six times the potassium). As a consequence, excessive sweating can lead to a significant loss of electrolytes, so in addition to potentially needing fluid therapy, horses might require electrolyte supplementation--especially containing sodium chloride and potassium.
Again, the degree of therapy depends on the degree of the problem. Some horses with a mild deficit might require only oral replenishment, whereas severely dehydrated and electrolyte-depleted animals might require more aggressive intravenous therapy.
Fluid and electrolyte therapy should be supervised carefully by your veterinarian, especially if potassium supplementation is administered since it easily can be toxic if used inappropriately.
One clinical sign that might indicate a serious electrolyte or acid-base disturbance in performance is called synchronous diaphragmatic flutter, or "thumps." This is a condition in which the diaphragm has a mild flutter or contraction the same number of times per minute as the heart beats. The general cause of this goes back to the electrically conductive property of the electrolytes and how alteration of electrolyte concentration in the body affects the electrical impulses.
In this case, the brain tells the diaphragm (the large flat muscle of breathing) to contract via a large nerve called the phrenic nerve. The phrenic nerve courses through the chest cavity on its way to the diaphragm and just happens to reside on top of the heart. In certain states of dehydration and acid-base/electrolyte derangements, the electrical signal causes the heart beat to "jump" out onto the nerve and "fire" it, causing the diaphragm to contract also. "Thumps" is one clinical sign that monitoring veterinarians at events watch for carefully.
Choke or esophageal obstruction in the horse can cause electrolyte derangements. These horses cannot swallow and, therefore, cannot consume water, which can lead to dehydration. In addition, they cannot swallow their own saliva, so that is an additional loss of water in the face of not being able to consume any. The horse, unlike other species, has a relatively high concentration of chloride and low concentration of bicarbonate in the saliva. Other species with an obstruction of the esophagus usually require bicarbonate therapy, but not the horse--it needs a fluid rich in chloride. If the choke is easily relieved, this fluid often is given orally via the nasogastric tube, or, in more difficult cases, the intravenous route is used to rehydrate the animal prior to attempting to relieve the choke.
Foals with a ruptured bladder have characteristic electrolyte abnormalities in addition to varying degrees of dehydration. Urine is relatively low in sodium and chloride and relatively high in potassium compared to blood. When the bladder ruptures, the urine is released into the abdominal cavity, where the electrolytes can be absorbed into the blood and vice versa. With the concentration of sodium and chloride being higher in the blood, they move into the fluid in the abdominal cavity; and with the concentration of potassium being much higher in the urine, it moves into the blood.
The blood typically is low in sodium and chloride and high in potassium. This exchange of fluid creates a problem, given the capability of potassium to be toxic at high concentrations. High blood potassium can greatly slow the heart and even stop it if the concentrations become great enough. These foals, in addition to needing fluids, are in need of sodium chloride, but definitely not potassium.
Another problem that affects water balance is a pituitary tumor. These tumors are slow-growing and typically occur in older horses. One classic sign of this problem in horses is excessive urination and drinking. The tumor secretes hormones that effectively increase urine production, and the increased water consumption is mostly to keep up with the water loss in the urine.
This is one reason to have an idea of how much water your horse normally drinks.
These horses usually keep up with their water needs as long as you provide them with the water. I've had some older horses with this disease that have consumed as much as four buckets of water a day. If they did not have access to the water, they would become dehydrated.
There is a plethora of electrolyte supplements on the market. Generally if a horse is eating a balanced diet and is not experiencing extreme electrolyte losses (heavy sweating or disease), the benefit of these supplements is questionable. If you expect heavy electrolyte losses, a supplement can be beneficial, but it is important to remember that the electrolytes are not stored in the body, so if they are administered when the electrolytes are in balance, the body most likely will view them as an excess, and the kidneys will get rid of them. It is important to give electrolytes while the loss is occurring (during long endurance rides, for example) or shortly after to replenish the loss.
It has been anecdotally reported for years that the use of an electrolyte supplement during the summer months can increase water consumption. I performed a study in 1987 that evaluated an electrolyte supplement's effect on water consumption on a group of in-training jumping horses during hot temperature conditions (Rhode Island in July). One group was fed a placebo in its grain ration and the other an electrolyte supplement based on the formulation of several commercially available supplements. After two weeks of having been monitored, the placebo group was consuming 25.9 liters of water per day and the electrolyte supplement group was consuming 26.2 liters per day; there was no significant difference between groups.
It was concluded that the feeding of an electrolyte supplement of the formulation and concentration compatible with numerous commercial products failed to increase the average daily water consumption of horses during hot weather.
Encouraging water consumption is extremely important. This often starts by making sure your horses have free access to a good, clean water source. For horses which travel extensively, it actually might be necessary to bring water from home; this can help keep them drinking on the van and get them started in a new location. Many people have had good success using various flavors of unsweetend Kool-Aid mix to mask the foreign taste/smell of a new water source.
In the winter months, there is no question that offering room-temperature water (from the house, not the cold tack room) during the cold can increase a horse's daily water intake by as much as 100%. The use of heated watering equipment in severe winter climates can be of great benefit. Keeping a horse's water intake normal during the winter months might help reduce the incidence of impaction type colic.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: Colic Surgery